Thermal mass - plaster vs drywall
 

Just wondering how much you guys know about plaster and drywall as far as thermal mass goes. I did some googling and found the quote below. It seems plaster would be a fair amount superior. As I think about moving forward with my office project, I'm also weighing going ahead with plaster vs drywall. The room currently is almost entirely plaster and I wouldn't mind keeping the thermal mass it provides. This is what keeps my house cool during the day in summer without A/C. However, the additional insulation I add will surely help.

Next up, has anyone done plaster work? Is it a huge pain to do? I was hoping to maybe keep things simpler and go with a cement board and just a thin coat of plaster over it to retain the thermal mass properties, but reduce the amount of work. Ideas are welcome.


Thermal Mass | GreenBuildingAdvisor.com

I've done plaster and stucco on straw bale houses, I've also done plaster repair and plaster takes skill, the up side is plaster is not perfectly flat! so when you notice a line from a seam in dry wall it's because the rest of it is flat and smooth, plaster is not so a variation blends in quite well!
If you do go with plaster, Durrabond is a nice base coat, it's not easy to sand, you scrape it to get bumps out, the Easy Sand dry mix is the way to go for a finish coat, it's a great dry wall joint compound as well because it drys faster and shrinks less, but once you mix it you are stuck using that or loosing it.

Blue Board looks like dry wall, but is grey/blue and denser, it has a course paper on it that is designed for a plaster like skim coat to give that plaster look and feel.
Cement board should work the same way but I've never used it.

You can also do blood lath, it's the metal lath that you nail to the studs, put a coat of mortar on as a base coat, then a brown coat that in plaster terms might be something like durrabond, then a sandable coat to finish it.

Does anyone know of just a more dense drywall type material that could be used? I'm not in it specifically for the look, more just for some added thermal mass.

There is drywall used for acoustic dampening purposes which is very heavy and dense. You can get 3/4" I believe also. Look for "quiet rock".

How about natural earth plaster? I've come across super-tight home construction projects that are using it for it's humidity-moderation characteristics, as well as thermal mass.


Can't beat the cost, & environmentally friendly qualities.

If you want to go high-tech, there's Phase Change Material that is encapsulated in plasterboard...


Look on the bottom of the linked page for thermal storage characteristics.

-AC

BASF Micronal additive for plaster PCM in bulk, 3/8" skim of plaster with micronal = 8" masonry! Phase Change Materials - BASF Dispersions & Pigments

Daox,
Having been in your Home and seeing the recessed floor, I would suggest a massive thermal mass of a thinset (self leveling) mortar layer under the Wood laminate floor you plan to install. This would solve the leveling issue while providing waaaay more thermal mass than plaster. I also recall the possibility of in floor heating ? Again, waaaay more mass than you have now.

US Architectural has a product called Versaroc, (picture osb impregnated cement board)
3/4" material is 5lbs/SqFt. Works well with normal woodworking machines. https://architecturalproducts.com/products.html

Further down in that GBA quote is:

A typical 2,000 SF home may have 9 tons of drywall. So doubling even part of that will add considerable mass that is well-coupled with the interior environment.

And:

Gypsum has twice the specific heat by weight of concrete or brick and 2 to 3 times that of stone, so in spite of its lesser density (~50 pcf), has nearly the same heat storage capacity as masonry per cubic foot (except for very dense stone such as granite, marble or soapstone).

I have experience with two different houses that were insulated to well beyond current Wisconsin code minimum. Both of these houses are on crawlspaces, and both have only the typical 1/2" drywall with no other thermal mass.

The inside temperature rises about two or three degrees per day when the outside temperature hits the 90's. The better the insulation and air sealing, the less need for thermal mass.

