What is Thermal Mass and How can it Make you Money?

Hey, what do you know, it’s September now. It has only been six weeks since I posted the controversial Air Conditioning Article, but things have changed quite a bit since then for residents of Canada and the Northern half of the US.

Back on July 18th, we were mired in the hottest weeks of the year, when day and night temperatures were both uncomfortably hot, especially out East.. In this situation, it’s hard to take advantage of temperature swings between day and night to keep your house cool and I received many complaints to that effect in the comments ;-)

But now September is here, and the amount of daylight is shrinking at its fastest rate of the year – we lose well over 2 minutes worth per day in September where I live at 40 degrees latitude. And the nights in autumn are much cooler, even while the days are still rather hot. For example, yesterday morning it was 57F(13.9C) outside while I was eating breakfast, yet over 98F(37C) at 3:00PM when I biked out to pick up the lad from Kindergarten. Large swings like this make it very easy to cherry pick the perfect temperature for home comfort by sucking in air at the right time of day. I stored up a whole load of chilliness during the night, so I had no idea how hot it was outside at 3PM until I opened the door… all without touching the air conditioning.

But how much coolness can you store in your house? Is it just dependent on the amount of air that is contained therein? Why do some houses stay cooler than others on summer days? Why do even large tents heat up almost instantly, but even small caves or basements stay cool year-round?

The answer lies in the title of this article: Thermal Mass. We can learn about it right now,  because it’s always fun to have a science lesson, and also because understanding this simple concept will save you money and energy for the rest of your life. So check this out:

Say you’re sitting in your living room right now. It’s a fairly big room: 15 x 15 feet, with a 9 foot ceiling. Here are a few interesting facts about it:

How much air is in the room? 15 x 15 x 9 = 2025 cubic feet.
How much does that air weigh? 2025 x 0.0807 pounds per cubic foot = 163 pounds.
How much energy does it take to heat that air from 60 to 80 degrees Fahrenheit?  0.018 BTUs per cubic foot per degree x 20 degrees x 2025 cubic feet =  729 BTUs.

Uh-oh, what’s a BTU? That’s something you should know, since you’ll see that term on every air conditioner, barbecue, water heater, and furnace you’ll ever own. It stands for British Thermal Unit, and it’s the amount of heat needed to warm one pound of water by one degree Fahrenheit.  So, if you have a 16oz mug of coffee, which contains about a pound of water, and you need to raise it from a chilly room temperature (60F) to boiling temperature (212F), you’ll need 152BTU of heat.

So let’s go back to your large living room. We know it will take 729 BTU of heat to warm the air in that room up from 60 to  80 F. But we just pumped 152 into your coffee. Does that mean we could warm up the whole room just by setting about 5 cups of coffee out on the floor?

Intuition tells us that would not be enough heat. And intuition would be right. The reason it takes much more than a few Coffees worth of  heat to warm up a room is the THERMAL MASS of the actual structure of the room and everything in it.

As it turns out, that room also has 765 square feet of drywall on the walls (which weigh 1224 lbs), 225 square feet of hardwood flooring (482 pounds), and a few pieces of furniture (400 pounds). Briefly ignoring that each material stores a different amount of heat per pound, we can still see that the room itself weighs 2100 pounds, drastically more than the 160 pounds that the air weighs. So to warm up that room, you’ll need not just five cups of boiling water, but somewhere north of Fifty of them.

Now you can see why a typical house can stay comfortable for at least the first few hours of a hot autumn day, even while a tent of equal size would start to sizzle within just a few minutes of sunlight exposure.

And the scientifically inclined might be starting to get some ideas about how to use this knowledge to save energy.

In a modern 2000 square foot US suburb-style house, there is about 20,000 pounds of drywall on the walls and ceilings. Since this suburban house has mostly carpeted floors (carpet has minimal thermal mass and acts like an insulator), the drywall represents about 90 percent of the usable thermal mass of the house. A house like this has a moderately stable temperature, but when you add a few humans and some sunlight, it will still warm up to uncomfortable temperatures during the course of a day, even if it starts out at 65F in the morning.

My house had similar construction when I bought it in 2006. But over time, as the carpets have worn out, I’ve replaced them with natural wood and tile floors. Not only do these add some useful thermal mass, they also bridge the heat into the underlying subfloor and framing better than carpet, furthering the effect. I have also rebuilt several bathrooms with an extensive amount of tiles and luxurious solid poured-concrete-under-mosaic-tile shower floors. . And I’m working on making the house even heavier, eventually planning to form some thick concrete countertops when I  renovate the kitchen, and even create a solid earth, stone, or concrete wall in the South-facing room which will capture immense amounts of solar energy on winter days.

So let’s put this all into practical application: My house now has about 15,000 pounds more usable thermal mass than it did when I moved in. With a few engineering tables I can see that amount of material, in my proportion of tile, stone and wood, will suck up about 5,000 BTU of heat for each degree the temperature in the house rises.

On a cool night when I run an outward-blowing fan, I can get the house down to 65F. The temperature at which my house becomes a bit uncomfortably warm is 82F. So we have a temperature swing of 17F.

During this 17 Fahrenheit heat-up on a hot sunny day, all of those 15,000 pounds of materials are fighting the temperature increase, sucking up the heat, and keeping me cool. By the time the interior temperature DOES finally reach 82F, they have absorbed 5,000 x 17 = 85,000 BTU of heat.

