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Stoop sheathing frost

Hello everyone,

I have an issue I cannot solve. About a month ago I was checking things in my attic and noticed some frost on my attic sheathing. The interesting thing is, the frost is only above my stoop; there is no other frost on the sheathing anywhere.  I crawled over and noticed the stoop is covered in a few inches of blown in insulation apparently on purpose. The stoop ceiling is soffit ventilation.  The stoop is facing the northwest so could it be wind blown moisture thru the soffit?  Should I seal the stoop soffit or add insulation to block the cold air? I can't figure this one out and need some advice...I will add that all major air leaks have been sealed and I have added insulation to reach R-50.

Re: Stoop sheathing frost

Frost forms in attics when warm, humid air accumulates and condenses on the underside of your roof sheathing. Depending on the temperature, you will have either frost or water collecting.  A little bit of frost forming is not unusual after really low nighttime temperatures, and it isn't a huge problem if it is able to melt and evaporate so that wood can dry.


It becomes a more serious problem when the quantity of frost isn't able to evaporate before more starts to accumulate. The longer this goes on, the greater volume of ice you will have and the greater volume of water you will have leaking down when it finally warms up enough to melt.


If you're further baffled by frost forming on one side and not the others, you're probably looking at the north side. The sun will sometimes warm the south side enough to thaw and evaporate moisture but leave it still coated on the north side. 


Reducing relative humidity levels in attics

Attic ventilation

Ceiling air barriers

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