I am beginning to install fiberglass insulation in my garage and want know if I have done the ceiling wrong. The issue is that the insulation touches the roof sheathing in the sloped area below that I marked in yellow. There are no soffit vents at the bottom of the sloped area. There are two triangular gable vents (one pictured, one on the other end) as well as a roof ridge vent. Do I need to remove the insulation I have installed and staple rafter vents to separate the insulation from the roof sheathing, or are they not needed in this area there aren't any soffit vents? Thank you.
Best Answer
This is old building code I am reciting, and I was an insulation installer for a while as well, for what its' worth.
You do not need soffit vents but it makes the venting more efficient. Code required 1/150 per square feet worth of venting in a given attic. My way of putting it may be rusty, but what it means is, if you have 150 sq ft of attic, to properly vent it, you will need 1 sq ft of "free area" for venting. If soffit vents are used, the equation increases to 1/300 or 1 sq ft of venting for every 300 sq ft of attic.
So check the free area equivalent for your gable vents, since the measured dimensions do not mean free area. The blades of the vents themselves offer restriction to the free flow of air and reduces efficiency of the vent. It may reduce the efficiency by as much as 30 to 50%.
About the insulation touching the roof. If the insulation is installed properly, there should be a 2" air space above the insulation to the sheathing. In all practical purposes, this is should be easy to do if the rafters are 10" deep and the insulation is only 8". If that is the case, the random touching of the insulation buckling up will not matter. If you have 8" of insulation in an 8" rafter, then heat will build up and in essence "cook" your roofing. A metal roof may be an exception, but standard composition shingles will degrade rapidly. I seen it.