Ok I wanted to start a thread for my roof after scouring forums I have an idea of what I want to and want to get opinions.

Chicago - Zone 5
Dew point is R-7.5 (from memory)
Cathedral Roof TJI 14" 24ioc (1.25" nailer depth, 3/8" web thickness)
Metal Roof by Berridge, standing seam concealed fasteners
code requires R38

What I am thinking to do is:

1) metal roof
2) #15 felt
3) water and ice around perimeter (required)
4) structural sheathing
5) 1.25" air gap
6) ripped 2" polyiso to 1' 11-7/8" width (1/4" wider than btwn webbings) wedged between TJIs (R10 greater than dew point control)
7) open cell spray foam 10" thick (~R35)
8) gypsum with latex paint

This is an R45 cathedral with an air vent to allow the sheathing under the metal roof to breathe but the rigid will minimize airflow draw from within the home. Will this be ok?
An alternate would be to install standing seam metal roof that is "ribbed" and just shoot the underside of the sheathing with 2" ccSF to seal it and control dew, and then use unfaced batts for the rest.

but the rigid will minimize airflow draw from within the home. Will this be ok?

How are you going to count on sealing up the edges of the rigid? This looks labor intensive.
Did you consider spraying a layer of closed cell directly on the underside of the sheathing and filling the rest of the cavity with something blown?

If i do what you suggest, if the sheathing gets wet from dew or something, how would it dry with ccSPF under it and metal roof over it?

How will it get wet from dew with metal roof and felt over it?

Yes I read an article at building science that discussed this and says the deck sheathing will never hit dew point because the temp difference is minimal...

Okay considering the fact that spraying the roof deck is trouble later (in case the sheathing needs to be replaced, etc), I am going to do this:

SS metal roof
#30 felt or synthetic
Sheathing
TJI 14" joist cavity 24" o/c
2" rigid polyiso board ripped to 1' 11-3/4" wide wedged between each TJI leaving 1-3/8" vent (R-12)
1" closed cell SPF for air seal (+R-7 for R-19)
HD batt or spider to fill the rest of the cavity which is 9-1/2" left open
gypsum

R38 is required so 38-19 = R19 batt

With the 9.5" though I can use a R30 HD batt which is only 8.25" thick for a total of R49. leaving the lower nailing span gap alone.

My closed cell (2") is sprayed right up on the interior of the sheathing.

What about future work on the roof? Do you feel that is a good long term solution?

A metal roof should be a 50 year roof.

Posted By ICFHybrid on 26 Nov 2013 06:24 PM
A metal roof should be a 50 year roof.

Maybe, unless you have a metal roof with exposed fasteners.  If that is the case and the fasteners are installed on the flat part of the metal as per the manufacturer's recommendations, then you will likely have leaks at the fasteners in 15-20 years due to thermal expansion and contraction of the sheet metal wearing and loosening the fasteners over time.  I've seen it happen on several homes and shops.  I would expect a standing seam metal roof with concealed fasteners that allow movement of the metal to perform much better, but in my experience, the cost of a standing seam metal roof is approximately 3x more (~$300/sq vs. ~$100/sq for materials).

There are hidden clips for that.

Anything done improperly will not be worth the money you pay.

Its a just in case thing. Nothing is final but it wont be a big deal to rip the rigid and give myself he vented cavityvwhich will provide a safer long term solution allowing the roof to dry more easily in case. So i'll probably do it...

Will this work ok?

Chicago - Zone 5
Dew point is R-7.5 (from memory)
Cathedral Roof TJI 14" 24ioc (1.25" nailer depth, 3/8" web thickness)
Metal Roof by Berridge, standing seam concealed fasteners
code requires R38

What I am thinking to do is:

1) metal roof
2) #15 felt
3) water and ice around perimeter (required)
4) structural sheathing
5) 1.25" air gap
6) ripped 1" EPS to 1' 11-7/8" width (1/4" wider than btwn webbings) wedged between TJIs (R4 greater than dew point control)
7) open cell spray foam 11" thick (~R40)
8) plastic membrane, gypsum with latex paint

R4 EPS with R40 ocSPF will be R44 total. The 1.25" air gap will provide drying for any moisture that gets in. The EPS allows the foam assembly to dry outward. The plastic prevents moisture from inside to get into the cavity. 1in thick EPS can be easily wedged. The 1in EPS will also provide a "backing" to spray the ocSPF to instead of the sheathing...

My other idea is basically the same as this except without the EPS, and instead run Tyvek over the TJI roof rafters before sheathing is installed. THen spray the ocSPF directly to the Tyvek/sheathing (the Tyvek will prevent the ocSPF from sticking to the sheathing)...

With EPS on the exterior, having a layer of poly on the interior adds risk (reduces drying potential and increases moisture load during the summer), with little/no gain as i can see it. what is the rational for the poly? For the most part, interior poly only has a place in northern Canada and Alaska, not climate zone 5, especially if the home is expected to ever be air conditioned.

