Hi, I've learned a lot in the short time I've been following these forums, and I'm trying to put it to use.

I have a split level home built in 1960, in zone 5 (southern PA).  The knee wall between the attic over the main level and the top half of the upper level is 2x4s on 16" centers, with fiberglass batts.  The batts have kraft paper on the occupied side but after half a century it's deteriorating.  There is nothing on the attic side of the batts; one thing I've learned is that the insulation should be contained on all 6 sides.  I've installed blocking between the knee wall studs at the attic floor as a first step to containing the insulation (still need to foam it in).  Should I just put plywood over the whole thing to complete the containment, or should I put foamboard up first?  If foam, can I use foil faced polyiso or does it have to be EPS to pass vapor?  What thickness?  Is plywood required if I use foil faced foamboard?  The attic floor has fiberglass batts between 2x8s.  There's no insulation on the roof.  The only ducting in the attic is from exhaust fans - no AC or forced air heat. We will be re-shingling the roof next year, and we may have cellulose blown in over the fiberglass.

on closing in the knee wall - the most important part is getting it airtight. You can use any of those, although it seems like the right place for foam backed plywood. Tape the joints with a long lasting tape, or caulk them with a urethane caulk. With your attic floor, air seal it before doing anything else. Move the batts aside and spray a layer (1-2")of two part foam on all the seams and joints and holes in the ceiling. You can have this done professionally or buy froth paks (double cans) and do it yourself. It can be done with cans.

Thanks Bob. A couple questions:

> I was planning to seal around the ceiling penetrations with 1-part foam when I do the blocking; any reason why I should use froth pak instead for this?
> I've been reading recently that if you put foam on the outside of an exterior wall you need a certain thickness to ensure that temperature hits the inside dewpoint within the foam instead of on the studs. If memory serves it was 5" over insulated 2x4s in zone 5. Is this a concern in my case? Can I safely go thinner than that since the attic won't be as cold as outdoors and the insulation between the studs isn't to modern standards?

Will

one part foam works, but buy a foam gun rather than using the individual DIY cans. You have much more control and waste far less foam. The advantage of a froth pak is that you can do an overall coating rather than just a bead. When you are doing a ceiling from the top, you should do all the joints between the drywall and the joists, an overall coat at every partition etc, as well as penetrations.

I don't know the details of how much foam to use on the exterior side of your wall in your location (dana1 can tell you that), but given the attic location and the probable smallish area, I wouldn't be overly concerned. (unless you have a high moisture source (bath) on the interior side) EPS is pretty vapor open, XPS slightly less so, and foil faced polyiso not at all. I suspect the foam backed plywood is vapor permeable; would be smart to check the specs.

Unfortunately, one side of the knee wall is against the bathroom. We run an exhaust fan while showering and for 20-30 minutes afterward but it still gets pretty steamy.

I'm thinking of just foaming the ceiling penetrations and covering the knee wall for now, and when I have a Contractor in to blow the cellulose next year I'll have them do the joists etc with professional equipment. By the time I've pulled all that half-century old fiberglass out of the way the batts will be disintegrating and I may well go through winter leaking more heat than if I'd done nothing. That and I'd have to shave my beard so I can wear a respirator.

The more time I spend in that attic the more I appreciate how underpaid insulators are.

Posted By Will17109 on 03 Dec 2013 05:56 PM
Thanks Bob. A couple questions:

> I was planning to seal around the ceiling penetrations with 1-part foam when I do the blocking; any reason why I should use froth pak instead for this?
> I've been reading recently that if you put foam on the outside of an exterior wall you need a certain thickness to ensure that temperature hits the inside dewpoint within the foam instead of on the studs. If memory serves it was 5" over insulated 2x4s in zone 5. Is this a concern in my case? Can I safely go thinner than that since the attic won't be as cold as outdoors and the insulation between the studs isn't to modern standards?

Will

Maybe you need to buy some new memory?   (I know need to!)

It's R5 on the exterior side of 2x4 construction, not 5" for climate zone 5, and that's only if there is wood sheathing on the exterior side of the studs, with no ventilation gap between the siding and the sheathing. 

In your case you have no wood sheathing on the exterior side of the studs (check), and a huge air space (the mini-attic space) between the studwall and the exterior layers (the roof deck & roofing.)

The only way to screw this up is to put a vapor barrier like polyethlene sheeting or vinyl/foil wallpaper on the interior, then put foil-faced foam on the other side.  For best R/inch go with foil faced iso (R3 at 0.5"), and not add anything super retardent on the interior side. (Krumbly kraft facers aren't a problem anywhere in the stackup, or even perfect kraft facers, since they become vapor open when their moisture content goes high.)

Thanks Dana1! I knew there was a 5 in there somewhere...

So well-taped foil-faced iso over the knee wall studs, and I guess I have to put plywood over that? Or does the dire warning printed on the foil only apply to living spaces?

IIRC the "5" was what I scored on my last I.Q. test. :-)

In crawlspaces and kneewall spaces that aren't used even for storage you only need an ignition barrier, not a 15 minute thermal barrier. Dow Thermax polyiso uses a fire-rated facer that meets spec as an ignition barrier, but would need something else to meet spec as a timed thermal barrier. (Half inch gypsum or half inch OSB would be cheaper than half-inch ply, if you needed the full thermal barrier.)

http://building.dow.com/na/en/products/insulation/thermaxsheathing.htm

http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_089d/0901b8038089d660.pdf?filepath=styrofoam/pdfs/noreg/179-04015.pdf&fromPage=GetDoc

In many places (but not all) an application of intumescent paint would be a code-acceptable alternative to a more massive thermal barrier such as half-inch gypsum/OSB/plywood.

OK; thanks. It's a full-blown attic (about 20'x30') so I'd probably better put facing on it. When I asked the local codes inspector about covering the XPS I put on the rim joists in the basement he told me that I could use 3/8" plywood or gypsum on that.

7/16" x 4' x 8' sheets of OSB are about the cheapest rigid panels you can find around here, ~$9/sheet. That is way less than 3/8" plywood and even less than drywall.  Plus, they are very strong and flat and with the smooth side to the interior can be painted and look pretty good.  I lined the interior of my entire workshop (30' x 42') with it a few years ago.

Thanks Arkie6; I'll check that out.

Here's another wild thought: I put 1x2 furring over the polyiso at every other stud and then screw OSB/plywood/gypsum to the furring. This way my fasteners won't go directly from attic air into the studs, I'll get an extra ~R1 from the gap, and the foil facing on the polyiso will be looking at the air gap so I'll have a radiant barrier in summer. Think it would be worth the extra effort?

I doubt it's worth the extra effort- the furring still has to be through-screwed to the studs anyway, how is that different from through-screwing the OSB, other than the R0.6 air gap?

Since it's not structural you don't need to screw the OSB on a very tight fastener spacing the way you would need to for structural sheathing. Hitting every other stud, 24" o.c. on the verticals using bugle-heads would be just fine. It's mission as a thermal barrier would still be fulfilled.