Condensation and Mold on OSB

Old 12-30-03, 09:23 AM
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Question Condensation and Mold on OSB

I am looking for some advice on a problem that we have in our basement wall. I am finishing our basement and I came across some mold and condensation on an exterior wall that is located in a portion of our basement. We have a half wall that is wood framed in our basement so similar to a walkout construction with 2 windows. I have attached a picture of one of the areas. I did clean all of the areas with bleach and it is drying nicely but I am afraid once I put all the insulation back up and the vapor barrier back on that it will happen again.

The house is 5 years old here in MN. The basement walls were insulated and a vapor barrier was placed over the fiberglass insulation by the builder during construction and was not disturbed until recently within the last 2 weeks. The moisture/mold was in 3-4 locations but not between every stud location. The house has vinyl siding and no air barrier or foam sheathing on the outside. Is the vapor barrier causing the problem by trapping the moisture?

The cement block walls have a vapor barrier behind the framing and on the warm side of the wall. I have done a great deal of searching the web for information and have seen everything from not having a vapor barrier on the warm side of the exterior wall to needing it. I am sure code is to have the vapor barrier but I don't want to have the house rot from the inside out. The basement has always been dry and the sump pit has only had water in it once in the five years.

I have also read that since new homes are so tight that an air exchanger should be installed to keep the pressure equal and clean air circulating. Should I install an air exchanger? Expensive?

Thanks in advance for any help.

Last edited by dverdick; 12-30-03 at 12:56 PM.
Old 01-02-04, 09:32 PM
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There is a great deal of information on this subject and everyone seems to have an opinion. The problem I have with answering you is that the moisture may come from a variety of sources. You hit on a couple of them like not having an air barrier on the outside. However, moisture problems in this case are more associated with cooling than heating.

The builder merely applying the vapor barrier over the insulation could also be a source. In this case the vapor barrier may have not been ar tight, allowing warm air from inside the basement to enter the insulation and condense. The builder may have wrongly assumed that you were going to finish the basement shortly after purchase.

Pressure differences could also be a source. This is especially true if you have combustion appliances in the house, like a furnace, boiler and/or fireplaces. But I do believe in your area fresh air supplies are mandated. Another cause for pressure differences could be leaky ductwork, especially on the return duct, if you have a forced air system.

Then you have capilliary action on the cut ends of the OSB as a possible source. It maybe true that new homes today are considerably more tighter than older homes, but for the mold to occur, there must be a source for the moisture. So the key to any solution concerning mold, is moisture control.

As far as an "Air to Air Exchanger" or "Heat Recovery Ventilator" is concerned, it will not solve your problem with mold growth in your situation.
Old 01-03-04, 07:10 AM
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It is time for you to play detective. Now what do we know for sure. It takes cold air hitting hot air to make condensation. Saying that, you indicated that this is located in only a certain area rather then the whole basement. So in that area is your problem. I think you are looking too big. Look small. Why is it just in that area. Is there a window nearby ?, cold air exchange, was the osb wet when it was installed, is there a leaking water pipe ? What. Answer this and your problem is solved. I know the MN codes well. It sounds like your builder did it correct. Good Luck Let me know what city you live in. I run through MN alot.

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