What does frost on the attic roof sheathing indicate? Insufficient venting? I have a problem with a room over a garage in which there is frost on the attic sheathing. The room is living space but heated only when needed. There is a good vapor barrier and 10″ of cellulose. There are two large gable end vents (24″X 24″) but no soffit vents because of the gambrel rafters being treated as walls. Has anyone ever experienced this frustrating situation before or can offer some help?
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You should get many responses on this one. There was a long thread this week about a three-season porch that was heated intermittently and it seemed that was a big part of the problem. The warm air had lots of moisture that condensed when the room was closed and cooled. If everything else is well insulated and properly vented, this could be your problem.
Al Mollitor, Sharon MA
I think the consensus hewre is that gable vents are pretty much worthless. A combination of soffit vents and ridge vents is much better.
You say you have a vapor barrier, but moisture is getting up there somehow. What about can lights, or other electrical penetrations? Are they sealed up well.
Bob Walker has mentioned many time that when he sees attic moisture problems that there is often a wet crawl space or basement. The moisture finds its way up through the structure somehow.
Adding ventilation sounds to me like the most likely cure. With your gambrel roof, the whole attic needs vented - Both sides and the top.
Do Lipton Tea employees take coffee breaks?
This problem is an indication of inadequate ventilation. You want about one sf ft of venting for every 100-150 sq ft of living space. and it should be in both soffits and in atic, whether gable, ridge or pot.
But you report having VB and decent insulation. There is theory for not nneeding any vents when the house is sealed well. That theory only works when it is a perfect seal. Obviously, yours has leaks such as at the attic access and light fixtures. Possibly on the sloping gambrel walls too. Were you present when the VB was placed and do you know if all subs were taught to respect it?
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