Option to add thermal mass to 480 sqft addition
Hello all.
Here is an interesting holiday question, with some background first:
We are in the (slow) process of building a 20’x24’ stand alone addition to our small house, which will be connected by a ‘breezeway’ (which sounds basic but will likely now contain a bathroom and HRV unit). This build is located around 60degN in the Yukon.
The design is rather unique in that the vapour barrier (Protecto self adhesive wrap) is on the exterior of the 1/2” plywood sheathing, this will be followed by 2 layers (8” total) of Powerwool Rigidboard 80 (R 33.6). (Also, there will be no conventional vapour barrier (poly) on the interior walls). This amounts to plenty of insulation for the build and leaves the interior stud bays empty, as we do not specifically require insulation within the stud bays. The reason to leave them empty or minimize insulating from the inside is to allow the sheathing to remain warm and therefore avoid any potential condensation to form on the sheathing. Also, should any moisture develop in the walls, drying can occur either: a) from the exterior face of the Protecto out (to exterior) or b) from backside (interior) of the Protecto in (to inside). The ceiling does have poly vapour barrier on the inside with 2×4 strapping, the attic will contain at minimum 2’ of blown fiberglass.
So, here is the question: I have options, of doing nothing by leaving the stud bays empty and covering with drywall or some other finish, or completely fill the stud bays with cheaply sourced 2×4 lumber cut offs to add thermal mass to the structure, or something in between. Seems like there would be no harm doing either, the opportunity exists to try something interesting, any thoughts or advice?
I will be running through the thermal mass calculations to see if the 4080 square feet of 2×4 walls can accommodate an amount of timber to be useful in tempering temperature swings, make the inside more comfortable, and save a little on heating costs.
AST.
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Replies
yukonbuild,
That's a good wall!
- Don't forget that wood has an R-value too. If you are were leaving the cavities empty because you are worried about adding insulation there, the 2"x4"s would be adding about r-4.5 to the wall inside the sheathing.
- You have far more than enough exterior insulation for your climate zone to keep the sheathing from accumulating moisture, even if you fill the stud bays with batts.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
That's what I'd do, which will be more effective than increasing the heat capacity (thermal mass) of the structure.
This!
Add the cheapest batts you can in there and enjoy the super low heat loss.
Without even seeing the thermal mass calculations I'm a third vote for batts in the bays.
Another vote for batts. Mineral wool is easy to install well, but you have so much insulation in the exterior you don’t need to worry as much about getting the batts installed well. You could save money with plain R13 fiberglass batts and still do very well, with a wall getting up over R45 for your wall. You don’t really even need faced batts here, which might save you even more money.
Bill
Filling the bays with solid lumber will make future wiring changes or additions much more difficult.
Frank,
Packing the stud bays with drywall scraps used to be a pretty common practice here - ostensibly to help acoustical separations. It made any renovations so much more complex.
It cuts down on the dumpster bill too.
DC,
Which was the real reason it become popular. I learned some interesting new language from an ex-navy electrician trying to fish wires in a house with drywall scraps in the walls.
Hi. Yikes, filling the stud bays with used drywall should terrible.
I would never leave a random contractor unsupervised on my builds.
Hi. Noted regarding future wiring, i will consider this in my final decisions, I could leave chases.
Batts or blown insulation. Feel free to enter "thermal mass" into the search bar if you want to see dozens of conversations about it. Insulation (and air-sealing) wins in nearly every situation.
I will check into past conversations, thanks for the note.
The air sealing is mostly by the protecto, it is tied in to the ceiling poly, and spray foam around the windows, from the inside.
The mass could be helpful if you are heating with a wood stove and are not getting up to feed the stove at night but if you have an automatic heat source it is not doing much.
Consider installing drywall for fire safety if nothing else.
Walta
Walta, I haven't done the math but I bet the extra R-13 or so of insulating the stud bays, assuming relatively airtight construction, would do just as good a job of maintaining the indoor temperature as adding heat capacity. And it works all year long.
Hello.
Indeed thermal mass would cut the temperatures swings, we will not be using a wood stove, likely a propane fireplace.
Drywall or some equivalent for safety, of course.
My wild guess is that your gas fireplace will put about 30% of the BTUs inside the cabin and send 70% up the flu pipe while looking very pretty.
Walta