Thermal mass - plaster vs drywall

[Daox]

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?

[S-F]

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!

[Daox]

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.

[AC_Hacker]

Originally Posted by S-F

...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...

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

[AC_Hacker]

Originally Posted by Daox

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.

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

[Daox]

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.

[Daox]

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