Insulation code?????

[John_M]

Ok...here is the situation.

For my cabin, I need R-19 in the walls and R-38 in the ceiling. 

I have 12' walls overall, with 3 feet or so for the upstairs.  I then have a 12/12 roof with 2 x 8 rafters.  At 8 feet I have collar ties that make up the ceiling.

What does the approximately 45 degree angle created by the rafters count as....wall or ceiling?

Being that it is a 2 x 8...I can't get R-38 in there (12' of fiberglass).  I don't want to fir it out over 4 inches!!!

I'm hoping its still considered a wall and the inspector will let me put in R-19 (or I'll probably go with R-25)

Anyone have some info on this??

Thanks!!

[bayview]

   Your pic. isn't loading . . .

   You may need to insulate with a different material with a higher r-value per inch . . . Rigid foam insulation will have a higher r-value than fiberglass or blown-in cellulose insulations.  Styrofoam has a r-value of 5 per inch.  Blown-in foam has a high r-value but can be expensive.

[MountainDon]

I have a feeling the sloped portion counts as ceiling, just as it would if the entire space was open to the floor below, as in a cathedral ceiling. ResChek does have a setting for cathedral ceilings as well as flat, IIRC.

[pagan]

Hi-R foam board has an R value of 6.5 to 7 and comes in 4' X 8' sheets. You can cut it to pressure fit in the cavities, just make sure you seal and air gaps with low expansion foam. You'll also want to tape all of the seams. Spray foam has an R value around 6, but can be pretty expensive to have a company spray in for you.

[John Raabe]

Many energy codes make a distinction between flat ceilings and single rafter ceilings (sloped cathedral ceilings - which is what you have).

Here is WA it is R-38 in flat ceilings and R-30 in sloped. Do you have to have a vent space above the insulation? Ask your lumber yard what local builders are doing for cathedral ceilings. I like to have rafters deep enough for R-30 batts but that won't work in a 2x8.

Assuming you have to have a 1" vent baffle you could use R-21 high density batts in the rafter cavity and then 2" of blue or pink foam board over the rafters & under the drywall. That will likely be the most economical and provide good insulation.

[MountainDon]

Here is WA it is R-38 in flat ceilings and R-30 in sloped.

Interesting, and that would help explain why when I ran REScheck on my cabin design with a cathedral ceiling instead of a flat ceiling, but keeping the same R-44 value the rated performance was higher with the cathedral ceiling.