Water is dripping from the light fixtures and recessed pot lights in our vaulted ceiling in the living room. I removed a light fixture and some Fiberglas roof insulation and saw mould on the underside of the plywood roof decking. Do you have any ideas about how this problem occurs?

There are several possible causes. The most likely source of the water, provided your roofing is in good repair, is condensation from high humidity in your home. Moist air can leak through small breaches in vapour barrier seal around recessed lights and outlets and condense on the underside of the plywood roof deck, especially if there is no air space above the insulation to help carry the condensed moisture to the exterior. This condensed water then finds its way back onto pot lights and outlets and leaks back through the interior ceiling finishes.

Here are some solutions. First, install exhaust fans in every bathroom and over your kitchen range. Inter-connect at least one of these fans to a de-humidistat, centrally located on your living room wall. This will help remove any accumulation of moisture-laden air. Next, remove all your pot lights and electrical fixtures in the living room and throughout the ceilings of your home. Make sure the existing pot lights are designated airtight or install vapour boots over these lights and fixtures and seal the boots to the vapour barrier before re-installing them. Re-seal the boot flanges and the existing vapour barrier to the fixtures. Also consider reducing all sources of moisture in the building including exterior water near the foundation walls. This can be achieved with a good gutter, downspout and leader system on your eaves. Exterior water usually finds its way under foundations to the interior of the building causing high humidity in buildings.

 If the problem persists and there is no air space above your roof insulation here are other, more expensive measures that will prevent the movement of moist air through the roof cavity. Change the Fiberglas batt insulation to blown dense pack cellulose, which acts as an air retarder system. Then remove the existing roof shingles and install 2 1/2 “ rigid high density Styrofoam, plywood and new roofing shingles. This will move the condensing surface from inside the roof cavity to a point beyond where moisture can return through the roof structure. By implementing the least expensive measures first, you may find these additional measures unnecessary.