Proper Ventilation in a low-slope, car-deck roof with poly-iso foam

Guest User
I have a question about ventilation on my complicated re-roof project. Can you help ? My roof is a vaulted ceiling structure made of tongue-in-groove, car-decking (no roof rafters). It has a 3/12 pitch, multiple valleys, and interior gutters. In my proposed new roof we will install: 1.A 6 mil PE plastic vapor barrier on the car-decking 2.4" of poly-iso foamboard insulation-1 layer 3.1"thick furring strips to create ventilation channels eave to ridge with vents at both soffit & ridge 4.full conversion to exterior gutters 5. 1/2" CDX sheating top-deck 6. full layer of ice and water shield 7. standing seam metal roof on top. In my proposed new roof: NOTE that the CDX sheathing will be screwed down w. screws penetrating the poly iso foamboard, the plastic film vapor barrier, and then fixed into the car-decking. KEY QUESTION: The house generates a lot of indoor humidity in the winter and we have had a big problem with condensation. The walls and NOW the roof will be soon well insulated. The contractor posits that this structure will fully ventilate BOTH any exterior water vapor that condenses inside the roof structure BUT ALSO will fully ventilate winter, heated, interior humidity; THAT IS...that the water vapor inside the house in winter will rise to the vaulted ceilings, move thru the car-decking seams, and hit the plastic vapor barrier. This water-vapor will then vent EITHER by moving up the under-side of the vapor barrier, move thru the ridge channel and out the ridge vent OR will move thru the vapor barrier (at screw penetrations), then move up around the edges of polyiso, within the roof-structure, where it will likely condense in wintertime. However, it will then be pulled into the ventilation channels and exit the ridge-vent. Note I'm in Seattle& it doesn't get real cold in the winter. WILL THIS ROOF STRUCTURE VENTILATE WELL ?? SECOND QUESTION: Rather than a single layer of 4"thick polyiso foamboard, Would it be better to install two, 2"thick layers, with offset seams ?? I know it would be better because the poly iso board will shrink overtime and may crack or loosen the structure; two layers would help preclude this0 HOW CRITICAL IS THE DIFFERENCE BETWEEN THE TWO APPROACHES.??
Eric Novotny
An informed customer is the Best Customer!
You are getting ready to start a pretty big project here. I would do things a bit differently. Seattle is a heating degree dominated climate. In the technical sense, the vapor barrier should be installed on the warm side which would be just on the other side of the roof decking (given that the roof T&G this would be just on the other side of the roof deck) and the other side of the drywall. As a quick sidebar, what is the source of all of the humidity inside the structure. If you have unhealthy levels of humidity, those need to be addressed with proper ventilation and humidity control measures. Back to the roof. I would use the Ice/Water shield over the entire roof. Ice/Water has an adhesive and vapor barrier component that will do a much better job of sealing what is a leaky and permeable roof deck. You want to stop the moisture migration at the source. The 6 mil layer will still allow from some trapped moisture between the roof decking and the plastic. If you have any air leakage and a cold draft that contacts the plastic, condensation will occur and now you have a wet roof structure. Ice/Water is also self-healing/sealing and will seal around penetrations that are made into it, i.e. screws into roof deck. I like the Poly-Iso foam approach and do not think that the 2X 2" layer vs. 1X 4" layer is necessary. You may find that 2X 2" layers are cheaper from a material standpoint while being a bit more expensive from a labor standpoint. I have not personally seen any of the shrinkage issues that you are trying to prevent, however, staggered seams are a nice addition to the project and will help mitigate any air movement from between the panels. At 4" of Poly-Iso (R-24 to R-32) I am not sure that you will really need the ventilated aspect of your roof. You typically see a necessity for that type of application in those situation where the homeowner has very heavy snow loads and melting concerns. By addressing the air loss from the structure, you will be negating the primary culprit in roof melting and putting the foam on is completing the insulation equation. If you do choose to go with the ventilated roof, there are some applications that have a dedicated eave vent for some over deck ventilated approaches. Take a look at the first article in the list: [url][/url] If your primary concern is the humidity and indoor condensation, sealing the roof deck (Ice/Water) and insulating the roof deck (4" Poly-Iso) will probably do the trick.
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