Originally Posted by
Holmes Anderson
I guess i'm late to the party on this thread but you may have more to worry about than choosing insulation. If that was unconditioned space and you are converting it to conditioned space, then you should consider ventilation. You say there are no soffits and I don't see a gable vent. Is there a ridge vent? Spray foam insulation against the sheathing is great for stopping airflow and heat/cooling loss, but it is hell on the shingles. Without cooling from ventilation on the underside of the sheathing, the shingles get too hot in the summer sun. Maybe heat is not a big problem in Seattle, idk, but condensation could be and even rigid foam or glass batts without baffles for ventilation will shorten the life of the shingles. You could attach sheathing to the trusses to create an attic and then put insulation on the attic floor and add ventilation. Powered roof vents are easy to install and that would rid you of excess heat and moisture without killing your shingles. Also, spray foam is hella expensive so I wouldn't apply it to a roof that is in poor condition and doesn't meet code.
I have to disclose that at our previous residence, the local jurisdiction forced our contractor to ventilate the roof deck of our 2200 sq ft addition before the spray foam insulation envelope was installed. That resulted in a huge expense replacing the roof just a few years later because of condensation completely rotting it out to the point my foot went through the 50 year shingles while I was doing leaf maintenance up there. The foam was supposed to be on the deck by manufacturer and installer specifications, but alas, the (not pleasant) local inspector had his way and it cost me twenty grand. Roof and soffit Venting is not typically used with closed cell spray foam envelopes for this reason other than on high end structures that have a layered roof system, typically in the south...a double deck if you will. The living space is unvented other than ERV for air quality that does heat exchange to insure that the temperature (warm or cool depending on the season) is maintained while the humans have comfortable fresh air to breath. The shingles don't care if they are a quality product as Roger pointed out in his own reply to your comment.
--
The most expensive tool is the one you buy "cheaply" and often...