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Discussion Starter · #1 ·
I started a ht in the basement. Was going to be really simple but when I started researching insulation its getting down to a science.

Part of my basement is above ground with regular framing and osb. The builder had insulated with unfaced fiberglass and covered with plastic. I found mold growing in the stuff when I peeled it back. After much research I decided to use 2 inch xps in these areas and the rim joist for mold resistance. Building science has a nice article about using foam in the rim joist area as well.

http://www.buildingscience.com/docu...ic=doctypes/information-sheets/4-air-barriers

The down side is that the r value is less (for rigid unless you get really thick) and I've heard acoustical properties aren't that great. Then I started reading about formaldehyde free fiberglass. They says its naturally mold and fire resistant. I'm considering to use this in the ceiling and the back wall of my theater for better acoustics since its not on a external wall. I'm certainly not a expert but I'm guessing the foam on exterior walls could still be beneficial because it reduces air infiltration while still being semi permeable to keep the wall behind it dry.

I know that rigid foam against the foundation is the way to go there. I'm hesitant to use fiberglass again in a basement with all the mold issues they can have. I'm including a pic below. The short wall has 1 inch xps with foundation behind it. Above the shelf is above ground and I used 2 inch xps there. My original plan was to use all xps and not use fiberglass to fill in open areas.

I've also taken care of the moisture problems and running a dehumidifier now.

thanks and looking forward to any input.

 

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where the vapor barrier goes all depends on your climate. Condensation (and thus mold) occurs when warm air contacts a cold surface. So in the case of the guy who installed a vapor barrier room ward of the insulation, warm air was traveled to the cool side of the plastic sheet and condensed. The sheets can work if installed correctly but the often are not. Even the slightest air opening (outlets anyone?) will allow air to pass back and forth.

From everything I have read the rigid foam you used is one of the BEST ways to prevent mold formation as mold can't eat it and it will prevent the formation of a large temperature differential that causes condensation and mold.

If I were in your shoes I would find some thin (R9?) fiberglass batts to install and call it a day.
 

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The foam insulation also has a higher R-value per given thickness allthough it does come at a premium price compared to fiberglass.
 

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Discussion Starter · #4 ·
I believe buildingscience recommends 2 inches or less of xps to meet permeability requirements. After some more reading I might stay with the original plan and not use any fiberglass so moisture can escape without adding more layers of stuff to the foam.

I might be giving up a little r value or better acoustics but I think long term it will have less problems. I live in IL which gets very hot and humid during the summer and cold winters.

I also plan to stain the floor and only use area rugs. I think in basements sometimes less can be better. I've read threads where people spend years building a theater and some unexpected leak or flood requires them to do a total rebuild.

Edit: The place with the most mold was over by the sump pump where the vapor barrier was partially falling down.
 
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