Cool roof design
I have been designing a small retirement home for the high desert of southern California. I’ve designed and built several homes for myself in the distant past (all frame), the last more than 20 years ago. So, I have lots of research to do and no experience with some of these materials which I intend to use. My hope is to get through the process with a very “Green” house, low maintenance, and as close to net-zero as possible. My plans are for the home being insulated slab on grade, ICF walls, with SIP roof (borate treated), and 8′ walkaround porches. A “Dickey” style roof with 5/12 and 3/12 pitches. The “Under Air” portion of the house (5/12 pitch) will be 12-1/4″ SIPs and the porch roofs will be uninsulated. The finish roof covering will be solar reflective paint standing seam metal. My quandary is regarding a cool-roof which is mandated in California (a good idea really). I hope to find the least labor intensive or simplest cool-roof design possible. I found an underlayment (Delta-Trela), which seems an all-in-one product with waterproofing and air gap, which is vapor permeable. Through my reading here on the forum I’ve found differences of opinion in whether the underlayment should be permeable or impermeable over SIPs. The Delta-Trela seems the perfect solution, but I don’t want to screw this up. I am hoping anyone with experience using these products can reflect on my approach to the design. I can post preliminary design sketches if anyone would like. Thanks In Advance, Dune
Hello Again Martin,
Just an update of discoveries. Turns out the Delta-Trela has Perms ratings between 69-120 Perms depending on type of humidity used during testing (Low humidity 69- High humidity 120 Perms). In any event, the high desert of California tends to be very low humidity, and the Delta-Trela would seem a good product for my use. I did read a lengthy article written by Dr. Joe Lstiburek who states that the drainage component of the cool roof system is far less important than the vapor redistribution provided by the air space. Another conundrum! Is the space provided by the Delta-Trela sufficient for the vapor redistribution (I think you hinted your opinion on this earlier). I would think in an arid environment no problem, but in a higher humidity condition during the winter rainy season may be trouble.
The second part of my interest in this type design (metal roofing simply over Delta-Trela) would be the temperature differences or solar heat transmission through the roof assembly. Where is the condensing point in the roof assembly? I would think still at the backside of the metal roofing. I’ve gained some information through resources at West Coast SIPs where they tested another similar matting product called Enkomat 7020 over WRB under a dark metal roof (I assume not using reflective paint). Their testing resulted in temperature reduction of 43 degrees F from 194 degrees F mean. I would think this is good as the heat in summer high desert afternoon is daunting. I’ve yet to discover any testing in this regard using similar matting products and a metal roof with PVDF Kynar coatings. My impression is that the combination would be better than the 43 degrees mentioned. Would you happen to know where I may find literature on the subject?
Am I overthinking this?
I found this very informative and those considering the construction of a Cool Roof should read.
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