It's true that polyurethane and polyiso both have a higher service temp than polystryrene, and char in place rather than melting even when fully in flame.

The molten polystyrene core of the SCIP argument is a bit out to lunch. If there is sufficient heat on either side of the concrete hot enough and long to melt polystyrene through 2" of concrete, the notion that that the concrete itself would remain structurally intact, or that there's a house left to save is a dubious assertion at best, begging field verification, not a simple armchair analysis.

To be sure SCIP construction would be more ignition resistant than either a steel or OSB SIP, given the total BTUs it takes to get the concrete skins up to the kindling temp of an EPS core, which would also be starved of oxygen. If any portion of it got to that temp there would likely be some repairs in order replacing cracked concrete and melted EPS, but it would probably take a severe firestorm to do-in the whole house.

Given the long and generally good history of EPS being used for roofing insulation under black torch-down flat roofs (even in hot climates), I'm not too worried about EPS core SIP roofs having reliability issues related to operating temperature, especially on roofs with pitch higher than 2:12.

Posted By Dana1 on 16 Apr 2014 02:49 PM
It's true that polyurethane and polyiso both have a higher service temp than polystryrene, and char in place rather than melting even when fully in flame.

Given the long and generally good history of EPS being used for roofing insulation under black torch-down flat roofs (even in hot climates), I'm not too worried about EPS core SIP roofs having reliability issues related to operating temperature, especially on roofs with pitch higher than 2:12.

So in other words, the statements made by PU SIP companies about EPS SIP's is largely overstated "scare tactics" or in the sales world speak, a "pitch line." Funny how one PU company claims EPS is more damaging to the environment than polyurethane because in their words:

"Less Environmental Impact

Typically, EPS panels are created in standard sizes and cut at the jobsite to fit your home's requirements. The scraps usually cannot be recycled. In comparison, ********* panels are designed and constructed to your specific plans, so waste is minimal. The polyurethane we use acts as its own adhesive, for a strong, consistent bond across the entire panel. EPS manufacturers have to use separate glues, with the quality of the bond dependent on how evenly the glue is spread and how long it dries."

Dana, would utilizing a peel & stick Butyl roof membrane on top of the roof SIP provide a sort of "thermal break" between the SIP and the finished roof (be it standing seam metal or asphalt)?

Got a U-factor or R-value for the peel'n'stick, and it's thermal mass per unit area?

I'd be surprised if butyl membrane provided even 90 seconds of delay to ignition under standard test conditions, and under some conditions may even shorten the time to ignition given that the flash point of butyl rubber is only ~250C:

https://www.exxonmobilchemical.com/Chem-English/Files/Resources/butyl-safety.pdf

Compared to OSB's ignition point of ~200-260C: http://osbguide.tecotested.com/pdfs/en/tb115.pdf

Hardly the stuff of thermal breaks, from a fire safety point of view.

Scrap rates of EPS SIPs will vary, as will the amount of recycling that is done. The environmental hits of the disposal problems are not to be ignored- they must be weighed, but unless the PU is blown with a far more benign blowing agent, the HFC245fa elephant on the PU end probably does more lifecycle damage than a 15% scrap rate mouse on the EPS side would.

The allegation that scraps of EPS core can't be recycled simply has no basis. The material clearly can be recycled, but it's a matter of whether it actually does get recycled in places (where it isn't mandated to do so.) The cost of recycling scrap EPS exceeds the commercial value of the recovered polymer, and transporting EPS to a processing facility without compressing it first in a diesel or gas powered vehicle potentially imparts more environmental damage than disposing of it by more conventional (but still responsible) means. When the cuts & customization are happening at a factory rather than the field it's likely that some level of reclamation is more cost effective than diverting it to an industrial refuse stream, but I don't have any direct beta on that.

Dana1,
Permatherm Inc, recycles every piece of scrap, they manfacture their own foam and any cut off scrap gets recycled or reground and used into non structural EPS sheets or custom molded packaging
but for the end user like ourselves, if there are no recycle centers nearby, the EPS can be repurposed for things like arts and crafts projects for schools and churches, sold for retrofit insulation,
or an ad on craigs list for "free" EPS foam will see it quickly disappear
we also stip the metal skins and recyle for a salvage value of 8 cents a pound, not a real money maker ,but more importantly it is getting recycled and does not go into the landfill

Posted By Dana1 on 16 Apr 2014 02:49 PM

Given the long and generally good history of EPS being used for roofing insulation under black torch-down flat roofs (even in hot climates), I'm not too worried about EPS core SIP roofs having reliability issues related to operating temperature, especially on roofs with pitch higher than 2:12.

What does the roof pitch have to do with the surface temps of the roof?

Would installing a metal "cool roof" help to keep the steel SIP underneath cooler?

Venting works better with steeper roofs. Depending upon your latitude, a steeper roof may reflect more sun.

Posted By Lbear on 22 Apr 2014 12:35 AM

Posted By Dana1 on 16 Apr 2014 02:49 PM

Given the long and generally good history of EPS being used for roofing insulation under black torch-down flat roofs (even in hot climates), I'm not too worried about EPS core SIP roofs having reliability issues related to operating temperature, especially on roofs with pitch higher than 2:12.

What does the roof pitch have to do with the surface temps of the roof?

Would installing a metal "cool roof" help to keep the steel SIP underneath cooler?

Quite a bit, actually. Roofs cool primarily by re-radiating absorbed solar heat back the much cooler sky, but there is also a significant amount convected way to the adjacent ambient air.

Below 2:12 there is almost no convective cooling on the exterior since the superheated air films near the roof don't move very quickly. Above 2:12 the amount of convective cooling grows quickly & dramatically due to a bit of stack-effect type motion induced on the superheated air films up to a pitch of about 4:12, a bit more slowly after that.

Any cool-roof finish will reduce the peak temperature of the assembly, since it reflects most of the visible (and some of the IR) portion of the solar spectrum and re-radiates back at the sky pretty well in the deeper IR.