Replies

1. GBA Editor
   Martin Holladay | Apr 11, 2017 09:18am | #1

   User-6813968,
   In my opinion, the best approach (by far) is to install an adequately thick later of rigid foam on the exterior side of the roof sheathing. In most cases, this rigid foam insulation is supplemented by additional fluffy insulation (fiberglass, mineral wool, or cellulose) installed between the rafters, with the fluffy insulation in direct contact with the roof sheathing.

   For more information on this type of unvented roof assembly, see How to Install Rigid Foam On Top of Roof Sheathing.

   Although the method I have described is my preferred method, it's not the only way to insulate this type of roof assembly. For a complete survey of all of the insulation methods that work (and warnings about a few methods that don't), see How to Build an Insulated Cathedral Ceiling.

   --Martin Holladay

2. 
   user-6813968 | Apr 11, 2017 10:07am | #2

   Only the living room/family room will have cathedral ceilings. So would I install the foam boards only on the exterior of the roof over the section that has cathedral ceiling? I think it may be too expensive to cover the complete roof with foam. This house will be in zone 4 so would the cost be worth it to install foam boards over the complete roof? Using this method would I install the foam board directly over the rafters then plywood over the foam boards then shingle over the plywood? Can this be done on a hip roof? I guess are the energy savings in zone 4 worth the expense of installing foam boards over the entire roof?

3. GBA Editor
   Martin Holladay | Apr 11, 2017 01:34pm | #3

   User-6813968,
   If the cathedral ceiling portion of your roof is only a small fraction of your total roof, then you are correct -- it doesn't make sense to cover your entire roof with rigid foam. In your case, you would choose one of the other approaches listed in my article, How to Build an Insulated Cathedral Ceiling.

   1. If you are insulating from the interior, I prefer vented solutions to unvented solutions.

   2. Remember, a vented approach only works if you have a clear shot from the soffit to the ridge, with no interruptions -- no dormer, no skylight, no hip, and no valley.

   3. A common mistake is to skimp on the roof's R-value. In most areas of the U.S., the code minimum R-value for roofs is either R-38 or R-49.

   4. No matter what approach you take, it is essential that your ceiling be as airtight as possible. Avoid can lights and seal all penetrations carefully.

   -- Martin Holladay