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What type of roofing material is best above a SIP roof with PV modules on top?

Robert Walker | Posted in Green Products and Materials on

I am a home owner in the final design stage for a net-zero house. The house will be located in update New York near Albany at an elevation of 1360 feet and is probably closer to Zone 6 than Zone 5. The area can have heavy snowfalls and also ice storms.

The house will have SIP walls and an 8.25 inch R50 SIP roof. I plan to add 2 inches of polyiso insulation either to the outside or the inside of the SIP roof in order to increase the R-value to R62. The ceiling below the SIP roof will be a cathedral ceiling.

On top of the roof 44 PV modules and their racking system will be placed which will essentially cover the south facing portion of the roof except for the overhangs. This will tend to hold snow or ice on the roof.

The main question is what type of roofing material is best to use for this roof assembly and what would keep this rather unwieldy roof assembly with the PV modules on top structurally sound? I have considered either asphalt shingles or a standing seam metal roof. The metal roof seems better but it would also be twice as expensive.

Also should the roof be vented or can it be a non-vented roof? Is it possible that with a non-vented roof, the R50 SIP panel and an additional 2 inches of polyiso insulation added probably on top, that it would be enough insulation so ice damming would not be a problem as long as all air leaks in the roof area were sealed properly? Is there a way to add a drainage plane to a non-venting roof so that any water that does get under the roof or any condensation that occurs has a way to drain out?

If a vented roof is used, how deep should the vented space be? I have read that 2 inches is recommended or could it be less?

Also what would the roof assembly consist of? Where would vapor barriers or air barriers in addition to a waterproof underlayment be placed?

Related questions are does a metal roof prevent water from ice dams getting underneath the roof? Is it better to use the metal roof since the racking system for the PV modules can be attached to the seams of the metal roof without the multiple long penetrations down to blocking in the roof SIP panel that an asphalt shingle roof would require?

Also has anyone used a product called Cool-Vent which is a ventilated nail base polyiso insulation panel? The website for this product is at http://www.hunterpanels.com

As you can see I have a number of questions. Any help with the answers would be greatly appreciated.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Robert,
    If you can afford the cost, a standing-seam metal roof will last at least twice as long as an asphalt shingle roof.

    The most vulnerable part of a SIP roof is the exterior layer of OSB. Because of the possibility of OSB rot (especially in OSB roofs with air leaks at the seams), many SIP builders insist on installing ventilation channels above the OSB. The usual method is to install 2x4s on the flat, 16 inches o.c. or 24 inches o.c., to create 1.5 inch deep ventilation channels from soffit to ridge. You'll need to install a layer of plywood or OSB on the top side of this ventilation channel.

    Ventilated nailbase is certainly an option, although you should know that there are some controversial aspects to any roof assembly that involves installing a layer of rigid foam over or under a SIP roof.

    For more information on this topic, see these two articles:

    How to Make a SIP Roof Better

    How to Protect Structural Insulated Panels from Decay

  2. Nathan Kurz | | #2

    I've been researching similar issues for a SIP roofed house I'm planning to build. Noting the word 'planning' and the lack of actual experience it implies, here are some things I've found that might be useful.

    It seems universally agreed that you want to be perfectly vapor closed on the warm inside of the roof panels and vapor open on the outside to prevent condensation of inside vapor on the cold outer OSB sheathing. It also seems agreed that taping the interior seams with a high quality vapor impermeable tape is worthwhile regardless of how well you seal the seams by other means (foam or caulk).

    Beyond basic waterproofing, perfect sealing of the outside OSB seams appears to be less important. Everyone does seem to suggest that you should install your foam in two layers with non-overlapping taped seams to be as waterproof as possible. Since both the OSB and the foam are presumed to be partially vapor permeable, the vapor permeability of the tape here does not matter.

    While you want the secondary layer to be completely waterproof, there is some question as to how vapor open you want the outside to be, with the two positions being "as open as possible" or "5-20 perms to discourage solar vapor drive": https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/14145/durable-yet-breathable-roofing-underlayment.

    R-Control panels recommends the use of a particular 3/4" geotextile mat (Enkamat 7020) between their roof panels and all metal roofs: http://www.achfoam.com/media/docs/SIPs/Construction%20Detail/R-Control-SIP-Construction-Manual.pdf (search for Enkamat). I haven't found actual pricing yet, but looks to be about 50 cents per sq ft.

    You'll have an additional layer of foam in between, but might be an easier alternative to battens and plywood. There are many such products, although the rest I've seen are thinner: http://www.advancedbuildingproducts.com/residential/r-vent.cfm, http://www.benjaminobdyke.com/visitor/product/key/cedarBreather,
    http://www.keenebuilding.com/driwall-cdr-vent-complete-drained-roof.aspx

    It seems universally agreed that if using battens, you want the battens against the OSB aligned to allow easy drainage by gravity. Some but not all metal roofs can be installed over battens instead of over sheeting: http://www.recticelinsulation.be/be-fr/node/731

    Some people suggest that using single layer of battens/sleepers installed on diagonal works:
    https://www.youtube.com/watch?v=LM6zv3A4JjI https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/21016/proper-materials-moisture-control-sip-roof

    There exist horizontally notched battens: https://www.greenbuildingadvisor.com/community/forum/general-questions/25733/rainscreen-roof. This thread also casually mentions that you do _not_ want to use ACQ treated wood for your battens as it destroys steel roofs and fasteners when wet.

    Still others suggest using 2x4's flush with the second layer of foam as a nail base for the cross battens.
    http://www.danperkinsroof.com/pdf/metal-roof-venting-and-insulation.pdf
    I'd guess this isn't necessary with the thickness of foam you are planning to use.

    Like you, I'd also like to know how the depth of the gap affects ventilation. Metal Era offers an online calculator that might be useful: http://www.metalera.com/Resources.aspx (bottom right). It's primarily concerned with temperatures, but might be a good proxy for ventilation. They also offer some vents that might be useful: http://www.metalera.com/Hi-Perf/Products.aspx

    If you are mounting solar panels, metal shingles might be an interesting option, as it's possible to mount panels without needing flashing or relying on sealant: http://finemetalrooftech.com/products/metal-shingles/shingle-accessories/solar-panel-holders/. The shingles themselves seem reasonable in cost ($3/sq ft for Galvalume steel).

    Please keep posting information as you do your research. And I'd be particularly interested if anyone has experience with metal shingles or ventilation mats over SIP roofs.

  3. GBA Editor
    Martin Holladay | | #3

    Nathan,
    I see you've been studying! I admire your thorough approach to planning. Thanks for the links.

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