Replies

1. 
   enutis | Jul 28, 2013 12:07pm | #1

   I had the same issue with attic walls in my house. I pulled out the existing kraft faced r13 insulation from each stud bay. I then air sealed the drywall to studs, electrical holes, etc. I then replaced the insulation in the bays. I then covered the fiberglass with 2" XPS foam with all seams glued with PL premium and then taped with Dow tape. Removing the kraft facng kept the wall from having a double vapor barrier. The point is to keep air from getting to the fiberglass insulation. Fiberglass needs to be airtight on all six sides to work effectively. You could also use other materials for an air barrier, I used XPS to increase the R value. Adding ridgid foam to the access panels and weatherstripping the hatches is also a good idea. There is tons of great info on this site about attic kneewalls.

2. 
   Allen D | Jul 28, 2013 07:23pm | #2

   I see by this answer and others that additonal fiberglass insulation is not suggested, and foam is recommended, probably because it's easy and about the same cost. I am in zone 3 where vapor accumulation/condensation is not an issue, so I hope I can skip all the sealing steps. I presume
   another 1 1/2 - 2" XPS would be adequate mostly for a delta T of about 40 degrees hot and cold.

3. Expert Member
   Dana Dorsett | Jul 29, 2013 11:15am | #3

   Air sealing is always important for both thermal and moisture handling performance- don't confuse vapor barriers with air barriers. Vapor barriers are all about slowing or blocking the rates of water vapor diffusion through assemblies when there is a vapor pressure/humidity difference, whereas air barriers are about slowing or blocking the rate of air movement through the assembly whenever there is an air pressure difference (from wind, or stack effect, etc.).

   Putting 1.5-2" of fire rated foil-faced polyiso (eg Thermax) on the will do more for the cooling season thermal performance than 2" of XPS, and since the it's blown with a low global-warming-potential blowing agent, it's a much greener product. XPS is blown with high-GWP HFCs with more than 100x the GWP hit of polyiso.

   Foil facers are vapor barriers, so you have to be careful not to create a moisture trap by putting a very low permeance material like polyethylene sheeting, foil/vinyl wallpapers, etc. elsewhere in the assembly. Dry climate zone or not, this is a problem that needs to be avoided.

   Air sealing foil facers is dead-easy using FSK tape (aluminum duct tape), but you'll still have to use expanding foam at the edges where the cut edge of the polyiso meets other materials.

   A common thermal bypass in attics with kneewalls is where joist-bays cross under the kneewalls under the subfloor of the conditioned space. Some sort of air-dam needs to be installed at the kneewall to keep wind from blown air in the soffits on one side of the house and out the other. Corrugated cardboard stapled to the joists and foam-sealed with can-foam on all 4 sides often works OK, as does cut-to-fit rigid foam, foam sealed at the edges.

4. GBA Editor
   Martin Holladay | Aug 01, 2013 08:35am | #4

   Allen,
   You've received good advice. Use rigid foam -- the more the better -- and don't forget to air seal everything (foam seams, the perimeter of the foam insulation, and all electrical boxes and penetrations).