As I mentioned in an earlier article, my central air conditioning system can pump 36,000 BTU of heat per hour out of the house.

The new materials I have added are sucking up an equal amount of heat to running the air conditioner for almost two and a half hours each day! If I relied on air conditioning to replace what I get for free with this nice cooling feature, I’d be burning an extra 210kWh of electricity per month, almost doubling my bill.

Note that while the thermal mass does suck up heat, you still have to get it back out each night so it can repeat the process the next day – that makes the open windows and the cooling fan even more important.

This rough engineering calculation (which I had never done in detail before writing this article) nicely backs up my anecdotal experience with these renovations: When we first moved into this house, we felt the need to turn on the A/C for a couple hours each afternoon, despite my best efforts at night cooling. Once I had eliminated all the ugly carpet and fixed up the bathrooms, the need for A/C was gone!

Of course, I didn’t spend hundreds of hours installing wood, stone, and tiles just to save electricity. The primary reason was the desire to have a nice comfortable house with stylish natural materials inside. But the practical lesson is the same – when renovating for comfort, think HEAVY. When shopping for a house, if you have the option, keep an eye out for concrete floors, or interior stone or brick walls, and South-Facing windows if you live somewhere with cold winters. And if you really want to get the Mustachian Scientist award, you can make a point of putting heavy things into your house just for the temperature stability they add. A treasure chest full of gold? A large fish tank? A Medieval stone table and chair set? I’ll let your own creativity take the reins on this one.

In the winter, the same effect happens in reverse. Before thermal mass, my house would heat up to shorts-and-tanktop temperature from the relentless horizontal sunshine on winter days, then lose heat quickly at night. Now it stays at a more constant temperature. As I add more south-facing windows, I will add more weight, focusing on heavy but environmentally cheaper materials like reused tile, locally sourced stone or even a wall filled with crushed gravel. True eco-homes have concrete floors and even giant tubes of water in front of their solar gain windows. A few hundred gallons of water can store enough heat to get through an entire winter night, even while it costs only a dollar or two when poured from a tap. And it looks quite neat.

I’m just getting started on my Energy Independent House project. I’m sure some of you are years ahead of me on this, soaking up free sunlight,selling solar power back into the grid, and watering your vegetable gardens with house drainage. But it’s a very fun hobby  for me to grow into nonetheless.



  • Kevin M September 1, 2011, 10:08 am

    Wow, this is great info explained very simply. Thanks for the work that went into this post.

    And also explains why bathroom floors are always so cold at night!

    • MMM September 1, 2011, 10:56 am

      Thanks Kevin! .. But I think the Cold Bathroom Floor Effect comes from thermal conductivity rather than thermal mass. Your bathroom floor is usually the same temperature as the rest of your house. But ceramic/porcelain conducts the heat more quickly away from your feet than wood does, and much more quickly than carpet does. That’s why carpet feels “warmer”.

      But I still hate carpet ;-)

      The ultimate solution, as practiced in efficient homes, is to have the floors actually heat the house in winter, through tubes of warm water embedded in the subfloor. So you need no stuffy vents blowing hot air – just a universal and silent slab of warm floor warming your feet and the air in the room – as well as shining radiant heat onto your skin, which allows you to run at a lower air temperature for a given level of comfort.

      • Uncephalized June 10, 2012, 10:24 am

        I hate carpet too. It’s just a disgusting dust-and-dirt-and-hair trap. Plus I much prefer the feel of a cool, smooth floor under my feet.

        Add in a large dog and two cats and carpet is my worst enemy. Our last rental was all concrete floors and that was great, but unfortunately we had very little time to move (got a short-notice job offer in a different city after 6 months of unemployment), so we had to find a place in about 2 weeks 120 miles from where we lived, and we couldn’t find anything with all hard floors this time. :-(

  • PNW September 1, 2011, 10:52 am

    What do you recommend for window fans: blowing inside air out or outside air in?

    I just bought 2 ceiling fans on closeout at Home Depot and plan to install. I find that they can make a big difference any time of the year. This will make a total of 4 ceiling fans in the house.

    • MMM September 1, 2011, 10:58 am

      Usually, blowing heat OUT works better – especially if you have a two story house, since you can use the fan to assist the natural ‘heat chimney’ effect that already happens as soon as you open both an upper or lower floor window.

      Blowing outwards allows the cool air come in evenly from all of the other open windows – so the entire house will be cooled evenly rather than mainly the area in front of the fan.

      Plus, the 100 watts of heat generated by the fan motor gets blown outside rather than back into your house.

  • rjack September 1, 2011, 1:27 pm

    MMM – Have you ever looked at geothermal heating/cooling?

    • MMM September 1, 2011, 2:27 pm

      Yeah, it is a fantastic invention! Not cost effective in my own climate, since my cooling bills are zero and heating is almost free once I pop in my remaining solar windows. But further Northeast where winter is colder and less sunny, I have a friend who saves thousands per year with geothermal.

  • Dan September 1, 2011, 1:34 pm

    You say you open all (or most) of your windows at night. Should you be concerned at all about safety / thievery? My girlfriend would never ever let me leave windows open at night, especially on the first floor with us in an upstairs bedroom.

