Q. I am a self-employed building contractor and have followed your column for years. The problem that I am having is with two separate additions that I built within the last two years.
Both additions have cathedral ceilings with skylights. The construction consists of 2-inch by 12-inch rafters, 24 inches on center, with 5/8-inch tongue and groove plywood decking. The roofing material is 30-year asphalt shingles applied over No.15 felt paper, with Grace Ice & Water Shield applied around the perimeter of the skylights as well as to the bottom three feet of the roof. There are continuous 2-inch soffit vents as well as a continuous ridge vent on both additions. The rafter bays have plastic baffles for ventilation from top to bottom. R-38 faced-fiberglass insulation was applied below the plastic baffles and was stapled to the rafters. The ceiling is made of 1-inch by 6-inch tongue and groove beadboard applied to the bottom of the rafters.
The problem is that every time we have a real cold snap followed by a quick warming up period, I get dripping from the cathedral ceilings occurring on the inside of the additions. My first thought was that maybe the ridge vents weren’t performing correctly, so I tore off the existing vents and did them over. I noticed a considerable amount of condensation on the bottom side of the plywood as far down as I could see.
I contacted several architects I know as well as other people in the trade. The consensus was that because of the cold, condensation built up, and when the weather warmed the frost on the bottom side of the plywood melted, thus creating the dripping which took place. Hopefully you can provide me with some help in resolving this situation.
A. I have bad news for you. The people you contacted are correct in saying that considerable condensation occurred during the cold spells. The subsequent dripping comes from the fact that the condensation froze on the underside of the plywood roof sheathing and melted when the temperature warmed up.
But the condensation takes place because you used faced-insulation and tongue and groove boards on the ceilings of both additions but did not provide an air barrier. Moist, warm air convects through the joints between the ceiling boards and bypasses the kraft paper vapor retarder. When it comes in contact with the cold roof sheathing, condensation occurs. No amount of ventilation can overcome this scenario.
If you had used unfaced fiberglass insulation and carefully stapled on 6 mil plastic as a vapor retarder, and had no ceiling perforations such as electric boxes, etc., you would not have this problem.
Unfortunately, there are only three ways to remedy this situation. One is to remove the ceiling boards and carefully staple 6 mil plastic to the underside of the rafters. The second is to remove the roof and the insulation and to spray closed-cell polyurethane between the rafters. The third and most appealing option is to have an experienced foam insulator remove the fiberglass from the rafter bays with hooks they insert from the fascia and the ridge vent and inject closed-cell polyurethane.