How do you build a mold-free basement in a new construction house?

Charmaine Herzig
Updated: Sept. 9, 2020

I have a new construction project where one floor is subgrade. It has a cement wall on three sides that the contractor insured me was prepared to be mold resistant. He put special draining paper on it and put gravel up against it but it already smells like mold. It is just framed. Is it possible to finish the project in a way where I won't have mold issues? Thanks 

Responses (3)

Charmaine Herzig 4 years ago

Hi there! The house is located in the Silver Lake area of Los Angeles. Per code the wood does touch the top of the concrete wall that makes up the subgrade floor. It's pressure-treated. It's what we're required to do in Los Angeles. The material that is on the wall is drainage paper that is not really paper but plastic. It's meant for that purpose. I'm also curious what type of drywall to install in the sub grade floor. My contractor mentioned dens glass or dens armor. Any experience with any of these?

Emmanuel B. Cosgrove 4 years ago

I’m glad to hear that it is a waterproof membrane, the ‘paper’ threw me off there : ) That is standard practice in most locations to put a waterproof drainage matt or membrane on the exterior of foundation walls. Building a basement properly in Southern California is a lot easier than in cold and humid climates, so that is a big advantage for you there as you have less issues with moisture and no risk of frost heave.

The way they have done the outside seems fine, with a drainage membrane and backfill, so any issues would be on the interior. Before we move on to drywall, I’m still not clear on the stud wall – are there studs pressed directly to the concrete wall or is there any membrane separating them? And again, how long ago was it poured? You make no mention of insulation or vapor barrier, or drywall just yet, so is it at the stage of wood only?

Mold typically forms in basements when wood studs and insulation are sandwiched against the wall with a poly vapor barrier, so without that I’m wondering if it’s maybe not mold, but just a new more humid environment that you may be accustomed to. I’m not saying it isn’t moldy, but us know more about what is going on down there and we can figure it out.