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Old 07-09-12, 07:53 AM   #1
Daox
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Default 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

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Drywall can used as a thermal mass material, but only to a fairly limited degree. It has a density of 50 lbs/ft^3 and an R-value of nearly 1 per inch. This means that it can only hold so much heat/coolth, and you don't get a lot of benefit from multiple layers because the R-value is high (again, relative to context) so it will not tend to conduct heat to the deeper mass layers.

If you still think this is a good idea for your situation, I would recommend using a heavy-weight plaster over drywall. A cementious plaster can have a density of ~100 lbs/ft^3, and an R-value of 0.3 to 0.6 per inch. Both of these statistics make it a much better space-facing thermal mass material than straight drywall.

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Old 07-09-12, 11:33 PM   #2
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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.
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Old 07-10-12, 06:13 AM   #3
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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.
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Old 07-10-12, 06:56 AM   #4
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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".
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Old 07-13-12, 01:46 PM   #5
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Quote:
Originally Posted by Daox View Post
...Ideas are welcome...
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.

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Old 08-06-12, 01:02 PM   #6
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BASF Micronal additive for plaster PCM in bulk, 3/8" skim of plaster with micronal = 8" masonry! Phase Change Materials - BASF Dispersions & Pigments
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Old 08-06-12, 01:44 PM   #7
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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.
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Old 08-06-12, 02:13 PM   #8
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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
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Old 08-07-12, 07:34 PM   #9
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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.
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Old 08-07-12, 09:51 PM   #10
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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.

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