We have just insulated our shed and put vapor barrier inside the
insulation, with a layer of tar paper between the insulation and the
siding. We are noticing that one wall is sweating and moisture is
collecting between the insulation and vapor barrier. Berfore we
drywall, we want to eliminate the moisture. The attic of the shed is
vented, so I can not understand why we are seeing this moisture
still? Does anyone have any advice as to how to fix this and why we
would still be getting moisture? We would like to use the shed as an
office and have also installed heating. Are we still lacking proper
venting or is this likely because we have not properly sealed the
vapor barrier? Thanks in advance for any help. If it makes a
difference, we are in BC Canada, moist climate with a mix of cold and
You have a vapor barrier on both sides of the insulation, which is
incorrect. Since the installation can not be absolutely air tight, some
moisture will find its way into the insulation and must have an easy escape
path. In cold climates it is recommended that the vapor barrier should be on
the warm inside and the outside be ventilated. You need to remove the tar
Thanks for replying Don. Perhaps I used the wrong term, as the paper
is not truly tar paper...but rather the black pourous paper
recommended by the folks at Home Depot and appears to be what I see
builders using ont he new homes. Is this the wrong paper? and if so,
what should we have between the insulation and the exterior siding?
tarpaper is vapor permeable and is not considered a vapor barrier. in
fact, i believe tarpaper is more permeable than tyveck. I have yet to
see a vented wall, and if they were, the R value loss to wind wash
would be significant. is it possible that your wall got wet before
you insulated it? If it's just a small amount of condensation, it
should find its way out. otherwise, pull the vapor barrier off, and
run an electric or sealed combustion heater to dry it out.
Perhaps I should explain the layers we have installed...
We have the siding outside, with a layer of the black paper inside
that, then the insulation inside that and finally the plastic vapor
barrier on the inner most layer. We have dried the moisture and
resealed the vapor barrier, but it is now wet again between the vapor
barrier and the insulation.
I did not install the black paper..the initial builder did, but it
does appear shingled and attached directly to the outside of the
studs. The moisture appears to be vapor and not coming in from driven
rain(it has also been good weather recently). The wet wall also has a
fence directly outside it that would block driven rain. Only the
inside of the insulation is wet and the outside facing insulation,
black paper and studs are as dry as a bone.
The problem you describe can usually be attributed to having two vapor
barriers. For example, a poly vapour barrier on the inside and a
vapor resistant sheathing, such as OSB, on the outside.
However, since you state you "just insulated", it is likely there was
moisture in the insulation to start with, and the moisture is
condensing on the poly..
Recommend you slash the poly, let the insulation dry, then repair the
poly using tuck tape and see if the problem re-occurs.
Since you don't have sheathing, another possibility (remote) is rain
getting through the siding on the windward side and soaking the
tarpaper to the point of moisture penetration.
Gary in Royston, BC
If that barrior that protects the insulation isn't stopping the
moisture from penetrating through it, to the outside, I would install
another barrier over the inside. Some say 4mil but 6 isn't much more
and less likely to tear will installing.
Just a guess, since I can't see your setup.
tom @ www.Consolidated-Loans.info
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