cathedral insulation

[chrisbrowning]

Before I buy the insulation for the cathedral ceiling have a question. Have built the 12 x18 little house with 2x6 rafters. I know I am suppose to leave 1inch spacing so it will be properly vented. What thickness of batt insulation do I buy. If I buy the insulation to install between 2x6 will it be too thick?
thanks

[Ray_N]

Chris,

Your suppose to leave some room for air circulation, there are  foam "channels" that you can find sometimes at building supply houses to put between the rafters to allow some ventilation.  

I am going to post in the general forum a similar question since I have some "free" insulation and thought I use that for the ceiling.  Maybe that thread will answer your question.

[John Raabe]

There is some debate as to the need for ventilation above the insulation in a sloped ceiling. No question but that filling the cavity full will give you a better insulation package - especially since you only have 5 1/2" to work with.

If you are doing a taped and sealed drywall ceiling and you use a vapor barrier primer (and you haven't punched a lot of holes in the ceiling for can lights and such) then there should not be much moisture getting in there to vent.

Check with the local inspector or the energy office to see what is best for your climate.

If you need vents you can use foam trough vents and 5.5" R-21 hi-density fiberglass batts. That will be about the best you can do inexpensively.

[Ray_N]

I got an idea on this one that sounds promissing.  Before you put in regular 3 1/2 fiberglass, you use some foam insulation.  If you get 4x8 or 2x8 sheets of 1 inch thinkness (about R-5) and cut them to fit between the rafters.  Left over pieces can be glued to the foam to keep it "off" the roof sheeting when you install it (glue or nails).

Then put regular 3 1/2" in fiberglass,  you wind up with R-11 plus R-5 plus ventilation.  

[John Raabe]

Good suggestion Ray. Builds a hi-R foam baffle for ventilation and ups the total insulation level.

Where roof heat buildup could be a problem for a material like asphalt roofing, you can use the vent space to dump heat. Foil faced foam boards could do even more to reflect heat.