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Condensation under Vapor Barrier


Bob W's Avatar
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06-10-08, 07:33 AM   #1  
Condensation under Vapor Barrier

Location:
Upper third (up to the main floor joists) of west facing walk-out basement, framed wall. The walls are 2 x 6 with R-19 and have a thick, stapled polyethylene vapor barrier. The v b is not sealed with tape. The insulation is wet on the interior side, dry and hot on the exterior side. I think the exterior, cement board siding agravates the heating due to the hot west sun. Of course rock holds heat better than wood.

When:
Has shown up with the recent humid 90 degree heat wave. The basement is cool and no windows have been opened. The house is 4-5 yrs old so this has probably been going on since construction.

Questions:

Is the moisture condensing on the insulation due to
1. humidity infiltrating from the outside environment and meeting the cool interior environment? 2. Or, is the humidity likely coming up from the sill plate/slab and the interior unsealed vapor barrier? 3. Or, both?

If #1 more insulation? but how?
If #2 dehumidification and sealing the v b?
If #3 both of the above.

Future:
I will soon want to sheetrock and finish my basement but without resolving there is only trouble ahead. It makes me wonder about the insulation in the a.c.'d upper two levels of the house. Could there be moisture there too?

Bob W

 
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airman.1994's Avatar
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06-10-08, 07:52 AM   #2  
What is the RH in the basement?

 
Bob W's Avatar
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06-10-08, 10:21 AM   #3  
Relative Humidity

The RH is pretty high at 75% if my $5 humidistat is accurate.

 
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06-10-08, 11:00 AM   #4  
Id get the RH down to under 55% before you go any further.

 
Bob W's Avatar
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06-10-08, 11:35 AM   #5  
Dehumidification?

The basement area I am trying to turn over to living area is about 1000' sq. What's the most efficent way to dehumidify this kind of area? I have an old portable so I guess I could see what kind of dent this would put in the r h. I suspect it might be pretty localized. When I have the heat zone installed I'll ask about cost of a central dehumidifier. The AC unit would have little effect due to the already cool temp @ 65-67 degrees.

 
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Bob W's Avatar
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06-10-08, 05:02 PM   #7  
Thanks Airman, 1994

You're economical with the words but have been very helpful. These units look great especially since they operate in low temps.

 
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06-11-08, 01:15 PM   #8  
Don't like to talk much! They work in low temps, More RH removed per KW, and have twice the life as L cheap O units.

 
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