Work here in Mass, the code book specifies the insulation requirements for ceilings and walls, but I can find no references to what R value would be required in a sloped ceiling.
I a finishing a space above a garage and it will have about 8 ft of flat ceiling and then sloping ceiling(12 pitch) down to knee walls about 4 ft high.
Intuitively, I would think something less than the flat ceiling R value would be required. Does anyone know for sure?
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Replies
I would guess it would be up to the authority having jurisdiction - i.e. the inspector - to make a judgement call as to when a wall stops and a ceiling begins. But remember the insulation is there to cut down energy costs, so the homeowner either pays the insulation contractor up front or the energy provider (probably a lot more) later on. Just how much is the cost difference in providing the R-30? (or whatever) vs the R-21? (or whatever) required in your area?
Hey..In the GREAT state "Orygun" we require R38 throughout on flat ceilings, R30 is permitted for no more than 50% of the heated/conditioned space. As is mentioned in the other reply to this article..you really should check w/ the jurisdiction.
Good luck !
sometimes intuition is too busy watchin the girls go by to be able to think straight. Reason is my dance partner.
Think like a ball of hot air. Visuallize it like a helium balloon.
it rolls along the flat ceiling until it hits the upsloping cathedral.
Then it gets lost up there. Don't go anyplace else, does it?
So where do you need more insulation again?
Excellence is its own reward!
Guess I didn't explain myself very well.
First the sloped ceiling goes downward from the flat ceiling. So my visualization of the hot air ballon has it sliding up the wall really fast. Going up the sloped ceiling medium fast and then sitting up against the flat ceiling. Indicating that lowest R value in the wall. Highest in the ceiling, and something in between in the sloped section. But that is the question, what value.
As to spending the money and doing it right, independant of what the code says, I concur completely. I use the code book generally for good guidelines of the minimum requirements and over build from there.
But in this case my real question is spending the money wisely.
Lets say I would normally put R-38 on the flat ceiling and R-19 in the vertical walls. Putting R-38 in the sloped ceiling might not be the most cost effective. For example I could put r-30 in the slope and upgrade the flat section to r-44 or something for the same money. The same reasoning that tells you not to put R-38 in walls and ceiling.
Using reason, I would think that you would want to have the same effective inches of insulation in the vertical axis. So if(using fiberglass for example) I have 12 inches on the flat ceiling using r-38. on the 45 degree slope I would use .707*12 in or 8.5 inches of insulation roughtly R-27 or so. Make sense or am I missing something.