Another question on the build in NM at 9000'. 9000 degree days. One section of the house will have a barrel-vaulted ceiling using curved glulams as the beams. 8-10' on center. I am using 12 1/4" SIPSs Ice and Water shield, metal roof on top. I am concerned about moisture from below and we will install by taping seams and using sealant per the manufacturers instructions. Is that sufficient? I am concerned about vapor drive through the Interior OSB, and EPS. Is this a non-issue? Or should I think of sealing the OSB to reduce permeation?

I've seen several SIPS roofs where the OSB has rotted to paste from moisture; all were, of course, shingled roofs. With a metal roof you'll see nothing until it's too late and your metal is blowing off, so If I were you I'd add venting above the SIPS and below the metal. This is usually recommended by the manufacturers but of course you may have to read the fine print to see it. On the interior, definitely tape the joints and use the sealant, and do whatever else you can do to prevent moisture from leaking into the panel. Be certain that the tape you use is long lasting which means SIGA, Tescon (both European) or 3M 8067. Not a place where you should cheap out.

Bob Originally I was thinking of creating a space with purlins run horizontally above the SIPs. But as this is a curved roof, there really is no way to effectively vent the space as far as I can see. One side of the curved roof abuts a wall, the other end could be open, but there wouldn't be any circulation. Is that sufficient? And of it is, is felt then better than Ice and Water Shield?

Don't use ice and water. Use a permeable vapor product under the metal. Check the discussion on the SIPs thread on this site. Your headed in the same direction.

1x3 spruce strapping will follow your curve, or strips of plywood if the radius is too small for the 1x3. I'd suggest two layers, cross hatched and vented wherever you can.

Bob,
Following the curve might work and cross hatching might work better, as long as being able to tie down the standing seam effectively. I'll need to apply screen at the edges as well as extend a drip edge further from the fascia than normal ( to allow airflow and yet prevent snow from blowing up there). Probably will fasten to the metal roofing rather than the decking as normally would be done.

Smartwall, I'll use felt then. It was the discussions on the site than had me thinking about my design and posting the question.

Thanks to all!

Better alternatives out there than felt. Similar to Typar but made for roofing. Sheds water but lets moisture thru. It would get rid of the necessity for the extra strapping. Most metal roofs are not that tight that moisture would be trapped, it's the vapor barrier on top and faulty seam work that seems to be the culprit. Try to get your hands on Cossella-Dorken Trella product. It's made specifically for metal roofs. It will save you a lot of time.

I wonder if something like "Slicker Classic Rainscreen" would work in place of roof strapping. Once you have air sealed the SIP joints well, very little air flow is needed to remove moisture (ie, no need for a large gap).

GAF Deck-Armor also looks reasonable as a breathable underlayment.

Would be interesting to see data on how well just breathable underlayment + a metal roof (ie, no vent of any kind) can remove moisture. Even shingles (with a breathable underlayment) allow a little bit of moisture removal.

Using petroleum products instead of renewable wood products is becoming common; maybe a better use for oil? Anyway some of these plastic rain screens do not provide a solid nailing. the one time I used this (or similar) product on cedar shingles the result was a wavy wall. Might make fastening metal roofing complicated.

That's what the Trella product does. It combines a waterproof, breathable barrier with a fiber spun matrix that lifts the metal roof off the deck to create air flow. All in one roll , problem solved. Since it supports the whole roof you won't get oil canning like you may with strapping.

Smartwall,
Just reviewed the Cosella site and the Trela product. I think it's the solution to my problem. It's insanely expensive at $2/foot, but at 20+ squares, a small addition to the overall cost of the house and should provide the security I need against rotting SIPs. Simpler to lay than a lot of strapping, and can provide weather protection for several weeks if necessary. Thanks for introducing the product to me.

Apparently the product is vapor permeable; does it also have solid dimples or are they mesh? If they're mesh, I strongly urge you to discuss this with your roofer so they are comfortable with it.

Got involved with the product 10 years ago on an icf build. The roof set up changed from a faux rubber slate product to a standing seam metal roof. The homeowner was concerned about moisture under the metal. I happened to be using Delta dimpled for the foundation. Pulled out my product book and found the Trella product. Sales rep. showed up with a sample, architect and homeowner liked it. On it went. 6000sq ft of it. If you can, get a sample. It is hard to see how it's made from a picture. It looks like a bunch of unwound plastic springs in person. It really has a lot of resistance to compression. Bob, the roof went on with no problems. The screws holding the roof down were installed with cordless drills with the torque settings dialed back.

I would think that some metal roofing is self venting. Ie, ridges, corrugation or perhaps dimples would keep the metal above the underlying OSB.

I got hold of their western sales manager today. Was quite helpful and is sending me a sample out. Their website has a number of other products that I may consider for other apps. Some similar in function to the SIGA products for sealing around penetrations. And rain screens, waterproofing materials etc.

Smartwall,
Thanks again for the tip on the Trella. Received it as well as their Peel and stick and Delta 6000 drain board for the 26' subterranean walls. I was just going to use just a waterproof membrane, but decided to add the drain board to ensure my hydrostatic pressures remain low. First level ( 14 ft) was poured last week.

A 12" thick EPS Type2 SIP is pretty much vapor impermeable through the EPS part BUT the joints and connections are always the weak link. Air can escape through the connections and condensate on the cold side of the roof and create issues. Taping interior joints is a must.

On the outside of the roof, peel & stick directly on the roof is a bad idea and then the metal roof on top adds to the already present problem.

Get a breathable membrane like SIGA Majcoat (which is vapor permeable/breathable) and put that on the OSB on the outside. Then use purlins like a 2x to create a breathable channel. Then install another layer of OSB on top of the purlins. Then install the peel & stick and metal roof on top of the OSB. This way the SIP can breathe and dry and the "sacrificial" roof can breathe from the bottom side.