How can I add thermal mass to my house?

Question

We're part of a real-time pricing program with our electric utility company, and a good strategy for offsetting electricity use during peak demand times is to pre-cool the house. The more thermal mass that our house has, the more effective the pre-cooling will be.

I've read about phase-change materials and the promise of Thermal Core PCM drywall, but it seems like the development into a commercial product isn't going to happen any time soon. I've seen various vessels filled with room-temperature PCMs for sale, but I have no idea how much I would have to buy in order to make a noticeable difference in thermal mass of a house.

Is there a practical way to add thermal mass to my house? I've heard of people suggesting adding large drums of water, though I'd be leery about doing so due to the possibility of structural damage and/or leakage. I also considered loading up our side crawl with bricks, but I'm not sure how much I could safely add without again running into structural concerns.

Answer 1

I have about 50 55-gallon drums stacked in my basement, all filled with water. They are stacked one on top of the other (bottom row/top row) along our north basement wall. I have two in floor-mounted fans, one pushing, one pulling, mounted in the ground-floor joist cavity (from above they just look like floor mounted cold air returns). During the day, the fans circulate the air around the house. Our furnace has a bypass door on the cold-air return that opens a door into the basement cold-air return. This way, during the summer the furnace fan (on 365 days a year) pulls all the intake air from the basement. With so much air coming from the basement, our 2 to 3 ton AC units run maybe 6 days in the entire summer.

The system is cheap to run (about 1/4 of our cooling costs with just the AC units). I got all the barrels for about $200, and I have emergency water for years if i need it. Plus, my house won't blow away in a gale.

Answer 2

One boringly unexciting approach is double drywall (and don't choose the lightweight type) though that is more often done when building or rebuilding/remodeling than as a retrofit without some other rebuilding/remodeling going on. It easily and unobtrusively adds a significant mass to the interior structure.

Bang for the buck, and in most cases pound for pound, water wins. Much higher thermal mass than an equivalent poundage of masonry. While I'm not aware of many residential applications doing so, you could certainly store quite a bit of cooling by freezing ice during the off-peak hours and blowing air over it during peak hours. https://en.wikipedia.org/wiki/Ice_storage_air_conditioning

Aside from the expense, most room temperature PCMs are "wax-related" in a manner that means they are flammable. That becomes concerning when talking about enough to make a difference in your house, rather than a personal cooling vest application.

Answer 3

This article provides excellent information about maximizing thermal mass:

Phase Change Materials (PCMs):

There is growing interest in the use of PCMs as a lightweight thermal mass substitute in construction. All materials require a large energy input to change state (i.e. from a solid to a liquid or a liquid to a gas). This energy does not change their temperature — only their state. For that reason, it is called ‘latent’ (i.e. latent heat of melting or vaporisation). Phase change temperatures vary enormously between materials.

Phase change materials, or PCMs, may be a useful lightweight substitute for thermal mass.

Materials that melt between 25° and 35°C are very useful for storing passive solar gains. Any temperature increase over a desired thermal comfort level is absorbed by the PCM as it melts. This energy stays stored until the PCM starts to solidify again as temperatures drop at night. As it solidifies, it releases the stored heat.

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For example, the thermal capacity of a 13mm thick plaster layer with 30% microcapsule content is claimed to be equivalent to that of a 150mm thick masonry wall.

PCMs or water filled containers have much greater thermal storage capacity than masonry and can be used as a mass substitute. PCMs are much lighter than masonry. Water has double the storage capacity of concrete and because of convection within the container, penetration rates are substantially higher. Thus water can supply similar storage capacity to masonry with significantly less mass and bulk. Accordingly, both can be cost effective mass options for upper storeys because they require no (or less) additional structural support.

Photo: Mike Cleaver, Clever Design

Water filled balustrades provide abundant thermal mass as part of this mezzanine balcony.

Answer 4

The only good solution to for this that I know of is pumped geothermal heat storage. In this system, a thermal well us used to store heat underground, and a heat pump is used to move energy between the house and the ground. Theoretically, if your local geology permits it, this lets your air conditioning during the summer store heat that can be recovered during the winter.

Note that the thermal mass here may exceed that of the house itself...

I have friends who had such a system installed, but I haven't heard them either complain about it or enthuse about it. I can ask...