How to insulate kneewalls or interior gables?
I have a room with a vaulted ceiling. The interior gable wall obviously extends to the ridge. On the opposite side of the gable wall is a vented attic space with insulation on the attic floor. I would like to insulate this gable wall against the attic.
I need a way to hold batt insulation in the stud bay so they do not fall out into the attic space. What do people suggest? Staple fishing line to the studs? Secondly, is “air washing of this insulation also an issue? I can see air washing being an issue in cathedral ceiling rafter bays as vented air is supposed to continually wash over the top of the insulation as it rises to the ridge, but in a gable application as I describe there is far less air movement. If an issue, what can I use to create a barrier? It seems if I use ridge foam I might be creating a moisture trap in the wall.
A similar question involves my knee walls. I know many suggest insulating the rafters instead, but in my application the floor outside the knee wall is actually over a covered outdoor porch so it makes no sense to insulate the floor and rafter bays, but to insulate the knee wall insteadl. I suppose I could use structural sheathing, but that is heavy, hard to work with in the space, relatively expensive and offers no side benefit of added insulation.
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As you correctly guessed, you need to have a durable air barrier on both sides of any wall insulated with fiberglass batts. A wide variety of materials can be used, including rigid foam with taped seams, drywall with taped seams, OSB with taped seams, or carefully installed housewrap.
My favorite material in these circumstances is foil-faced polyiso, because it's the easiest type of rigid foam to tape.
Most building inspectors will allow rigid foam to be installed in this location without protecting the rigid foam with drywall; but make sure that you are in compliance with local regulations on this detail. It's possible that your local inspector will require any rigid foam to be protected with drywall.
I like the foam plan, but does this create a moisture trap, particularly if you use the foil=faced foam. Thank you so much for the reply!
The foam doesn't create a "moisture trap."
There are design guides for the minimum thickness of exterior rigid foam, and you can follow these design guides if you want. Here is a link to an article that explains everything you need to know about minimum foam thickness: Calculating the Minimum Thickness of Rigid Foam Sheathing.
The bottom line: Properly installed, exterior rigid foam does not create a "moisture trap." For more information on this issue, see Worries About Trapping Moisture.
One final point: When it comes to rigid foam on the back of kneewalls or walls facing attics, it really isn't necessary to meet the minimum R-value requirements for exterior rigid foam, because these spaces aren't at outdoor temperature -- they are generally warmer. Condensation problems in this type of wall are almost unheard of.
Martin - one more question
(Actually getting around to doing this job)
If I use foil face foam on the back sides of the studs, and use unfaced FG, do I or can I or should I use a 4 mil poly on the inside of the wall under the drywall?
Unless you are in a very cold climate zone, poly under drywall is not recommended. If your inspector requires it, try a product like Membrain which varies its vapor retardance based in the conditions.
Sorry, Zone 6 - north Idaho. Poly is common, but I wonder about the double vapor barrier. Martin, who I respect as someone who seems to know more than a thing or two, says the foam is a pretty safe bet, but I failed to see if that was qualified with "don't put poly on the inside". Seems like he would say don't do it in this case. But I wished to check as I do think it makes sense to do it in other applications in this zone.
It seems if I air seal the kneewalls where drywall meets the floor and all outlets, thus keeping most moisture out of the wall, what little moisture that does get in will dry to the inside.
Don't install any polyethylene on the interior. As you guessed, walls with exterior rigid foam are designed to dry to the interior. An interior air barrier (usually provided by the drywall, along with attention to air sealing at electrical boxes) is much more important than an interior vapor retarder.
That was my plan. Thanks for confirming!