Replies

1. GBA Editor
   Martin Holladay | Mar 10, 2019 04:56am | #1

   Andy,
   If you want to install 7 inches of EPS insulation, you want the EPS to be a continuous layer. That way, the EPS interrupts thermal bridging through your rafters. So your EPS belongs on the exterior side of the roof sheathing.

   You don't want to cut the EPS into narrow strips and insert it between your rafters.

   That said, 7 inches is a lot, and complicates the use of fasteners to penetrate through the rigid foam. You might want to consider using a thinner layer of rigid foam.

   For more information, read this article: "How to Install Rigid Foam On Top of Roof Sheathing."

   In Climate Zone 4 A or 4B, all you need is R-15 of rigid foam above the roof sheathing, with the rest of your R-value between the rafters. That means about 4 inches or 4.5 inches of EPS, not 7 inches -- making it easier to install a second layer of sheathing or purlins above the rigid foam.

1. 
   AndyWilliams | Mar 12, 2019 04:53pm | #2

   Martin, thank you for your reply. I couldn't help notice that you posted it at 4:56 AM on a Sunday morning. My, you are most certainly an early bird!

   I would indeed prefer to move some of the insulation outboard to prevent thermal bridging. One of the issues that I am running into, however, is that the conditioned living space is attached to an unconditioned, uninsulated garage/shop/storage space; the roof planes are continuous; ie., no valleys. How do I deal with the 4 1/2" (or 5"nailbase) difference in deck heights? The garage/storage/shop is larger than the living space, and it also has a 200fs patio overhang so I need to address all of that large, unconditioned roof area in an affordable manner.

   What are your thoughts on the following in-board insulation plan, and would you please address any issues that might arise concerning condensate on the underside of the deck?

   2.5" of closed cell at R 17.5 on the underside of the deck;
   closed cell down the sides each top chord and across the bottom of the top chord;
   5 1/2" of R23 rockwool batt.

   My trusses have 6 places where the webs attach to the top chords, so there would still be some thermal bridging.

   Thanks for you thoughts!
   Andy Williams

1. GBA Editor
   Martin Holladay | Mar 12, 2019 05:41pm | #3

   Andy,
   It sounds like you are now contemplating the flash-and-batt approach -- and that makes sense, considering the need to keep the roofing on the same plane.

   For more information on the flash-and-batt approach, see this article: "Flash-and-Batt Insulation."