Published on September 19th, 2012 | by Henri


Moisture trapped under old roof sheathing

Q. We own a ranch that was built in the early 1950s with a flat roof. Somewhere along the line, a pitched roof was added, leaving intact the original roof deck with several layers of tar paper, tar and pebbles. The roof overhangs 30 inches on the front and back of the house. The original flat roof rafters are 2×10’s on 16-inch centers.

I had the soffits replaced last summer. The old ones were vented with a continuous strip of ugly wire screen so the cold air could move between the rafters over what was once probably 3-1/2-inch kraft-faced fiberglass, now very compressed. As the insulation was poor and there were 6 to 8 inches of space between the insulation and the old roof (one could look right across the inside of the flat roof from front to back between the old rafters), I decided to have cellulose blown in. The job was done by a reputable company. I quizzed them about moisture build-up, and they said there was no problem, as did another building contractor. Everyone seemed to think it was a non-issue, especially since we have plaster ceilings.

We capped off the newly insulated bays with rolled bats of fiberglass on either end over the front and back walls of the house, leaving the 30-inch overhang space above the soffits uninsulated, drilled lots of 3-inch holes through the old roof into the attic space above the soffits and ran a continuous strip of standard vent the full length of the new soffit to let air flow up under the pitched roof to the 2 gable end vents.

Still concerned about the potential for moisture build-up in the cellulose, I recently drilled 3 test holes through the old roof, but after a year’s time it was hard to tell if it was damp as the material was cold, and I didn’t have a moisture meter.

Should I drill a pattern of holes through the old roof deck or would stripping some of the roofing material to expose the 3/8-inch plywood under it suffice? Or should I just keep an eye on it?

A. If there are convective paths around the plastered ceilings (recessed ceiling light fixtures, bathroom and kitchen fans, cracks at joints of ceilings and walls, etc.), the potential for condensation under the old roof sheathing does exist. This would, in turn, wet the cellulose which, under the circumstances, would never dry. Removing the old roof covering entirely would have been preferable, but the plywood sheathing still represents a vapor retarder, albeit one not quite as effective as the built-up roof; the plywood should also have been removed either in large part or totally.

At this point keep on eye on things although you may not see signs of condensation on the cellulose in the holes you have drilled because there is ventilation there. One way to deal with this is to cut larger holes in the old sheathing so you can slide your hand under the plywood to feel for moisture. If you do find that the cellulose is moist, you should remove more of the old roofing and sheathing to allow the moisture to dissipate into the new attic.

It would also be best to have a continuous ridge vent and seal the two gable vents, as they are not the best outlets for the continuous soffit vents you installed.

Soffit and Ridge Vents

Air flow with soffit and ridge vents

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