Heating Your Home: Thermal Mass

Pyramid Author’s note: the following article on home heating is the fourth in an eight-part series.

The previous article discussed the disadvantages of using forced air to heat your house. Another approach is, literally, to heat your house –- not the air cycling through it. Why would you want to do this?

Well, for one, when you heat the building itself, you can open all the doors and windows, let all the warm air escape, close everything back up, and, instantly, be warm again – without having to add more heat.

The Empty Fridge

Warm masses heat you like the sun does: by sending you radiant heat. As I explained in Heating Your Home: Heat 101, heating and cooling differ only in perspective. If you can get your head around that, then the Empty Refrigerator Effect explains why heating air is an inefficient way to heat a home.

Like it says in the fine print, refrigerators achieve their rated efficiencies only when they’re full. Every time you open the door, warm air enters. If the fridge is empty, the inside temperature may go up five or ten degrees. On the other hand, if the fridge is full of food when you open the door, warm air still enters, but the food absorbs the heat and gets warmer by less than a single degree. It takes many such door openings before the fridge has to turn on. With proper design, your house can be a full refrigerator working for you.

To make your house a full fridge, it has to have substantial thermal mass. Thermal mass is usually heavy, but that’s not the point: things with high thermal mass absorb lots of heat before they get warmer, and lose lots of heat before they get cooler. More traditional building materials – stone, brick, and earth – have high thermal mass. Concrete (and water) also have high thermal mass. Unfortunately, drywall, studs, paint, fiberglass insulation, and the other flimsy stuff we generally use to make houses in the U.S., don’t.

To do you any good, the mass has to be on the inside of your home – ideally, with insulation behind it. Unfortunately, few walls are built with adequate thermal mass, and of these, fewer still have insulation behind that mass. This is not so in northern Europe, Scandinavia, and various cold, but otherwise enlightened, countries.

Heating Floors: Radiant Floor Heat

If not in the walls, then where do you put the mass? The floor. Slab floors are a pragmatic and very effective way of adding thermal mass to houses utilizing contemporary stick framing. Typically, tubing, or less commonly, plenums (duct-like voids) run through the slab. The slab can be heated by pumping warm water through the tubing or warm air through the plenums. This setup with water circulating through PEX tubing is called a hydronic floor heating system, or a radiant floor. For weight reasons, the heated slab for radiant heat has to be included in the original design for a suspended floor/crawlspace system. Thin slabs, sometimes poured, self-leveling gypsum concrete (gypcrete) – with correspondingly less mass – are more typical in retrofits because many suspended floor systems can support them with little or no framing changes.

Overall, radiant floor heat is amazingly efficient and effective. People and pets find they are comfortable at a lower ambient air temperature when standing (or laying) on a warm surface. This means you can maintain the same level of comfort with a lower thermostat setting, saving energy and money. Radiant floor heating systems generally use natural gas or another non-renewable fuel to heat the water.

Upcoming articles in this series talk about wood heat: why fireplaces and woodstoves are inefficient and pollute, and what to use instead to get clean, efficient heat from wood.

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