    • MMM September 1, 2011, 2:16 pm

      You don’t need Mr. Money Mustache’s expertise to figure out the crime risk. Read your own weekly police beat report! My ‘hood has fairly low crime, and I don’t collect
      Jewelry so I don’t even think about robbery and such. If it was an issue, I would add stops on my windows so they only open 6″ wide. I wouldn’t give up on fresh air out of fear of the unknown. Be brave, son.

      • Dan September 1, 2011, 2:21 pm

        What if the criminal only has a 5.5″ inch wide head?

      • Dan September 1, 2011, 2:24 pm

        I was just kidding. Thanks for the window stops idea!

    • EarningAndLearning April 18, 2017, 12:44 pm

      My brother lived in Burbank (neighbourhood in Los Angeles) and he left his downstairs windows open and front door UNLOCKED all the time. Once I asked him, “What if someone comes in while you are away and takes your couch/TV/kitchen stuff/furniture?” He replied, “You mean someone coming in and taking away all my shit without me having to PAY him? That would be awesome!” Ha ha (He’s a super minimalist & his wife wasn’t.) That perspective on stuff always makes me laugh, and the fact that he felt so safe in LA makes me feel doubly safe up in Canada!

  • Dan September 1, 2011, 1:36 pm

    PS all this talk about thermal mass only really matters in places where there is a significant swing between nighttime and daytime temperatures, right? Nighttime has to drop to below 70 to meaningfully cool a house in the few cool evening hours… perhaps thermal mass effects are less useful in Arizona and Nevada.

    • MMM September 1, 2011, 2:24 pm

      You are right on the concept – you mainly benefit in summer if the nights are comfortable enough that you want to let the air in. But even in Florida summer, you could run your air conditioning more at night and less in the day, if you have a nice heavy house. This lets you take advantage of lower night time electric rates in areas with smart billing.

      In winter, thermal mass is useful any time you get sunshine and want to store the heat. That means anywhere cool enough to require some winter heating. Arizona and Nevada are mostly desert, which means large day-night temperature swings year-round. So thermal mass is a westerner’s friend. Note the adobe house construction methods of the ultimate Mustachians – the Native Americans. Heavy clay walls which smooth out the large temperature swings in what is now the US Southwest.

      • Dan September 1, 2011, 2:26 pm

        Night time electrical rates aren’t lower enough that a standard AC unit should be run at night cooling down the home thermal mass. Though I do know that this works in big industrial applications (for example the U of Arizona in Tucson freezes literally tons of water at night with cheap electricity and then uses that thermal mass for campus central A/C). Anyways overall good points thanks for brainstorming / sharing

        • MMM September 3, 2011, 11:56 am

          I’d have to disagree with you on the calculations for day/night cooling in a house. It depends on your electric rates. In Ontario, I see that peak electricity is 9.6c/kWh and off-peak gets as low as 4.2 cents – less than half price! In Wisconsin, these rates are even more widely spread. Of course, my own town is not advanced enough to have time-of-day billing yet, and much of the US is in the same boat to this point.

          But let’s say a Southerner does have time of day billing for the sake of this discussion.

          By running a central A/C system at night, you get the half-price power, AND your outside condenser benefits from slightly cooler ambient air being drawn through its coils. You can super-cool your house by running the system straight through the cheapest hours (10PM to 7AM in Ontario). Then balance that by leaving the A/C off for the peak hours (11am to 5PM) or some subset thereof.

          Shifting 5 hours per day of a big Florida-style 4,000 watt air conditioning system from 9.6c to 4.2c would save about $32 per month in electricity. Under a Wisconsin or California pricing scheme, this savings could be over $60/month. Even more important to me, I’d be pleased to be helping out the country by reducing the need for additional peak-hours-only power plants.

          The only disadvantage to this plan is that there would be a slightly faster heat leakage into the house due to a larger inside/outside temperature differential for part of the day. But the larger the thermal mass, the lower you’d need this differential to be to maintain comfort. Leaving out some more calculations, this leakage is still much smaller than the electricity savings achieved by getting your juice at half price.

          • Mike June 23, 2014, 2:46 pm

            I was wondering about exactly this issue while reading and am glad to see it raised in the comments. So bottom line, what would be your advice for a locale where summer nights are still uncomfortably hot and there is no “smart” electric billing? Thermal mass is awesome for exploiting temperature swings, but what would you do in a Florida summer?

            • Mr. Money Mustache June 23, 2014, 3:42 pm

              Having lived through a couple of weeks of hot summer at my brand new house, I can say that the light-colored metal roof and excellent roof/ceiling insulation have been amazing performers. It resists solar heat so well, I don’t even need to open the place up at night anymore. In Florida, a building like this would just need very light A/C use to keep the humidity and temperature a few degrees cooler than outside.

            • Mike June 23, 2014, 3:52 pm

              I can see how those would keep the heat down for sure. My question was more that wouldn’t the high thermal mass work against you under those conditions? Just as it’s hard for the house to heat up, wouldn’t it take much more air conditioning to cool it down? Is that just a higher expense you have to suck up for a few weeks that is more than offset by the high-mass savings the rest of the time?

            • Randy January 5, 2020, 4:41 pm

              Having lived in S FLA for eight years, I have two suggestions.