Other than removing the poly, you might consider using blown cellulose rather than ocSPF. As the 1" of EPS is not enough for dewpoint control with that much interior insulation (you need around R-14-15ish based on the IRC table 601.3.1, assuming about 2x12 equivalent of vapor permeable insulation). Now you don't have to worry about wet sheathing rotting as there is no wood sheathing exposed to dewpoint (other than the TJI webs, which is going to be the case unless you fill the cavity 100% with ccSPF). But cellulose has a rather large capacity to take in that moisture without suffering R-value degradation. Maybe ocSPF is able to do this as well..... maybe someone else can comment on that. I would assume that blown cellulose is also less expensive than spray foam, and is almost as good at helping retard air flow. But then 1" of eps might not be stiff enough to deal with the pressure of dense packing the cellulose, ocSPF does not have the same pressure issues. Maybe 2" EPS and cellulose is cheaper than 1" EPS and ocSPF.

Things to consider at least.

Is there a reason you can't use scissor trusses or maybe parallel cord trusses. They usually work out cheaper and will give you more insulation space to work with.

SolarOH, as I understand it, 1in of EPS has a perm rating greater than 1 so it will breathe to the outside 1.25in airgap under the sheathing. The EPS is there to put a backing for the ocSPF. What are you referring to with the term "poly"? I am not using polyiso in this assembly. Are you referring to the plastic membrane? I would use the intelloplus or certainteed MemBrain product which is a "smart" plastic. The thought there is that moisture won't enter the cathedral roof assembly because it is stopped by the MemBrain hence dewpoint won't happen because there is no moisture present.

FBBP, it is TJI rafters. All cathedral. The design is final. I'm looking for an economical way to insulate this.

I read plastic membrane as poly in my mind, as in the impermeable polyethylene of old. my mistake.

The only way the "smart" membrane will offer true protection is to ensure that the ceiling plane is airtight. I know you're plan is to make the outside plane of insulation airtight with spray foam. But if that fails over time (i personally would not put stock in closed ccSPF staying adhered to the TJI's over the years of constant thermal expansion/contraction), the airtight approach at the inner plane (membrane/drywall) will act as a backup at the least. Your other posts indicate a mind towards long term reliability, and making the drywall/ membrane airtight in installation will help with that, and with little/no added materials to boot. Thermal movement will be at it's worst at the outside plane of the insulation, and at it's least at the drywall plane.

Well actually per Dana's other post in the "Retrofit, Adding Foam to exterior" topic:

http://greenbuildingtalk.com/Forums/tabid/53/aff/22/aft/82155/afv/topic/Default.aspx

He states 2in of EPS is plenty permeable, "If you added 2" of Type II EPS (1.5lbs/cubic foot density, R4.2/inch @ 75F/24C, R4.5/inch @ 40F/4.4C- increasing as temps go lower). A 2" it would have about 1.5 perms of drying capacity toward the exterior, which is plenty, and it would bring your whole-wall R up to about R23 during the winter. "

So couldn't I just use regular plastic sheeting on the interior side of the TJIs? The EPS will let the ocSPF breathe to the outside airgap. you mentioned ccSPF I plan to use ocSPF by the way. To reiterate for clarity:

1) metal roof
2) #15 felt
3) water and ice around perimeter (required)
4) structural sheathing
5) 1.25" air gap
6) ripped 1" Type2 EPS to 1' 11-7/8" width (1/4" wider than btwn webbings) wedged between TJIs (R4 greater than dew point control)
7) open cell spray foam 11" thick (~R40)
8) plastic membrane, gypsum with latex paint

Sorry on the ccSPF, it was in there on a previous stackup. But open cell foam also is at risk of separating from the osb webbing of the TJI's over time, someone with more experience please correct me if i'm wrong. The airtight drywall is a belt and suspenders anyways. Worth taking the time in my opinion, at least if it's DIY scope.

And permeable or not on the outside, polyethylene is not a good idea on the inside when you plan to air-condition the space. The smart membranes are ok, as they will let themselves dry to the inside if/when they start condensing hot humid air during the cooling season (admittedly a very small amount of condensation, as long as the air sealing of the foam holds) or if there is ever a small roof leak.

@surfsup

Take a look at the materials and methods they used in the roof design of this passive home design.
http://www.youtube.com/watch?v=2ZX4hvN5k_s

Problems with all polyethylene is they do shrink, and can separate from the framing leaving gaps. If moisture gets trapped in a wood framed home sprayed with foamed, its guaranteed to cause serious structural failure. I'm sure years into the future many of these spray foamed homes will fail in certain areas just as EIFS did in the 90's

Cellulose you would only have some settling issues, settling on Horizontal planes are not much of an issue as the vertically stacked planes. But like all building materials there are pro's and their are con's so your left choosing the less evil of the two.