This has been on my plate for several months now. I think I have it under control, but am seeking feedback.
The City of Houston recently incorporated energy performance requirements into the energy code which required “Cool Roof” performance on low-slope roofing membranes. This has complicated working on LEED projects since we really need documentation on both SRI (required for LEED) and solar reflectance and thermal emittance as measured by the Cool Roof Rating Council’s criteria in CRRC-1. While I have assumed there is a correlation of some kind, no one has been able to explain it to me. By examining product literature, I have found that the SRI on products meeting Cool Roof requirements is about 0.88 (10 points higher than LEED requirements). Energy Star seems to have been left behind in this discussion.
This would be a slam dunk if everyone was using single-ply membranes; however, most of the roofs that I specify in this area are modified bit, and there are a number of local government and institutional clients which have “design standards” requiring modified bit.
My research indicates that there are several strategies for getting a “reflective” roof membrane with a modified bit system:
(1) Field-apply a special acrylic emulsion to the installed cap sheet material. The installed cost in this area is approximately $1.00 per square foot. The product has been around for at least 10 years. Acrylic resins will oxidize at a predictable rate under exposure to UV radiation so the life of this coating in our area is about 7 years when it is applied in the thickness recommeded by the manufacturer. If owners would recoat with a good grade of acrylic house paint every 5 to 6 years, the useful life of the roofing system may be extended significantly.
(2) Factory-applied acrylic coatings applied to cap sheet product (offered by Johns Manville and GAF). Not sure what the cost premium over a basic cap sheet product is for these products. The manufacturers are saying that these require maintenance, but are not very specific about the requirements.
(3) Factory application of a film to the cap sheet product (offered by Derbigum and Soprema). The manufacturers are very cagey about the composition of this film, but it seems to have a significant acrylic component. Again, the manufacturers will tell you that it will require maintenance, but are not very specific. Soprema has some very good data about the extended reflectance.
(4) Cap sheet with highly reflective mineral granules (look more like flakes) which has been offered by Siplast for about 5 years and has just become available from Firestone. The white color is inherent in the mineral granules so that degredation of reflective characteristic will be minimized.
This 4th strategy seems to me to be the best approach.
Another stategy undertaken by roof consultants in this area is to install two plies of modified bit sheets and cover with a TPO (or in one case, a KEE) sheet.
I am seeking input and feedback from anyone who has done some research or has some experience in this area. Shared experience through posting to 4specs is appreciated or if you would rather be somewhat discrete, please e-mail me at jpjordan@jordanconsultants.com.