Cathedral Ceiling Moisture Problem

[Larry Samberg]

Not an uncommon topic…

Until recently our cathedral ceiling was open == just decking over 4×8 rafters.

We had it closed in. The contractor installed a ridge vent and soffit and put styrofoam channels from soffit to ridge between the rafters. He then filled the rafters with fiberglass insulation, covered that with one inch of Thermax over the rafters. He then put furring strips along each rafter (over the Thermax) to support the 1×6 t&g pine ceiling.

Looks great and the insulation is excellent. The only problem is that one day last winter we had a very large amount of water dripping down the front inside (north facing) wall of the house.

We hadn’t had any precip in at least two weeks and there was no snow on the roof.

Needless to say, this has ruined an otherwise great project for us…..

[Dodgeman]

That could possibly be the cause.

1) Is the fiberglass insulation the kraft faced type? If so, two vapor barriers have been installed. One is the foil faced Thermax, and the other is the kraft paper. This would trap moisture between them and possibly cause a moisture problem.

2) the syrofoam channels from the soffit to the ridge have to be perforated to allow the air space between the rafters to breathe and allow the moisture to vent to the ridge. If they are solid styrofoam, there is no way for the dead air space to breathe from the insulation side to the vent side.

I am assuming you have continuous soffit venting and ridge venting.

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Dodgeman

[Author]

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