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

## By Mr. Money Mustache

## Thu, 01 Sep 2011 11:52:13 +0000

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.