Replies

1. GBA Editor
   Martin Holladay | Mar 04, 2017 06:12am | #1

   S.G.,
   For a full discussion of all of the ways to achieve R-49 in a ceiling framed with 2x12s, see this article: How to Build an Insulated Cathedral Ceiling.

   Most methods will require a thicker roof assembly -- in other words, you'll have to add rigid foam above the roof sheathing, or add rigid foam below the bottoms of the rafters, or you'll have to fur down the framing in some manner to provide enough room for the needed insulation.

   You should check with your local code authority to see whether your local code includes exceptions for cathedral ceiling insulation requirements; some code authorities allow builders to install R-30 or R-38 insulation in cathedral ceilings. The language for these exceptions varies from jurisdiction to jurisdiction.

   Finally, note this exception in the 2012 IECC:

   "R402.2.2 Ceilings without attic spaces.
   Where Section R402.1.1 would require insulation levels above R-30 and the design of the roof/ceiling assembly does not allow sufficient space for the required insulation, the minimum required insulation for such roof/ceiling assemblies shall be R-30. This reduction of insulation from the requirements of Section R402.1.1 shall be limited to 500 square feet (46 m2) or 20 percent of the total insulated ceiling area, whichever is less. This reduction shall not apply to the U-factor alternative approach in Section R402.1.3 and the total UA alternative in Section R402.1.4."

   -- Martin Holladay

2. GBA Editor
   Martin Holladay | Mar 04, 2017 06:41am | #2

   Q. "Is there a good WRB/Air barrier system for the renovation of a house with board sheathing?"

   A. There are lots of options. The best option would probably be to install a layer of Zip sheathing over the board sheathing, and to tape the Zip sheathing seams with Zip System tape. (Zip sheathing is a WRB as well as an air barrier.)

   I'm in favor of rigid, durable air barriers.

   If you can't afford to install Zip sheathing, you could use a heavy housewrap. For example, Tyvek CommercialWrap is thicker and more durable than ordinary Tyvek. Of course, you would need to tape all of the seams with Tyvek tape.

   The problem with this approach is that ladders can rip the Tyvek, and all of the fastener penetrations may reduce the effectiveness of the air barrier.

   -- Martin Holladay

3. 
   user-1072251 | Mar 04, 2017 08:32am | #3

   If you use the ZIP sheathing, which I also recommend in your situation, be sure to caulk the bottom and top of the wall between the old sheathing and the ZIP , where you won't be taping. This is the best, and most bulletproof of the various methods.

4. 
   smgarch | Jul 23, 2017 09:00am | #4

   We received pricing back from the G.C. and the plan for the wall is in the order below. What do you think?

   - Drywall
   - Existing 2x4 and new 2x6 stud cavities with Roxul Batt insulation
   - Existing board sheathing
   - New Tyvek Drainage Wrap WRB
   - 1" High density type II EPS rigid insulation with taped seams (R-Value of 4.76)
   - Black building paper
   - Siding

   *the windows will be installed at the sheathing/Tyvek drainage wrap layer

5. 
   user-2310254 | Jul 23, 2017 09:28am | #5

   You don't need the building paper over the EPS, so you could save on the labor of installing that material. Generally, you want to create a gap between the siding and the rigid foam layer to promote drying.

   Here is an article on how and why this detail is desirable: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing

   Also... You could potentially save a lot on the foam cost by using reclaimed material. It is often one-half to one-third the cost of new product.