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Roofologist
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Discussion Starter · #1 ·
So I have been thinking about some of the newer insulating practices that we have been seeing of late. The conditioned attic space. (This really isn't a new practice but I have see more and more of it here lately. I guess I should say that it's becoming more popular.) Spray foaming the underside of the roof sheathing.

I know how we roof these but am curious how you guys would go about it.

Do you install straight over the OSB or PLY with felt then shingle or metal? What kind of felt? #30 or Synthetics? Why?

Do you figure out a way to get some breathing room under the roofing materials? If so, how?

How will this affect the manufacturer warranty?

Any and all discussion welcome!
 

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My thoughts would be to install as normal. The spray foam on the underside of the sheathing, if installed thick enough, is usually also the vapor barrier, so condensation on the underside of the underlayment shouldn't be a major issue in my mind.
 

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I'm not a big fan of insulating directly to the underside of the deck, no matter the type of insulation. Ventilation, not only for heat but for moisture, is a best practice that gets thrown to the wayside when installing spray foam because it is seen as a 'miracle product'.

As with anything there is the human factor where the installation isn't perfect (either with the roof or insulation) and without that best practice of ventilation there is a possibility of damage (leaks, condensation, etc).

As for retrofitting roofs that have no ventilation, vertical strapping (from ridge to eave) over the existing decking, then new OSB would provide an air space.
 

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Roofologist
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Discussion Starter · #5 ·
SIP homes are a very popular form of building in my area and also represent the question at hand. The SIP roof structure is a fully insulated structural panel that has absolutely ZERO breathing space.

Knowing a lack of ventilation will void any blistering or accelerated aging issue's I called the manufacturer of our favorite brand of shingle to verify. They affirmed my suspicion and informed me that installing their 50yr SBS shingle shouldn't present a problem, BUT, if the above mentioned problems arose they would not warrant them. However, the manufacturer did say they would cover any wind related issues which was a major issue with this house.

I informed my customer of the warranty issues that may arise and went forward as normal.

Now knowing that a shingle manufacturer will not cover their products without ventilation what products would you use?

Shaz suggested installing vertical strapping and sheathing which is feasible.

Is there something else you would use? How and why?

Would you use the same practices with a metal roof?
 

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With the proper warning to the customer, I would run shingles over 30# or synthetic. If the shingles are hotter, they will probably have less life. I would prefer to install metal though. With both systems, moisture beneath rotting the plywood and molding would be my concern.

That said, we have done a bunch of vertical strapping with sheathing over it to create a vented roof. Works great!
 
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