More in Insulation

Insulating a Roof

While building a house, should I put Sheetrock on an open beam ceiling or do I need roof vents to stop condensation?

Ask This Old House Crew
Photo by Matt Kalinowski

For the last four years, my wife and I have been building a house.
She decided on open beam ceilings in the living room, so we put up
insulation. But, before we could put up the Sheetrock, winter came.
From the heat and the cold mixing, we have condensation between the
insulation and the roof. If I put the Sheetrock up, will that stop the
dripping, or do I need roof vents?

— Sean, Corbett, Oregon


Tom Silva replies: When insulating any roof, you need an air space of at least
two inches between the underside of the roof sheathing and the top of
insulation. Proper vents can be stapled to the roof sheathing to create
an uninterrupted airflow. The soffit and ridge should also have venting.
Install a good vapor barrier under the insulation — on the warm
side — making sure there are no holes. Any seams, if needed, should be
overlapped by at least 16 inches. Foil taping or polyurethane
caulking the seams won't hurt. The insulation should be R-30.
(R-value refers to a material's ability to resist the transfer of heat.
The higher the R-value, the more insulating power the material has. )
Make sure you don't compress the insulation, which makes it less
effective, if it doesn't fit correctly in the space. Cut a new piece


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