GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Potential condensation concerns with unvented roof assembly

Sco44 | Posted in General Questions on

I had a post earlier this year about a unvented roof that I planned to have installed. I couldn’t figure out how to continue that post/system wouldnt let me reply on it. However, this post should contain most all the relevant information.

Having had this roof installed in August, The house has been very warm, and now requires very little to heat it compared to last year (much colder winter this year too), so it’s great from an insulation standpoint. However, I have what appears to be a condensation related issue.

A few things changed with the roof design. The cavities were dense packed with cellulose, and the bay’s were closed up with 1.5″ polyiso and canned foam at the top plates.. One layer of 3″ polyiso on the roof, with taped seams and good attention to tight seams and foamed penetrations.

House has 4′ overhangs. We ran the 3″ polyiso about 2.5′ past the walls. Thinking this would help with heat under the soffits, and against house during the summer. I have noticed moisture originating around the screws that hold the polyiso to the sheeting (this is past the walls in the open air soffits) Not all the screws are doing this, but some even drip moisture, and it seems to wick out from the screws. Sometimes 4-6″ out parallel to walls. But I can check a wet spot a day later and it’s almost completely dry.

The screws/areas doing this are about 1-2′ past the house walls, exposed to cold from below. It has been dry, no rain. Outside humidity around 60-70%. Outside temps have been in low to
Mid 30’s. My biggest concern, is that this could be happening within the sealed roof/building envelope?

I pulled blocking in a bay to look for moisture, seems nice and dry, but being packed with cellulose, i couldnt see any screws. I’m not happy with the blocking the insulation company did, they wedged the polyiso in fairly tight between rafters and tried to foam the edges or small gaps. I plan to closed cell spray foam to seal the existing blocking once the temps warm up enough.

It is possible that some heat escapes passed the blocking in some areas, but I don’t see this causing the moisture 1-2′ past at these screws, where they penetrate the sheeting. Is heat getting past the blocking and top plates, reaching the cold roof decking and condensing? Would this not be happening within the closed roof system also?

Any ideas?


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. GBA Editor
    Martin Holladay | | #1

    It's hard to visualize the situation you are describing. A photo or sketch would help.

    If I understand correctly, you are talking about long screws that were installed (presumably with buttons or washers) to hold the 3-inch polyiso to the roof sheathing. Evidently these long screws completely penetrate the roof sheathing and poke through the other side. Moreover, it sounds like your roof sheathing is visible at the overhangs, implying that you have open soffits (without any trim) allowing you to see the underside of the roof sheathing. Is that correct? (That's odd, by the way, because that type of soffit is unsightly. One might have expected the roofers to place their screws at the rafters.)

    I also assume that the polyiso is covered on the top side by a second layer of roof sheathing and new roofing. Is that correct? If so, what type of roofing do you have?

    Assuming I'm visualizing this correctly, what's happening is that the screws are becoming condensing surfaces under some weather conditions. That's not too unusual. It sounds like you are describing dew.

  2. Sco44 | | #2

    Martin, yes you are visualizing correctly. I can get a photo posted as well though. I can see the screws because the soffit trim is still pulled down. It was pulled down to install the blocking at the top plate/rafter cavities. Once the soffits are re-covered the roof sheathing isn't visible.

    Yes, Polyiso is installed with large plate washers on top side. I'm not sure why some of these screws are not into rafters. When I was on the roof looking at the job, they had lines snapped on the ISO board to mark rafters. Perhaps these screws are where two sheets of Polyiso meet and it isn't on a rafter. Or they added more screws where the Polyiso stops on the overhang.

    The single ply membrane is versico 60mil TPO. There is no cover sheet above the Polyiso.

    I did some more inspecting last nite. On the opposite side of the house there is no evidence of moisture at any of the screws that I could find penetrating the sheeting in the same manner. The only thing that I can see being different is a better job of the rafter blocking install. Specifically, being installed flush with the outside edge of the top plate. On the side of the house with moisture on the exposed sheeting the blocking, in spots is 1/2-1.5" back from the edge of top plate.

    Could warm air be escaping from these areas and condensing on the cold screws and sheeting? If it is dew as u describe it, would it not being doing this at every exposed screw I can find?

    I will get a couple photos to show what is happening.


  3. GBA Editor
    Martin Holladay | | #3

    You are worrying unnecessarily. This is an outside phenomenon. It is no different from dew on your bird feeder or on a bicycle that you left in your back yard.

    The microclimate on one side of your house is different from the microclimate on the opposite side of your house -- different enough to affect how much dew you see. Again, this is totally normal.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |