First Understand, then Destroy, your Home Heating Bill

A $40 insulation job on the garage door of my previous house saved hundreds of dollars of fuel. And made the garage a much more pleasant place to work.

We’ve all read the tips in the newspapers about how to reduce our heating bill. Turn down your thermostat and get a programmable one. Seal your air leaks. Add insulation to your attic. Blah blah blah.

It’s boring, but it’s powerful advice, because US households spend an average of about $960 each per year keeping themselves warm. That’s three months of grocery money for a family, pumping heat into a leaky sieve that could actually have been built to require much less, or even no added heat, depending on the location and climate.

So while the USA Today-style heating articles are good enough for those readers, they’re not good enough for us. I don’t just want to lower my heating bill, I want to Destroy it. My eventual goal for any house I settle down in for the long run is to make it require no fossil fuel energy to keep it warm all winter.

To understand heating, you must first know what you’re paying for. Heat is often measured in BTUs, British Thermal Units. 100,000 BTUs is referred to as a “therm” on my Natural Gas bill, and a therm currently costs about $1.00.

Then you must understand how you are losing your heat. The most important form of heat loss from your house is something called thermal transmittance, also known as U-factor. This is really just a number that means “how leaky is this material”, and it is based on the leakiness of a single pane of glass.

A single-pane glass window has a U-factor of 1. In English units, this means that each square foot of glass leaks out one BTU of heat per hour, for each degree Fahrenheit your house is warmer than the outside air.

So when you’re looking through a giant 10×10 foot window (100sqft), from a 70F room out into a 0F deadly-cold wintry blizzard, your window is leaking out 100 x 70 = 7,000 BTU per hour of heat into the night. That’s  7 cents per hour or  $50.40 per month of heat from that single window.

The “R-value” you have probably heard about is simply the inverse of the U factor. That means 1 divided by U equals R, and 1 divided by R equals U. You’ve heard of R-13 wall insulation, right? The U-factor of that is 1/13, or 0.08.

With that knowledge, we can move up to understand the heat loss of an entire house. A 1000 square foot house probably has an outside surface area of about 2000 square feet including the roof. If it has standard R-13 wall insulation (and no windows for this example), and the temperature inside is 68F,  it loses how much heat on a 32 degree day?

2000 square feet x 0.08 U-factor x 36 degree temperature difference = 5760 BTU per hour = $1.38 per day.

“Hey, that’s a pretty cheap gas bill for a whole house in what sounds like a pretty cold climate”, you say. But wait, it gets even better.

Imagine this household uses the same amount of electricity as me, about 299 kilowatt-hours per month, equivalent to 415 watts of average continuous use. How much heat does this electricity use add to the house? The answer is 1416 BTU per hour.

“What? You were just talking about home heating, and now you are talking about electricity.. aren’t they two different things?”.

No, it turns out they are actually directly related. Almost all of the electricity you use in your house ends up being converted to heat, with 100% efficiency. Your light bulbs give off heat and light, and the light bounces around the room and eventually gets absorbed by the walls and turns into a tiny amount of heat as well. Likewise for your appliances, your computer, etc.

What this means is that any electricity you use in the heating season provides some of your heat, and reduces the amount your furnace needs to provide. The only exception to this rule is electric appliances that vent their waste heat outside, such as a clothes dryer or exterior lights. But we don’t use clothes dryers, right?

So let’s go back to the 1000 square foot house. It needs 5760 BTU per hour to stay warm, and it gets 1416 of those BTUs just from the electricity you use while living there. So you’d still need the furnace to stay warm.

But what if we increased the wall and roof insulation to R-53? That quadruples the insulation, which cuts the heat loss in four. And guess what – you have a self-heating house!

Practical Example: the attic of this house is 1000 square feet. We want to quadruple the insulation from R-13 to R-53. To add the extra R-40, you’d simply have to blow in about 12 inches of cellulose insulation. This would take 61 bags of the stuff, which costs $610 before tax credits.

Adding $610 of insulation to your attic is saving you about $15 per month of heat in this situation – $60 per year over a four-month heating season. It will also cut your air conditioner use. So it’s at least a 12% annual return on investment, forever.

The self-heating house is the holy grail of home heating, and this is the way houses in many parts of the US could be built to destroy their winter heating bill. With proper design, the cost of the extra insulation would be small compared to the energy savings. Changes like this are really coming – the 2003 and 2007 international building codes, which most cities and towns adopted for all new construction, have much higher insulation standards, meaning lucky homebuyers in the future will have much lower costs.

Now, my example was kind of silly, because the house was just a simple 25×40 rectangular box with no windows. What if we add windows?

Good double-pane windows have a U-factor of about 0.3. If the sample house has eight 3×5 windows (a total of 120 square feet of glass), this adds a heat loss of about 1296 BTU per hour. D’oh! There goes almost all of the free heat we get from our electricity! How can we recover? Luckily the windows come with some good news too.

