I recently installed insulation in my garden shed and covered it with a plastic vapor barrier. This was done in the summer when the weather was warm and sometime humid. Shortly after I'd finished this installation I removed the plastic from the walls so I could install some electrical. when I pulled down the insulation I found black mold. My question is why would mold be present and why so fast? I thought I insulated correctly.

  • 2
    Was your shed climate controlled? – The Evil Greebo Oct 28 '13 at 18:12
  • 2
    Do you have a weather wrap on the outside of the shed (under any siding material, but typically over the sheathing). – BMitch Oct 28 '13 at 18:56
  • 1
    yes the shed was wrapped with black paper and then the cedar siding over that. – Debbie Oct 28 '13 at 21:45
  • 1
    climate controlled? no not then. Since cleaning the mold I stuck a heater in it to dry things out again. – Debbie Oct 28 '13 at 21:46
  • 1
    it was primarily in the middle of the walls except for the main weather wall where it was at the top as well. I thought there may be a leak but haven't been able to find a leak so far. there are no eaves on the unit yet and I spray foamed and closed up the openings under the shed roof. do you think sealing it up like I did would cause this issue? – Debbie Oct 28 '13 at 22:10

Vapour barrier goes on the cold side of the insulation.

The problem here is that for an unheated, Un-airconditioned shed, the outside will always be the cool side.

Insulation won't stop heat transfer, it only slows it down. A closed shed, no matter how well insulated, will always turn into a sauna.

Now you have a hot, steamy inside, and a (relatively) cool outside (especially at night). If you put the vapour barrier on the inside, sandwiching the insulation in, the condensation will occur within your insulation. Hence, the mold issue.

You're better off without a vapour barrier in this situation. This way the batts can dry out. Even better would be some rigid foam insulation, but I really think that insulation in this situation is a waste of money.

  • 3
    Second that. Insulating a non-climate controlled space causes more problems than it solves. – The Evil Greebo Oct 28 '13 at 18:49
  • 7
    Agree with everything but the "cold side" sentence. The idea of the vapor barrier is to keep warm moist air from getting into the insulation where it can reach condensation temperatures. So you want the barrier on the conditioned/heated side since condensation in the winter time is the main problem. – BMitch Oct 28 '13 at 19:00
  • 1
    so do you think just installing drywall (without the insulation behind it) will cause any problems? aside from being a bit chilly! – Debbie Oct 28 '13 at 21:42
  • Sure. Leave a gap at the top and bottom to allow air flow. – Chris Cudmore Oct 29 '13 at 14:52

It may not be a condensation issue.

Since the structure was built as a shed, it is likely that tar paper or tyvek or ... was not installed on the exterior. Because of this, the sheathing/studs may simply be damp. Because air circulation within the cavity is now severely restricted, such damp sheathing/etc would raise the humidity in the wall cavity sufficient for mildew/mold to grow on the damp surface.

This, plus warm summer temperatures, is consistent with the very rapid appearance of your mildew/mold. To get a final answer, you would need to provide more information about the walls, and about the shed's design, location and history.

For a more general house-oriented discussion:


Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow website:

Things get wet from the inside, the outside, and they start out wet. When the rate of wetting exceeds the rate of drying, accumulation occurs. When the quantity of accumulated moisture exceeds the storage capacity of the material, problems occur.

Ideally, building assemblies should be designed to dry to both the interior and exterior. In heating climates, the primary drying potential is to the exterior.

The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow. As such, energy conservation has the potential to destroy more buildings than architects.

  • 1
    thank you both for your comments. A little more detail regarding my "shed". It is a cedar shed and black paper was installed on the outside then cedar siding. It was warm and a bit more humid than usual when the insulation was installed. I live on the Westcoast so we typically have mild winters and summers. I intend to use the shed as my workshop which would be heated intermittently by use of a kiln, torch and heater (if necessary). The insulation I used was Roxul ComfortBatt R14 rating and never appeared wet. I installed it in the roof as well and did not find any mold there. – Debbie Oct 28 '13 at 21:40

Sounds like circulating air from the outside caused the mold. Any wind/air that gets in from the exterior will carry moisture with it at some point. Vapour barrier always goes on the warm side, so please ignore previous cold side comment. Good luck with every thing.

  • Also, not sure what the black paper is or if its breathable but "tyvek" wrap would be a good option. I have a feeling the black paper is a weave type wrap that unfortunately lets through a little too much air. Ideally, a tyvek wrap should be used which provides a water and air barrier on outside of structure. – Brian Oct 17 '14 at 10:27

the black tar paper under the shingles was your vapor barrier, putting another barrier inside traps condensation, thus naturally occuring moisture from temp changes cant escape, you cant have 2 vapor barriers.

Not the answer you're looking for? Browse other questions tagged or ask your own question.