Another Insulation Question

[JohnK007]

Going to insulate my ceiling of my garage by using fiberglass rolls between the roof rafters. Rafters are 2X6 - 24" OC. I was going to use R-19 Kraft faced 5 1/2" thick but got to thinking. This would put the fiberglass right against the interior of the roof sheathing. I have read that there should be an air gap so air can circulate in hot weather otherwise the roofing (asphalt shingles) can fail prematurely.

If this in fact a cause for concern, should I instead use R-13 3 1/2" batts instead of the R-19? Hate to loose that extra R- value but on the other hand I don't want to have to reshingle a 6 year old roof in a couple of years either.

Garage is in northeast Illinois where it gets damn cold in winter.

 

[stevejh82]

A point for you to ponder, is the warranty on lighter color shingles any different than the warranty on black shingles? Absolutely not, even though the black shingles get hotter. I spray cellulose insulation, and we routinely dense pack cathedral ceilings in homes where that drywall touches the rafters, per the company that manufactures the insulation's directions. They have specified this method years and there have been tests done that show that the temperature difference is negligible. If you are going to cover the insulation with some type of material (drywall, osb, etc.), you would need some type of ventilation along the roof deck as the batts will allow air to move through them, thus allowing condensation to form.

 

[walrus]

. Any ridge vent or soffit vents? I think a hot roof will only happen with foam up against the sheathing, air moves thru fiberglass.

 

[JohnK007]

Thanks for the response Steve. At least initially I don't intend to cover the insulation. Just staple the kraft faced batts between the rafters. I too have a cathedral ceiling in the family room but I don't remember how they insulated it. Those rafters are either 2X10 or 2X12 and I think they just stuffed batting in the cavity and drywalled over it.
I suppose I should just go for the greater R- value and call it a day. Sometimes I overthink things I guess.

 

[JohnK007]

There is a static ridge vent I put in when I re-roofed the garage a few years ago. In hindsight I should have put a couple more in instead of just one. No soffit vents per se but the soffit isn't exactly built super tight. Cutting in a couple on each side wouldn't be all that hard.

 

[stevejh82]

Here is a response from pink panther's website that should help answer your question.

http://saveenergy.owenscorning.com/2008/03/question_how_do_i_install_raft.html