Green Design Pyramid

Published on May 25th, 2008 | by Chris Schille

14

Heating Your Home: Thermal Mass

PyramidAuthor’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.

Previous Articles in this Series:

Related Articles:

Photo credit: lyng883 via Flickr




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About the Author

Chris Schille discovered his true passions early in life when his parents moved to a piece of bare land in Humboldt County, California. There they built their own passive solar home, planted a huge organic garden, and joined a community striving for self-sufficiency. It was there that Chris developed a life-long love for the natural world and rural life. Chris holds degrees in mechanical engineering and computer science. After a ten-year stint in software, he left to design and build his own passive solar home (in Humboldt). A love for all aspects of building, and concern for its environmental costs, led him to start his own residential building business, Rustic Precision. He lives with his wife and daughter in Cupertino, California.



14 Responses to Heating Your Home: Thermal Mass

  1. Anna Hackman says:

    I have radiant in certain parts of my home and love it. You can’t keep the temperature lower but at the same time be warmer. The only thing I don’t like is the price to put in radiant. It is expensive.

    What is your feeling about staple up vs gypcrete with cost versus efficiency? Anna http://www.green-talk.com

    Unfortunately, the simple answer to your question of cost is that it depends on whether or not you need to do anything with your existing floor. For example, if you have carpet that you intend to tear out and replace with tile, that makes the gypcrete option significantly more affordable, though it still may not be as (installation) cost-competitive as staple-up. If you’re happy with your flooring the way it is, staple-up is likely the way to go.

    As to performance, that depends on the use of the area. The less thermal mass you’re heating, the more “responsive” the system; that is to say, it reaches the desired temperature more quickly. For example, if you like to sleep in a very cool room, but want a little heat to help you rise in the morning, a bedroom floor that heats relatively quickly might serve you better than a floor that takes an hour to warm but can hold heat for much of the day. That said, continuously occupied spaces are most efficiently heated with more thermal mass (a thin slab, like gypcrete, or a deeper slab).

  2. Anna Hackman says:

    I have radiant in certain parts of my home and love it. You can’t keep the temperature lower but at the same time be warmer. The only thing I don’t like is the price to put in radiant. It is expensive.

    What is your feeling about staple up vs gypcrete with cost versus efficiency? Anna http://www.green-talk.com

    Unfortunately, the simple answer to your question of cost is that it depends on whether or not you need to do anything with your existing floor. For example, if you have carpet that you intend to tear out and replace with tile, that makes the gypcrete option significantly more affordable, though it still may not be as (installation) cost-competitive as staple-up. If you’re happy with your flooring the way it is, staple-up is likely the way to go.

    As to performance, that depends on the use of the area. The less thermal mass you’re heating, the more “responsive” the system; that is to say, it reaches the desired temperature more quickly. For example, if you like to sleep in a very cool room, but want a little heat to help you rise in the morning, a bedroom floor that heats relatively quickly might serve you better than a floor that takes an hour to warm but can hold heat for much of the day. That said, continuously occupied spaces are most efficiently heated with more thermal mass (a thin slab, like gypcrete, or a deeper slab).

  3. weee says:

    Another great way to make new buildings energy self-sustaining.

  4. weee says:

    Another great way to make new buildings energy self-sustaining.

  5. Evan Chekas says:

    I have recently purchased a wood pellet boiler for my my baseboard heating system. I would like to hook it into a thermal mass to improve its efficiency. Does anyone have any suggestions on how I could do this or a resource I should investigate? Thanks for any help.

  6. Evan Chekas says:

    I have recently purchased a wood pellet boiler for my my baseboard heating system. I would like to hook it into a thermal mass to improve its efficiency. Does anyone have any suggestions on how I could do this or a resource I should investigate? Thanks for any help.

  7. Rob Clifford says:

    I’ve seen a thermal mass constructed in a basement to sustain the energy produced which could be used in any slab. The floor had a 2m.x4m. hole cut in 1m. deep. The cavity was well insulated from the surrounding ground then lined with concrete. then the cavity was filled with coiled pex sand and gravel then concrete sealed over the top. Using sand and gravel heats quickely but still has a great thermal mass.

  8. Rob Clifford says:

    I’ve seen a thermal mass constructed in a basement to sustain the energy produced which could be used in any slab. The floor had a 2m.x4m. hole cut in 1m. deep. The cavity was well insulated from the surrounding ground then lined with concrete. then the cavity was filled with coiled pex sand and gravel then concrete sealed over the top. Using sand and gravel heats quickely but still has a great thermal mass.

  9. Roy Killam says:

    I’m thinking of using the earth under my slab as a thermal mass. I plan to lay insulation under the slab two feet in from the outer walls and up the knee walls also. Will this work ?? The ground is very dry and there is a ledge to the surface in spots also. Thanks, Roy

  10. Roy Killam says:

    I’m thinking of using the earth under my slab as a thermal mass. I plan to lay insulation under the slab two feet in from the outer walls and up the knee walls also. Will this work ?? The ground is very dry and there is a ledge to the surface in spots also. Thanks, Roy

  11. PEX tubing says:

    I mean continuously occupied spaces are most efficiently heated with more thermal mass

  12. Pingback: Adventures In Earthship Living | Emergency Survival Network

  13. Dave says:

    I am also thinking of going for wood pellet boiler can you give me any advice on this ?

  14. Greetings! Very useful advice within this article! It’s the little changes which will make the largest changes. Thanks a lot for sharing!

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