Couple of points:

A 2x4 is $2.89  and a 1x4 is $1.92 in my neck.  A savings on material there.  Ripping plywood to 3.5" came to $3.73.  Agree a 1x2 would be subject to splitting, not so much with a 1x4.  Lets keep the analysis as apples to apples as possible.

No need to take it personally.  You seem to think the only "proper" SIP venting detail is the one you present.  If that is the case, then my conclusion is that a SIP panel is not a cost effective or practical roof.  I think I would rather stick with some of the advanced truss and insulation schemes that will produce a comparable roof in structure, insulation, weather tightness, venting, and air sealing as your detail.  Not sure how that all stacks up cost-wise, but a technique more familiar to contractors and tradesman, and my guess, cheaper.

as far as cost effectiveness, 8 1/2 r-38 or so sips in upto 8x24 panel's cost under $5.50 a sq'.

a typical house with 1500 roof should take 4 men 4-6 hours to construct, get in place glue/foam splice etc.

What would other systems cost?

even if we but some type of batten system on, (which by the way tech support says is not needed on a roof.)
By the way vaproshield is placed directly on the OSB no felt or battens

A sips has to be very cost effective, tight highly insulated build IMO.

Posted By 3cityblue on 06 May 2015 05:40 PM

You seem to think the only "proper" SIP venting detail is the one you present.  If that is the case, then my conclusion is that a SIP panel is not a cost effective or practical roof.  I think I would rather stick with some of the advanced truss and insulation schemes that will produce a comparable roof in structure, insulation, weather tightness, venting, and air sealing as your detail.  Not sure how that all stacks up cost-wise, but a technique more familiar to contractors and tradesman, and my guess, cheaper.

It's not "my" venting detail. It was created and recommended by the greatest scientists in the building science field. They have the PhD's and did all the scientific research. So I take their word on it.

One would have to create a spreadsheet to compare the costs between a SIP roof vs a Truss system that would equal the air tightness, strength, R-Value and speed of install of a SIP. Can't neglect the labor costs. A SIP roof can be installed in 2-3 days and one has a completely sealed, sheathed, insulated and structural roof in place, along with an "instant" roof overhang/soffit in place. A truss system would take many weeks to install the trusses, roof sheathing, sealing, interior ceiling drywall, taping, attic insulation, framing out the overhangs & soffits, sheathing the soffits and fascia, etc.

Posted By gosolar on 06 May 2015 05:53 PM

By the way vaproshield is placed directly on the OSB no felt or battens

On a standard truss attic roof with OSB sheathing, Vaproshield would work. I question it's ability to work properly on a SIP roof, not a standard truss roof with OSB sheathing.

Where I think you are missing the point is that on a standard truss roof build-up the OSB roof sheathing bottom side is exposed in the attic. So if the OSB roof sheathing gets wet and moisture is trapped between the metal roof and the OSB sheating, the OSB being exposed in the attic can dry to the inside of the home. Not ideal but it can dry.

A SIP cannot dry to the interior. Not possible. So when the exterior OSB skin on a SIP gets wet, it can ONLY dry to the exterior but if you install a vapor impermeable surface on that exterior (metal roof, asphalt roof, non-permeable membrane, etc), the exterior skin of the SIP cannot dry. It will rot and fail.

One can get into the perm rating of the SIP panel, being a polyurethane closed-cell core which at 4"+ would basically be vapor impermeable or a polystyrene core at 6"+ that would basically for all intents and purposes be vapor impermeable and unable to dry to the interior.

I think OSB has a perm of 1

the membrane is breathable, it has a perm of about 220.


take a look at the picture

Posted By gosolar on 06 May 2015 06:55 PM
I think OSB has a perm of 1

the membrane is breathable, it has a perm of about 220.

Maybe I am missing something. Please explain to me how water vapor is able to mitigate through a metal roof?

I am NOT doubting the membrane is breathable. That is quite apparent. The question I have is how does it breathe through a metal roof since a metal roof has a Perm rating of ZERO?

