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Insulate exposed sub-floor

Posted: Thu Feb 21, 2019 3:45 pm
by stonesy

I wanted to share my plan for addressing a zero-insulation issue that I hope to fix myself. Purchased a new home at the end of 2018 and the bedroom is downstairs, over an exposed area. The previous owners never bothered to put any insulation under the bedroom sub floor, as a result, the bedroom floor is very very cold, and the bedroom itself is colder than I would like,

My plan is to:
1. Use an application of foam insulation to seal the edges of each void between the joists
2. Wrap the pipes that are exposed
3. Use batts of R-30 or R-38 fiberglass insulation to fill the voids, with the foil side facing “up” against the sub-floor
4. Protect the insulation from rodent invasion by sealing each void with polyshield insulation blocks that extend over the joists to prevent thermal bridging.
5. Seal it all with a layer of plywood, probably 1/2” or so

Is this overkill? Underkill? I am in California and it rarely freezes, but has lately. Any of these steps redundant? Advice appreciated.


Re: Insulate exposed sub-floor

Posted: Thu Feb 21, 2019 8:11 pm
by Shannon
Sounds like this is a room on stilts or over a crawl space?
I would think that either the spray foam or the fibreglass would be enough in your area but I could be wrong. Spruce is our Cali. guy so he will have some first hand knowledge from closer to home for you. I'm sure he will be along here shortly.

Re: Insulate exposed sub-floor

Posted: Mon Feb 25, 2019 7:07 pm
by stonesy
Hi Shannon,

Yes, the room is up on stilts, but not over a crawl space, it's over a completely open area. Roughly 8-10 feet of open air between the sub-floor and the ground. I've attached a photo, and you can see the wide-open subfloor behind the concrete support pillar...

altavista_from_below_skinny.jpg (68.07 KiB) Viewed 162 times

Re: Insulate exposed sub-floor

Posted: Mon Feb 25, 2019 9:41 pm
by Shannon
I figured that must be the case .
I think adding all that insulation is likey way over kill for your area?
Let me see what Spruce says.

Re: Insulate exposed sub-floor

Posted: Mon Feb 25, 2019 11:06 pm
by A. Spruce
Wow, that's quite the house!

I've dealt with a similar situation. I filled with fiberglass batt insulation then sheeted with 3/8" no-groove siding. Satisfied the inspectors. Oh, I did insulate the plumbing as well.