nailable ISO and vented attics

I am looking at a project in a subdivision full of cape cod style houses with 2x6 rafters and minimal insulation. As they are finished on the second floor levels, I am considering options to increase the r-values of the attic areas. I have looked at Atlas and Hunter panels...4 1/2 inches of ISO with a layer of 7/16 OSB with /or without venting spacers. also, these houses have zero overhangs so a design using a fascia/soffit board tied into the project adds to the positive options in the system. I plan to strip to the sheathing, and attach the ISO panels directly using 6 inch screws. I have two engineering/ technical questions.....applying asphalt shingles over this system with or without the ventilation may or may not breach warranty specifications. so that a ridge vent in an unvented ISO system is useless, requires adding gable vents for the attic space?...also, would I need a vapor barrier between the roof sheathing and the ISO?

thanks in advance David

Dave, We reroofed many large commercial buildings here in Florida, most of the customers wanted to use ISO board for the added insulation, after checking around with various shingle mfgs it was advised that the only way to install shingles over the ISO board was to install wood sheathing over the ISO then dry in and shingle, this was to disperse the build up of excess heat and down this way that old Sun beat down every day. The mfg stated if we were to go over the ISO with shingles the shingles would cook from the heat.

We have done a lot of cold roofs in Kalamazoo. Here are some pictures without captions of one we did on a house about 4 years ago. We are carpentes that roof or roofers that are good at carpentry, or something like that. The problem you will have with the panels is marrying them to the structure at the bottom to get the overhang and the ventilation started at the bottom. You will have to tie it back up the roof a ways so that it doesn't pull off when the snow builds in the winter. It would be cheaper to just install ISO board directly over the deck in about 3 layers of 1.5" and then screw down sleepers and rafters over top and install a 3 1/2 inch "attic" over it. The panels cost about twice as much as just ISO and this way would be a LOT more efficient. Call mre if you have any questions. JMHO

https://picasaweb.google.com/crookston.john4/ColdRoofInKalamazoo?authkey=Gv1sRgCOTnivPelaiZGg#5357746115389222610

i am looking at a similar project,100 yr. plus church converted to condos. 2x6 t and g deck, no attic,leaving the wood exposed, asking for 6" vented Hunter panels, need to build-extend fascia,with hardie board, asking for complete ice and water,hunter panels, triflex 30, and inspire tiles,ridge vent. no drawings as yet on the fascia detail,they want one contract to do roof and carpentry, need a substantial fascia, as 40 ft 12 12 rafter, need a vented hardie board soffit. The tile rep has no issues with the design, but its a big sale for him.

Last time I checked, GAF wouldn't approve over nailbase ins. and Certainteed would. I have a feeling the vented nailbase would not allow them to renege on the warrantees. Not sure about using the six inch fasteners on a steep pitch. They worked fine on lower pitches, but you should verify number of fasteners, placement pattern, and acceptability for steeper pitches. Bldg insp. requires vapor barrier under the foam package when I do them. Not a big deal to put one there. OS is totally right about new surface planes swallowing up existing detailing. That can either be a set of pleasant challenges or a deal-breaker, depending on who, what, when, and where. Up in the Sierras they're big on the spray foam, solid fill with self-adhered underlayment above the sheathing, but I am afraid that even though they swear they work fine, they are violating shingle manuf. specs. I know GAF turned me down on that one.

David, you are dealing with 3 attics here, one on either side and the one on the top. You will also have to re-side the dormers because the "new" roof with be way above the old flashings. You will also have to build a window well at the front of the dormers because the roof will be higher than the windows.

In regards to the insulation, that could become a pickle. You could just cut a hole at the top of the old ridge and make sure that the air from the top attic mixes with the air coming in from the new fascia at the bottom. I would consider sealing off the two lower attics if you use a vented board, because in effect your new "attic" is going to be the vented space on top of the ISO. It is tricky. It might be better to just remove the roof sheating and urethane the walls and such from the outside with spray foam. I have done a lot of that, so you can call me if you have any thoughts.