Replies

1. 
   AndyEngel | Sep 17, 2013 03:07pm | #1

   This detail is executed all the time with no problems. Things to consider, though.

   Be sure there are no pathways (recessed lights, for example) that would allow inside air to bypass the foam and hit the roof deck.

   Make sure the shingle manufacturer warrants the shingles on an unvented roof. Some do, some don't.

   Be very sure the roof is water-tight. It can be devilishly hard to find a leak in a spray-foam insulated roof. Could be a minor leak never gets noticed and the roof just rots.

2. 
   ozguy | Sep 23, 2013 10:00am | #2

   Thanks Andy,

   I intend to put the new roof in place, and then wait at least a month or more before spraying in the foam so i can be as close to possible as sure that the roof doesn't leak.

   I'm avoiding putting any cans or holes in the roof line that are not skylights or plumbing vents, to minimize the chance of ice damming.Thinking of track lights hanging from the collor ties, and suspended lights from above the collar tie to add some drama.

   Another spray guy, is advocating open cell foam everywhere. Will save a ton of $'s but I'm not 100% sure on what he's saying; "It's the air barrier that is most important, the R rating is secondary". You could obviously pinpoint leaks easier with open cell, and I get the air barrier, but isn't the R value just as important as well?

   Is 1" of closed cell, and the rest open cell foam a good idea? The You get the water proofing, as well as lower overall cost, albeit sacrificing some R value.

   He also said that ventless roofs are being approved in IRC for 2015.

   Thanks for the info on the shingle warranty. I'll check that out with the roofer. Timberline HD is what we'll likely use.

3. 
   Perry525 | Sep 25, 2013 12:07pm | #3

   A very healthy old roof - due to be ???

   If you search on the net, you will read some of the horror stories, where spray foam has gone wrong. Not sticking, shrinking, cracking.

   The success of spray foam is entirely down to the skill of the opperator and his ability to judge the correct mix for the time and day.

   A lot has to do with the temperature of the receiving surface and the humidity of the day.

   As mentioned elsewhere, using foam as indicated will make it almost impossible to track down any leaks in future years, the mere existance of the foam will hide any leak, which will then not show itself until it is of catastrophic proportion.

    You have already made the point that the roof planks are in perfect condidtion after 135 years.

   Of course their condition has been the result of good design based on lots of air circulation and temperature rises to dry out regular doses of condensation.

   You are proposing to change all this and to move them into an unfriendly enviroment, where any water vapor that finds its way into the wood will build up and lead to wood rot and roof collapse.

   Keeping in mind that water vapor is created inside the building by people and that it is programed by nature to head for the nearest cold surface, the nearest surface that is below dew point.Then that inner roof surface is target number two following any windows.

   You job is to insulate the roof, while keeping the water vapor away from the roof. You can only do this by installing some form of water vapor shield, either a membrane or a form of coating that will prevent the molecules of water vapor from entering the fabric of the wood, or by providing some guarenteed ventilation that will enable the wood to dry as it has always done up to now.

   You could consider placing the insulation on the outside of the roof, this will enable the wood to keep warm, above dew point, and will direct the water vapor to condense on the windows or some other cold surface, where (hopefully) it will do no harm.

1. 
   ozguy | Oct 03, 2013 02:17pm | #4

   Baffles it is then.

   Thanks Perry25,

   With that input I have reinforced the decision to put baffles into the rafter bays, to create an air space so the roof deck can breath, and so any leak in the roof will hopefully drain out of the soffits or evaporate out of the ridge vents.

   I wish I could insulate the top of the roof deck, however the house is in a strict landmark historic district, with no exceptions for roof height or thickness.

4. 
   soffitman | Nov 15, 2013 10:33am | #5

   Saftey concerns: Site-built baffles using 1" Rigid Polyiso

   1" rigid insulation  is often mentioned as an option for site-built baffles. Articles include;

   "A Crash Course in Roof Venting" by Joseph Lstiburek

   https://www.finehomebuilding.com/how-to/articles/a-crash-course-in-roof-venting.aspx

   Attic-Insulation Upgrade by Mike Guertin in Fine Homebuilder

   I plan to use this method but  am concerned with the potential fire hazard  of leaving the rigid foam exposed. My attic is not be used for storage and there is no HVAC or other equipment located in the attic. My interpretation of the code confirms that the baffles would meet code.

   1"  rigid foam is otherwise a great solution.  I have not found any discussion on the potential fire hazard.. The black smoke created during a fire is causing me to rethink my choice of material for the eave baffles.

   Comments would be greatly appreciated.