Insulating and Ventilating a Garage Roof
This Old House magazine
Q: I am in the process of insulating and enclosing my garage that has
a cathedral ceiling with 2x10 roof rafters. What is the proper way to
obtain my minimum one inch of air space between the roof sheathing and the insulation? What is the best type of insulation to use?
— Arnold, Anaheim, CA
A: Tom Silva replies: First, you should install proper vents against the underside of the sheathing from the soffit vents to the open space of the attic.
Ventilation is crucial in today's well-insulated homes, helping to
insure good indoor air quality. Also, this is the time to install a
ridge vent if needed.
As far as insulation goes, I'd use high-density, eight-inch fiberglass batts covered with a four- or six-mil poly-vapor barrier. Vapor barriers keep moisture from penetrating and condensing in the insulation. Be sure to seal every seam in the vapor barrier after insulating. Moisture can penetrate through even the smallest gap.
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