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Old 05-21-2012, 07:31 PM   #1
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Knee wall insulation with foam panels


Moved into a new (to me) house with a bedroom over the garage. On both sides are 8ft "knee" walls. Faced R19 insulation is installed between the studs and the previous owner installed FOAMULAR 250 Extruded Polystyrene (XPS) Rigid Foam Insulation over the back of the studs. My questions are as follows:

Does this create two vapor barriers?
Some online reading says this is okay so long as the stud cavities are sealed, what does this entail?
Can I keep this setup if the foam facing the roof is covered to NYS specifications (thermal barrier)?
Should I remove the foam ASAP?!?!?!?

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Old 05-22-2012, 08:49 AM   #2
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Knee wall insulation with foam panels


Technically it does create two vapor retarder levels. Not ideal but can be okay if you seal the foam up well.

Make sure you seal all the seams in the foam tightly and seal the baseboard on the inside as well.

It requires an ignition barrier but I would not remove it. A suitable ignition barrier would be additional non-combustible insulation like fiberglass or roxul or drywall.

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Old 05-22-2012, 06:48 PM   #3
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Knee wall insulation with foam panels


Here are some images.

There are SMALL gaps between the top of the foam panels and baffles. At the bottom the foam is sitting on top of insulation running between the floor joists with another 16" x 16" piece of unfaced insulation laying at the bottom. Would you say this is sealed properly?

As you can see I need the ignition barrier.

Thanks for the help
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Old 05-22-2012, 11:38 PM   #4
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Knee wall insulation with foam panels


Make sure the foam goes down and seals off the joist cavities at the bottom of the kneewall. This will help seal off the underfloor and stop that draft.
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Old 05-23-2012, 04:08 PM   #5
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Knee wall insulation with foam panels


The foam goes past the bottom of the knee wall but does not seal off the joist cavity. Would you suggest I cut the insulation and extend the foam to the sheet rock which is screw to the underside of the floor joists and add my ignition barrier?

Also, why wouldn't the two vapor barriers cause an issue in my case? Not trying to question your logic, I would just like to have a better understanding.

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Old 05-23-2012, 09:26 PM   #6
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Knee wall insulation with foam panels


That works.

As long as the bulk air movement is stopped from both sides, vapor movement is minimal. Combine that with the fact that the foam raises the internal temperature of the wall stud to a point where condensation is extremely unlikely, it does not normally present itself as a problem. Like I said, not ideal but if you seal up everything, you should be fine.
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Old 05-26-2012, 01:20 AM   #7
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Knee wall insulation with foam panels


Air-seal the electrical boxes and run a bead of caulking under the drywall to floor (add backer-rod if needed) behind the baseboard. Part of the new Code: http://publicecodes.citation.com/ico...002_par021.htm

The f.b. is rated at +-0.75 perms, the paper facing is variable; http://www.buildingscience.com/docum...vapor-barriers

To add to WoW's correct explain: https://docs.google.com/viewer?a=v&q...4EyM707m0gPdXQ

Great you added to the R-19 (R-17 when compressed to 5-1/2"), down to R-? with exposed side to wind-washing in the attic, down more R-value; due to inherent convective loops in that low-density batt insulation; The "biggest loser" in fiberglass insulation....

Still going down.... by 28% loss due to "thermal bridging" of whole-wall R-value vs. "clear wall'. Not now, bonus!

You may not need an ignition barrier, check with your local AHJ. Because....

No attic storage, no air exchange, etc. #4.2.1 a-f: http://commercial.owenscorning.com/a...579af27e5d.pdf This may be why it is not covered, check before as said, or cover for "peace of mind", if worried.

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Old 06-13-2012, 02:07 PM   #8
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Knee wall insulation with foam panels


Quote:
Originally Posted by GBR in WA View Post
You may not need an ignition barrier, check with your local AHJ. Because....

No attic storage, no air exchange, etc. #4.2.1 a-f: http://commercial.owenscorning.com/a...579af27e5d.pdf This may be why it is not covered, check before as said, or cover for "peace of mind", if worried.
The Owens Corning, "A Guide to Home Insulation" (found here) states:

1. Once the batt insulation has been installed flush with the underside of the framing, install FOAMULAR C-200 or InsulPinkRigid Insulation boards against the framing, using nails and washers.

2. Install a continuous and sealed polyethylene air/vapour barrier over FOAMULAR C-200 Insulation. Install gypsum board over wood strapping. FOAMULAR C-200 or InsulPinkcan act as the air/vapour barrier with sealed joints. Consult applicable building code.

Last edited by Lascaux; 06-13-2012 at 02:09 PM.
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Old 06-13-2012, 10:50 PM   #9
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Knee wall insulation with foam panels


That link is from O.C. in Canada with a poly on the inside, under the drywall, not outside the foam board, in attic. If interested in f.b. in attic and crawlspace, three pages, not too long ago: follow the links I gave; crawlspace walls

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Old 07-17-2012, 01:22 PM   #10
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Knee wall insulation with foam panels


Perhaps, it would help if I provided the correct link to Owens Corning's A Guide to Home Insulation (pdf).

The relevant section is "Attic, Below Rafters" on page 43. It states,

FOAMULA R C-200 or InsulPink boards may be installed below the attic rafters to achieve maximum assembly thermal resistance while maintaining ventilation space above the installed batts. It is a good practice to fasten strapping through the foam boards to the rafters for easy attachment of drywall.

Yes, it is from Owens Corning in Canada.

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