Frost in the Attic


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Old 02-10-06, 01:16 PM
T
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Frost in the Attic

Hello,

I have posted a similar question before but I am totaly stumped. I had a new roof put on last spring. I added plenty of soffit vents and one large roof vent. I also added insulation giving me an R 35 value. I went in the attic yesterday morning after the temp dropped to about 19 F. My roof sheating in my attic had frost on it and all the nail heads were frosted over. In my previuos post it was recommended that I close the Gable end vent because they were likely short circuting the soffit and the ridge. I did do that but obviously I am still having a problem. My attic is sealed to what I think is tight and I have no idea how that much hot air would be getting in to cause this problem. The soffit vents seem to be un-restristed. Is there anything I can do to correct this or is this just an engineering flaw in the house? BTW my house humidity is only at around 36%-38%. Please help I am totaly baffled.

Thank You,

Tom
 
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Old 02-10-06, 02:28 PM
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Interior house humidity is at a reasonable level. Could be lower, but does indicate that interior humidity can only be the source if the air-tightness of the ceiling joints / wall separations / penetrations are ridiculously poor. The first step is therefore to air seal the ceiling plane.


Next, it might just be morning frost. What was the temperature the night before? As outdoor air temperature increases, it picks up humidity from exposed water sources. It carries it into your attic. But your roof might still be cold from over night. Or maybe there was still outdoor air humidity in your attic, and then you hit that cold snap. So the underside frosts. This sort of dew formation is more common in warm conditions, but whatever.

As long as it is not a long term frost accumulation, there should be no damage or mold formation on the underside of the roof.
 
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Old 02-10-06, 04:16 PM
Lowongas
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turn your humidifier off completley for a few days and watch the frost and condensed moisture dissapear. I tell my customers to look in the attic as you have, especially on the cold side, north and east. If it is frosty and wet, turn off the furnace humidifier and watch. If you have no humidifier but have your heat ducts under your house in the crawl space you could be heating up your ground water.More diffacult to remedy.
 
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Old 02-13-06, 07:47 AM
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It's not a reasonable expectation for people to keep their humidity circa 20%; you should be able to live in the house with the humidity at 36%. If interior humidity IS the culprit, well then it's pretty easy to fix.
 
 

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