Replies

1. GBA Editor
   Martin Holladay|Apr 02, 2014 10:31am|#1

   Rolf,
   Q. "Are there any negative effects in using readily available uncoated extruded polystyrene with a perm rating of 1?"

   A. No, other than the usual drawbacks to the cut-and-cobble approach.

2. 
   Rolf Reiss|Apr 02, 2014 10:52am|#2

   Q. By usual drawbacks are you referring to the fact that studs and headers are not insulated?

3. GBA Editor
   Martin Holladay|Apr 02, 2014 12:11pm|#3

   Rolf,
   The answer to your question can be found in an article calledCut-and-Cobble Insulation. In that article, I wrote:

   "Cut-and-cobble has some disadvantages:
   To address thermal bridging, it’s always better to put the rigid foam on the exterior side of the sheathing. However, in an existing house, homeowners are rarely willing to demolish their siding just to install a layer of rigid foam.
   This insulation method is very time-consuming. If you want to do a good job, it will take longer than any other insulation method.
   Wood framing expands and contracts with changing humidity levels, raising the possibility that attempts to seal the perimeter of the rigid foam (whether with caulk, spray foam, or tape) will fail over time. Anecdotal evidence suggest that this danger is real, especially for cut-and-cobble cathedral ceilings."

4. 
   Rolf Reiss|Apr 02, 2014 02:24pm|#4

   Martin,
   I just sat down and read your insightful " cut and cobble" article as well as the responses. Thank you for passing it along to me. My project is new construction with all of the windows, doors, trim and cornerboards installed. After being shocked by the cost of sprayed icynene I had to alter my plans to the cut and cobble option. Being new construction I can use my table saw to cut the polystyrene and create reasonably tight fits between the wall studs. Do you have any recommendations for caulk that would be likely to adhere to both the polystyrene and the studs over the long haul. Silicone? urethane?
   Thanks again