New Construction Advice

Linda Whitaker
11/4/2017
We are building a new home, and I would love to get some feedback on the recommended roof and insulation specifications that our builder and roof installer are proposing. We have a gable roof with a 9/12 pitch transitioning to 4/12. The 9/12 is mostly over a second floor with roof trusses. The 4/12 pitch is about 16 feet in length, with ½ of that over climate controlled attic/storage space and ½ over an un-insulated porch roof. The roof is not currently vented and is planned to be insulated with open cell foam sprayed directly against the underside roof decking. We will have a snap-on 1 inch standing seam metal roof over 5/8 inch OSB decking (no purlins). We currently have titanium UDL over the OSB. The roof installer seems worried by the roof pitch change from 9/12 to 4/12. There will be transition flashing, (z-flashing) over the lower panel, and in addition the roof installer has recommended removing the titanium underlayment and replace it with ice and water shield over the entire 4/12 pitch, in case ice dams form. We live in Western North Carolina, zone 7a or 6b. Average of 8 inches of snow per year, but a once a year snow of 10 inches is not uncommon. Any feedback on the roof installation and/or the insulation plan would be greatly appreciated. Thank you very much.
Eric Novotny
An informed customer is the Best Customer!
11/5/2017
If you are going to seal the attic and have a comprehensive insulation schedule (i.e. sufficient levels of Spray Polyurethane Foam - SPF), the incident rate of ice dams as a result of unchecked heat loss (i.e. the one that are mostly problematic) is basically nil. Covering the lower pitch part (depending on the square footage) in Ice/Water isn't total over kill, but I would still cover it in Titanium UDL as well. I don't like going directly over the Ice/Water as they can be a bit sticky at the overlaps in the rolls and hinder panel expansion and movement. Just make sure you have enough foam on the underside of the roof deck and you will be 100% fine.
Linda Whitaker
11/5/2017
Hello Eric, Thanks so much for the fast response. My husband and I have been reading about the risk of moisture becoming trapped in the OSB, which is sandwiched between the Ice/Water membrane and spray polyurethane foam. In addition, because the attic space is unventilated, humidity from inside the house will ultimately permeate the open cell insulation and likewise become trapped in the OSB. The moisture-laden OSB will eventually mold and deteriorate. We also read that in this application we should also use rigid foam board insulation under the metal roof and over the OSB. We've thus been learning towards having a vented, uninsulated roof and using the open cell foam insulation on the walls and ceilings. Then perhaps we could avoid the cost of removing the UDL, adding the Ice/Water and putting the UDL back on (which will cost us a lot). Thanks for helping us wade through all of this.
Eric Novotny
An informed customer is the Best Customer!
11/6/2017
Rigid foam, if you can incorporate it, is a great addition on the top side of the roof framing/sheathing. Most moisture issues are because of air transfer and less so because of diffusion.

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