Solar gain at work in MMM’s room. Shutters open in day, closed at night.

Let’s say the smart designers of this house put two thirds of the windows on the South side of the house. The sun shines an average of five hours per day in this region, delivering 1000 watts per square meter (90 watts per square foot) mostly onto the side of the house. The windows let a certain amount of this heat into the house, based on their solar heat gain coefficient, which is usually around 0.3, or 30%.

There are 80 square feet of glass on the South side, getting hit with 90 watts per square foot of light, and 30% of the heat makes it into the house. This is 2160 watts of heat for five hours, or 36,850 BTU of heat per day shining in. Averaging this over the 24 hours of a day, your house now has an extra 1535 BTU per hour of heat coming in, just from the South-facing windows.

The lesson is: having 2/3 of the windows facing the Sun more than made up for all of the house’s total heat loss through all of its windows!

Rule of Thumb: Each 3×5 window facing South gives you about $2 per month of free heat (in a moderately sunny climate).

You can drastically improve the performance of your windows by using shutters or curtains on them. Open them when it is sunny, and close them at night and even on cloudy days if you wish. A tight-fitting curtain or honeycomb blind, or better yet a set of interior shutters, can cut heat loss through the windows in half again. Just remember to open them back up when the sun is shining in the window.

The final easy thing under your control is the interior temperature. As we’ve seen above, the heat loss from your house depends entirely on the difference between the interior and exterior temperature.

Supposed that I lived in San Francisco, where the average winter temperature (day averaged with night) is a moderate 50 degrees Fahrenheit. If I keep my house at 50, I will need to add absolutely no heat to it to keep warm. If you keep your house at 70, you will need some heat, and if someone between us likes 60 degrees, she will use exactly half of the amount of heat you use.

So when you drop your interior temperature, it saves you some cash. Even if you just let it fall for a few hours while you go out for a walk or while you’re at work, it still saves heat because you are lowering the average difference between interior and exterior temperature.

In my climate in January, the day/night temperature averages to 30 degrees. If an average person keeps his house at 72F (a 42 degree difference), and I keep mine at 67F (a 37 degree difference), my gas bill will be about 12 percent lower than his. If I further lower my average by dropping the temperature at night to 60F, I can save an additional 8%.

Lesson: Running your house at 67F during the day and 60F at night will save you about 20% on your gas bill compared to a house that runs at 72F around the clock.

So we have covered the big three: insulation, solar gain and window loss, and interior temperature. With the right combination of these three things, and enough thermal mass to keep your temperature nice and constant, you can have a self-heating house.

The MMM family house is not a trivial one to heat, so it has no yet attained this status, but I am getting there. It’s about 2600 square feet in size, with 1800 of that above the ground. There are about 30 exterior windows and doors, some of them pretty big, and they are not (yet) all that well optimized for capturing solar heat. And most significantly, it houses a lady of Indian descent and a young boy, who both become quite unhappy if the interior temperature drops below 67 degrees Fahrenheit (19.5 Celsius) during the day.

I have made these changes so far, with the approximate annual dollar savings listed alongside them:

Added 20 bags of blown-in insulation to the attic: $25
Insulated the steel double garage door: $25
Using 10 insulated shutters to close second floor windows each night: $50
Opening shutters on 6 large south-facing windows to capture solar gain: $50
Covering large rarely-used basement windows with removable insulated plugs during winter: $20
Running lower interior temperatures, especially at night: $80

The total savings so far are about $250 per year, meaning the heat bill was $650 per year when we moved in here, and now it is $400. That’s a 40% drop. Some of the remaining 60% can be wiped out like this:

150 square feet of solar gain glass on South side (nearly free from recycled building materials store): $80
Much more insulation in the part of the attic I didn’t get to yet: $50
Insulated shutters for the large remaining windows: $50
Solar collector on the roof for domestic hot water and  radiant solar heat: $50.

That cuts the bill down in half again to $200 per year. But after that, I’m out of ideas. Our current house is not an energy-efficient design to begin with, so it is difficult to get it fully self-heating. More changes could be made, but they would become more exotic and thus cost more than they delivered in savings.  A future home of ours someday, however, will learn from the limitations of this one and have natural heat built in right from the drawing board. Perhaps yours will too!

Update: This article was written way back in 2011. A couple years later, we moved to a “new” house (actually a 1959 house which I bought then rebuilt nearly from scratch). It is a smaller place with a much better orientation to the sun, and I used all of the principles in this article. As a result, it is much closer to optimal, getting nearly all of its heat for free and requiring no cooling in summer. An dream come true for both engineers and nature lovers, since the view is all sunshine and trees from most rooms.

Bonus Points: After I skimmed the surface with this article, the readers posted a bunch more useful energy-saving techniques in the comments below. Check them out for more learning, especially Brian M’s comment about sealing leaks in an older house. Air leaks are even more important (and cheaper to fix) than insulation, so if you still have any of these around, fix ’em.