Bldg/construction Question

[logan97]

Have a basement wall partially below and some above ground. Local bldg code requires a 2x4 insulated wall to be erected.Going to put faced batts of insulation, my question is should I install some kind of vapor barrier on the block wall or just insulate the 2x4 wall and then sheet rock it. My concern is moisture ,possibly mold growth.

 

[randy41]

everything i've read says not to use fiberglass insulation against a concrete wall in a basement.

 

[Gary Fowler]

Do you know if a waterproof membrane was installed on the outside of the block wall? If it was then fiberglass would be acceptable because no moisture would be getting inside. CMU blocks are very porous and water/moisture is going to go thru them quite easily if not sealed. You can also seal them from the inside with a special water proof grout but it is kind of pricey. You may want to consider using a spray on expandable foam for your insulation. It would completely seal the blocks and every crevice behind the 2x4 wall and provide the best quality of insulation. Likely a wash in cost difference if you have to install a water proof sealant for the fiberglass.

 

[logan97]

there was a good waterproof spray on the outside wall up to grade.

I could slip some tyvek in-between the wood wall and block.

 

[dave1949]

there was a good waterproof spray on the outside wall up to grade.

I could slip some tyvek in-between the wood wall and block.

I wouldn't rely on the water proof spray, and it doesn't cover the above grade exposure anyways.

The thing I have seen with basements that are good, dry, well-drained, etc., is condensation forming on the inside during warm humid weather. The spray foam would probably stop that by isolating the cool wall from the warm humid air, but I don't think tyvek would, or is intended for that use either.

 

[JDgreen227]

Have a 36'X40' addition on my house that was done back in 2002, I installed a stud wall in the entire new basement and left a gap of about 1/2" between studs and block wall. Insulated the walls with fiberglass and then drywalled over that, and I recently cut out an area at the bottom of the wall to add a new electrical outlet. There was no sign of condensation or mold after almost 12 years and I think leaving the gap between the stud wall and the blocks made a difference. Still, I am sure much of that was because of waterproofing the new block foundation with tar, something the local codes required.

 

[dave1949]

If this is air conditioned space, that would really help prevent mildew and mold.

 

[JDgreen227]

If this is air conditioned space, that would really help prevent mildew and mold.[/QUOTE

Dave1949...please clarify...are you stating that air conditioning will help prevent mildew and mold, or will the insulation help prevent mildew and mold? For what it may be worth, the space I described in my earlier post DOES have air conditioning outlets in the ceiling, but they have always been blocked off as the basement is not a living space. Given that there are no air returns built into the ductwork I doubt if air conditioning would make much difference in the humidity level. I do run a large dehumidifier in the space about 8 months of the year.

 

[randy41]

you can insulate using rigid foam products and build your wall in front of it.

 

[dave1949]

If this is air conditioned space, that would really help prevent mildew and mold.[/QUOTE

Dave1949...please clarify...are you stating that air conditioning will help prevent mildew and mold, or will the insulation help prevent mildew and mold? For what it may be worth, the space I described in my earlier post DOES have air conditioning outlets in the ceiling, but they have always been blocked off as the basement is not a living space. Given that there are no air returns built into the ductwork I doubt if air conditioning would make much difference in the humidity level. I do run a large dehumidifier in the space about 8 months of the year.

Air conditioning, or the dehumidifier, both reduce the humidity that is needed for condensation to form on a surface that is below the dew point.

The dew point is a fairly high temperature for warm and humid air. Think about in summer how dew will begin to set on grass not long after sundown for example.

The insulation isolates the block wall, a relatively cool surface, from the warm and humid air in an unconditioned space. I don't think most areas out of the south have enough humidity and warmth in winter to cause condensation. In the summer months though, without AC we can have 80 degree air with 70% humidity. Allow that to contact a 65 degree wall surface and you have enough condensation to grow mildew.

This chart shows when condensation will form (dew point) for a range of temps and humidity.