Maybe if I ask you some questions we can get to a better understanding:

1 - Do you know that a SIP is basically vapor impermeable?
2 - Did you know that the OSB skins can only dry ONE WAY and that way is to the exposed side?
3 - If you wrapped plastic on one side of an OSB skin and then got the OSB wet. Which direction would the OSB dry to?
3 - Using the above example and you got the OSB skin wet and then wrapped it on BOTH sides with plastic. Could it dry and if so, how?

Posted By gosolar on 06 May 2015 07:15 PM

The metal will heat during the day and help evaporate moisture as well?

Yes, the metal will heat up during the day but it is IMPOSSIBLE to evaporate moisture through a metal roof which has a ZERO perm rating. Water vapor CANNOT pass or evaporate through a metal roof.

Ha maybe were both missing something, I assumed the moisture when forming droplets just rolls thu the membrane by gravity?

The metal will heat during the day and help evaporate moisture as well?

Maybe I need to call these folks again

Standing seam metal is not air tight unless you solder very seam and seal the edges to the roof deck. Water vapor CAN get out via convection, with even fairly modest vent space between the metal and the roof deck- even the full coverage mesh-type rainscreen products like John ObDyke would get you there. (John ObDyke rainscreen meshes come in both 6mm (1/4") and 10mm"(3/8") thicknesses.)

Or, 3/8" furring ripped from OSB or 1x furring works too. The fast drying rate of the vent cavity keeps the furring from rotting. It's good to put a mesh material at the edges to limit the risk of turning those empty spaces in to critter-condos. Roll ridge vent mesh commonly used under shingles is fine.) If you go to 2x furring you'd have to double-layer the ridge vent mesh stuff or use a product like Cor-A-Vent as critter-inhibitor.

Thanks Dana. I thought there might be more than one way to skin this cat. Assuming one were to use 3/8" OSB (cost is now reduced to $0.64 per 3.5" x 8' strapping) what do you suggest the stack-up be? Does 15# or 30# felt have any place next to a SIP OSB skin in this situation?

Also, any concerns with holding power of fasteners in this instance, assuming that some of the strapping is going to fall within the SIP field and thus no structural member to screw/nail in to?

Spoke to obdyke.com, they recommended none of their rain screens, they said it would compress and fail.

their recommendation was felt and strapping on sips.

-----

One of my friends has access to tons of scrap OSB 1/2" material.

So I can do the strapping for just the labor of cutting and installing.

Must admit a low cost option, and several SIPS manufactures recommend felt.

If I went with that option and placed the strapping horizontally wouldn't that have drainage issues?

I would think a 45 degree or herringbone pattern would facilitate drainage better, the roof is 9/12

Gosolar, from my point of view the primary purposes of the strapping is to provide the air gap to allow drying of the SIP OSB, and to provide a fastening surface for the plywood, barrier, metal stack-up of the top layer of roof. I would still put the strapping vertically to enhance air flow from bottom to ridge vent and to allow drainage in the rare instance that moisture accumulates under the main water shedding top roof.

Only question I have now is the holding power of OSB strapping to SIP OSB skin. If Dana provides any insight on a good stack-up and alleviates any fastener concerns, you can take it to the bank IMHO.

3city,

I just was thinking of the membrane, straps, metal, not building a deck with plywood on the straps.

Not certain Y you're suggesting that, the metal can support itself on 12-18" spacing, we have no snow, and 5-10" of rain a year

GoSolar, Your location in Georgia is fairly forgivable in that the rate of wetting is fairly low. I question your annual rain fall. Did I understand you to say 5" per year? In that range, your capacity to dry is very good given the temperatures and amount of sun. If I were building in that area with a metal roof, I would either use a multi-crimped roof (self-ventilating) over a standard synthetic underlayment such as Titanium or with a standing seam metal roof, I'd add a 3d poly weave material such as Enkamat 7020. This detail promotes drying and provides the added benefit of sound reduction.
If you were building in a harsher environment such as a zone 5 or 6 and especially in a marine area, I would strongly recommend a fully vented roof using strapping as earlier discussed. The strapping I typically use is a 1x3 (stock item at Lowes or Home Depot)
The trick is to match your systems capacity to dry with your locations capacity to get wet. It is NEVER a one size fits all detail.
As for your siding detail, a corrugated siding is a slam dunk for promoting drying (And durability) It is one of the many claddings that is referred to as "self-ventilating" IN your area, I, again, would use a synthetic underlayment.

