# Knee wall insulation with foam panels



## steviecg2 (May 21, 2012)

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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## Windows on Wash (Aug 30, 2011)

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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## steviecg2 (May 21, 2012)

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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## Windows on Wash (Aug 30, 2011)

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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## steviecg2 (May 21, 2012)

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.

Thanks


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## Windows on Wash (Aug 30, 2011)

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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## Gary in WA (Mar 11, 2009)

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/icod/irc/2009/icod_irc_2009_11_sec002_par021.htm

The f.b. is rated at +-0.75 perms, the paper facing is variable; http://www.buildingscience.com/documents/reports/rr-0004-air-barriers-vs-vapor-barriers

To add to WoW's correct explain: https://docs.google.com/viewer?a=v&...iZ9vwo&sig=AHIEtbRlhNBCWID8xGZ54EyM707m0gPdXQ

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; http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/

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/assets/0/144/172/174/5deb06d2-43b4-44b2-b4ec-20579af27e5d.pdf This may be why it is not covered, check before as said, or cover for "peace of mind", if worried.

Gary


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## Lascaux (Apr 17, 2012)

*Owens Corning - A Guide to Home Insulation*



GBR in WA said:


> 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/assets/0/144/172/174/5deb06d2-43b4-44b2-b4ec-20579af27e5d.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 InsulPink™ __Rigid 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 InsulPink™ __can act as the air/vapour __barrier with sealed joints. Consult applicable building code._


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## Gary in WA (Mar 11, 2009)

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; http://www.diychatroom.com/f103/crawlspace-walls-129582/

Gary


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## Lascaux (Apr 17, 2012)

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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