Flat vs vaulted roof insulation

[ericm]

I'm looking at the code where our retirement home's going to be built and it says that flat ceilings need R49 but vaulted ceilings only need R30. Why would that be?

 

[AC-WC]

My SWAG-Because the rafters are likely 2X8 and that's all that can be fit in there. Otherwise you would need 2X10 or 2x12 or larger which makes it costly and way overkill for structure.
I raised my ceiling in the garage to vaulted with the original 2X6 rafters and R19 was the highest I could get to fit. (no, I'm not living in there and it's not heated all the time).
With a flat ceiling you can always add on top to get the r49 and it doesn't weigh enough to worry about.

Definitely not an expert on foam but you might be able to squeeze a few more r's vs fiberglass? Way more expensive.
Just depends on how long you stay for the payback.

I did see an episode on This Old House where they did a cathedral, insulated like normal and then on the roof side installed 2" foam board with OSB exterior. I think it gave them an extra r10? Of course they didn't say how much that cost and they were making an extra tight/efficient house. I really wish they would give a breakdown of their actual costs but they would probably lose some viewers

 

[billconner]

The IRC, I think basis for California residential code, does have some specific allowances for ceilings without attics, as might be the case for your vaulted ceiling.

N1102.2.2 (R402.2.2)Ceilings without attics.​

Where Section N1102.1.3 requires insulation R-values greater than R-30 in the interstitial space above a ceiling and below the structural roof deck, and the design of the roof/ceiling assembly does not allow sufficient space for the required insulation, the minimum required insulation R-value for such roof/ceiling assemblies shall be R-30. Insulation shall extend over the top of the wall plate to the outer edge of such plate and shall not be compressed. This reduction of insulation from the requirements of Section N1102.1.3 shall be limited to 500 square feet (46 m2) or 20 percent of the total insulated ceiling area, whichever is less. This reduction shall not apply to the Total UA alternative in Section N1102.1.5.

It's about practicality. You can also reduce the R49 in flat roof if the insulation extends over top plate.

N1102.2.1 (R402.2.1) Ceilings with attics.​

Where Section N1102.1.3 requires R-49 insulation in the ceiling or attic, installing R-38 insulation over 100 percent of the ceiling or attic area requiring insulation shall satisfy the requirement for R-49 insulation wherever the full height of uncompressed R-38 insulation extends over the wall top plate at the eaves. Where Section N1102.1.2 requires R-60 insulation in the ceiling or attic, installing R-49 over 100 percent of the ceiling or attic area requiring insulation shall satisfy the requirement for R-60 insulation wherever the full height of uncompressed R-49 insulation extends over the wall top plate at the eaves. This reduction shall not apply to the insulation and fenestration criteria in Section N1102.1.2 and the Total UA alternative in Section N1102.1.5

I think the vaulted ceiling will also require spray foam, as it would likely be a "hot roof" design.

 

[ericm]

It's in zone 4B in Oregon (2021 IRC). The ceiling will be vaulted with 12" rafters. They leave a space between the insulation and roof sheathing and vent the soffits so it's a vented roof rather than a hot roof. I think that's good as it can get hot in the summer. Our current house has some hot roof sections that are probably R19 and you can feel the heat radiating from the interior. I'll ask what type of insulation they're planning and maybe see if its worth the cost of upgrading to a higher performing type like rock wool vs fiberglass if they wern't already doing that. It'd be difficult to do spray foam and keep the space between insulation and sheathing for ventilation.