              1 A radiant barrier. Basically heavy duty aluminum reinforced with with nylon. This works well in areas where you need cooling year round. The barrier is stapled to the bottom of the 2 x 4’s supporting the roof. It reflects the radiant energy coming through the roof back to the roof. Less effective the further north you live.

              2 Insulation. Make sure it is to the top of the 2 x 6’s. If it is, roll out an insulation blanket at a 90 angle over the top. R25 or so.

              We had a 1200 foot one story home. Cost for these upgrades were about $700 ($1000 minus 30% tax credit) with me doing three overnights in the attic. Highest electric bill after was $99 and the wifey likes it cool (72F). Neighbor’s bills in similar homes were “$200+ . Paid for itself in 7 months.

      • churchie March 6, 2016, 5:49 pm

        I read where adobe walls were designed to be just thick enough to keep the searing heat from penetrating to the inside of the house (analogous to a dam being just high enough to contain the lake behind it). But the genius, the trapped heat then released out of it overnight when temperatures dramatically dropped – keeping the house from getting cold. Wonder how they worked that out 1000s of years ago.. . .

    • Amanda February 13, 2016, 7:01 am

      It’s even more important! I live in the desert and we regularly have 40 degree or more temperature swings, especially in the summer. In mid-July it’s common for people to bring a light cardigan with them if they are going to be out late. In fact, despite living in the desert, using this technique means I use my swamp cooler less than 2 hours a day throughout the summer.

  • Brooke Trout September 1, 2011, 2:18 pm

    My new experiment is making sure to unplug all my appliances that I absolutely can after reading about phantom power drains. Can’t wait to see my next electric bill to see the difference.

    • MMM September 3, 2011, 12:00 pm

      It’s a good experiment to do! With a $20 energy meter like the kill-a-watt or the Ryobi one you can get at Home Depot, I like to measure the actual phantom power drains before deciding which ones are worth unplugging. For example, my computer on standby mode is less than 1 watt – not significant enough to give up the convenience of standby. But television systems with DVR machines can be over 30 watts. Definitely worth turning off.

      But DVRs take a long time to reboot from scratch, so the best thing to do is get rid of your DVR and TV service altogether, since TV is ridiculous anyway :-)

      Ultimate electric savings are attained by good old-fashioned hang-drying your clothes if you don’t already do so.

      • Mona September 7, 2011, 9:50 am

        Our local library lends out the kill-a-watt meter for free so I would check your local library to see if you can borrow one rather than purchase it.

        I wanted to let you know how much I enjoy reading your blog MMM, your writing makes me feel like we are old friends just chatting about a shared interest. I was already implementing some of your ideas prior to finding you, but I am more confident about the changes I am making in our household and inspired to achieve more now that I’ve seen what your family can do even with a lot less than I have! Thank you!

        • MMM September 10, 2011, 9:48 pm

          Thanks Mona, I feel like we are old friends too :-)

  • Bamboozle September 1, 2011, 8:55 pm

    Wow MMM, I love your blog even more. When I was at design school, we worked on a sustainable passive solar house where the thermal mass is stored underneath the house. it works by having the entire thing enrobed in light absorbing panels angled to face the sun at all times. The house was well, uh, all metallic black. and from these angular panels, the heat absorbed travelled down insulated copper ribs into salt water cells underneath the floors. They get HOT. The basic benefits are there like any passive solar designs but it was also able to generate enough electricity from temp differences (peltiers + a small turbine spinning off excess heat) to be completely off the grid with no PV cells present. As we were working on it, everybody was like. soooo… no electricity bills, pretty awesome. why don’t we have more of these houses in America?

    • Rachel September 13, 2011, 11:40 am

      How much would it cost to build a home like this? It *sounds* expensive, and that is a big barrier to a nation-wide roll-out. Make it affordable and lots of folks (me included) will opt in.

      • MMM September 13, 2011, 12:31 pm

        It actually costs no more than a normal house to build one that is more energy-efficient! It’s just that in our country, home buyers do not traditionally consider energy efficiency when buying, so builders don’t consider it when building. For thermal mass specifically, a house could simply have one solid concrete wall somewhere inside somewhere near a South-facing window. Total cost of a few thousand pounds of concrete – less than a hundred bucks at construction time! Other materials like earth and crushed stone are even cheaper, if they are designed in by the designer.

        • Bamboozle September 13, 2011, 12:46 pm

          exactly what MMM said. Although what we worked on sounds expensive (designed and bespoke cells & panels), but thermal mass is pretty darn cheap. I don’t know what costs less, crushed stones or salt water? The basic principles are still the same and can be done frugally if you consider it well. In both this post and in the book I mentioned below, By pairing thermal mass to a window, adding or removing openings to your house already has large benefits by itself without being fancy.

        • Rachel September 13, 2011, 6:01 pm

          I have a two story house with a walkout basement, and even though we’re south-facing, having that part of the house underground seems to keep the house cooler. However, there is a crawlspace where the furnace is and another one on the east side of the house that get very hot, and the rooms next door to them get very warm as a result. What would you recommend? Also, is having an exhaust fan or fans installed in the attic an economical move? It’s baking hot up there.

  • Bamboozle September 1, 2011, 9:06 pm

    This book on the subject is also good. I have it, and it showcases ancient houses from korea, japan, and other indigenous tribes whose ingenuity on passive heating and cooling were miles ahead of our current civilization. Raises the question if this is a kind of a lost art / intelligence? http://www.amazon.com/Solar-House-Passive-Heating-Cooling/dp/1931498121/ref=sr_1_2?s=books&ie=UTF8&qid=1314930270&sr=1-2 But do get it from your local library of course!

