Roof venting
datum
| Posted in General Questions on
I have a building with a completely closed ceiling/roof. This building is 70′ long with a 12″x48″ center beam and 2″x12″ rafters (insulated) on 2′ centers and has no soffit vents. Moisture has created a problem since I painted the ceiling.
How can I vent the roof without adding 35 vents per side of the beam. Thank you , Tom
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Replies
Datum,
Where are you located? What type and how much insulation do you have installed? What do you mean “moisture is a problem?”
Datum, We are 15 miles south of Portland, Oregon. Roof has about 8" of bagged insulation. The beam is sweating and sheetrock is showing signs of moisture also. This only happened after we cleaned and repainted the ceiling. Tom .
Installing air permealable insulation against the sheathing can be a problem. Is this something you did recently or did you inherit this installation? Is this an attic space or cathedral ceiling?
Datum,
When you see interior moisture on an unvented cathedral ceiling, here is the usual mechanism for the problem.
1. Like most homes, your home is imperfectly air sealed. Interior air is leaking into the insulated roof assembly.
2. The warm, moist interior air is encountering cold surfaces (usually roof sheathing). Moisture is accumulating on the sheathing, or (in some cases) frost is forming. This frost melts when the outdoor conditions change.
3. The water drips down from the damp sheathing and stains your ceiling.
To read about the correct ways to insulate a cathedral ceiling, see this article: How to Build an Insulated Cathedral Ceiling.
If you think that there is an air gap between the top of your insulation and the underside of your roof sheathing, you could install soffit vents and a ridge vent, and see if that solves the problem. However, this is unlikely to work, because (a) your rafter bays probably lack ventilation baffles, and (b) your rafter bays may be full of insulation.
Moreover, if you adopt this solution, you'll still end up with a poorly insulated ceiling, with outdoor air flowing past the fibrous insulation -- a method of venting that degrades the thermal performance of the insulation.
The best solution isn't cheap. The best solution is to install an adequately thick layer of rigid foam insulation on the exterior side of the existing roof sheathing. This approach requires new roofing, which is why it is so expensive. For more information, see this article: How to Install Rigid Foam On Top of Roof Sheathing.