I am in central florida and buiding a sfh. I am wanting to avoid interior sprayfoam and looking at ISO on roof. Plan is 5/8 plywood, Peel&stick, 2" ISO, double bubble Foil, 1X4 stringers, Exposed fastener metal roof. Should I be concerned about the multiple vapor barriers here? What is the best way to configure this? I want to have unvented attic space and plan to insulate under the deck if necessary, what would be best for that? Thanks Henry

The bubble foil stuff is worthless (as in "worth less than it costs on a performance basis"). In order for it to hit anywhere the advertised "R16-equivalent" or whatever they're claiming it has to be a dead-flat roof with an enclosed 2.5" air space cavity on BOTH sides of the bubble foil. That is exactly how you were planning to install it, right? Without both air gaps in unvented still-air cavities it's performance plummets pretty quickly to about R1.5-R2. With only one air-gap only 3/4" deep that is also vented to the exterior under the metal roofing you're looking at R2.5-ish, best case, probably only R2. You'll do better than that from a peak-cooling load performance point of view using a high SRI cool roof finish to the metal roofing, but it still wouldn't meet code.

At 2" the polyiso on it's own is nowhere near code minimum. For US climate zone 2 you need a minimum of R38 if between joists or rafters, or a calculated U-factor less than U0.030, which is the equivalent of a continuous R33. With the R value of the polyiso + roof decking you're looking at a bit less than half that. But with 2" of exterior iso you could install R23 rock wool batts between 2x6 rafters 24" o.c. under that roof deck, and with half-inch gypsum below that it would squeak by code minimums on a U-factor basis per TABLE N1102.1.3 :

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

That's probably do-able and affordable(?).

There would be no dew-point issues to worry about with that stackup. Use a standard latex paint (or nothing at all) on the interior gypsum. Avoid vapor retardent finishes like foil/vinyl wallpaper etc, or it'll create a moisture trap.

What does "sfh" mean, in 'merican dialect?

Single Family Home?

Good guess!

Why not use EPS? - more R for less $.

EPS between the rafters?

That would create a potential moisture trap at the roof deck since 5.5" of EPS is only about 0.5 perms. EPS is not cheaper per-R than R23/5.5" thick rock wool batts, nor is it higher R/inch. Rock wool's R/inch is comparable to Type-II EPS, higher than Type-I EPS, and it's completely fireproof. EPS on the interior would need either in intumescent coating or a thermal barrier against ignition to meet code.

Cutting up EPS to fit between rafters also has a high scrap rate, since it's not manufacture to fit in framing bays, and it would need to be sealed with can-foam at the edges, whereas rock wool is a friction-fit.

Polyiso runs about the same $/R as EPS, but has more R/inch- it would take 3" of EPS to achieve the same performance as the 2" High-temp derating for EPS & polyiso are about the same.

In US climate zone 2 (central FL) it's perfectly fine to install fiber insulation snugged up to the underside of the roof deck in an unvented attic as long as there is at least R5 of insulation above the roof deck. With 2" of iso it's going to be performing at about R12-R13 during the winter and shoulder seasons, maybe R10-R12 at high noon under the scorching sun in summer (depending a bit on the SRI of the metal roofing), so it'll be just fine with 2" of iso above, and 5.5" of rock wool below.

See:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

Great responses, thanks so much. So scrap the double bubble. Shouldn't I still have at least a foil facing to reflect for what it is worth? Would using 2X stringers help with that air gap enough to be worth it?. I saw where someone was running their stringers diagonally down the roof instead of horizontally, what is the purpose of this and what would be best? It seem like the vented space might die at the valleys if just horizontal. I guess there is no problem with the peel and stick under the ISO (double vapor barrier?), is this the preferred method? Thanks again for your input.

EPS between the rafters?

No, replacing the polyiso - I agree with other things (perhaps cellulose) between rafters. Around here, EPS is $1.95 per R and polyiso is $2.46 per R (both in 4x8 sheets).

Posted By bigblueearth on 14 May 2015 04:54 PM
Great responses, thanks so much. So scrap the double bubble. Shouldn't I still have at least a foil facing to reflect for what it is worth? Would using 2X stringers help with that air gap enough to be worth it?. I saw where someone was running their stringers diagonally down the roof instead of horizontally, what is the purpose of this and what would be best? It seem like the vented space might die at the valleys if just horizontal. I guess there is no problem with the peel and stick under the ISO (double vapor barrier?), is this the preferred method? Thanks again for your input.

A 1.5" air gap with only one low-E facer facing the gap adds about R1 to the performance.  Is it measurable? 

Indeed it is!

Is it "worth it"?  

Depends on just how much R1 by any other means costs relative to the up-charge of foil-faced goods (if any.)

Posted By jonr on 14 May 2015 05:17 PM

EPS between the rafters?

No, replacing the polyiso - I agree with other things (perhaps cellulose) between rafters. Around here, EPS is $1.95 per R and polyiso is $2.46 per R (both in 4x8 sheets).

Most comparisons are normalized to cents per R per square foot to get around the fact that not all insulation is sold in 4x 8' sheets (in fact MOST insulation is sold in something other than 4x8' sheets.)  Blown cellulose is typically less 5 cents/R-ft² installed,  sprayed closed cell polyurethane foam is typically more than 15 cents/R-ft2  installed.  Neither of those is commonly sold in 4 x 8' sheets.


These cost statements are for 32 square foot sheets, so make that :

EPS at 6.1 cents/ cents/R-ft²

Polyiso at 7.7 cents/R-ft²

Installation & scrappage typically raises the overall cost by something on the order of 2-4 cents/cents/R-ft^{2 ,} but it varies by thickness being applied, the scrapped fraction, and the local labor rates. For budgetary purposes ~10 cents/R-ft² isn't usually very far off. The material costs vary by a bit by local market too, and EPS costs go up a bit with density.  In practical terms the installed costs of the two products are pretty similar, if not exactly the same in every project or market. XPS is usually slightly more, about 12-13 cents/ R-ft², but that too varies.

A 6.1 cents/ cents/R-ft² price point sounds like Type-I (1.0lbs per cubic foot) density stuff, which comes in a bit under R4/inch as opposed to 1.5lb density Type-II EPS at R4.2-ish per inch.