  • Enginerd September 3, 2011, 9:25 am

    I believe this is the same concept that dictates that you should keep your refrigerator and freezer as full as possible to reduce load on the appliance. Both work in a state with the compressor on (high power draw) and the compressor off (low power draw), so the more thermal mass in your fridge/freezer = more stable temperature = running the compressor less = less power used by fridge/freezer.
    It is easy to fill the unused space in the freezer with ice if you don’t have a lot of frozen foods. I’m not sure what a good moustachian method would be for extra stocking of the fridge if you are single or don’t go through refrigerated foods quickly. Reusing a couple plastic gallon milk jugs by filing them with water would help. Illustrates a good reason to “right size” your fridge/freezer when you are buying one.

    • MMM September 3, 2011, 11:34 am

      You’re right in that adding ice jugs to your freezer increases its thermal mass. But it’s different than my “night cooling” concept because you aren’t taking advantage of any natural temperature swings.

      In a fridge, adding the ice blocks saves energy because your fridge will tend to turn on and run for a long time, then turn off and stay off for a long time. During the power-on cycle, there is an initial period of waste where the compressor starts pumping and vaporizing the coolant, but no actual cooling is happening. Then the cooling system slowly gets up to peak efficiency over several minutes. So by spending longer in peak efficiency mode and less time in charge-up duty, you reduce the energy used by your fridge.

      It would be neat if a fridge could be designed to behave like the University of Arizona – run for hours at night to stash up several kilograms of sub-zero ice, then use this up during the day to reduce compressor run-time during the day. This would lower peak power demand during the day time, and also take advantage of cheaper night-time power in areas with variable billing.

      • bob June 10, 2012, 2:18 pm

        I think the idea of more thermal mass in your refrigerator or freezer makes a lot more sense when you take into account how often you open the doors and what temperature the air is when you open the door. As you open it up you will let out some of the cool air and replace it with warmer room air. If you have more thermal mass, that mass can help cool the new warm air so your compressor nay not kick on or will run for a shorter period.

      • Dkah March 1, 2014, 9:43 pm

        Already has been done. I remember reading about a guy that made an ice cooling unit by using an absorption cycle system. It would heat up in the sun cause a change of state and freeze the ice during the day and at night the ice would melt change states again and set up for the next day. I couldn’t find the original link but here is one on absorption cooling fridge that is ran off of solar back in 1935.


  • Jeremy September 11, 2011, 8:48 am

    This is one of the foundations of the earthship: http://www.earthship.com/

    The founder, Michael Reynolds, began building them near Taos, N.M., in the 1970s. Their walls are constructed out of old tires rammed full of earth, which is basically a 300-pound brick.

    There’s all sorts of information on them at the link above.

  • LJ September 13, 2011, 1:32 am

    Good advice :), my grandmother house which is completely build of stone and timber (dates from the 17th century) and keeps an stable 20C temp in summer w/o AC and once it is warm you can mantain the warmth without using much energy even if it is freezing outside

  • Robert September 27, 2011, 11:55 am

    I was thinking about installing a window A/C to use on our bedroom for sleeping and turning off the central air. I live in Florida and I like it to be really cold when I’m in bed, but setting the central unit very low would be costly. I wonder if the unit’s cost and power consumption would be worth it. It’d probably take a few years of use to negate the costs and actually be saving money.

    • MMM September 27, 2011, 12:55 pm

      That is actually a FANTASTIC idea for Florida! A window A/C for a single room will use less than 500 watts on average, since its compressor will not need to run continuously to chill just one room. Contrasting to the use of a full-house system in a hot place like Florida in summer, you’re probably saving 1000-2000 watts through the 10-hour sleeping period – 10-20 kwh/day, which is $1.20 – $2.50 per day. A little $100 window air conditioner would pay for itself within only a few months, even if my estimates are way off for your house!

      In fact, this technique would be cost-effective even for people in other hot humid areas in the summer, like the whole US East Coast. There is no benefit to cooling the whole house while you sleep.

    • Marcus August 13, 2013, 8:30 am

      Live in NY and just started using a window a/c in the room at night and turning off the central…works like a champ!

  • RubeRad October 27, 2011, 10:47 am

    I lived in England for two years, and they had a system where night-time electricity rates were MUCH less than daytime, so our flat had a common heating system to take advantage of this. It was an electric heater filled with a pile of ceramic bricks (heat-banks). You set a timer for when you want the heater to run overnight (to fill the bank with heat), and then during the day you adjust the opening on a vent that lets heat escape from the bank. It takes a while to figure out the right settings, but not too long, and then it works great!

  • Admiral Anderision October 31, 2011, 12:48 am

    Have you ever heard of Earthships? Total off-the-grid living, thermal mass plays a big part in the construction of these ‘Uber-Eco-Friendly’ Houses.

    In fact, these earthships have so much thermal mass, that they don’t even need an air conditioner or heater, they stay at a comfortable 20℃ year-round.

    • Admiral Anderision October 31, 2011, 12:48 am

      oops, didn’t read all of the comments before i posted…..

