Thanks for this great website. I looked, but didn’t find an answer to my specific situation so here it is:

The house is a log structure built in the 1850s. It’s a 1 1/2 story as it typical in our area with 2×4 rough sawn lumber rafters supported in the middle by a rough hewn log about 7″ square. After discovering some of WR Grace’s finest tremolite asbestos in some existing vermiculite insulation we had the upstairs rooms gutted and cleaned by professionals. Since it was about 2001 or so, and I’d already used an Icynene contractor on another job, I had the rafter cavities filled with Open Cell foam – more in the center of the building near the peak as that area was not struck off. The roof boards were standard rough sawn 1 by’s and I could see a layer of cedar shingles through the spaces. I knew we were going to have to replace the roof someday, I stapled roofing felt over all those cracks so I wouldn’t be ripping out the insulation when the roof was stripped, which I plan to do this fall. To cover the insulation on the inside, I used 1 x 3’s to strap the rafters and installed 3/4″ of rigid foam between the strapping. 5/8″ drywall was installed over that and painted. So now it’s time to reroof and I want to complete the insulation to at least R 40 or 50. I’m estimating I’ve got the roof to about R-20.

I’ve identified several problems with my work so far which I don’t want to make worse. The first is I’m coming to believe in the near impossibility of completely sealing the inside plane — 1st – I left the rough hewn log exposed which is pretty hard to seal but looks really nice and 2nd the log walls go right to the existing roof decking which I’ve been sealing the best I can with the Permachink products and a lot of Pur Foam, but I don’t think it will be perfect or stay perfect. So since my choice of open cell foam seems to have been a bit misguided given your current analysis, the question now is How to move forward with additional insulation during re-roofing. It looks to me like my addition of the taped rigid foam layer created something of a vapor barrier which I gather means that I should not be considering Polyisocyanurate foam or Pink board as a new roof insulation as that would trap the existing roof deck between two impermeable layers of insulation.

As such I was looking for permeable insulation and what I found was Roxul Toprock DD or Monoboard. I can get 2″ Roxul Toprock DD (R 4.3/” at 25°) which is used commercially on roofs and I was thinking of adding 4″ or 6″ of this to the roof deck along with another layer of plywood or Zip Panels etc, and roofing felt and a near white asphalt shingle from Certainteed or Owens Corning. Roxul says their board products are vapor permeable, but I’m not sure there’s much drying to the outside would happen if I laid the asphalt shingles directly on the deck with no ventilation. To address this I thought I would ventilate the roof with strapping over the existing rafters using ridge and soft vents.

So here’s the questions: Will the Roxul boards be an improvement over the rigid foam alternatives?
With Roxul board or rigid foam, should I ventilate the roof (involves two valleys and a chimney near the peak so that will be hard)? If it will help to ventilate, should there be some kind of rain screen fabric placed over the Roxul boards, maybe before strapping? Would a steel standing seam roof change require and changes?

I appreciate your response.

http://www.roxul.com/files/RX-NA_EN/pdf/Brochures%20and%20Sell%20Sheets/Building%20Envelope/BEDR%20Roofs%20Sell%20Sheet.pdf