I’m currently hanging joists beneath what will be a 3′ X 5′ hooded bay window extending outside the foundation, and wonder how others vent the joists and under floor areas that hang out in space.
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The two 5 X 12’s define the bay window’s base and 2X12’s will be extended outwards at 45 degree angles to the adjacent rim joists to create a space for three windows. I’m using only DF heartwood in the framing members outward of the rim joists and am painting it with copper napthanate as I go. The underside will eventually be sheathed in 1/2″ CDX, but with the subfloor above it creates a dead air space between those joists. I thought perhaps I might drill some holes top and bottom in the rim joist and the one beyond it to allow some air flow in and out of the basement, although once I place insulation batts in that space I’m not sure how effective the holes would be. How would do it? |
Edited 10/11/2007 9:26 pm by BobSmalser
Replies
Venting allows moisture in vapor form to dissipate. Where would the moisture be coming from?
Air-dried Doug Fir in my climate rises to 15%-17% EMC during the winter rains, even under cover. It's been an unusually wet summer here and the wood in question is already at 15% instead of the 12% typical for this time of year.The threshold for mold spores to germinate is 20% EMC and 55 degrees F. I figure even minimal condensation from the heated space above the subfloor of that cantilever could raise the moisture level to the 20% threshold by the time late spring temperatures sufficient to grow fungus kick in.Even tho DF heartwood has moderate resistance to rot, over the long life span of the structure I expect eventual problems. I hadn't considered a complete seal using foam, and suspect that would work if I waited til next summer to seal it. Other builders have suggested installing vents in the soffit beneath the cantilever.
Edited 10/12/2007 1:22 pm by BobSmalser
Simple. Do not vent it, foam it. Solid.
Spheramid Enterprises Architectural Woodworks
"If you want something you've never had, do something you've never done"
Don't worry about it.
It's not going to be an issue.
I've put together many similar bays and venting was never an issue since most times, unless it is blocked off, the space is open and accessible from the basement to insulate later.
Enough of my "swag"... and 2cents