  • Brooke Trout January 3, 2012, 4:06 pm

    Just wanted to say thanks again for your advice on this! I got back the results on my phantom power experiment. I slashed my bills to half of my average expense and this was during the hottest and coldest seasons of the year when I expected to pay triple the average. I used to keep things like curling irons and cel phone chargers and toasters plugged in all the time, not to mention entertainment and computer centers. Now everything gets unplugged if its not being used. Really incredible what a very big difference this makes when you multiply the savings by a year’s time. I’d much rather have that money in my mustache!

  • Heath May 10, 2012, 9:26 am

    Hmm… I definitely get the concepts in this post, but I’m curious as to how I can apply them to my specific circumstance.

    I live in the metro-Phoenix area (AZ) and I’m considering buying a house (once I save up 20% down, of course!) in Tempe. I want to know what to look for when buying, and how to modify my house as I live in it (with respect to thermal mass).

    The summer is clearly the biggest issue in this area, especially when combined with the urban heat-island. In other words the city only cools off (below 80) in mid-summer at around 4:30 in the morning. Then it begins to heat up quickly after that with the sunrise. I figure tall trees on the southward side, or perhaps some vines (hops?) along that side of the house could help minimize the heat’s impact during the day. And naked cement floors will act to hold in the cold from our cheaper-energy-at-night AC runs. (One commenter from above also opened my eyes to a single-bedroom AC unit for nights, but then the whole house might NEVER cool off, and the thermal mass would be working against me…)

    But in the winter, these fixed-shade and cement floors will only make it colder! And it get’s pretty cold here, I’d say. Though that may be me whining because I’m still a Noob-Mustachian, and haven’t gained high enough badassery. Any suggestions for mid-city Phoenix winter, with those fixed anti-heat elements in place? Perhaps a single-room space heater at night?

    • Gerard July 11, 2012, 3:53 pm

      Heath, it looks like the average winter night temperature in Phoenix is in the high 40s or low 50s (fahrenheit), which is actually excellent sleeping weather, if you have a decent comforter or quilt. Here in cold cold Canada I set my thermostat back to 50 (its lowest setting) at night, and sleep like a stone. (And, of course, your house will not drop to the outside temperature unless you sleep with all your windows open.)

      In terms of shade, take advantage of the seasonal differences in the angle of the sun — deep porches and overhangs that let in the sun in winter, but block it in summer.

      In July and August, uh…. I got nothing. Summer in Newfoundland?

      • Captain and Mrs Slow November 24, 2012, 11:36 am

        50 is 10 celisu (been so long since I’ve used F. I have to google it now). My wife has bad asthma so the temp can’t go below about 18 (65F)

        The only time I mind the bedroom being cool is when I have to go pee in the middle of the night!

    • bunnykick2000 January 31, 2014, 12:43 am

      Hi Heath. I know this comment is late to your game, but it might be useful for others that come along later.

      There are a few options to look at when buying a house in the desert. Make sure the orientation of the house is east-west. You could care less if it is pointed at some nice mountains in the distance. Plant some deciduous trees on the south and west sides of the house if possible. A car port on the west side of the house would save TONS of cooling costs in the summer from the blazing hot setting sun. Make sure the eaves have enough over hang to shade the house from the sun in the summer. Also, you need to get into the attic and go crazy sealing all the duct work connections with mastic and then insulate the duct work and attic to the max.

      Sorry to tell you this, but most houses in Phoenix are not build for energy efficiency. If you are looking for a well built home there, you’ll probably have to build it yourself or be very diligent in your search. Best of Luck.

      • Heath February 4, 2014, 6:51 am

        Thanks for the good advice! I’m still in the early saving stages, so I’m nowhere near my own home purchase. But I’ll keep your suggestions in mind :-)

        I was sad to hear that almost none of the houses in Phoenix are built for efficiency. This would seem like the PERFECT place for it, with the extreme temps and very low humidity…

        • Bunnykick2000 February 5, 2014, 11:12 am

          AZ is a beautiful place to live. But I always think of how we let a great opportunity go since the 70’s. Imagine if every house in AZ was built with solar panels and a little bit of thought since then. We could be an exporter of energy…

          When you finally get ready to buy, look for Energy Star rated homes.

          Hope your Mustache is getting long.

    • Sevy March 3, 2014, 9:27 pm

      Two words… attic fan. A lot of these have a thermostat that you can set to turn on at a certain temp. In hot weather heat gets built up in there with only small vents to escape. Even if you have to pay someone to install it $400-500 you’ll make the money back in no time with not needing the A/C as much or at all.

    • Chad July 5, 2015, 7:16 am

      You’ve got thermal mass a bit backwards. It operates both ways, heating in winter and cooling in summer. You just need to find a way to charge it with heat (sun) during the winter. Alternatively, as mentioned elsewhere, if you have cheaper electric rates during the night, you can run your heater through the night in winter to charge up the thermal mass that will then release the heat slowly over the day. Remember: A lot of these things aren’t anti-heat elements, they’re anti-temperature change elements.

  • LT September 25, 2012, 11:20 am

    I am interested to learn more about the tubes of water in front of windows. I tried to Google it, but couldn’t really find any information.

    • Gerard October 30, 2012, 7:50 am

      LT, google “factor ten house chicago” for an example of a house that uses hundreds of 2-litre pop bottles filled with water to do this. And the house does a lot of other cool things, too.

