GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Condensation on Interior Side of Polyiso Board Insulation

CanadianBuilder14 | Posted in General Questions on

Hi everyone im new to this page and i have a question. I am in the process of finishing my basement. I adhered 1.5 inch polyiso board all over the interior of my basement and left a 1/2 gaps all around the bottom and top of the board and spray foamed to seal against the cinder block wall. I framed a 2×4 wall infront tof the polyiso and insulated with roxul batts. All this was done in November. I live in Canada and the winter can get down to -35 degrees celcius. So i went to do some electrical work and peeled back the roxul and noticed that there was condensation and some small spots of mould/mildew growing on the polyiso board. I peeled some roxul back where the polyiso board joins on the wall and the blue Tuck tape that i used has mouisture on it as well. The board does seem cool to touch. The humidity in my basement is around 40%. I am wondering what i did wrong and what do i need to do to correct this. My basement is about 8 feet tall and only 4″ is under the grade. I replaced the windows as well and i find there is alot of moisture building on the cinder block sills as well. I need to figure this problem put before i drywall. Im glad i caught it before i hung my drywall and sealed the wall up. Thanks in advance everyone.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Michael Maines | | #1

    Your mineral wool is keeping the indoor warmth from reaching the face of the polyiso so that face is cold enough to condense water. You can increase the proportion of polyiso, decrease the amount of mineral wool or reduce the indoor relative humidity. But simply adding an interior air barrier that also slows vapor movement is usually all you need.

    In steady-state conditions, meaning if the indoor and outdoor temperature and humidity remained constant, you can calculate what the temperature of the inside face of the condensing surface--the polyiso interior face--will be and compare that to the dewpoint, the temperature at which moisture will condense out of the air.

    Concrete has essentially no insulating ability. Polyiso is about R-5.5/in, and mineral wool is about R-4/in. So you have a total of R-9 polyiso, R-14 mineral wool, for R-23 total. 40% of that is in the polyiso so in steady-state conditions the inside face of the polyiso will be about 40% of the difference between indoor and outdoor temperatures. If it's 16°C inside and -4°C outside, the difference is 20°C. 40% of that is 8°, so the polyiso face will be about 4°C. (-4°C + 8°C). 16°C indoor air at 40% RH will condense on a surface that is about 2°C--lower than the polyiso face when it's -4°C outside. But if you raise the indoor temperature to 20°C and keep the relative humidity at 40%, it will condense on a 6°C surface. Or as the outdoor temperature drops, the polyiso temperature will also drop.

  2. Expert Member
    BILL WICHERS | | #2

    You're getting condensation because the inside face of the polyiso is dropping below the dew point of the indoor air. You have a few options:
    1- Add more polyiso to get a higher ratio of rigid foam to batts.
    2- Remove the mineral wool so that the inside face of the polyiso stays warmer.
    3- Try a vapor retarder before you put up drywall.

    Note that for #3, you'd need to be sure the wall was reasonably well air sealed. The first two options work by keeping the first condensing surface above the dew point. The third option works by limiting how much moisture can get into the wall assembly. Which option will be most effective depends on your specific site conditions. #3 is probably easiest to implement with no performance loss for the wall, and if you only have a minor moisture problem it will probably be enough.


  3. maine_tyler | | #3

    What average temperature is your basement?

    Also, If you are in Zone 7 or 8, I believe you are right on the edge of meeting the code recommended ratio of impermeable exterior insulation to interior permeable insulation.
    Normally for a basement, I would imagine you have some cushion on these ratios, but since most of your wall is above grade, I'm not sure that cushion is there.
    (Did you actually mean 4 FEET is below grade, or only 4 INCHES as noted by ")?

  4. CanadianBuilder14 | | #4

    Thanks everyone for your quick responses. The condensation is very minor so i think my best option will be to go with is #3 adding a vapor barrier on the interior of the wall.
    Do you guys have any tips on air sealing my interior wall so when i close it all up with vaper barrier on the inside i wont have to worry about this again?
    -Tyler my wall is 8 feet high and roughly 4' of it is above grade.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |