This Minnesota winter has been an especially problematic one for frost in attics, and it seems that most people don't understand why it happens or how to fix it. There is a lot of mis-information floating around about why frost gets in to the attic and how to fix it, but I'm going to give you the real story.
This winter we experienced an unusually long cold spell in Minnesota, with temperatures never climbing above freezing for about a month. This means cold attics. When moist household air is allowed to escape from the house to the attic, it condenses on the roof boards in the form of frost. When this happens for a long period of time, a lot of frost can accumulate. When the frost melts, it soaks the insulation in the attic, stains the ceiling, and can even leak out through windows (I talked to someone last week who was having this problem).
I've heard several homeowners tell me they called their roofer out because they thought the stains on their ceiling were caused by a leaking roof, and of course it wasn't a roof problem. Nevertheless, the roofers typically suggest adding more ventilation to the attic space. This will not fix the problem. To suggest adding more ventilation to a roof to fix frost in the attic is like telling someone with a leaking water pipe to install a dehumidifier. Ventilation is not the way to fix frost in the attic.
The fix for frost in the attic is sealing attic bypasses - these are passageways from the conditioned household space to the attic. Bypasses are what allow moisture in the attic to begin with, and this is what needs repair. While bypasses are certainly the most common cause of frost, there are other ways it can happen, and these are much more obvious. Disconnected bath fans can be particularly problematic, and disconnected furnace flues can be catastrophic. The photo below is one of my favorites - when the roof was replaced, the furnace flue came loose in the attic, which quickly turned the attic in to a winter wonderland.
If you've had frost in your attic, wet insulation in your attic, or stains on your ceiling, find your attic bypasses and seal them. If you're not sure how, read my related posts or give me call.
Reuben Saltzman, Structure Tech Home Inspections