How to best use fiberglass batts for insulating a suspended concrete slab
I live in western Wyoming, zone 7. I am building into my uninsulated barn to add a bedroom and mudroom onto my house. The barn is on an existing uninsulated slab. We want to suspend another concrete slab on joists with hydronic heating in the suspended slab.
In preparation for the addition we are tearing down siding and removing existing batt insulation and have a bunch of fiberglass batts left over. I don’t want to throw them out, so I was thinking about using them to insulate under the suspended slab. (we are replacing old batt insulation with flash and fill with 2″ of polyiso board with reverse board and batt siding) Is there a way I can well insulate the floor using the batts, or should I chuck them and use spray foam?
PS. Thank you to everyone for your informative Q & A’s. I have learned so much from this website. Everyone on here helped me design my addition from the insulation, siding, heating, windows, etc…
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Replies
Justine,
Is this suspended slab going to be for a first-floor room or a second-floor room?
If it's a first-floor room, I would skip the joists. Just install one or more layers of rigid foam insulation on top of the existing uninsulated slab, and pour a new slab over the rigid foam.
If it's a second-floor room, you could use your left-over batts to insulate between the joists if you want. But they won't be very effective. You'll get much better performance if you include a continuous layer of rigid foam -- either under the joists or between the plywood subfloor and the new slab.
That makes sense. One more question. I've read that polyiso is a good alternative to xps board to use as sheathing with flash and fill. I want to use it, but the guys who sell it here say they don't like it. Any thoughts? I also read that I should not use foil faced polyiso in this application. Is this correct? And if it is can anyone recommend a brand that sells unfaced, because our distributor said they don't know of any companies making it.
Justine,
What do you mean by "flash and fill"? Most people use the phrase to describe spraying a thin layer of closed-cell spray polyurethane foam against the interior side of wall sheathing, followed by blown-in fiberglass insulation to fill the rest of the stud cavities.
If that's what you mean, it has nothing to do with polyisocyanurate, which is a rigid foam product sold in sheets.
Sorry about the confusion. I am insulating the walls with 2" of spray foam, 3" of blown in fiberglass, osb, and 1" of rigid foam, then reverse board and batt siding. I had read that polyiso board would be a good alternative to xps rigid insulation. The distributor was recommending using the polyiso. I hope this clarifies my question
Justine,
Polyiso is a good product, so I have no idea why your supplier "doesn't like it."
However, I don't recommend that you sandwich your OSB sheathing between two layers of foam (polyiso on the exterior and closed-cell spray foam on the interior). It's much better if your wall sheathing can dry in at least one direction.
Choose one location for your foam. Don't make a foam sandwich. Choose to put your foam on the exterior if you want -- that would be polyiso. Or if you prefer, put your foam on the interior -- that would be spray foam. But don't do both.
Sorry about the confusion. I am insulating the walls with 2" of spray foam, 3" of blown in fiberglass, osb, and 1" of rigid foam, then reverse board and batt siding. I had read that polyiso board would be a good alternative to xps rigid insulation. The distributor was recommended not using the polyiso. I hope this clarifies my question
I think I understand. I'd like to ask one more question to make sure I have this right. If I chose to use the closed spray foam on the interior then I should not use rigid insulation at all? Would it be sufficient in zone 7 to use the spray foam, fiberglass fill, osb, and then siding? Is there anything else you would suggest?
Justine,
I advise you to have an R-value target for your walls. The 2012 IECC and IRC call for R-20+R-5 insulation or R-13+R-10 insulation in your climate zone.
However, many energy-conscious builders prefer to aim for R-40 walls.
It's up to you to determine your target for your wall R-value. Once you know your preferred R-value, you can design a wall to meet your goals.
My own preference would be for a 2x6 wall filled with cellulose, protected by 2 or 3 inches of exterior polyiso. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.