DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Remodeling (
-   -   condensation behind vapor barrier of walkout basement wall (

fusion_ta66 07-23-2012 11:51 PM

condensation behind vapor barrier of walkout basement wall
Hello. Here's the situation. House was built in 1995, basement walls are studded, insulated, and covered with poly sheeting. Basement is a walkout, and the walkout wall is totally above ground.

When we bought the house last year, during the home inspection I noticed small dark spots (possibly mold), behind the (likely original) clear poly sheeting, but ONLY on the wall that has the walkout (100% above ground). Didn't look bad at all, but I had the sellers replace all the insulation and sheeting along that wall anyway. Studs appeared to be totally fine. Poly sheeting was and is currently sealed with adhesive along all edges, fully taped and appears to be sealed very well.

This summer has been very humid here in MN. We have been running the A/C and the basement has been nice and cool. Last week, I noticed quite a bit of condensation behind the poly sheeting. This was not present all winter. Since there were small mold spots during the home inspection, I am assuming this has been a problem before.

Since the condensation is behind the sheeting, I am assuming that the hot, humid air from outside is somehow penetrating into the wall (which is 100% above ground), and flowing up againt the cool poly sheeting inside the house, and condensing.

The question is, what to do? I am planning to sheetrock the basement this summer. Will that help at all? I can slit the sheeting and let it dry out and reseal, but I won't be able to do that next year after the drywall is installed.

Can I expect the moisture inside the wall to diffuse back outside during the fall and winter?

I don't want to totally remove the sheeting along that wall for fear that I will have moisture build up in the winter along that wall.



gregzoll 07-24-2012 08:20 AM

It is because they are not breathing. The basement walls need to breathe, which means getting rid of the plastic sheathing, so that the system works properly. In other words, the insulation and building materials are not working as a whole to help keep the space warm.

fusion_ta66 07-24-2012 12:07 PM

Thanks for your reply. So if I remove the sheeting, what is to prevent condensation build up inside the wall when warm, moist air from inside meets the cold outside wall in the winter? I thought that was the point of a vapor barrier?

gregzoll 07-24-2012 06:32 PM

Read the info over at and it will explain further about how to properly insulate and maintain a conditioned barrier. A vapor barrier can be anything. It can be XPS, Kraft Faced Fiberglass, plastic sheathing, Tyvek, Drywall, etc..

trauma2 07-24-2012 08:10 PM

The poly on your basement walls is there to prevent moisture from the exterior to enter the living space. The poly should be to grade ( ground ) and run under the bottom plate of the framed wall next to the block wall. Concrete is very permeable to moisture and thus needs a moisture barrier if you will use your basement as living space. Also Not everything is a vapor barrier ie Tyvek ( this is a wind barrier but allows moisture to easily pass).

gregzoll 07-24-2012 08:19 PM

trauma2, no tyvek is not a wind barrier. And Placing a plastic shield that does not breath is a recipe for disaster. That is why it is no longer a good reason to use, due to you want the walls to breath in a way, that moisture will not congregate and build up inside the walls.

Following the info at is about as best as good info out there, due to it explains how the structure should work as a whole.

trauma2 07-24-2012 08:45 PM

Please read the label on Tyvec. it is not a vapor barrier, it is an air infiltration barrier. If the interior poly has moisture behind it then there are two choices. 1 to control moisture on the outside by diverting water away from foundation. 2 - Take down the poly and dry the walls with fresh air or dehumidifier the use waterproof paint on the block. Then put the poly back and insulate.
Framed exterior walls Require a impervious vapor barrier on the warm side of the wall and a pervious material on the cold side( exterior)

gregzoll 07-24-2012 08:51 PM

trauma2, suggest you go through the info at and read the info from 3m on their Tyvek product. And no, using a imervious vapor barrier inside the home is a old way of thinking, that has been thrown out in the past ten years, due to it was found to not work, as some off the hip engineeer & architects thought that it would.

If you go through this board, you will find that everything you just stated in your two posts has been shot down as FUD, due to the info stated over and over by one of our regular's, who does this stuff for a living, will tell you that impervious plastic vapor barriers are crap and need to go away.

fusion_ta66 07-24-2012 09:43 PM

There is no cement or concrete behind the wall in question. It is 100% above ground.

gregzoll 07-24-2012 09:50 PM

Then having the vapor barrier inside is a problem. That means that your home is not breathing as it should, to not allow moisture to build up inside the walls.

Gary in WA 08-01-2012 10:14 PM

For your location, poly (plastic) is not recommended, above or below grade; pp. 18, Fig. 5:

You are seeing the outside air condensing on the poly which should be removed and replaced with Membrain* or asphalt paper-faced batts, above grade. No poly below grade.

Tyvek is a water resistive barrier. If the laps are taped it is an air barrier, if not, the bulk of it is a wind barrier (except the laps) with a cladding over it. It has a permeance rating of 58, very open; to allow moisture to pass through in either direction.


All times are GMT -5. The time now is 02:24 PM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1