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Discussion Starter · #1 · (Edited)
I was approached by a friend with a situation regarding the insulation in the floor joists of the addition on their house.

The addition is a bar room that is approx. 13'x14' and is sitting on pier foundations, somewhat like a deck or sunroom, and is essentially sitting over a crawl space (which is open access from three sides, essentially, unconditioned). Currently, they have faced insulation (most likely R-30) that is laid in the floor joists (paper side up) and is exposed to the ground on the unfaced side. About 18"-30" of clearance. They also complained of this room being cold constantly.

Their plan is to rip all insulation down and replace with new. The idea of tacking 2" rigid foam insulation in the joist bays and sealing with the appropriate tape was brought up. Also mentioned was doing a double layer of 2" rigid foam, and bottom layer of R-19.

I'm assuming that I don't need to redo the vapor barrier because the existing insulation was stapled in and batts should pull out leaving the paper. Then again, I may be wrong on that. Should the paper come out as well?

Essentially, if the above idea is going to be ineffective, what other options do they have?
 

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I was approached by a friend with a situation regarding the insulation in the floor joists of the addition on their house.

The addition is a bar room that is approx. 13'x14' and is sitting on pier foundations, somewhat like a deck or sunroom, and is essentially sitting over a crawl space (which is open access from three sides, essentially, unconditioned). Currently, they have faced insulation (most likely R-30) that is laid in the floor joists (paper side up) and is exposed to the ground on the unfaced side. About 18"-30" of clearance. They also complained of this room being cold constantly.

Their plan is to rip all insulation down and replace with new. The idea of tacking 2" rigid foam insulation in the joist bays and sealing with the appropriate tape was brought up. Also mentioned was doing a double layer of 2" rigid foam, and bottom layer of R-19.

I'm assuming that I don't need to redo the vapor barrier because the existing insulation was stapled in and batts should pull out leaving the paper. Then again, I may be wrong on that. Should the paper come out as well?

Essentially, if the above idea is going to be ineffective, what other options do they have?
How about just closing it up (after filling in any open spots? Then check the walls and behind window casings, to make sure they're properly insulated.
 

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Discussion Starter · #4 ·
MarkJames said:
How about just closing it up (after filling in any open spots? Then check the walls and behind window casings, to make sure they're properly insulated.
Closing in the area under the addition, or do you mean closing in the joist bays with plywood sheathing?

If so, would moisture then become an issue?
 

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Sean
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If you do spray foam it (closed cell) you should still cover the underside of the joists with OSB or plywood

I would also recommend that even if you fit foam panels in place then FG it (cut slightly smaller & foam around the edges). One other option is use the foam panels, add the OSB / Plywood & then consider blown in FG or cellulose - just make sure you seal the seams good

As for the paper there - rip it out, both ideas listed above are vapor barriers / retarders already
 

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Discussion Starter · #7 ·
Thanks for the input so far.

If I take out an R-30 batt insulation, and put in a double layer of rigid foam, which is only rated like R-10 or 12, wouldn't I actually be taking a step away from making the space warmer?
 

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Open foundations require underpinning with plywood here. You could build a p.t. skirt and put batts or foam in. Otherwise the floor framing is exposed to high moisture levels with batts or foam and the underpinning is required.
 

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diplomat
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If the FG doesn't touch the subfloor, it can get cold from air infiltration or just the cold rim joist letting heat out. Often the FG will have a small gap above it, especially when installed from below.

I've done many floors on pier type foundations and the two tricks that work well and don't break the bank like foam are to have the fiberglass touching the subfloor and have 3/8 ply underneath. I assume the subfloor was glued really well and is fairly air tight. You could foam the 3/8 ply seams if you're worried about air infiltration.

I usually overstuff the fiberglass. 13" R38 for 2x10s, or two layers of whatever.

FG is better per inch when compressed, not worse: http://numsum.com/spreadsheet/show/21111
 

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Can't really compare foam(in any decent form) to fiberglass in r-value. You are not comparing apples to apples as the foambaord also has air sealing principles that get you the r-value effectively. If you just slap r-19 in with a pressure boundary, you are drastically cutting down your insulating value.
 

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diplomat
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Can't really compare foam(in any decent form) to fiberglass in r-value. You are not comparing apples to apples as the foambaord also has air sealing principles that get you the r-value effectively. If you just slap r-19 in with a pressure boundary, you are drastically cutting down your insulating value.
Absolutely true but if you have the space to get to the r value point of diminishing returns, then FG can be ok.
 

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Box Builder
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It sounds like they're real problem is not insulation but air leakage. R30 isn't bad insulation, it is just that fiberglass sucks at creating an air barrier. I would rip all FB and accompanying kraft paper out. I would then spray the joists and subfloor with a borate type product like Timbor. Then I'd close cell spray foam the bays to full depth. This would have to be done carefully with multiple layers. Then I would skin the underside of the joists with rigid foam at least 1" thick. I would also air seal the foam as best I could with caulking and tape. Then I would skin over the foam with some sort of painted osb or plywood. Probably ply as I hate osb. Seal the ply joints as well as possible by adding batons or something. You would get good R-value, a thermal break, and air sealing out of it. The closed cell should stiffen the floor and if it cracked (from floor deflection) the air sealed ridgid foam should help air infiltration.
 

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Discussion Starter · #13 ·
Thanks for all the ideas guys.

We ended up replacing whatever insulation we had to, skinned over with 1" rigid foam, and then a layer of half inch ply.

We'll see if the room stays warmer and that damn animal stays out.
 
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