Attic ventilation question for Joe
I have an area above my detached garage that I am trying to finish off into an apartment. I am looking to get some input on the best way to go about venting the attic. The problem I am having is that on one side of the building the roof has a fairly steep pitch (45 degrees), and is used as the interior wall up to the ceiling height at which point it opens up to an attic. My current plan is to use furring strips and 1/4″ plywood to make 2″ deep vents under the roof. I was then planning to fill in the rafters under this with R21 cotton batt insulation. I then thought I would put foam board on the inside with drywall over that. Do you think I will have a problem with this setup?
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Replies
Mark,
My name isn't Joe, but here is my advice: your proposed assembly will work, but only if the total R-value of the insulation at least meets minimum code requirements for your climate zone.
For more information on insulating sloped roof assemblies, see How to Build an Insulated Cathedral Ceiling.
Thanks for the reply Martin, is the R requirement that of a wall, or ceiling in this case where the roof is acting as a wall? I live in central Virginia. I am concerned with anything in my design that might cause moisture problems.
Here is a picture of what I have done so far, the plywood will continue up to the attic and I will fill in small pieces at the eaves. My thought in using the foam board is to bump the R value, prevent thermal bridging, and to be a vapor barrier.
Mark,
If it's sloping, it's a roof, not a wall.
Central Virginia is in Climate Zone 4, where a roof needs a minimum R-value of R-38.
What would be the negative ramifications if I can only come up with around R30?
Mark,
Assuming that you have no legal requirements to comply with the building code, the main negative ramification is that your house will require more energy for space heating and cooling than a code-minimum building.
Martin, thanks for the reply, my biggest fear is moisture issues. I am thinking with my current strategy I should be able to seal this area with the foam boards and then drywall. Do you think I am at risk for condensation issues? Do you think there would be a benefit from a perforated radiant barrier on the roof side of the plywood baffles?
Thanks