Replies

1. GBA Editor
   Brian Pontolilo|Oct 13, 2019 11:08am|#1

   Hi Beverett09.

   A vaulted ceiling can be insulated with lots of different materials. What's more important to work out is the assembly. For example, a vented assembly can be insulated with fibrous insulation, but it may be tricky to reach R-49, a good minimum target R-value for your area. An unvented assembly typically has at least some air-impermeable insulation, either above or directly below the roof deck, and these assemblies are often built with a combination of materials. You also need to consider dew point control, vapor control, and air sealing. In other words, these are complex assemblies and are very important to get right. I suggest reading this:How to Build an Insulated Cathedral Ceiling

1. 
   Beverett09|Oct 13, 2019 12:48pm|#3

   Local code R30 value for vaulted ceilings. The roof is currently vented in the soffit. 4/12 pitched roof.

2. Expert Member
   Zephyr7|Oct 13, 2019 11:32am|#2

   If you use rigid foam, you want to put it on top of the sheathing. 2x6 rafters don’t give you enough space to build this as a vented assembly and get sufficient insulation to fit. Even with a full fill of closed cell spray foam you’ll barely be able to get to R38, and there will be a lot of trimming needed — normally you don’t do a full fill of ccSPF to avoid the need for trimming.

   I would put rigid foam above the sheathing and use batts between the rafters inside. The article Brian linked shows how to essentially retrofit a vent gap if you want to go that route.

   Bill

1. 
   Beverett09|Oct 13, 2019 12:48pm|#4

   Local code R30 value for vaulted ceilings. The roof is currently vented in the soffit. 4/12 pitched roof.