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.
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.
The reason you need ventilation is because warm air (vapor) will condensate when it travels from a warm environment to a cool environment. This process is called “reaching the dew point”.
In your attic the inside air (vapor) will travel through the insulation. When it reaches its dew point it will change from vapor to condensation. That condensation needs to be vented in order to remove the condensation. The Code requires “cross ventilation”.
Attic Ventilation shall be “cross ventilation “ and shall be 1/150th the area that is to be vented. (See ICC R806 Vents.)
However, there is one exception: The attic ventilation can be reduced to 1/300th the area to be vented provided one of the following items are met:
1) a Class I or II vapor retarder is installed on the “warm-in-winter” side of the ceiling, or
2) not less than 40% and not more than 50% of the ventilation is located in the upper portion of the attic. The upper portion is defined as: “Upper ventilation shall not be lower than 3’ below the ridge or highest point of the roof. “ (See ICC R806.1.2.)
Also, the Code says you shall maintain a 1” minimum clearance between attic insulation and framing.
Summary: So, you can see there is clearly a benefit to having ridge vents, gable vents, etc. in addition to soffit vents, but in a proportion that allows COMPLETE CROSS-VENTILATION to the attic. If you don’t install a soffit vent in each joist space, then the air cannot be vented...in each joist space. That is to say, just having a ridge vent does not provide “cross ventilation”. (You need an “in-ee” and an “out-ee” in each joist space.)