Replies

1. GBA Editor
   Martin Holladay | Sep 30, 2011 10:40am | #1

   Louie,
   I'm not sure I understand your question. Are you installing fiberglass batts in the walls or the ceiling or both?

   Are you installing rigid foam insulation on your walls or ceiling or both?

   Is the rigid foam insulation on the interior of your wall or the exterior? (Same question about the ceiling.)

2. 
   user-981606 | Oct 01, 2011 11:05am | #2

   Hi Martin

   We have vaulted ceilings, with wood planks for the roof deck. so i installed 1/2" rigid foam board (interior) only in the ceilings to cover all the seams, gun foamed the rigid board, and then installed the fiberglass batts without paper, and now I will install the 1/2" drywall. The roof shingles are in good shape and when the time comes to reroof,

   I am trying to air seal the ceiling, before i build the interior walls, I am planning to build soffits to run the ERV unit, and recessed lighting.

   The exterior walls are the same with the wood planks for sheathing, and i will be installing new siding, so i decided to install the rigid foam board on the outside, with a rain screen.

   This project is small, and a great opportunity to try out some of this new (to me) building concepts, thanks martin

3. GBA Editor
   Martin Holladay | Oct 01, 2011 11:10am | #3

   Louie,
   Whether or not your plan will work depends on two factors:
   1. Your climate.
   2. The type of rigid insulation you are using.

   It sounds like you are cutting the rigid foam insulation into narrow strips to fit between your rafters, and installing this rigid foam insulation directly on the underside of your roof sheathing. In other words, you are creating an unvented insulated cathedral ceiling.

   In a cold climate, 1/2 inch of foam is not enough to avoid problems with condensation and moisture accumulation. So, we need to know where you are located, and what type of foam you are using (XPS, EPS, or polyiso).

4. 
   user-981606 | Oct 01, 2011 11:11am | #4

   and when the time comes to reroof , i will install the rigid foam board on the roof deck or not since it is in the ceiling.

   thanks, I reread my post and left this infomation out.

5. 
   user-981606 | Oct 01, 2011 11:20am | #5

   I live in Eureka, CA, and the foam board is EPS (R-TECH)

6. GBA Editor
   Martin Holladay | Oct 01, 2011 11:38am | #6

   Louie,
   According to the DOE climate zone map, your house is in climate zone 4 (c), otherwise known as zone 4 (Marine).

   You can learn more about the building science principles and code requirements behind unvented cathedral ceilings here: Creating a Conditioned Attic.

   Here are the code requirements, based on building science principles: According to the 2009 IRC (Section R806.4), it’s possible to build an unvented roof assembly using a combination of air-impermeable and air-permeable insulation in unvented rafter bays, as long as the minimum R-value of the air-impermeable insulation that is "applied in direct contact with the underside of the structural roof sheathing" meets the requirements for condensation control shown in Table R806.4. The table calls for a minimum of R-10 for Climate Zone 4C.

   So, in your climate zone, you need at least 3 inches of EPS, 2 inches of XPS, or 1.5 inches of polyiso. If you use just 1/2 inch of EPS, your roof assembly can get very wet in cold weather due to condensation.

7. 
   user-981606 | Oct 03, 2011 11:58am | #7

   Thank you martin,