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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

Asked by dena platis
Posted Tue, 04/24/2012 - 02:29

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4 Answers

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1.
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Dena,
I know that spray-foam contractors routinely spray foam from above in the manner you describe. If this work is done properly, it's a good solution to your dilemma.

Of course, you'll need to check with your local building inspector to see if this solution is acceptable in your jurisdiction. While the installation is a technical code violation, I see no building science reason why it won't work, as long as the contractor is conscientious and there are no air leaks.

Existing light fixtures should be protected by sturdy boxes; as long as these boxes are big enough to avoid overheating issues, everything should work.

Your contractor is right that existing ductwork can be encapsulated with foam. Just be sure that the ducts are sturdy, undamaged, and properly connected, because once the foam is sprayed, it will be difficult to make duct repairs.

Finally, choose a week when the long-term weather forecast predicts several clear days without precipitation.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 04/24/2012 - 07:42

2.
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Thanks so much for your reassurance we are on the right track. We're hoping the contractor will do a thorough and good job, he has never done a retro before, only new construction from below. He seems like a conscientious worker and understands how critical it is for us to fill each cavity with no voids around the edges.

One other question, this is a split shed roof. The upper story shed roof is having the moisture problems, the lower story shed roof has had no indication of a problem but built the same, no continuous roof ventilation and with traditional fiberglass insulation inbetween the TJI's (running perpendicular to the slope of the roof). The lower story has lower relative humidity levels and is not showing signs of moisture buildup. I hate to waste the resources if I don't need to but am nervous that something will happen over time. Do I rip off the lower roof now and re-insulate it with SPF, while I have the foam contractor out here, thinking it too will eventually fail? Or could it possibly be fine forever?

Lastly, just as a totally different approach to the solution, we had someone suggest leaving the fiberglass batting and sheathing down and lay 2" of rigid foam over that, then frame in 2" air baffles/channels for stack effect air flow under the roof, and then put on another layer of sheathing and then lay down the metal roof. A kind of hybrid approach: an unvented ceiling assembly with a vented roof over it. Anyone have any feedback on that approach? - I felt like it is more complicated.

Thanks again for your time and consideration, Dena

Answered by dena platis
Posted Tue, 04/24/2012 - 23:47

3.
Helpful? 0

Dena,
Q. "Do I rip off the lower roof now and re-insulate it with SPF, while I have the foam contractor out here, thinking it too will eventually fail?"

A. That depends on your checkbook balance, I guess. Repairing both roofs would be the right way to go.

Q. "We had someone suggest leaving the fiberglass batting and sheathing down and lay 2" of rigid foam over that, then frame in 2" air baffles/channels for stack effect air flow under the roof, and then put on another layer of sheathing and then lay down the metal roof. A kind of hybrid approach: an unvented ceiling assembly with a vented roof over it. Anyone have any feedback on that approach?"

A. That approach is used all the time. However, you'll probably need more than 2 inches of rigid foam (depending on your climate). For more information on this option, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 04/25/2012 - 05:01

4.
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Thanks again for your prompt and thoughtful responses. Dena

Answered by dena platis
Posted Wed, 04/25/2012 - 10:56

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