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Help! Condensation on inside of fiberglass insulation

Posted by microxl (My Page) on
Mon, Jun 9, 08 at 23:11

Our 5 yr old house has a walkout west exposure. The 2 x 6 walls were nsulated with R-21 fiber glass and covered with a poly sheet vapor barrier. While stapling some loose corners of the vapor barrier today I noticed considerable condensation on the inside of the poly and on the insulation itself.

When I removed the poly sheeting I found the insulation wet on the basement side but dry and hot on the outer side of the fiber glass. With the 90 degree heat and humidity outside and the cool basement air inside I guess I am not surprised. I suspect the cement board siding may actually contribute to the problem by heating up more than wood siding.

The question is, what can I do to resolve the problem? That is, short of heating my basement in the summertime!
I am in the process of converting the area to living space and will soon want to sheetrock.


Follow-Up Postings:

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RE: Help! Condensation on inside of fiberglass insulation

Q.) are you sure you don't have a leak into the cavity space and that you aren't seeing water leakage into the wall and not condensation?


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RE: Help! Condensation on inside of fiberglass insulation

Dehumidifier in the basement should minimize that problem if it is only due to the heat differential and basement humidity..

There is one other thing than can cause the moisture---if a cooling/heating register is close to that area and cool/warm air is directed at the area. If that is happening, simply add a deflector to prevent the air from blowing directly on that wall.


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RE: Help! Condensation on inside of fiberglass insulation

A poly sheet is not enough over any insulation in any wall cavity.

First, poly sheets (as are paper faced 'vapor retarders') fully flammable.

Building codes require that any flammable paper or poly materials be covered with at minimum 1/2" gypsum board.

Hang and finish your insulation and exposed vapor retarder with the rwquired 1/2" minimumgypsum board and your problems will be resolved.


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RE: Help! Condensation on inside of fiberglass insulation

The moisture is coming from the outside and condensing in the cooler insulation and on the exterior side of the vapor barrier.

For cooling season the vapor barrier is on the wrong side.

Since you are in NH, you have predominately a heating season and the barrier is towards the warm (interior side) of the insulation.
In predominately cooling areas, the vapor barrier goes on the outside (the side that is warm most often).

Since there is no practical way to move the vapor barrier for the seasons, the longer season is used to decide the vapor barrier location.

In the off season there may be limited condensation.
Running a dehumidifier inside the house is not going to remove moisture that is on the other side of a vapor barrier.

Be SURE there are no leaks from anything.
The small amount of condensation on the outside of the vapor barrier should dry to the outside.


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RE: Help! Condensation on inside of fiberglass insulation

I have same issue in our 6 years old house which we bought 2 months ago. First I noticed small (5 cm in diameter) paddle of water close to one of the walls, water was dripping from the bottom edge of vapor barrier sheet. This wall is facing south and backyard. Insulation was wet inside along whole wall, but only this wall, others were dry. I pulled it out in few places - it was wet only where it touches plastic vapor barrier sheet. Wall behind insulation was warm and dry. No leaking pipes. I think it's condensation, because it's quite cool in the basement.

brickeyee, thanks for the explanation. I'm curious if there is a way to minimize amount of condensation. Has anyone solved this problem?


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RE: Help! Condensation on inside of fiberglass insulation

"I'm curious if there is a way to minimize amount of condensation. "

Control the indoor humidity.
For residential use this generally means a dehumidifier.


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RE: Help! Condensation on inside of fiberglass insulation

brickeyee, as you have noted "The moisture is coming from the outside and condensing in the cooler insulation and on the exterior side of the vapor barrier." therefore controlling indoor moisture won't make any difference. Again, I interpret this as moisture is coming through the ground, then basement wall, then fiberglass insulation and then it condenses on the poly VB sheet because air temperature inside basement is cooler than air temperature inside fiberglass insulation covered by VB sheet. Out of 4 walls in the basement only south facing one has the most condensation, some condensation also exists on the adjoining 2 walls, about half way through. South facing wall is a lot warmer, therefore air inside fiberglass insulation is warmer too and can hold more moisture.


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