Everything seems to be impossible! I have been looking for 40 minutes on line to find a supplier for 4'x8'x3" sheets of polyiso. Does someone happen to have a source for this? I am in Illinois. all the box stores have 2" max thickness

its primarily a commercial roof insulation product, so look for commercial roofing supply companies in your area. You may have to special order it. Or you can buy 2" and 1".

North Coast Supply in Glendale Heights, Ill sells Hunter Panels; they make polyiso and will make 3" in either paper faced or foil faced; paper is the default commercial product so you'll need to specify foil if that's what you want.

Atlas Roofing Corp has a polyiso insulation sales office in East Moline, Il per the following link:

http://www.atlasroofing.com/general.php?section_url=301&menu=407

Awesome thanks so much!

Bob, obviously more of my time will be required to stack/sandwich two panels instead of one, but i have two concerns with using two sheets instead of one thicker one.

I read an article recently where a guy stacked some polyiso sheets in a basement a greenbuilding talk (i think dana responded to the guy there) where he got condensation between the sheets.

I also posted an article here from the dow website that said dont use tape because the polyiso sheets shrink and expand, so with two sheets, the seam area will let air in and the dew point can be reached between the boards.

Any comments on this?

I have a closet area with a low slope shed roof where I tried three different unvented insulation system several years ago. The 2nd attempt was a 4 layers of XPS, foamed at the joints. A couple of years later I checked the roof sheathing and it was all wet, so I removed all the foam, let it dry and had it spray foamed, and installed a solid layer of foam sheet on the inside of the rafters. Not a big proponent now of using foam sheets in between-stud stackups, since i know the downsides. If you do it, I would definitely tape all the joists with SIGA tape which is flexible (so will handle the expansion and contraction) and it will have a decent chance of staying pretty airtight. But it's not an ideal solution as I fortunately discovered before my roof rotted.

Sounds like an argument for using something more breathable - perhaps 4" EPS.

Well looks like I'm back to the drawing board on all this...here's what Dana said - using 3" polyiso rigid with oc SPF under it should be fine - and what I came up with:

Quoting Dana: "You're adding complication and steps that add cost. Skip the c.c. foam, put in ~R20 iso (3-3.5"), sealed at the edges with FrothPak or can foam, and either blown fiber or o.c. foam for the rest of the cavity fill. If you're going with o.c. foam, skip the FrothPak- o.c. foam done right will provide as-good or better air sealing. You'll be at about R50 center cavity with that stackup."

OK So a summary, ZONE 5 ROOF:

SS metal roof 3:12 pitch light gray color
#30 felt or synthetic
Sheathing
TJI 14" joist cavity 24" o/c
3" rigid polyiso board ripped to 1' 11-3/4" wide wedged between each TJI leaving 1-3/8" vent (R-19.5)
6" open cell SPF (Demilec Agribalance R=4.5 per inch = R27)
gypsum

R 19.5 + R 27 = R 46.5 total

Bob and jonr, please keep in mind this is a vented roof (the rigid polyiso is between the webbing, not the nailplate of the TJIs. So there is a 1.375" gap at top for venting.

Hello to everyone on the forum, I'm new and have been reading for a few weeks. I've been very impressed with the thoughtfulness and knowledge around here and look forward to learning from everyone on the site.

Hi Surfsup,

I hope I haven't missed something, advanced apologies if I have. Reading through the thread, it looks like there are two things I could add to the conversation. You mention that this is a vented roof, but it looks very close to an unvented assembly to me. If you would consider going unvented, check out section 806.5 of the IRC for guidance. I'm a fan of unvented assemblies (if done right) because they are simple. Also, Building science has an excellent article called "rocks don't burn" that discusses the merits of rock wool. I like rock wool for the non-shrinking, fireproof, non-toxic benefits. You can get it at home depot or lowes on special order. Good Luck!

Laurie
modernhouse-plans.com
architect/LEED AP BD+C/certified passive house consultant

Consider taped EPS under the joists and damp sprayed cellulose between the joists. Simpler, less expensive and it breathes well.

I'm interested in hearing more about Bob's case. I assume that air was leaking past the spray foam? Not surprising for a thin gap between wood and foam.

Laurie, there is a 1.5" vented channel. I am leaning to this method because an unvented roof, if there are problems, will require a lot of expense and work to fix with sheathing attached to blown in insulation.

Jonr, if i use EPS, i gather i need a vapor barrier at the inside under the gypsum stapled to the inside of the TJIs, yes?

I think a single sheet of 3" poltiso ripped to 24" wide will still be easiest and most effective...

if i use EPS, i gather i need a vapor barrier at the inside under the gypsum stapled to the inside of the TJIs, yes?

Most people find interior EPS+paint enough of a vapor retarder with cellulose. But code may say otherwise.

I don't know if you completed this project (probably) but you can get Rmax polyiso foil faced sheets via the Home Depot Pro Desk.  I priced them a few months back.  There was a slight premium per inch for the 3" product but it would certainly be exceeded by fastener/adhesive costs, let alone labor.  It isn't something that is stocked in plentiful quantities so you might have a little lead time.  In my case, I could have had ~10 sheets within the week but more would have required a longer lead time.

Hope that helps.

Posted By Surfsup on 07 Dec 2013 04:42 PM
Laurie, there is a 1.5" vented channel. I am leaning to this method because an unvented roof, if there are problems, will require a lot of expense and work to fix with sheathing attached to blown in insulation.

Jonr, if i use EPS, i gather i need a vapor barrier at the inside under the gypsum stapled to the inside of the TJIs, yes?

I think a single sheet of 3" poltiso ripped to 24" wide will still be easiest and most effective...

No, you don't need a vapor barrier. In a fully vented assembly (soffit to ridge on every bay) you don't need a vapor barrier for zone 5. 

Foil faced polyiso IS a vapor barrier (the foil is a Class-I vapor retarder.)  Almost all fiber-facers on polyiso are also class-II vapor retarders (< 1.0 perms) and would meet code in assemblies where interior vapor retarders would be required.

At 3" thickness or higher unfaced Type-II EPS is a Class-II vapor retarder.

In all roof & wall assemblies air tightness to the interior is more important than vapor retardency.  If using 4x8 sheet EPS or fiber-faced polyiso, seal the seams of the foam with acoustic sealant or duct mastic, and stagger the seams of the interior side gypsum with  those of the foam.

I am about to break ground so I have 10weeks to figure this out. My newest plan of attackis this.

Metal roof
Roof sheathing
TJI rafter 14"
1/4 in plywood ripped to be glued/nailed btwn the TJI webbing at bottom of top plate
Blown in applegate cellulose approx 12in thick
Gypsum

The plywood is easy to cut and place inside the TJIs to form a top vented channel 1-3/8" high (thickness of the top plate). It also provides a backing plate to blow in the applegate cellulose. Applegate recommends no vapor barrier with this application. I can cut the plywood and since it is 1/4 thin, it's easier to place up in there and caulk/screw in. I believe the plywood would be a good, cost effective solution to still get me the vented channel for future moisture issues if they arise without having insulation blown directly to the underside of the roof sheathing. I'll still get 12" of insulation. At 3.8R/in that's an R45 installation with the cathedral ceiling.