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Moldy basement wall.

I have an 18 month old house with a walk out basement. The walk out is vinyl sided, Tyvexed, sheathed with OSB, insulated with fiberglass batts R-19, 2x6 studs with a plastic vapor barrier. The basement is unfinished and unheated, though the temperature this winter was always 55 or above and the humidity has been hovering at 45 to 55%.
Recently I noticed condensation between the vapor barrier and the insulation. I pulled down the barrier and the insulation to find that 60% of the stud bays were molding on the upper 1/3rd of the OSB and on some studs. The insulation installer said that they shouldn't have put on the vapor barrier until the house cured.
Is there any way to solve the mold problem (assuming I get the moisture under control) short of removing the molded OSB?
If I'm seeing mold on the interior, would it have affected the exterior of the OSB?
The insulation installer said that once the walls were dry, he could blow in insulation and if we drywalled it would solve the problem. Is this right or is the problem just hidden?
Thanks for any input.

Re: Moldy basement wall.

Gary .... This is a common dilemma created because of the current desire to make our homes as energy efficient as possible. In many cases it's a local code issue requiring foundation insulation on new homes.

With new houses these problems are unique to the nature of the building materials used for construction. Most of the materials in wood framed homes have fairly high moisture content when installed.
These include not only the wood framing & sheathing materials but also the concrete in the foundation & basement floors also the drywall , taping compounds and paint used to finish the inside the home. Some of this moisture is quickly released into the home as the drywall compounds and paint dry. The other materials like concrete typically release moisture slower as it cures .... sometimes for the first couple of years.

Some of the moisture from the concrete foundation curing process will be released to the exterior above grade in the warmer months. This moisture evacuation is unfortunately limited during cold winter months as the structure above grade reaches the freezing point or below. During this time of year the only area for this moisture to escape to is the warmer indoor basement. When the new foundation is covered with fiberglass insulation and a plastic vapor barrier the escaping moisture is being trapped in the insulated wall cavity. There also is additional moisture inside this wall cavity from the drying wood framing .

This will explain the moisture you are seeing and if cold enough you would see this moisture freeze and change to frost.

In your situation you would need to remove the vapor barrier and insulation from the entire basement level and left uncovered for a period of time to allow the moisture to stabilize before re-insulating. This can be done with ventilation or a dehumidifier or a combination of both in the moderate season.
If the interior of the foundation is left uncovered there may be some minor moisture issues but are generally not much of a problem.

This will also allow you to clean the mold that has appeared. There are several solutions available from household ingredients to commercially available ones.
As for the extent of your mold it's difficult to say from here but it may only be surface mold confined to the interior materials .... depends.

It may be a few months down the road .... depending ...... when the moisture issue stabilizes then go ahead with re-insulating.
Ideally placing rigid foam sheets completely covering the exterior of the basement level behind the studs would be beneficial. This would provide a continuous insulation barrier as well as a thermo break for the studs.... and would likely eliminate the need for a vapor barrier. Since the studs are in place already and may remain there then closed cell spray in foam would be the next best choice.

If you choose to re-insulate with fiberglass or blown cellulose then a properly applied vapor barrier must be installed.
If the insulation and vapor barrier are not properly installed forming an air tight seal this will also greatly contribute to this issue.

My apologies for the long winded post .... just the mood today:D

Hope this helps. :)

Re: Moldy basement wall.

Thanks, this helps. I've been running a dehumidifier to dry things out and it doesn't look quite as dire as when I first opened things up.
If I decide to use fiberglass again, would you recommend starting with fresh insulation or could the non-moldy parts of the old batts be used?

Re: Moldy basement wall.

Mold requires moisture and organic material for nourishment, as well as other growing requirements that vary by mold genera and species.
Having said that .... fiberglass itself is not food for mold rather a place for organic debris like sawdust or insect fragments to become trapped within the fibers and allowing the mold to feed on the organic debris.
So perfectly clean, dry fiberglass is unlikely to harbor a problem mold buildup.

In the case where the fiberglass is in contact with wood studs that have become moist and mold has appeared to feed on the wood..... it's likely that insulation would have mold spores ... you might consider not re-using that insulation.

It's impossible to tell over the internet how extensive the issue is there.
It may very well be ok to re-use the insulation that hadn't had any exposure to areas where mold was discovered.... depends.

It becomes a judgement call on your part.

Hope this helps. :)

Re: Moldy basement wall.

Things are starting to come together on this project. I think we have to mold licked and are deciding on the insulation. We are thinking about blown in Optima Fiberglass in the stud cavities, closed cell foam in the sill cavities and a vapor barrier. The contractor is suggesting using something called Membrane Smart Vapor Retarder as the "vapor barrier." He would install 6mil poly if I insist, but only if I drywall within a few weeks of the insulation job. Is the Smart Vapor retarder a better choice than poly?

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