Re: Basement insulation and vapor barrier
Just throwing 2 cents in this.
There is plenty of confusion and misconceptions as to the role of the vapor barrier.
Vapour barriers together with air barriers prevent moisture from escaping into the walls.
Moisture gets into walls and attics by two paths ...... air infiltration through cracks or gaps and vapour diffusion through the wall materials themselves. Vapor from air infiltration can be 100 to 1 times greater than from diffusion.
Water vapour manages to filter its way through most building materials ....... which is referred to as diffusion. Although all building materials slow diffusion to a certain extent, only those that almost totally block it are officially classified as vapour barriers and deemed acceptable for new construction standards.
Permeability is the technical measure of how easily vapour will get through a material and only materials rated as less than 1 perm are considered vapour barriers. Aluminum foil, polyethylene films, aluminum paints, latex vapour barrier paint, vinyl wallpaper, and several coats of oil paint all qualify ....with varing degrees of effectiveness.
A similar technical measure is given to the permeability of materials as being an air barrier.
The polyethylene films are the preferred materials for vapour barriers because they come in large sheets, have few joints where air can leak by, and can be efficiently sealed around openings such as windows, doors and electrical boxes.
If we completely seal a polyethylene sheet on the warm side of the insulation, we call it a combined air/vapor barrier.
So .... they create an air barrier while also meeting the code requirements for a vapour barrier.
Keep in mind this is referring to the vapor that exists from the warm moisture laden air within the basement.
The method of only applying a vapor barrier to the foundation wall would and should never be used up here and in my opinion applies to all cold climates.
The vapour barrier should be placed on the warm side of the dew point on the coldest day of winter. The objective is to prevent the vapour barrier from getting cold enough to allow condensation on its own surface.
Having the vapor barrier on the cold side of the insulation will trap moisture inside the wall.
The method jkirk mentions is used when the foundation walls have a dampness issue ..... not dripping wet.
With that method there is a continious vapor barrier that is on the warm side preventing moisture from the living space.
katemap .... you mention this is a new block foundation .
It's unknown as to the method of damp proofing or waterproofing that was done to the exterior in preventing any chance of moisture or dampness entering from the exterior.
First thing that comes to mind .... you mentioned this is a new foundation.
I would recommend waiting till after spring before closing in the foundation walls.
There is plenty of moisture from the new blocks and mortor that needs to evaporate. If you close these walls in right away you run a good chance of trapping this moisture before it has a chance to acclimate.
Besides .... it will also give you a chance to see how moist the walls will become from spring thaws.
Last edited by canuk; 01-25-2009 at 07:53 PM.
"" an ounce of perception -- a pound of obscure "