Best Practice/New Build

K S
10/2/2017
Hello All We are about done with the plans on our house to be built in the mountains in NC. I have attached a pic of the rendering. I want to make sure I get this house right as I plan to leave it only in a pine box many years from now. :) Before I go to speak with the roofing contractors, I would love to hear the collective wisdom here chime in on what would be best for this home. The two shed roofs you see are 2:12 pitch, the one will have solar panels, the other will be over an uninsulated garage. The main roof is 3:12. Trusses will be used for all roof framing, with plywood sheathing. The underside of the plywood will be sprayed with 3 inches of closed cell foam and then another 7" of open cell foam. I'd love to hear your opinions from plywood out. From reading, it sounds like 24 ga. metal standing seam would be best, and with our pitch, mechanical only? How about underlayment? I am fine with the extra money for ice and water shield over the whole deck if that is the best solution. Options there? Is it fine to go straight onto plywood sheathing with no ventilation with an unvented attic? If a client came to you with this house and wanted your best install, what would you suggest? Thanks!
Info @windowsonwashington.net
An informed customer is our best customer.
10/2/2017
Have you considered a SIP style roof decking? This will give you a complete thermal break and should eliminate the expense of the spray foam in this case. You are married to standing seam in this case and once you get into the 2:12 pitch, many will require a mechanical lock. You can definitely run snap lock at 3:12 and there are plenty of solar array type clips that can be incorporated to allow for a penetration free mounting of any solar panels.
K S
10/2/2017
I did consider SIPs, but they are ruled out by my "NO OSB" mantra. I also do not fully trust SIPs--installs have to be absolutely meticulous as far as air sealing and if not, it is a huge issue to correct. I looked into nail base with plywood and foam to go over the decking, but it was far more expensive and labor intensive than spray foam on the underside. Closed cell is not that much more expensive(per R) than open cell, and open cell is actually cheaper than dense-packed cellulose. With a truss roof, I can get an almost complete thermal break from the inside.
Info @windowsonwashington.net
An informed customer is our best customer.
10/2/2017
Only issue with SPF is the bridge of the roof framing, but that is mostly a non-issue with truss framing as you noted. You are fine with the CC SPF and OC SPF combo.
K S
10/2/2017
Any thoughts on underlayment? Need for ventilation?
Info @windowsonwashington.net
An informed customer is our best customer.
10/2/2017
You don't "need" ventilation in this application with a sealed roof deck. Does ventilation allow for some convective cooling...yes. Will it make your solar panels work a bit better if the roof is cooler...yes. There is a trade off at some point. If your climate has some definite heating degree days and you don't have a bunch of snowfall, some radiant heat gain is a good thing. The point is, you can't go wrong with your assembly based on what you are doing and your material selections. As far as the underlayment, just use a high quality synthetic that is recommended by your panel manufacturer and you should be fine.
K S
10/2/2017
Here's a twist! Talked to the builder's favored roofer today about all this. He said go with 24 ga. material, mechanical lock, with a double layer of ice and water shield under the 2:12 portions, and regular synthetic underlayment under the 3:12 portion. That all sounded great. then he threw in a "By the way....." (uh oh)"....we don't warranty installations on 2:12 pitch and below." What? I mentioned that Englert was more than fine with installation of their product on a 2:12 roof and has a warranty--why would they not warranty the install? He mentioned something about snow and driving rain and wind(mountains of NC--not really your bastion of snow, or wind, or driving rain compared to many parts of the US). He says they do them all the time, but they never warranty them. I was shocked. Time to find a new installer. This guy was not a MRA member.
Info @windowsonwashington.net
An informed customer is our best customer.
10/3/2017
Not sure. We put mechanical double lock on all kinds of low slope stuff and have never (knock on wood) had any issues.

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