Q: We live in Kingston, NY and cathedral ceilinged the main living space of our 1925 raised ranch. The rafters are 2×6’s. we used R-21 fiberglass batts (5 1/2 ” thick) and 1″ pine tongue and groove boards. The roof is an unvented shingle roof. Is there an affordable solution to prevent humidity build up? – Kate
A: In cold climates, unvented roof cavities can lead to moisture problems. If moist air gets into the roof cavity, the moisture can condense on the underside of the cold roof sheathing and damage the sheathing or roof framing. Homes with high interior moisture levels are at risk, especially if there is are easy route into the roof cavity – for example, through recessed lighting fixtures in the ceiling.
Many contactors use closed-cell foam for unvented ceilings, because the foam, if properly installed, blocks moist air from entering the roof structure. Fiberglass batts, however, will allow moist air to easily pass through and reach the sheathing.
The best solution is to create an effective air barrier on the interior face of the roof structure. However, this is nearly impossible to do with tongue-and-groove boards.
Ideally, you would have installed an air-barrier material prior to installing the wood finish. One of the new “smart” vapor retarders, such as MemBrain or Intello would be best as these one-way membranes prevent air and moisture from entering the roof cavity, but allow the roof cavity to dry to the interior of the building if moisture builds up. That is, they allow moisture to pass one-way into the living space.
Another option would be standard drywall, carefully sealed at seams and edges to block airflow. You could leave it unpainted or use a standard latex paint and install the wood over this. As long as you don’t use a vapor-barrier paint, drywall is highly permeable to water vapor. You never want to use foil, plastic, or any impermeable material on the underside of an unvented roof as the structure cannot dry out if it gets wet from interior moisture or a roof leak.
Also, keep household moisture levels under control in the winter – no more than 50% RH. Sources of excess moisture may include wet basements, uncovered dirt in a crawlspace, firewood drying in the basement, and kitchens and baths without exhaust vents. Small, tight houses with many occupants and lots of plants will tend to have much higher moisture levels than large, leaky houses with fewer occupants and a lone cactus.
If you’re seeing moisture condensing on the interior of insulated-glass windows, your moisture levels are too high. Or you can take the more scientific approach and buy a humidity gauge. If moisture levels are too high, first try to reduce the sources. If that doesn’t do the trick, increase ventilation levels. A whole-house ventilation system may be necessary in a small, very tight home. – Steve Bliss, BuildingAdvisor.com
Read more on how to insulate Unvented Cathedral Ceilings.