You mentioned concrete, concrete is very heavy though. I talked with a builder in Wisconsin at the Midwest Renewable Energy Expo that builds low energy buildings that they aren't using furnaces in. They build the entire house from basement to the top floor with insulated concrete forms. He claimed that the concrete does a good enough job with moderating temperature swings in the winter that they skip installing ductwork and dealing with natural gas and just install Daikin heat pumps. I asked about backup heat, he says that Daikin has some units with backup strips but that they are good below zero so their use is minimal. I'd assume with probably 5/8" drywall plus wrapping the house in concrete and efficient windows could be a good option. Not for retrofit, but for new construction.

I am thinking about this again as my office remodel is getting pretty close to needing walls.

I am a bit more concerned than ever about thermal mass. When I did the demo work, we remove over four trash cans full of plaster from that one outer wall. That is a LOT of thermal mass now gone.

Anyway, I was thinking perhaps a nice compromise would be 1/2" cement board behind 1/2" drywall. Thoughts?

Did you look at Quietrock? Your cement board idea sounds interesting. I wonder what the ROI will be when the cost of material and time is considered. If there is enough mass the wall will buffer changing temperatures and then the idea of ROI goes out the window in favor of comfort. Maybe two layers of the cement board? I don't know what the cost of that stuff is or hat your budget is. Ultimately a lot of the eco renovations are comfort related as the ROI is a number of years greater than any human life expectancy. That said, if I were building myself a new house I'd probably put CDX under the sheetrock for strength and sound insulation.

I'm really enjoying reading about your progress. You have my vote of confidence!

Well, the thermal mass really is my main means of A/C in the summer time (by cooling it down over night). Without it my house would overheat pretty quickly. The ROI is there by not having to install any A/C system.

I know adding a bunch of insulation and air sealing will help a lot. Its just hard to know how much is enough thermal mass. I just see what I took out and it seems like a TON.

http://energysmartalternatives.com/w...Oil-Prices.png

S-F,

Your statement is a very sweeping one. Also it is a statement that could severely dampen the initiative and gumption of readers of this entire forum.

Especially unfortunate if your statement was not true.

Can you back it up?

-AC

I am interested in various "overlayments" (newly coined word meaning a layer over a radiant floor sandwich, but under the final surface material) that might be used in a radiant floor. So, I did some testing of thermal transmissivity on various cementitious building materials like Hardibacker, Durock, Wonderboard, and Sheetrock (white and green). Transmissivity is not exactly the same as heat holding capacity, but it follows along similar lines.

I found that all of the materials had quite similar characteristics. For my purposes, Wonderboard was the best, but it was made of concrete interspersed with other stuff, including styrofoam particles which probably wouldn't hold up well to foot traffic. Durock had the same constituents but sligtly lower transmissivity. Also probably would be bad for foot traffic. Hardibacker is made up of treated paper stuff and concrete, and is very strong in compression, but its transmissivity was the worst. But for floor use, would be the best of the boards tested.

Sheet rock was somewhere in the middle of the pack transmissivity-wise, but is very attractive in a price per square foot basis.

So I'd say to go with double layers of Sheetrock.

If you're going for thermal mass, you might consider covering both sides of your interior walls with plywood and filling them with sand... just make sure you have structure that can take the extra load.

Water is a wonderful thermal mass medium. As I recall, it holds about 5x the heat that cementitious material would per volume.

-AC

I went to the home improvement store on Friday night and did the incredibly unscientific method of picking up each walling sheet and seeing how heavy it was. I ended up picking picking up 1/2" concrete backer board (with NO foam beads in it), and 1/2" high density drywall to go over it. The concrete backer board has a density of 2.9 lbs/sqft. The high density drywall I got has a density of 1.75 lbs/sqft, and is labeled 'basement board'. The lightweight drywall they had at the store is 1.2 lbs/sqft.

So, my ~1" combo here is roughly the equivalent mass of nearly 4 inches (3.875") of lightweight drywall.

The cost was roughly 3X as much as just going with 1/2" lightweight drywall as well. That concrete board is not cheap.

I also have to redo the ceiling of the office. I think for this I'll use the 'basement board' / higher density drywall.