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conquistador
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# Posted: 21 Jun 2015 05:46pm
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I've been working on my 12x16 cabin for a little over a year now. All has gone great until I was at the cabin today finishing the stain on the exterior. With 105 degree heat index readings I cranked on the window unit I recently installed and was able to come inside from time to time to cool down. I looked up into my loft and realized I have condensation on the underside of my rafter insulation at the peak of the ceiling. It is enough that the paper backing on the insulation has darkened significantly but the actual insulation and roof behind it is bone dry.
I have a metal roof installed over osb with felt in between. I am assuming I have a issue with air flow. When the ac is running, there is at least a 20 degree difference in temp between the floor and the loft level.
How do I vent the rafters if that is the problem. Do I need the pink rafter vents that go between the insulation and roof? Any help would be greatly appreciated!!
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Don_P
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# Posted: 21 Jun 2015 07:45pm
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I can't speak to whether you have adequate insulation from this. My take on it is you had air with a very high dew point, the warmest, most saturated, was near the peak. The AC got the kraft paper cooled to below the dew point and the moisture from the air condensed on that cool surface. The ac had, I'm guessing, not run for long enough to dehumidify the air. It had cooled the air without yet conditioning it.
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conquistador
Member
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# Posted: 21 Jun 2015 08:08pm
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If it helps, I overlapped the roofing felt from one side of the roof to the other. I left a gap in the osb sheathing under the metal but could the felt be restricting the airflow?
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Don_P
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# Posted: 21 Jun 2015 09:57pm
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There should be an unobstructed slot ~3" wide at the ridge with a ridge vent over it. There should't be tarpaper covering that vent. There may be a poly or foam baffle in the ridge vent itself.
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conquistador
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# Posted: 22 Jun 2015 08:03am
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Thanks for the replies. My father in law told me to get the bigger AC unit, guess I should have listened. I don't think it is big enough to pull the moisture out of the loft parts of the cabin. The floor level gets cool, but the upper parts never cool down.
I am going to try to cut out the felt at the roof ridge. I think I can get at it from the inside, will probably take some effort and a few choice words, but I believe I can open it up for ventilation.
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PA_Bound
Member
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# Posted: 22 Jun 2015 11:20am
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Instead of larger AC unit, a ceiling fan may be better. Because warm air always rises and cooler air settles below, it is very difficult to cool the air at the ceiling- especially when the ceiling is vaulted (look at how many cabins have issues cooling loft spaces). A ceiling fan will move the air around at all levels and do a much better job at mixing cool and warm air- which may reduce/resolve your condensation issue.
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conquistador
Member
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# Posted: 22 Jun 2015 11:34am
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Good deal on the ceiling fan. We are in the process of finishing the wiring which does include a ceiling fan. I may go ahead and mount it up and see if it helps with the issue.
Thanks!
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Don_P
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# Posted: 22 Jun 2015 10:38pm - Edited by: Don_P
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One problem many homes have is actually too big an AC. To actually "condition" the air you want the unit running almost constantly pulling the moist summertime air over the cool coils and condensing the moisture out of the air there. When a unit is too big it cycles alot. It cools the air, and surfaces, below the dew point but doesn't run long enough to remove the moisture from the air for the temperature it has created. A setup for a sick house. I don't think you ran the unit long enough to know whether it can dry it out. A cheapo digital temp/ humidity guage can help diagnose stuff like this. PA bound is right, move the air around, it wants to "stratify" and you'd rather it didn't.
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