To condition (the attic) or not to condition, that is the question!
After reading and reading and reading (including lots of GBA articles), I thought that I had this figured out. Our plan was to have a non-conditioned attic space, with continuous soffit ventilation along with ridge vents. After further reading, especially Dr. Joe’s Top 10 List of Dumb Things to do in the South, I am confused. Please share whether you would vent an attic or not with the following considerations in mind:
– located in western North Carolina, zone 4, mixed humid
– one level, slab on grade home, approximately 2100 sq. ft.
– hipped roof, 3 1/2 : 12 pitch, energy heel trusses
– asphalt shingles
– passive solar
– heated and cooled with ductless minisplits
– no mechanicals/ductwork in the attic
After reading more, I’m wondering if the following conditioned attic set up would be preferable in any way:
Exterior to interior:
shingles, #30 felt, Roxul, plywood, trusses/rafters, AirKrete Insulation, air-tight magnesium oxide board ceiling
Thank you!
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Replies
Vented attic assemblies have a proven track record in all climates. Now, there are any number of reasons why one might wish to bring the attic into the conditioned space envelope - ductwork in attic, future plans for add'l living space, difficult to insulate knee walls or cathedral ceilings, multiple ceiling penetrations such as can lights, etc. Absent such reasons, traditional attic venting works.
Joe Lstiburek points out problems with humid climates potentially causing condensation to form in attic in the South in the article you reference. Zone 4 is not, it would seem, the hot-humid climate zone where thse issues should worry you. That's my take, anyhow.
Stacey,
1. A vented attic in your climate will work fine, especially if you pay attention to ceiling airtightness.
2. In all climates, attic ventilation matters less than ceiling airtightness. For more information on this topic, see All About Attic Venting.
3. Your suggested alternate assembly -- with Roxul above the roof sheathing, and asphalt shingles nailed through the Roxul to the plywood below -- won't work. Asphalt shingles need to be installed on solid sheathing, so your suggested assembly would require a second layer of roof sheathing above the Roxul. In any case, Roxul is rarely installed on the exterior side of roof sheathing when the roof has a significant slope.
Thanks for the reassurance, Kent and Martin! We'll plan to have a ventilated attic ~ another decision I can check off the list.