Why is there frost build up on basement walls? (poured concrete)


Why is there frost build up on basement walls? (poured concrete)

William Hackett April 7, 2022Last updated: April 7, 2022

Recently (spring time) noticed the carpet in my finished basement was wet.  I removed the drywall, plastic vapour barrier and pink insulation and noticed a huge amount of frost build up in the corner of my poured concrete wall.  House is approximately 70 years old and live in central Canada (Winnipeg) where it gets to minus 30 celcius in the winter.  Never have moisture problems in the summer.  What is the proper way to insulate and finish the walls to prevent frost build up from reoccuring?  Do I need the plastic vapour barrier, getting mixed recommendations from different contractors.  Thanks.

Responses (1)

Mike Reynolds April 7, 2022, 1:25 p.m.

The main reason basements turn moldy is the inclusion of a vapour barrier on basement walls. Older homes will rarely have had exterior membranes to protect foundations from moisture, so they will always absorb ground moisture and stay wet. If you have vapour barrier on the inside of a basement wall then anything behind the vapour barrier will also stay wet. Learn more on this page – 

Why basements are moldy and how to prevent mold when building and renovating basements.

If you assume the basement wall is wet and build accordingly, you can avoid mold issues. See our best practices for basement construction here, but here are the basic principles: put vapour protection directly on the inside surface of the concrete to keep the moisture from entering your basement wall assembly in the first place. That can be a sheet of 6-mil poly vapour barrier, or rigid foam boards.

With no moisture able to penetrate into the basement, you can then safely frame a wall and install insulation.  And there is absolutely no need for a vapour barrier on the inside. The only thing it could possibly do at this point is harm, by trapping moisture. That can come from any flaws in the barrier against the concrete, or even just from the moisture in wood, since lumber arriving for a renovation project is likely quite wet and may have even spent time sitting in the rain either at the lumber yard or your front yard. So, the only place that moisture can go is into the air of your basement, which is easily handled by a dehumidifier. Don’t try to stop it with a vapour barrier, and don’t listen to anyone that tells you should, or in a few years you may end up right back where you are now.