vapor barrier on basement walls??
randy12345678
| Posted in Energy, Heating & Insulation on
Which is the correct placement of the vapor barrier when finishing a basement, between the concrete and studs or between studs and drywall? New construction, 8″ poured concrete walls with asphalt tar sealing the concrete to the grade outside and 2″ DOW foam on exterior of concrete walls. Basement walls 8′ tall with 6 to7′ underground. Draintile under floor with no moisture problems (besides the normal dampness of a basement which is eliminated by the use of a de-humidifier). On the inside of the basement we will be building 2×4 walls and insulating them with fiberglass friction fit, then attaching drywall and painting. Or should the poly vapor barrier be eliminated?
Replies
The proper place for a VB in a basement is outside, adhered to the foundation. Otherwise, you shouldn't use one. See http://www.buildingscience.com.
Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Ditto
Thanks much, just got back from building science, very, very informative.
They have one of my favorite sites. To see their principles put to practice, see the article on basements in FHB 169.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
The proper placement of a vapour barrier is on the warm side of the insulation.It is meant to stop air bourne moisture from reaching the insulation,condensing and turning to water within the insulation.The proper placement of a moisture barrier is on the outside of the foundation wall to stop the penetration of water from reaching the foundation and therefore being directed to the perimiter drainage system.
In a basement application, no drying can occur to the exterior. Drying can only occur to the interior. Basement waterproofing systems are de facto vapor barriers by their very nature. Adding a vapor barrier to the interior will insure that any moisture that gets inside the wall structure will be unable to dry. Dual vapor barriers are a bad idea in residential construction.
The practice of putting an interior vapor barrier in basement walls has caused many, many failures. If no water ever gets into the wall cavity, then there is no problem. That is a big if and a bit like russian roulette with multiple bullets.
As Ray and Andy say.
Think of concrete as a wet sponge. It wants to give up it's moisture to the inside.
A slight improvement over your design would be to have a thermal break between the studs and the concrete wall (with the moisture barrier on it). This could be a thin layer of fanfold foam or insul-tarp (or similar), or just 1 inch rigid foam. You could in fact use these option AS the moisture barrier if you wanted.
The reason is that with a fiberglass wall cavity, it is possible that warm air will get in and condense on the surface of the moisture barrier in the summer months. You have already reported that humidity levels can be high.
Finishing these walls may serve to lower the humidity levels somewhat.