yes not much rain 5-10", max in 24 hours 1.5 "

Posted By gosolar on 07 May 2015 05:38 PM
Spoke to obdyke.com, they recommended none of their rain screens, they said it would compress and fail.

their recommendation was felt and strapping on sips.

Furring channels should run vertically, not horizontally.

If the manufacturer recommends NONE of their rain screens, then that is your clear answer. That means if there are problems you are on your own since they stated their product will not work properly and all warranties are VOID, including the SIP warranty.

In the end, you will be holding the ball if and when anything goes bad. I am not a structural engineer nor is anyone else here who is giving you advice. If they are, then they should volunteer their engineering stamp but they won't.

My recommendations come from the Building Science corporation. They are the belt and suspender approach and scientifically proven to work. Can you find a cheaper, faster and less labor intensive approach? Possibly, but you will be the Guinea Pig and if things go bad, like I said above, you are left holding the ball and the astronomical repair bill.

FASTENERS: As far as fastening the roof to the SIP. The SIP OSB skins are usually 7/16" or barely under 1/2" in thickness. OSB has a tendency to not be too friendly to nails as they tend to pull out, especially if the OSB gets wet. Screws are the way to go but typically metal roof companies want to see 1/2" minimum on the sheathing but they prefer 5/8" thick roof sheathing. Once again, it falls back down to what the manufacturer states. Call the metal roofing company and ask them if they will warranty attachment into a 7/16" OSB panel. If they say "no" then that is the final answer.

This is what happens when poor details are used and drainage planes are ignored or improperly done:






That is why the best practice for a SIP roof is as follows:

Not pretty, the fisrt pic may be a non breathable membrane, second pic I saw before I think they said it was felt, but the failure was due to bad seams in the panels not sealing the inside air from escaping to the outside.

Maybe this type will work with less hassle of strapping battens  for air space


very little contact with any membrane plenty of air can pass.
Galvanized G60/G90,
Painted Siliconized Polyester,


 

Posted By gosolar on 08 May 2015 03:35 AM

Maybe this type will work with less hassle of strapping battens  for air space

That is an aesthetic question. In my opinion that type of metal roof belongs on a barn or shed, NOT on a home. Best check with the local HOA or ACC board if they would even allow such a roof on a home.

Not really the subject is vapor barrier not style,

this type of metal is used on new high tech home design it can be made in numerous colors.

Now does it have good air space on a sips roof?

Posted By gosolar on 08 May 2015 07:05 PM
Not really the subject is vapor barrier not style,

this type of metal is used on new high tech home design it can be made in numerous colors.

Now does it have good air space on a sips roof?

I agree, it is an aesthetic issue but as taken from an online description of that type of metal corrugated roof panels:

"The corrugated panels are generally used on commercial buildings and commonly used on roofs for barns and sheds."

Most HOA's would NOT allow a corrugated metal roof on a home. Again, if they are allowed it then becomes an aesthetic choice.


As far as functionality goes, yes, there is an air space but there is a problem since you can't leave that channel completely open. If you do, birds will make their nests in those open channels and other critters like squirrels, bats, roof rats, etc., will make their housing up in those channels and then you will have other problems. One would have to place a screen on each and every one of those open channels. That is very labor intensive and a difficult detail to do properly. It will probably cost you more labor wise to block those channels properly than it would to do a proper furred roof.

It's apparent that your budget will not allow for a properly designed roof furring detail. Why not just go with a different building product instead of OSB SIPs? Maybe a standard truss roof with blown in cellulose? That would be the least expensive and you can attain R-50 quite easily. Maybe a Steel SIP roof would be better for you? Have you looked into those options?