Insulating a Cold Floor
I am insulating the ceiling of a tuck-under garage as well as running new circuits to my garage. I’ve read this article but I have a few questions. I am in southern NH climate zone 5/6. https://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor
1. Can I use polyiso insulation here on the underside of my floor joists? I know the perm rating is much lower than EPS. Do I need to be worried about moisture getting trapped?
2. The rigid foam layer (if properly detailed) is the air control layer right? So I don’t need to worry about making the drywall air tight?
3. Does anyone have good examples of air sealing electrical penetrations through a rigid foam layer?
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Replies
1) Floors are not roofs or walls so permeability of the rigid doesn't matter in this application. I would use foil faced as it is easy to tape.
2) You need one solid air control layer, could be the foam or drywall. Both work just as well.
3) Get device boxes with gasketed flanges (sometimes called vapor tight). Offset the flange from the stud face the thickness of the rigid so it will mate against the back of the drywall. Install the rigid with a gap around the device box and foam the gap with canned foam. You can also tape the flange to the rigid for a bit of extra sealing.
You can also strap out the rigid with 2x3 / 2x4 on flat and use shallow device boxes the fit into the 1.5" space.
Thank you for the reply and for the great information! That's great I'll just use the foil faces polyiso.
I think I would like the strapping underneath the foam to make the drywall easier to install and to make potential modifications (provisions for EVs) easier to make in the future. The article calls out using 1x4's for strapping. Do you think the 2x4's would make the drywall ceiling look lumpy?
The thickness of the strapping doesn't matter, it won't effect the drywall finish at all. If anything, the 2x4 will be straighter so a flatter ceiling overall.
Thanks for the info
The nice thing about 1x4 or even 1x3 is you can hold them with one hand while using a screwgun or nailgun with the other. 2x4 is a lot harder to put up.
That's definitely fair, but air sealing the electrical penetrations will be way easier if the entire electrical box is sitting below the foam layer, which could only happen if it's a 2x and not a 1x
If you strap the wall as Akos mentioned, you can use standard 1.5" deep "4 square" boxes, and mudrings to give you device openings (one or two gang) up to the surface of the drywall. These are standard parts used commercially all the time, and they're available at the box stores and every supply house for reasonable prices. You also get more room for wiring when doing things this way instead of using single gange boxes -- especially shallow single gang boxes!
The white fiberglass hardboxes are another option that are better about air sealing, but you have to be more careful with those since they don't have gaskets. What they offer is a lack of leaking holes in the box itself. I use these and tape the air barrier to the perimeter of the box using flashing tape.
I would still detail your drywall airtight as a secondary air barrier, but the taped polyiso alone would be sufficient.
Bill
Thanks for the reply and for the great information! That's a good idea to use the 1.5" deep metal electrical boxes and attach them to the 2x4 strapping. I was originally planning on doing this and then making the foam my air barrier. How would I make the drywall air tight with these types of boxes?
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