How do I properly insulate and vent a cathedral ceiling in Middle Tennessee?
austinbecton
| Posted in Energy Efficiency and Durability on
I’ve read numerous articles on here, alas I’m just confused by what exactly I need for my area. Initially, a contractor friend told me to install R-30 insulation rolls, baffles, and 6-mil poly (vapor barrier). However, this seems to not be the proper method based on what I read. Could someone provide me specifics for what is needed for my zone? One BIG note, I’m doing T&G. Was told to attach to rafters (with vapor barrier in between).
Another issue I have is that I do not have soffit vent between every rafter (only about 15-20%).
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Replies
Austin,
Everything you need to know can be found in this article: "How to Build an Insulated Cathedral Ceiling."
Based on the information you have provided, here is what you need to know:
1. The first step is to determine whether your roof is best insulated with a vented approach or an unvented approach. A clue is in your statement that "I do not have soffit vent between every rafter." It's hard to tell what that means. It you want to consider the vented approach, it's essential, at a minimum, to have (a) Soffit vents at the base of every single rafter bay; (b) A ridge vent at the top of every single rafter bay; (c) Ventilation baffles that create an air gap between the top of the insulation and the underside of the roof sheathing -- again, in every single rafter bay.
Since these elements need to be in every single rafter bay, it quickly becomes apparent that you can't use the vented approach if your roof has valleys, hips, dormers, or skylights.
2. If the vented approach is impossible, you'll have to create an unvented cathedral ceiling. This requires either (a) a sufficiently thick layer of closed-cell spray foam on the underside of the roof sheathing, or (b) a sufficiently thick layer of rigid foam above the roof sheathing. Option (b) requires a second layer of roof sheathing and new roofing. Details on both approaches are provided in the article I linked to.
3. Unless you insulate your rafter bays with spray foam, you'll need an air barrier directly behind (in other words, above) your tongue-and-groove boards. Polyethylene isn't a good choice as an air barrier. The best approach is to install drywall with taped seams first, and then to install your T&G boards on the interior side of the drywall. The drywall acts as your air barrier.