How can I turn my pseudo vented roof into a true unvented roof, with cathedral ceilings?
We are living in a new house with a low pitch 1.5:12 metal split shed roof and cathedral ceilings. There was a misunderstanding when the house was built and no ventilation was framed in under the roof. We found out when we had condensation issues dripping on us over the winter....So now we want to go back to the architect's original intention and turn it into an unvented roof. In order to avoid the huge mess of taking the drywall ceilings out and spraying foam in from below, we want to know if we can do it effectively from above? My contractor has suggested the roofer remove the metal standing seam roof using a special tool, remove the roofing paper and sheathing, expose the 24" wide x 12" deep TJI bays to remove the existing 10" deep fiberglass insulation batting, leave the vapor barrier in place and blow in the foam. The foam contractor told us we don't need more than 6" of SPF, providing us with R36, more than enough to meet code (for central WA- zone 5). I am confused by this solution since IRC 806.4 requires air-impermeable insulation be in direct contact with the underside of the roof deck. The proposed retrofit does not meet this requirement, the TJI cavity is 12" deep and we would have a 6" void between the SPF and the sheathing. Is this a problem? Even if there is no/ minimal moisture permeating the ceiling/insulation, could the sheathing be susceptible to rot from either below or above. The winters have intense moisture and cold for many months here. Is there a potential for the dew point to form under the sheathing? Also, is it reasonable to expect the foam contractor to fill each cavity 100% and have absolutely no voids for moisture to creep through. How can I know this has been accomplished - will a visual inspection be adequate? And along the perimeter of the house the blocking (between the TJI's) was notched 2" to allow air flow, would it be necessary to close up the notches and/or fill the foam up the sides to seal it all up?
To help visualize the construction, the 12" TJI's (joists) are spaced every 24" OC perpendicular to the slope of the roof, they are sitting on top of the glu-lam rafters that are exposed from below, drywall ceiling inbetween them. So each TJI creates a wall or barrier for air to flow from the bottom to the top under the roof. There is no stack effect, or continuous flow ventilation.
Is there another solution I should consider? I like the concept of the unvented roof except for the risk of not finding a roof leak until perhaps way too late.
Also, my contractor said it was absolutely fine to blow foam around the flex duct used for the exhaust fans (since they weren't insulated to begin with), I am concerned it will degrade their structural integrity?
And he thinks we can encapsulate the steamroom light in foam to minimize any leaking of moisture from there, I am worried it will over heat?
Noone said anything about the safety of inhabiting a house with SPF, especially within the first 24 hours - any take on fumes and health hazards?
Any thoughts, insight, words of caution, information.... would all be deeply appreciated.
Thank you, Dena
Posted Tue, 04/24/2012 - 01:29
Other Questions in GBA Pro help
Since drywall has a permeance of 0.02 liter/sec - m at 75 PA is ever possible to have sheathing dry to the interior of the home?