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This past week I’ve been going back and forth with an Anchorage homeowner who did the right thing- insulated his 1960’s home- and now has an unexpected situation.  It turns out that up in the attic he is getting a build up of frost on the inside (downside) portion of his roof after beefing up the amount of fiberglass insulation.  With a vapor barrier under the original fiberglass bats, somehow moisture is frosting up in the attic space.

Usually this would be a ventilation problem, incurred from a restriction of circulation through the attic.  It has louvered vents on both ends, so it is a ‘cold roof’ (that is, outside air is free to come and go in the attic area).  One has some frost yet is accessible while the other looks fully frosted up.  That may restrict a flow of cold air coming through.  I asked about moisture possibly entering from the top of the roof- but no, no new leaks known of.

As it is a shallow roof, at first it seemed like possibly the new fiberglass could be blocking air intakes under the eaves but he even increased the open area with new cuts.  He also had ‘baffles’ inserted, so that the pieces of tightly fit cardboard would allow space between the underside of the roof for air to come up from under the eaves as a direct avenue.  The other possibility is a new moisture source from possibly a broken PVC stack pipe that may be entering condensation from escaping air out the top of the roof (such as from a sewer stack).

At this point, acknowledging that the weather is much colder than usual and that Anchorage tends to have moist ambient air, I am not sure what could be causing the frosting as it is not uniformed throughout the upper part of the attic- only on the underside of the baffles between the gables and roof trusses!  I am conferring with the Cold Climate Housing Research Center, yet have any of you readers had similar circumstances? Please reply if so!

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