Can you solve this month's stumper?

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Permalink Reply by Ron Clark on February 22, 2013 at 9:28pm

The moisture problem with the home in question that Bo Jespersen's company, The Breathable Home is encountering in the Stump the Chump article this month is likely due to warm moist air from inside the house (as described in the article) coming in contact with a cold roof deck.  Frost is probably not even forming.  When this happens, water vapor in the air condenses and causes the described situation where moisture drips down into the room below.

The fix would be to create access to the air gap above the the rigid insulation and completely fill that gap with foam as well as seal the gaps at the top of the rigid insulation near the ridge beam. That would eliminate any possibility of warm moist air from contacting the cold roof sheathing.

Permalink Reply by Matthew Dahlhausen on February 28, 2013 at 9:43am

The solution to this problem is to prevent warm moist air from contacting a condensing surface.  This is difficult, because it is nearly impossible to create a perfect air barrier – especially one that maintains a seal over time in a climate with large temperature swings and between materials that have different coefficients of thermal expansion. 

One solution would be to pull up the roofing material (singles, slate, whatever), and add rigid insulation underneath, overlapped (2-3in), then add lathe and sheathing (the space between the lathe creates an air channel) and then add the roofing material.   The insulation will need to be thick enough so that the air under the sheathing won't reach the dew point even during the coldest nights.  This option is expensive.  

A cheaper option would be to remove the rigid foam, spray the underside of the roof deck with foam, and then place the rigid board back in while the foam is still nice and gooey, essentially as a continuation of the foam.  That will get rid of the air space where condensation can occur.  It would be nice if the foams had the same thermal expansion coefficients, and there weren’t air bubbles at the interface, but that is probably inevitable.   The risk here is that there isn’t enough vapor permeability through the foam and that any water that gets under the roof deck from roof leaks will stay there.  This is a risk with all under roof deck foam applications, and demands a good job maintaining the roof once the under-deck insulation is installed.    

To just solve the water issue, a ridge vent could be installed and the soffit area could be re-done to create an air channel under the roof deck, but this would mean continued air leakage.  Far from ideal.