Metal Roofing w/Insulation Options

Justin Paniagua
11/19/2014
I'm considering my options for installing a metal roof. I have open cell spray foam on the underside of my roof deck at R-19 5-6". The roof will be either a standing seam or Decra panel (probably dependent on my wife's astethic taste) directly onto a solid substrate. In addition to this I was thinking of adding insulation between the roof deck and the metal roof in one of two ways. Brining it as close to R-38 as possible. Option 1: 1. roof deck 2. vapor barrier 3. Hunter panel that has polyiso bonded to OSB on one side. Probably 3" R-15 http://www.hpanels.com/images/stories/pdfs/lit_prod_color/english/H-Shield-NB.pdf 4. Underlayment 5. Metal Roof Option 2: 1. Roof Deck 2. Underlayment 3. SR2 Standing Seam Roof Panel 3.25" R-26 http://www.metalsales.us.com/sites/default/files/docs/SR2_Metal%20Sales%20IMPower%20Series%20Data%20Sheet.pdf I know it's a commercial product so it may not be cost effective. My concerns: 1) In option 1 could I see moisture problems develop without ventilation between the roof and the substrate? 2) Is option 2 even viable for installation on solid substrate? Could there be moisture problems here as well? 3) Is adding this insulation even necessary or would it just be overkill? If I do option one directly onto the roof deck w/underlayment could I see moisture problems? Your opinions would be greatly appreciated. Justin
Info @windowsonwashington.net
An informed customer is our best customer.
11/19/2014
While I appreciate your wanting to get to code, the reality is that the OC SPF and some rigid foam the exterior will do just fine in this case and vastly out perform a traditional insulation and even more so when you figure in the performance of metal. Ultimately the insulation approach may be slightly modified by the selection of what metal system that you use. You have solid decking already. No need to put underlayment on it at that level if you are going to put rigid foam on it. I would put the amount of foam that you want down and either put your decking right down to that or created and above deck style vent. The reality is that metal is not absorptive to the water and the sheathing doesn't really need to "breathe" as it will be getting zero moisture into it. I get the thought process but there will be basically zero moisture coming from below and none coming from the top. Other wood applications with plywood don't require breathability if they don't get wet. If you want to err on the side of caution, put down your rigid foam, underlayment, purlins (creating a vented space), plywood, underlayment, and metal. Best of both worlds at that point I guess but likely overkill.
Justin Paniagua
11/20/2014
Thank you for the thorough and thoughtful response. I didn't think moisture would be an issue especially considering the climate we have here but it seems almost everything I see or read about metal roof install is for areas where humidity is an issue e.g. south, east coast, etc. The way I understand it I could just put the foam board on the current roof sheathing followed by OSB or plywood, underlayment and finally metal roof. If I want to be really cautious I could add purlins but it wouldn't be necessary. Basically it would look like the attached photo.
Info @windowsonwashington.net
An informed customer is our best customer.
11/20/2014
That will work fine and is what I suggested as a primary solution. If you are worried about venting, that is when the purlins and above deck venting (also helps with convective cooling and thermal bridging) come in .

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