Re: no existing attic insualtion vapor barrier
There is plenty of confusion and misconceptions as to the role of the vapour barrier.
Originally Posted by cjwittmeyer
Moisture gets into walls and attics by two paths ...... air infiltration through cracks or gaps and vapour diffusion through materials themselves.
Vapour 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 varying 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/vapour barrier.
This is important …… in order for an effective air and moisture barrier it needs to be a monolithic sealed envelope.
So .... they create an air barrier while also meeting the code requirements for a vapour barrier.
Keep in mind this is referring to the vapour that exists from the warm moisture laden air from the living space …. in this case.
The problem with paper faced insulation is the paper facing has a higher perm value than 1 …. So is considered a vapour retarder. What this means vapour diffusion will be slowed but will eventually pass vapour once it reaches saturation.
Another down side to faced insulation is properly sealing preventing air infiltration. Simply installing the faced batts and tacking the tabs doesn’t provide a continious sealed envelope.
Here in Canada the use of faced batts has been discontinued for the last 30 years.
The main reason was the paper face increases the risk of fire. The secondary reason was research done here showed poor performance as being a air/vapour barrier.
Having said all that ……….
Chances are if you live in an older home it’s likely several layers of oil and latex paints have been applied to the walls and ceilings over the course of the years. This has created an effective layer to prevent vapour diffusion …… a vapour barrier on the warm in winter side of the insulation.
As mentioned earlier …… vapour from air infiltration is 100 greater than diffusion.
This is a major step many people overlook when they decide to increase the insulation performance.
Taking the effort of sealing any air infiltration from penetrations between the attic and living space with things like electrical , plumbing , HVAC , etc. before stepping up the insulation will greatly reduce issues of condensation and heat loss.
By the way ….. this also is beneficial for the hot weather conditions.
Hopefully this makes sense and helps.
"" an ounce of perception -- a pound of obscure "