Adding doam insulation on roof

[cathode bud]

I'm pricing a remodel, and the homeowner with cathedral ceilings wants more insulation. Since we're re-roofing, I considered placing 2 inches of foam board above the deck, then strapping and sheathing. With 10–12 inches of fiberglass already, is that enough? Also, would soffit ventilation need redirecting? Has anyone done this successfully?

 

[CircuitGeek]

I'm pricing a remodel, and the homeowner with cathedral ceilings wants more insulation. Since we're re-roofing, I considered placing 2 inches of foam board above the deck, then strapping and sheathing. With 10–12 inches of fiberglass already, is that enough? Also, would soffit ventilation need redirecting? Has anyone done this successfully?

yeah, you need to check the R value is. With 10-12 inches of fiberglass (R-38 to R-40) and 2 inches of rigid foam (R-10 to R-12), you're looking at a total R-value of around R-50 or R-52.
That should easily meet or exceed building codes for most climates and will be a huge improvement for the homeowner's comfort and energy bill. Just be sure to check your local code to confirm the specific requirement for your zone

 

[JonJon]

Polyiso has the highest R-value but when it gets cold, it can start to lose its effectiveness.

 

[furg0y]

Adding 2" of foam above the deck definitely helps, but with cathedral ceilings you'll want to be sure you're not blocking airflow. The soffit to ridge vent path has to stay open, otherwise you'll need baffles or to redirect the venting. I've done similar builds and it worked fine as long as the ventilation detail was sorted

 

[slvr22_bd]

I'm pricing a remodel, and the homeowner with cathedral ceilings wants more insulation. Since we're re-roofing, I considered placing 2 inches of foam board above the deck, then strapping and sheathing. With 10–12 inches of fiberglass already, is that enough? Also, would soffit ventilation need redirecting? Has anyone done this successfully?

How's this project going? I think adding 2 inches of continuous foam board above the sheathing is a smart way to boost R-value and eliminate thermal bridging. With the 10-12 inches of fiberglass already in the bays, you'll have a very high-performing assembly. The exterior foam will act as your main thermal and moisture barrier, which is key. This setup allows you to create an unvented roof assembly, so you won't need to worry about directing airflow from the soffits. This is a very common and successful approach for cathedral ceilings.