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Wet sheathing


Denray
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I was looking at a roof this morning with my wife, the claims adjuster, where there was ridge venting only. The comp shingles were on top of OSB that had 6 inches of Styrofoam type insulation sandwiched between another layer of OSB on the bottom. Across the field you could see where all the rafters were as if zinc had been dripping down from above. Many rafter bays were a bit sunken and the bottom edge of the top sheathing was moist and rotting. The interior drywall of the cathedral ceilings showed no damage. I wonder what is going on. No photos.

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You might want to take a look at the the Structural Insulated Panel Association website at http://www.sips.org/ .

Everything there seems to suggest that the interior of SIPs roofs should be conditioned and should not have the venting typically found in normal attics. As least, that's my read.

See the ridge detail diagrams at http://www.sips.org/technical-informati ... on-details and the "Juneau issue" at http://www.sips.org/technical-informati ... roof-issue. The following is also from the FAQs section...

What about roofing? Does a building with a structural insulated panel roof need to be ventilated?

A: The area inside a SIP building envelope is considered conditioned space and will be ventilated by the building?s HVAC system. There is no need to provide a vented attic beneath a SIP roof, and doing so would compromise the conditioned space of the building. Most roofing manufacturers specify how to attach their product to SIPs. Please contact the roofing manufacturer for application instructions.

Does a building with a structural insulated panel roof need to be ventilated?

A: The area inside a SIP building envelope is considered conditioned space and will be ventilated by the building?s HVAC system. There is no need to provide a vented attic beneath a SIP roof, and doing so would compromise the conditioned space of the building. Research conducted by Building Science Corporation on test homes in hot climates demonstrates that including the attic in the conditioned space allowed for more energy-efficient space conditioning and less probability of moisture related issues. Some building science experts, such as Building Science Corporation Principal Joe Lstiburek, have advocated venting the roof by providing an air space between the SIP roof panels and the roofing material (known as a ?cold roof?). This practice is not a requirement for SIP buildings, but an extra measure to improve the durability and moisture resistance of the building.

I'm certainly no expert on SIPs homes, and you should probably defer to a SIPs expert, but it does seem that the ridge venting might be creating unwanted air-flow and causing the problems you observed.

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No one else mentioned it so I will, I think you have a case of ridge rot and it's condensation running down those panel joints and not rain water. The standard ridge vent was probably an attempt to fix it. Go to buildingscience.com and search out 'ridge rot'. Dr. Joe has a decent fix for it on there somewhere, but it ain't gonna be cheap.

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