Moisture inside vapor barrier for a new construction
heyingh
| Posted in General Questions on
I am building a new house in Canada. After the insulation was done, we noticed there was some moisture inside the vapor barrier beside basement windows. The builder can’t figure out where it was from and why. Please have a look of the picture and gave your comments. Originally there was a smartboard belly band installed between the basement and main floor. Later I changed the deck size and the smartboard was replaced by the deck ledger board. Do you think the water is leaked through the deck ledger board?
Thanks,
Henry
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Replies
Henry,
Let me understand correctly: You insulated the interior side of your basement wall with fiberglass batts? And then you installed interior poly? And then you took a photo of it (shown below)?
If the answer to all of those questions is "yes," then the moisture is no surprise. Concrete basement walls are wet, and you should never insulate a basement wall with fiberglass batts and poly.
For more information, see How to Insulate a Basement Wall.
.
Last time I visited, Canada was a pretty big place- can you narrow down the geography ( and thus climate) a bit tighter?
Ground moisture, dew, or rain will wick into a poured concrete foundation, and when it warms up during the day some of, that moisture is driven off, some of it into the wall cavity. When the dew point of the air in the cavity reaches the 18-20C, it will condense on the poly. The first and most important thing to do is to be sure bulk water drains away from the foundation rather than allowed to seep down the soil/foundation interface, and avoiding direct rain-wetting of the above-grade portion of the foundation.
Even though interior side vapor barriers are required by code in Canada, it's usually a bad idea when applied to a basement studwall, since it traps the susceptible wood into a space that has high exterior side moisture loads. Replacing the poly with a "smart" vapor retarder like Intello Plus or Certainteed MemBrain relieves much of the moisture load by letting the foundation dry toward the interior when the foundation moisture content was high, yet is still a class-II vapor retarder during the winter when the air in the basement has a higher dew point than the temperature of the concrete and exterior stud-edges. It's becoming more common to use air-impermeable rigid foam of sufficient R-value for wintertime dew-point control at the stud edges of a batt-insulated basement studwall, and skip the interior side vapor retarder altogether, to avoid exactly the sort of summertime condensation problem you're currently experiencing, eg:
http://www.buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/content/high-r-foundation-9-polyisocyanurate-insulation-2x4-framing-with-cellulose/images/highr09_figure_01.jpg
The amount of air-impermeable insulation required for dew point control at the above-grade section is highly climate-dependent.
Hi Martin / Dana, thanks for your reply.
Henry
Hi Martin / Dana,
The answer s to Martin's questions is yes. But, the basement is a walkout basement. Only the walkout side has the moisture problem, where is not concrete wall. The other three side, either half is buried, or totally underground, is no problem. They are insulated in the same way, fiberglass batt plus vapor barrier. The moisture was gone during the daytime. When I check in the evening 8:00PM, it is coming back again.
I lived in Calgary, Alberta. The recent temperature is about 26C highest, 8C lowest. Calgary is dry most time, but recently is not. I check the weather, the outside humidity is 40-80% in the past week. I can imagine the condensate is vaporized during daytime, and condensed during evening. But where is the condensate from? The outside is wrapped by building paper. Is the moisture dried outward gradually?
The other question, if the moisture problem is existing for long time, will it rot the frame?
Thanks again,
Henry
Henry,
Interior polyethylene is usually a bad idea. Now that you know you have a problem, you can understand the solution, because you have seen evidence with your own eyes that justifies the recommendations that building scientists have been making for many years.
If you want to install an interior membrane on your walls -- and in Canada, many building inspectors insist on seeing one, even if there are reasons to avoid interior membranes -- you should choose a "smart" vapor retarder like MemBrain. So the easiest solution to your problem is to remove the poly and install MemBrain (or a similar product imported from Europe) instead. (A smart vapor retarder has variable vapor permeance. When the environment gets damp, the permeance of the material increases. That way the water vapor can escape from your wall cavity when necessary.)
The moisture you see is probably held in the framing lumber and wall sheathing. The moisture content of these components is often high, especially the first year after construction. (Eventually, if all goes well, the lumber and sheathing dry out.) The driving force that pushes the moisture toward the interior may be sunlight; this phenomenon is called inward solar vapor drive. If your basement is relatively cool, so is the polyethylene; that gives the moisture something cool to condense on.
Hi Martin,
If the condensate appear whenever the weather is hot, is it cause mold, and eventually damage the frame in future?
Thanks,
Henry
Henry,
The type of condensed moisture you are seeing on the back side of the polyethylene is definitely a cause for concern. It is a sign that your stud bays are damp, and that the moisture is unable to dry to the interior of your home.
If the walls stay damp, your walls could easily develop mold or even rot. Mold and rot aren't inevitable -- it's possible that your damp walls will dry to the exterior before you get rot or mold -- but why roll the dice? If this were my house, I would rip out the poly and put it in the dumpster.
Hi Henry:
Calgary this year has had its share of ground and rain problems and we are seeing houses that never had previously reported problems suffer terribly. One possible cause is that the wood got so wet during the rains and which when insulated and vapour barriered trapped moisture.
Excellent forum as usual Martin.
I am in Calgary and investigate these problems ([email protected]) - give me a call Henry.
Randy
ATLAS Infrared Inc.