Hello There,

I’m relatively new to the Building Science space and thought of building my knowledge base towards improving the energy efficiency/comfort of my residence along with fixing ongoing issues.  Still, have found the depth of products today quite extensive.  I currently reside in Toronto, Ontario…Zone 5.

I’ve had an issue for quite sometime with several skylights installed on a poorly insulated curb sitting on a low-slope flat roof with torch down membrane.  The significant condensation issues have contributed to leaks, ice-damming and skylight seal failures…In short, I now have to replace all the curbs and skylights with an insulate curb which makes sense for our climate.

Haven’t been able to find a qualified roofer who can build me the required insulated curbs so have taken on myself to build the curbs, and once built, have the roofer install (secure and torch membrane to the deck) along with the skylight install. 

Attached I’ve included a design of a curb which aims to maximize the R-Value while eliminating/reducing any thermal bridging, condensation, ice damming, etc.   Towards this end, the skylight curb specs are as follows;

      1. Skylight supporting frame is from 2×4 spruce to provide the
          structural support for skylight, installation, etc.
      2. Skylight curb cavity will contain 3.5″ Roxul Comfortbatt Insulation
           (R-14) with a CertainTeed Membrane and Drywall on the interior
           surface.
      3. Exterior 2×4 curb will have 1/4″ OSB/Plywood Sheathing applied
           followed with an Ice/Water Membrane for a complete air/vapor
            barrier seal.   This will protect the R14 Insulation and potential
            cold bridging.
      4. Upon completing step 3, a 2″ inch Comfortboard R8 Mineral Wool
           will be installed  on the curb exterior vertical and horizontal surfaces                   which will act as the thermal bridging insulation. Total R-Value: R8.
      5. The Comfortboard R8 will be protected by mechanically fastening 
           a 3/8 Plywood + 1/8 IKO Protective Board to permit the installation of             the Peel+Stick Base Cap and Top Torchdown Membran (2-Ply  System
       6. Upon securing/waterproofing the curb to the roof deck, the skylight
            flashing will be installed, followed with the skylight.

The design shown required a Double Vapor Barrier (with Comfortboard R8) sandwiched in the middle providing the thermal bridging barrier.  Anticipating that if the thermal bridge is properly enclosed…moisture won’t be an issue and will provide the desired condensation protection, etc.

This design presents the following challenges for which I’m unsure whether
there’s a more simplistic approach…Your feedback would be appreciated.
 
        1. The cold poor performance of the PolyIso product line has led me
             to use the Roxul Product Line.  Unfortunately, have been unable to
             find a local EPS product that can sustain the torchdown membrane
             temperatures which led me to the Roxul.
             Considering the application/climate would PolyIso (and its inherent
             limitations) still be the more appropriate?

          2. Is a sheathing/air+vapour barrier installed between the R14 & R8
               really required (ie: condensation and dew concerns) or does it 
              present more issues?

           3. The torchdown membrane provides a complete Air/Vapor Barrier
                Seal…From your experience, will the CertainTeed Membrane
                provide the required Vapour Resistance to resist interior room 
                 moisture along with permitting the wall cavity to breath towards
                the interior space if required?

 Your feedback/expertise towards arriving at the desirable design which addressed the historical issues (and ensures a proper design) would be very much welcome.

Thanks.