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Community and Q&A

SIP rot and remedy

Markatmanta | Posted in GBA Pro Help on

I have had to recently address a SIP panel issue, installed 7 years ago, on a timber frame building. The issue being, that the spline joiners between the panels, seem to have had a “chimney ” effect resulting in moisture/heat travelling to the peak area. Combined with a butyl membrane encapusulating the entire exterior surface, the SIP’s had zero permeability for the panels to breathe. The equation ended up with roof rot of the exterior SIP’s OSB panel, and rot of the joining headers. This was found all along the peak area, for an average distance of 2 feet down from the peak ridge, running the full distance horizontally. The prime areas of notice, were predominately where the splines where located.
To rectify the problem, several steps where taken. First was to remove the butyl membrane by hand, as best as possible. Then remove the rotted OSB/headers (headers- 2x material used to enclose the ends of the SIP’s) and try to save the remaining integrity of the SIP’s. This was accomplished by removing the rotted OSB to the nearest foot, all by hand (circular saw blade at 4″ intervals and putty knives). Then the complete removal of all splines from the exterior, keeping the interior bottom OSB intact as well as the major portion of the SIP. The next step involved replacement of the OSB with new boards, glue lamming them down with a foam adhesive. The exposed spline areas were reshot with expanded spray foam from the exterior. The foam used was one that could achieve proper flex and bonding, as to compliment the EPS used to construct the SIP’s. Icynene and other urethane foams, although better for R values, are to rigid and may result in cracking between the panels once again. The panels were then covered over with a membrane once again, this time with a Delta product, that had a permeability rating of 50, and a water barrier aspect to it as well. The exterior this time was strapped on the vertical, then on the horizontal, to facilitate for the fastening of the metal roof. This allows for a much needed and improved air flow from over top of the panels.
Interior wise, all the visible joints were caulked as to reduce any possible vapour and heat movement. Three horizontal acoustic butyl caulking strips made, and then plastic vapour barrier taped on top. The timber frame beams had been installed previously with vapour barrier overlaps on top, a 3/4″ plywood spacer and then the SIP’s. This was to plan for the installation later of the complete vapour barrier encapsulation of the interior, as well as the slipping in of a finished 3/4″ T&G pine ceiling. What has been accomplished now, is the limiting of heat and vapour movement through the ceiling structure by having a continuous vapour barrier present. The 3 acoustic butyl strips per bay and per purlin space help accommodate for finish fasteners for the T&G that would otherwise compromise the vapour barrier envelope.
I believe all these strategies taken, has helped minimize an otherwise costly situation, and has reestablished a proper roof/ceiling envelope. Hope this gives hope and clarity for a viable solution from such a common problem.

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  1. GBA Editor
    Martin Holladay | | #1

    Thanks for sharing your story. The problems you describe happen too frequently for comfort.

    In my article on the topic (Air sealing SIP seams) I emphasized two points:

    1. To seal SIP seams, installers need not only spray foam at the joints, but interior tape. Exterior sealing (as used in your case by the installation of an exterior peel-and-stick membrane) cannot be substituted for interior tape.

    2. In cold climates, a ventilated cavity between the top of the SIPs and the roofing is essential.

    You've learned these lessons the hard way.

    Another point: This failure is due to air movement, not vapor diffusion. Your plan for the "the installation later of the complete vapor barrier encapsulation of the interior" is somewhat misguided. You don't need an interior vapor barrier. You need air sealing of the SIP seams on the interior.

    Finally, and this point is crucial: The OSB facing on SIPs is structural. Any time that one of the OSB facings of a SIP rots -- especially when we are talking about the facings on a roof SIP -- the structural integrity of the panel has been reduced. Gluing a piece of OSB in place of the removed rot does not restore the SIP's structural integrity. Whenever SIP rot is encountered, it's essential to get an engineer on the team to review repairs and to ensure that the roof is still capable of supporting the loads and resisting the forces to which it will be exposed.

  2. Expert Member

    To the casual observer the rate of failure of SIPs is disturbing. I'm surprised any building technology with those sorts of systemic problems continues to be used.

  3. SwitchgrassFarmer | | #3

    I would be very interested in hearing more about how such SIP damage is repaired, particularly the part dealing with maintaining structural integrity.

    I live in a SIP home. Hopefully our extensive air sealing and venting (foam, SIGA tapes, internal and external flashing, cold roof, rain screen, HRV etc) means we will never face this problem, but you just never know...

  4. Markatmanta | | #4

    First, I'd like to thank you for your response. Your note on the vapour barrier in interesting. The reason why i not only caulked the seams, but also covered the interior with 20 mil. poly and tape, is that the building code in Ontario doesn't recognize OSB or EPS foam as being sufficient to withstand vapour penetration. The first time the roof was installed with the high temp butyl membrane on the exterior combined with the direct lay of the steel roof, was in accordance with recommended building practice from both the steel manufacturer and the SIP manufacturer. This is of coarse is absolutely inaccurate as you also have agreed upon. Lastly, with regards to structural component properties of the SIP itself, I agree that considerations should be recognized. The project of concern has an approximate span of 17 ft from ridge to soffit where by the SIP's run the complete distance, 4 ft on centre. The SIP is supported by two rows of purlins, a ridge beam and a rim beam. Notice was given to the structural integrity of the SIP, as it was compromised only on the upper OSB exterior, with the conclusion that the interior OSB/EPS foam component was still in perfect shape and also that the interior finish of the 3/4 solid pine would assist in some support, as they are sandwiched between the pulins/beams with relatively short runs. Also the two layers of 3/4" strapping act as bridging between the purlins/beams and thus would also assist in helping reserve the SIP structural demands. Finally, due to a 12/12 pitch and a standing seam metal roof, little to no load is encountered to the area of concern, which further supports the approach taken to resolve the SIP rot problem. I hope other readers are able to take these notes and digest both your suggestions and my approach taken, and arrive at a decision which both rectifies the problems at hand and assist in easing cost expenditures.

  5. Markatmanta | | #5

    It is important to note, that once removing the OSB from directly on top of the joining SIP "spline", I was able to observe the remaining OSB of the spline and how proximal it was to the adjoining SIP panels, to both the right and left. It appeared to be fairly tight to the SIP's EPS foam. Once the spline's OSB was removed, a whole different view appeared. There was seen at least a 1/4" gap in places, with the added rows of expanded foam during its assembly, having separated or even cracked away from any EPS foam. This observation concludes then that there was a compromise in the R value rating at this juncture, and as well, offered a space of travel for any heat/ moisture to follow upwards to the high point of the panels. If photos allowed for me to show, one could easily see a discolouration in the panels on the exterior, primarily at the top/following downwards from the peak along the SIP's seam. The discolouration indicating OSB rot. At this point, any concerns of remaining SIP structural integrity, would have been already compromised and/or noticed perhaps in other visible ways, should structural failure be happening, or perhaps water infiltration be now occurring.

  6. GBA Editor
    Martin Holladay | | #6

    I understand that you have to satisfy the building code in Ontario. I also understand that many building code officials in Canada haven't studied building science since the late 1970s, and insist on interior polyethylene.

    The fact remains that the rot you observed has nothing to do with vapor diffusion. There isn't much vapor diffusion through OSB and rigid foam. What happened is that interior air entered the poorly sealed SIP seams; and some air may have escaped near the ridge; and interior moisture condensed on the cold OSB. This phenomenon has been observed repeatedly; it can even occur when there is no air escaping from the ridge, due to convective loops in the poorly sealed SIP seams.

    A large cluster of SIP failures attributable to poorly sealed SIP seams occurred in Juneau, Alaska about 15 years ago. By now, every SIP installer in North America should have heard about those failures, and should be familiar with the need for both spray foam at the seams and interior tape at every seam. But many SIP installers are still ignorant.

    Again, thanks for sharing your story. Anyone contemplating the use of SIPs should know about stories like yours.

  7. GBA Editor
    Martin Holladay | | #7

    The short answer to your question about structural issues surrounding SIP rot is: get an engineer on board. These issues are serious. When in doubt, don't take chances.

  8. Expert Member

    I think you were mislead as to the requirements of the Ontario building code. It requires a vapour retarder close to the interior less than one perm. Many buiders interpret this as having to use poly, but as you can see from the chart I have linked to OSB of a certain thickness also works and thinner OSB can be painted to work.

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