Blocking gable vents after ridge vent installation

[mwestra1]

I had a new roof installed 15 months ago. Because I switched to a ridge vent, I blocked my 2 attic gable vents with plywood nailed up over the vents from the inside. The 2 pieces of plywood are now covered in spots of black/grey mold. Any ideas on what could be causing this?

At first I thought it was a general attic moisture/vent problem. But there are no other signs of mold or moisture on the roof sheathing or trusses, or any wood in the attic. Only the plywood blocking the vents. Both bathrooms and the kitchen fans are vented up through roof vents and were replaced when I replaced the roof. My roofer swears I have enough soffit vents for adequate ventilation. Should I drill out some more soffit vents, anyway?

Then I thought maybe the plywood was already contaminated somehow. When I replace the plywood, should I install insulation on the inside and housewrap on the outside? What about painting the plywood with a good outdoor primer and paint?

Thanks!

 

[JohnD]

I think the temperature inside your roofspace will generally be warmer than the air outside.

The piece of ply will be in contact with both, and will therefore have a relatively cold surface exposed to the air inside the roof.

This will cause condensation to occur on the inner surface, in the same way that a glass window will experience condensation when the air outside is colder than inside.

I do not know what your roof construction is; in the UK (this is a UK website) we mostly have tiles or slates on waterproof felt on timber, and sometimes get condensation on the felt if the roof is not adequately ventilated, but our roof spaces are mostly cold, as the thermal insulation is usually placed on the ceiling of the room below, not under the roofing.

If you change the ply for a double-skinned panel with insulation inside, I think the condensation will stop.

It will do no harm to paint it.

 

[jeds]

I agree with JohnD. Condensation due to temperature difference. An interesting fact is that mold (mould) will only grow on fresh water (i.e. pure water) so mold is a sure sign that condensation is occuring.

I wonder why you want to block these vents - why not just leave them open? You can't have too much ventilation into your roof.

 

[mwestra1]

Thanks for the info. That explains it! I'll look for some kind of double-skinned panel with insulation inside.

FYI, my roof (Seattle area) sounds similar to UK constructin... asphalt tiles on felt underlayment on wood sheathing on trusses. The insulation is on top of the ceiling with a relatively large open attic space.

I blocked the gable vents because they can "short-circuit" the airflow from the soffit vents up to the ridge vent. It's common here, and describd in the ridge vent manufacturer's documentation. However, I haven't run into anyone else who has used simple plywood and experienced mold build-up.