I have a 1921 1.5 story bungalow in Minneapolis (climate zone 6).   There are three separate accessible attic areas with knee walls.  Two out of the three have HVAC ducts in them, along with some plumbing runs and access doors.  They are anything BUT air sealed, in other words.  When I purchase the house, the knee walls were insulated with fiberglass bat with the craft paper on the attic side, and someone had also added craft paper faced fiberglass to the roof deck.  The spaces were vented with one apx 8×8 vent and an electrically powered exhaust fan each.

I removed the power vents and had the accessible areas of the roof deck (and exterior walls within the attic spaces) sprayed with open cell foam, blocking off the vents.  This resulted in noticeable improvements in comfort and energy use, and reduced ice dams.  However, I still get some significant ice dams, particularly in the valleys on the south side of the house.  A few more details:  I have exposed rafter tails, 2×4 roof framing, and the foam was not shaved down, so I have an average depth of apx 5” ( I would say it varies from 4-8”), and the framing is covered with at least 1” of foam.  My asphalt shingle roof has about had it and need replacement.  The existing roof deck is original (planks with some spaces between them, some knotholes, etc).  I am going to get a new roof this summer, so I have an opportunity to make some improvements to help with both energy use and ice dams.  It’s a nice house, but I probably wont be here forever, and it’s by no means a high end home or in a top price market area, so expenditure must be considered.  Oh yeah, I’m not exactly swimming in cash either! J

Here are my concerns and questions:

1)  I don’t have a vapor barrier.  If I was doing this now (about 5 years later, and many FHB articles later), I would probably go with closed cell to get a vapor barrier and more r-value.  I would have asked for a thicker application as well. 

2)  I have thought about adding a few inches of rigid foam over the existing roof deck and then putting new sheathing on top of that.  Questions I have about that are what is preferred, XPS or Poly-Iso?  What would the min thickness be not to have condensation beneath the existing roof decking?  What if I was able to put some sort of VB over the currently installed open cell foam?  Could I then go with something thinner for the rigid foam?  I ask because I have exposed rafter tails, and I’m not sure how this whole thing would look if I made the roof deck 5”+ thick.

 3)  I have a few thoughts about how to add a VB and/or more r-value to the existing foam job.  Could I have closed cell foam sprayed on top of the open cell?  Or, I could shave it down flush with the rafters and add a layer of foil faced poly-iso.  Or, I could just spay a VB coating on it, but that would be difficult given how “bumpy” it is.

• 4)  Lastly, I am considering a steel roof, although I suspect it won’t be cost competitive.  Is steel usually applied over ridgid foam?  How thick?  Can you put it right over foam, or does it require a plywood deck between them?

Thanks,
Erik