  • Fawn November 14, 2012, 1:14 pm

    We live in a smallish (3 bd/2 ba, 1000 sf) apartment with no laundry, where no one actively uses electricity between 8am and 3pm or after 10pm, and we never run the AC or the heaters. Yet our electric bill is still over $100 a month. Could it all be power drain? The fridge? The ceiling fans? I am at a loss on how to lower the bill- and no we do not have cable service at all.

    • Mr. Money Mustache November 14, 2012, 3:11 pm

      Hey, that’s an interesting problem. What is your rate for electricity (or alternatively, the number of kWh you use per month)?. The dollars-per-month alone doesn’t tell us much, since electricity price can vary by 400% within the US alone.

      If your fridge is really old, that could use a fair amount of the bill. If you have an electric water heater, there’s another $20-$30 – more if it is old and poorly insulated (try adding an insulation blanket). Other than that, I can’t think of anything that would use much power, unless you have loads of incandescent bulbs.

      Also check the bill for how much the fixed component is – sometimes there is a big monthly fee regardless of your power use.

      • Fawn November 14, 2012, 4:43 pm

        From our bill: kWh used: 805
        Charges per kWh: $0.07983
        Misc other taxes and delivery charges: $10.97
        We do not have access to our hot water heater. Is in a tiny, locked, service closet under our oven. We do have electric cooking, and I do cook two meals per day from scratch (using the stove top, a mixer, rice cooker, oven or crockpot daily). We actually do have probably a 50/50 mix of old and new lightbulbs and will switch the bad 50% out tomorrow. We also have a MacBook that is asleep all day but turned on and plugged in. We run two ceiling fans nearly all day, and on very hot days (90+) we runaway window fan at night. This is all new to us, having moved into civilian housing at a new duty station after living in military housing fora long time. Should I just unplug everything except the fridge and stove/oven? Switching them out is not an option, as they belong to the apartment.

        • Fawn November 14, 2012, 4:48 pm

          Argh! Edited to add that is $0.07983 for generation charges. There is also $0.07478 for delivery charges. Each per kWh. So we are essentially paying $0.155 per kWh.

        • Mr. Money Mustache November 14, 2012, 5:06 pm

          Wow.. 805 kWh sounds a little bit fishy for the setup you described. You could always use a $20 “kill-a-watt” meter to measure the consumption of various things. A fridge is about 50 per month. The cooking wouldn’t take all that much (probably about 100kWh from what you describe). Hot water might be 200. And those are the biggest things. (You don’t have any always-on exterior lights, right?). Maybe your meter is inaccurate or someone else is using some of your power. You could ask a neighbor as a way of comparing too.

          • Fawn November 14, 2012, 5:40 pm

            No external lighting at all. I have asked around and although we have the largest apartment In the building, we do not have the largest bill (the smallest bill, a one bedroom downstairs, runs about $65 a month and the largest bill goes to a two bedroom upstairs at $225). I am starting to wonder if really old wiring could be the culprit and we will just have to deal with the cost. Could old wiring be “eating juice”?

            • alewyfe November 28, 2012, 1:41 pm

              Hm. Could be some fishy wiring… hard to know without digging deeper, but not impossible. We discovered some crazy wiring hi-jinks at our work-studio building when the power was shut off while my partner had been paying the bill… turns out our unit was cross-wired with another, and we’d been paying their bill and they, well, hadn’t. Took months to sort it out, meanwhile with us running an extention cord down the hall to a common outlet when we were there to do work until our rotten landlord could verify/fix the problem. What a nightmare! Hope your solution is simpler… but yes, check that common-area usage or other units aren’t being metered on your bill!

  • omes11 August 30, 2013, 2:15 am

    Great site Mr Money Mustache! i found it a couple weeks ago and have already implemented several tips and cut out a bunch of monthly expenses that were completely unnecessary but totally “ok” to my former non-Mustachain self (and have also turned a handful of other people onto the site). I have a question about this article: So adding the thermal mass to your house will help it warm up slower, however, on the other hand won’t it also take more energy (i.e money) to heat it up in the winter (not sure how cold the winters get for Colorado but assume a city that has cold, harsh winters such as Chicago)? Sorry if this is a dumb question but just wondering. If anyone else knows the answer that would be great as well.

    • Prairie Practicality January 22, 2014, 9:25 am

      The idea behind mass is to provide “cold sinks”. If you have thermal mass in the winter you get “hot sinks”. Basically it allows your house to store heat longer. Eventually all the heat is lost out your walls/roof/windows so internal objects don’t affect your overall heat requirements…unless you added more to the walls/roof that acts as layers of insulation (i.e. drywall, heavier floors etc.).

      If there was a way to get heat (or cold) cheaper at some points (i.e. outside air, sunlight) and store it (in the “sink”) for later then you’re in the money.

      We expect -40 here in the winters, summers get to 100.

  • Fintan Mac January 30, 2014, 9:29 am

    I live in Ireland where the Summer is rarely too hot and the winter average daytime temperature is about 6 celsius (43F). That said we would often have subzero nighttime temperatures. I use OFCH and was advised to leave the heat on at 12C (54F) all night and when out of the house and to turn it up to about 20C (68F) when we’re home. I was told that the reason for this is that it takes little enough heat to keep it topped up to 12C and upped to 20C compared with the amount of heat required to boost from subzero to 20C. I also know that we had a couple of very bad winters recently (-12C/ +10F) where people woke up to frozen pipes becuse they’d let the house freeze overnight where we had no problems with that.

