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

Trapped moisture on SIP roof

Edward Mallett | Posted in GBA Pro Help on

I knew SIPs were the Achilles heel of my 100% engineered house design, But because of inattention to detail, heavy rains at a critical time of my house build pushed water underneath the PVC membrane (applied on mechanically fastened DensDeck) onto my SIP. It happened the night before the adjacent wall could be spray foamed to properly seal off the membrane. Unfortunately, the spraying was later done to the walls and the underside of the SIPs without knowing there was water present. It took a lot of effort on my part to get the roofer to even understand there was a problem and the fix they performed, in hindsight, turned out to be woefully inadequate. The true nature of the problem only became apparent when carpenter ants showed up 11 months later in the ceiling–indicating a really big problem (I regard them as heroes actually for alerting me to the issue). My own probing up through the SIP gaps revealed moisture content above 25% on the underside of the SIPs’ upper ply. At least it’s exterior ply instead of OSB, but roughly 1000sqft are affected. The roof is shed-style with a 2% slope. Clearly the membrane needs to be peeled and Densdeck removed. but how the heck can one now let a bare roof air dry while still protecting the SIPs (now sealed underneath) from the weather. Perhaps placing a new roof layer on top leaving ventilation over the original SIP surface? Has anyone had experience with this kind of repair?

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. Peter L | | #1

    I am not sure I am following what's going on.

    You mentioned a SIP roof but then mentioned spray foam. Why would you spray foam a SIP roof? Do you have any details or pictures?

    Are we talking EPS core SIPs or Polyurethane core SIPs?
    What do you spray foam with (open or closed cell)?

  2. GBA Editor
    Martin Holladay | | #2

    Not a good situation. Probably you'll need to remove the roofing; install 2x4 sleepers, 16 in. or 24 in. on center; new roof sheathing; and new roofing. Install insect screening at the eaves and ridge to allow plenty of air flow, and hope for a hot, sunny summer.

    I would get an engineer involved, to make sure that the SIPs aren't structurally compromised.

  3. Edward Mallett | | #3

    Peter, They are poly core SIPs coated with BASF Walltite. all enclosing a steel frame.

    Thanks Martin, that is what I had been thinking. I am hoping the house system engineers can spec the add-on. Snow load is also something to deal with here. The annoying thing is, the facia profile will be thicker than the other roof sections. The SIPs are a pretty good design in my view--in theory. Gaps are left accessible from the underside, which then get foamed in. I will remove all the foam there too to help dry it from the underside. Soundness is important though--insurance will not cover this if there is rot or mold present. Joy, now on to the legal and insurance issues.......

  4. Andy CD Zone 5 - NW Ohio | | #4

    Edward, I'm really curious too about your original stackup here. I did a SIP roof recently, a high pitch residential, so I'm interested in the various approaches, particularly the failed ones. Can you say a little more about it?

    Are you saying your SIPS had an exterior plywood skin (NICE!) but were coated with a spray-on low-perm waterproofing, and then a rigid roofing board screwed directly on top, followed by a PVC membrane? Everything I've ever read about SIP roofs has emphatically discouraged ANY kind of low-perm treatment of the SIP itself, instead calling for a high-perm WRB and then some sort of air gap above. But we have here a commercial-style nearly flat roof, so I'm guessing the rules are different.

  5. Edward Mallett | | #5

    Hi Andy,
    Sorry if there is any confusion--there is no coating on top of the SIP, just underneath. Here is a diagram from the plans showing a wall-roof junction. The roof membrane comes up the wall 12", wraps horizontally and then is supposed to be sealed up when the wall section is sprayed (over 3" rigid EPS). I suspect rain hit the EPS, dripped behind the vertical 12" piece of ply, found the beginning of the SIP and then wicked under the DensDeck and into the SIP seams. I should also note that the roofer screwed up a flashing detail under a window and I actually observed water flowing in during a downpour. He said he dried it out, but clearly not. The other theory I have--at least for the damaged section 30' away from any wall, is that the SIPs were covered with clear plastic on some pretty hot and humid days, before the roofers arrived, basting the panels in moisture. I'm the amateur here, how did so many pros mess up?

  6. Expert Member
    Malcolm Taylor | | #6

    It's a bad situation and you have my sympathies. To step back a bit, you could look at SIPs as being a building technology that has might not have a level of forgiveness necessary for the way we have structured our residential construction industry. There just seem to be too many ways it can be installed incorrectly.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |