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Roof SIPs - Zone 4B - (Is Joe Lstiburek in the house?)

When doing an OSB roof SIP with a polyurethane core in a Zone 4B climate. If one tapes the interior seams with a building tape and uses a peel & stick Butyl membrane on the exterior OSB.

Does one still need a "cold roof" above the roof SIP?

I am under the impression that with both seams (interior and exterior) are sealed with a vapor barrier (interior tape and exterior peel & stick), one can place a standing seam metal roof directly onto the Butyl membrane & roof SIP without needing a cold roof. My concern is of course the "SIP rot" issue.

Asked by Peter L
Posted Mon, 04/28/2014 - 21:42
Edited Mon, 04/28/2014 - 21:51


2 Answers

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Peter L,
There is no single answer to your question. Some experts advise that SIP roofs should always include vent channels above the SIPs; other experts say that if you do an excellent job of sealing the SIP seams, you can skip the ventilation channels.

What should you do? The answer depends, in part, on how much risk you are comfortable assuming. If you don't like risk, install the ventilation channels.

Needless to say, you also need to consult the SIP manufacturer and comply with all of the manufacturer's installation instructions.

If you want to know Joe Lstiburek's recommentations, you can find them here: Complex Three Dimensional Airflow Networks. Lstiburek wrote, "How to avoid the problem of SIP ridge rot? Make the panel joints airtight – especially over structural supports such as beams and purlins – and provide a mechanism for moisture removal such as a vented over-roof."

Notice that he used the work "and." Lstiburek's recommended approach is less risky than the unvented approach.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 04/29/2014 - 07:40

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Thank you for the info. Something to think about before I make my decision. In a dry 4B zone climate the water damage risk is very low. Interior air infiltration is the other issue but if the SIP joints are airtight, I don't see that being a problem.

The one issue that is not addressed is that in a wildfire area, embers are more dangerous than the actual flames. A wildfire study showed that most homes caught on fire from the embers landing in the soffit, attic and gable vents. My one concern in creating a "cold roof" is that it creates an area that embers can make their way into and land on the OSB and start a fire. Especially in a convection-driven scenario where the embers would be driven into the cold roof area and thousands of embers would land on the vulnerable OSB. That is part of the reason why I wanted a SIP roof; so I don't have gable, soffit and attic vents.

I will have to weigh my options...

Answered by Peter L
Posted Tue, 04/29/2014 - 14:48
Edited Tue, 04/29/2014 - 15:08.

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