    It seems to make sense alright, but I’d like to know for sure and particularly if it’s really more economical. Our house is about 10 years old and holds the heat reasonably well.

    Thanks for any advice you can give me on this.

  • Elizabeth Johnson March 16, 2014, 11:12 pm

    Hey! Redding and Longmont are 2.83 miles apart- latitudinally speaking.

    Our summers often don’t cool below 80 degrees overnight. Mustachians here use swamp coolers and they feel great. But I rarely run the A/C. My winter bills are higher than my summer bills because I love the heat and wearing tank tops and shorts round the clock. I calculated expenses and opted no swamp for now.

    Unfortunately, many Reddingites over condition their houses as do businesses. I’ve lead many campaigns at previous jobs turn that A/C up (down?). “The coldest winter I ever spent was Redding indoors in the summer.”

  • Zoltan August 13, 2014, 6:22 am

    Here in Europe everybody talks about insulation and passive houses (zero-energy) but no-one mentions the environmental cost of all the plastics and other industrial products with high embedded energy that build up a modern, highly insulated house. Thermal mass is another topic that is neglected.

    To the whole truth I have to say that where I live most of the houses, old and new are made of bricks, concrete foundation and floor, monolithic concrete ceiling and tile roofing. So from a thermal mass perspective we are OK compared to most of US houses. However, in the last decade or two, the houses got lighter and lighter due to modern light-weight construction materials and significantly thinner (still load-bearing) walls (you need the space for thick insulation).

    As a result, more and more modern houses require air conditioning during te summer heat wave, which was almost unknown before. The well insulated but light houses basically act as heat traps.

    As a enthusiast of traditional and environmentally conscious architecture, when I had the opportunity to build my own house, I knew that I will try and build a house that has as little embedded energy as possible by using natural materials as wood, earth and lime, minimizing the usage of concrete, cement, plastics and other industrial products, paints with high VOC etc. The other design principles were high thermal mass, good solar orientation, generous roof overhangs, large windows southward, usage of traditional building techniques. I also wanted to make it as low-tech as possible but I sort of gave up on that and still used pellet central heating and solarpanels for the boiler.

    The result speaks for itself. The house is very heavy, I’ve got two feet thick rammed earth walls which weigh 230 tonnes (1600 square feet house), the foundation is concrete but we did not use cement above it only lime mortar, wooden ceiling, natural tiles on roof, one feet recycled blown-in paper insulation above the wooden ceiling. The house is very comfortable, very cool during the summer and nice warm during winter. There are only natural materials used inside like lime wash, naturally oiled wooden floors etc so the indoor air quality is excellent and the humidity is extremely even during the whole year, always around the ideal 50-60%.

    To cut it short, thermal mass is extremely important factor, thanks for raising the attention to it.

    • Mr. Money Mustache August 14, 2014, 10:20 am

      230 tonnes! I am jealous of your house – nice job.

    • CuriousK March 28, 2017, 9:58 am

      Hi Zoltan,

      Your reply is a very interesting read to me. I am not in the architecture or construction industry, but I’d love to read more about this. Could you suggest several books or websites? Thanks ahead!

  • churchie March 6, 2016, 2:44 pm

    MMM, check out ‘trombe wall’, since you are very handy maybe you can build one of these in the future. Coincidentally, I’ve read about a couple folks who have these in Colorado. The thermal mass of a large masonry heater itself acts like an effective heat sink even when not in use. http://www.tempcast.com

  • MKE January 26, 2017, 11:54 am

    I checked through the comments, and no one mentioned trees. Could I look through the fifty-thousand MMM posts and find an article about trees?
    Trees keep sun off your house in the summer, keeping it cool. In the winter, the leaves have fallen off and the sun heats it. Neat!
    The first thing most Americans do when they buy a house is clear-cut the property and take away all the trees. Big mistake with both obvious upfront costs and later hidden costs.
    I know this post is about thermal mass, but talking about heating and cooling a house cheaply and not mentioning trees is like talking about getting around cheaply and not mentioning bicycles.

    • Drew September 6, 2017, 7:37 am

      I grew up in an old house under 14 huge maple trees, never had A/C. I don’t think we would have survived summer without them. When my mother moved, the new owners cut them down.

  • Sky February 28, 2019, 2:40 pm

    I’m surprised you don’t mention improving insulation anywhere possible. I understand the importance of thermal mass, but if you want to minimize the amount of heat-leak/heat exchange, this is hugely important. The house will block heat in longer in the winter and block heat out longer in the summer.

  • JonesArt February 15, 2021, 1:28 am

    I have a new build in which a large open plan room (9m x 7m) is a 7m (30cm x 20cm) solid concrete beam, I was going to clad it with insulation but should I actually leave it as is to absorb heat as a solid heat store in the winter when we will have a wood burner running?

    • Mr. Money Mustache February 15, 2021, 3:30 pm

      Yeah, that sounds ideal. As long as the beam is insulated on the OUTSIDE, so that you aren’t bridging heat through into your room (concrete is fairly conductive), it is much better to leave the concrete exposed on the inside for exactly the reason you mention.


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