I live in southern Oregon and am planning on installing a t&g finished ceiling on a scissor truce gable roof. We are planning to insulate with blown in cellulose insulation (R-50) after the ceiling is installed. What’s the best approach to addressing the need of a vapor barrier in this situation? The inspector told me to use 6mil plastic, yet the only time I’ve ever seen a moisture problem was with plastic use on the interior side of a ceiling?
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Replies
The vapor barrier prevents moisture from entering the attic area of the truss so you have to have a ventilated house to exhaust the moisture.
The ventilated attic space takes care of any small amount of moisture that escapes through to the attic. Moisture and mould are a problem when you have lots of moist air accumulating up there in the attic with no way out.
You are using blown-in insulation in a sloped roof. How are you providing an air space for ventilation?
Joe,
If the Oregon code requires the installation of a vapor retarder, you have several options other than poly, for example: 1. plywood, 2. kraft paper, 3. vapor-retarder paint.
But don't forget -- regardless of what the code says, the most important detail is the air barrier, not the vapor retarder. That's where many T&G ceiling installations fail. The best installations are over taped drywall, to ensure an air barrier. Don't forget to foam penetrations like vent pipes and wiring penetrations, and whatever you do, don't include any recessed can lights.