Rigid foam to catch drips from metal roof without decking
I’m building a small, one-story house with a simple 4:12 gable roof in Climate Zone 2, semi-desert Southwest with maybe 2 inches of snow per year. I want to use exposed-fastener, white-painted steel roof panels on purlins 2′ oc on trusses 2′ oc without any OSB or plywood decking. The panels are approved for such use. The attic space will be unconditioned, with R-38 insulation and a moisture barrier in the ceiling. The panel manufacturer does not recommend conventional underlayment, because it may sag and collect water, and I would prefer to avoid it anyway, because it would be a major pain to install without decking.
The solution I have in mind is to cut 2-foot wide strips of 1/2″ thick, foil/poly-faced EPS to fit between the purlins 1/2″ below the metal roofing. I would seal the EPS against the purlins with a high-grade, permanently flexible caulk or spray-foam insulation. The rigid board material costs roughly 23 cents per square foot. I would leave 1/2″ gap between the EPS and the underside of the steel roofing to eliminate any risk of galvanic or capillary action and to create long ventilation channels running parallel to the eaves. The main purpose of the rigid boards would be to catch any drips from leaks in or condensation from the metal roof panels. The reflectiveness and R-value of the EPS would also reduce the temperature above the ceiling insulation. Drips from leaks or condensation shouldn’t amount to much water here, and virtually every day, the sun will warm the roof temperature enough to dry any moisture sitting atop the EPS, especially with the screened ventilation channels running from end to end of the roof.
Does this plan seem sensible?
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Replies
Jeff,
Your proposed solution would probably work, but the detail is fussy to install and inelegant.
The way I see it, you have two choices:
Do nothing -- just leave the metal roofing on purlins and don't worry about the condensation. I've seen this approach implemented in Vermont many times. A small amount of condensation occasionally drips on the insulation -- ideally, you will install cellulose -- and evaporates harmlessly. (Many Vermont attics get a fair amount of snow that filters through the ridge vent. The snow can look alarming on top of the cellulose, but it almost always evaporates or sublimates harmlessly.)
The main disadvantage of this approach is that you will find it difficult to remodel the attic into conditioned space if you ever want to do so.
The second alternative (if you are worried about condensation) is to do it right: install a layer of OSB or plywood roof sheathing, followed by asphalt felt and rake-to-rake purlins to support your metal roofing.
Thank you for your reply, Martin. I've learned a ton from your articles. Would you happen to know of any studies or other documentation that might help reassure my building official that drips will evaporate harmlessly from the insulation? I would love to skip my fussy installation if I could. If I have to install something below the metal roofing, at least I can install my solution after the metal panels are in place, and I can therefore install the metal panels by standing inside the building on a ladder or short scaffold and reaching over the 3' wide panels to screw them down, which I expect to be easier, working alone, than getting sheathing up on the roof and standing atop the roof to attach everything.
Jeff,
What pitch is your roof and how big is it?
I installed the steel panels on my roof single handed, working off a 6/12 plywood deck with rake-to-rake purlins like Martin described:
http://ourhouseuponmoosehill.blogspot.ca/2012/04/roof-sheathing-and-cladding.html
Having also done it the way you're suggesting (working from between trusses), I'm not sure I would recommend proceeding with your plan.
Especially if your roof has fairly large trusses - 6'+ at the ridge, 3'x16'+ steel panels...
And even if you do work from between the trusses there will be all kinds of reasons to have to climb up on to the roof anyway.
The purlins make good "ladder rungs"...
And invest in a good harness.
Nice blog and house, Lucas.
My house will be a lot smaller and simpler than yours: 16x48 with a simple 4:12 gable roof. Given that, what reservations, if any, do you have about working from between the trusses?
What relatively little power the house needs will be all electric, and all of the plumbing and its venting will be in one area, so I plan to have quite minimal roof penetrations and therefore essentially no reasons to climb up there that I can think of, but if I'm not thinking of something, please let me know.
Thank you.
Jeff, thanks.
With longer panels I find it easier to drag the sheets upward over the eave which is made easier by being able to walk up the roof deck.
Before you begin fastening, you'll want to lay out the first three sheets or so and fiddle with them until they're square with each other and in-line with the roof - this is much easier if you're able to work from above (recommend punching a small hole with an awl for each screw rather than self-tap).
Not sure what brand/profile roof panel you have selected, but if they are more than 30" wide you will not have an easy time fastening the far edge of each without working from above.
The last sheet on each side and ridge cap will force you to work from above.
4/12 pitch isn't bad to walk on even if it's steel underfoot, but choose a day when there is little or no wind blowing - even a light wind can be extremely difficult to deal with by yourself.
Thank you for the advice, Lucas.
Jeff,
No worries, it's only advice.
Best of luck with your project sir.