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?
Best Regards,
Dune
I found this very informative and those considering the construction of a Cool Roof should read.
Dune
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Replies
Dune,
The design of Delta-Trela provides for some air movement above the sheathing (and therefore some outward drying of the exterior OSB). In that case, you want to choose a vapor-permeable underlayment like Perma R Products PermaFelt, GAF Materials Deck-Armor, Cosella-Dörken Delta-Maxx Titan, VaproShield SlopeShield, Cosella-Dörken Vent-S, Nemco Industries Roof AquaGuard BREA, or Cosella-Dörken Delta-Foxx.
Thanks Martin. My intent is to make the process as simple as possible. Forgive the question, but are you saying one of those underlayments under the Delta-Trela? I had assumed the Delta-Trela would do the job alone.
Dune,
Sorry. My answer was confusing. The Delta-Trela is, indeed, a vapor-permeable underlayment.
SIPs can certainly end up with damp OSB facing on the exterior, so an assembly that allows some outward drying, however slowly, is certainly preferable to an assembly that provides no outward drying.
Thanks again Martin, and for all you do educating us in our endeavors to build better. I think a cool-roof can be built cheaper, but look to save labor and end up with as good or better result. I have to do a little more research on permeability and thermal issues before I decide how to proceed. Maybe in the end I'll try to advise on a cost analysis with each design I consider.
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?
Best Regards,
Dune
The cold roof system on top of SIPs panels typically requires 15# felt directly over the Panel to help remove any moisture that does get into the panel so it can dry to the outside. Inside face of the panel seams needs to be taped and air sealed. After the felt, 2x4s on the flat 24 oc and then top layer of 5/8" OSB with your normal roof assembly. I personally am using Rhino Roof over the top of the upper layer of OSB and then Standing seam for my house. I can share my detail drawings if you would like to see them.
Thank you Zdesign for the offer. I am still trying to figure out how I will accomplish the design and would appreciate you attaching a file.
From doing a lot of reading on gba and other sites I think this method (effectively a roof on top of a roof) is fine, and safe, but probably not the least expensive, or easiest with standing seam metal roofing.
I'm doing similar (high desert, sip, slab on grade, standing seam) and plan on doing sip osb, vapor permiable wrb, purlins on 24" (even 2x4 parallel with ridge is safe) then, standing seam over that.
What I understand is sips have problems when they: don't adequately seal interior seams, use a vapor impermeable membrane , or don't have a way for condensation under the metal to escape.
I'm still figuring out the requirements for ice shield around the edges, the necessity of sealing nail holes for the strapping, and how a desert environment effects sip drying.
Dear Chris,
All questions in my mind as well. My original thoughts around a cool roof was to build a roof over a roof as you say. I thought that may be the least expensive way to go, but I got thinking about labor intensive. I've seen writings and videos illustrating the vertical purlins not being covered with OSB and just putting a metal roof on. That doesn't make sense to me with the thin metals I've seen out there. They are nothing like the old corrugated steel pole barns I built in my youth on the farm. Recently, I saw a video by Matt Risinger describing Delta-Trela and thought if I could get the idea past the Inspectors that would be the way to go. Expensive stuff, but I'm thinking when I break down all the costs and then the labor savings maybe they'll be close enough to fit in the budget. If there's a better mousetrap out there I'm sure willing to listen. As far as the interior sealing, I intend to use more sip seal along the seams and tape. That should keep things to a minimum. My major question is regarding the roof underlayments vapor permeability. It does not make sense to me for the barrier to be impermeable.
You may have issues with using SiPs for the roof diaphragm with the ICF walls. This would be due to the seismic design requirements. There are prescriptive requirements for anchoring concrete walls to wood roof diaphragms so they won't separate in a earthquake. Check with an engineer to see how this would work with SiPs.
Thanks for the comment Tim. And yes, I would have to get the local code inspectors to agree to the design of the ICF to SIPs attachment detail. My research tells me I can with a 3" PT sill, bolted, with SIPs sealed and screwed to the sill. Of course the bolts will be heavier and closer, along with the SIPs screws. I am doing this as an owner/builder so I will have to exceed any applicable code. But you are right and I will get advice from an engineer.