Dear Forum Members,

I could use some more advise than I've read and acquired from talking to two energy auditors and four Sw Florida Insulation Installers.

I'm building an ICF home (I've hired a GC whose build ICF homes since 1992), in SW Florida.  It will be approx 1600 sq ft under air.  The initial plans are back from the drafter and the GC has contacted the local A/C calculations specialist (this is all this company does is engineer A/C layouts).  They expect a little more than 1.5 tons..but will know more when the final plans with all window and door specs are known.

The roof will be a metal roof on a peel & stick film that's on 3/4" plywood.  I want to seal the attic, so I am going with CCSPF.  Now, here's where it's getting confusing.

I'm not made of $$ and on somewhat of a budget..in that I want to evaluate the ROI on any added item to see if it is justified or will be monetarily proper.  I've read that teh code requires R38 in the attic in zone 2 (SW Florida).  The DOE recommends R30-R60.  I've also heard that in Florida, since the delta between outside average temperature and interior temperature is only about 30 degree's at most, that insulation R-Values are not as important as in more Northern climates where the Delta could be 70+ degrees.

The two energy auditors and four SW Florida insulation company owners all say 3 inches of CCSF in the attic under the roof deck is all that is needed.  That anything else is overkill...ROI on A/C electricity wFill be longer than I will be alive.  That the current R-Value codes are based on fiberglass and do not take into effect the sealed attic concept with the near zero air transfer.

My question is...in a nutshell..are they correct?  Am I wasting my time looking at blowing cellulose overtop the 3 inches of CCSPF?  Will the cost outweigh the benefit?
What would be a cheaper option if needed?

Any direction and clarification would be beneficial

Thanks

I would be concerned with trapping moisture in your roof decking by putting peel-and-stick on the top side and closed cell spray foam on the underside. This can lead to rotting of the wood decking if any moisture gets in there. Every screw that holds that metal roofing on will be penetrating the top membrane providing a potential leak path.

Here in central Arkansas, the cost to blow R38 loose fill cellulose (~11" settled depth) in the attic is ~$1/sq ft. That is about the same cost as spraying 1" (~R6) of cc spray foam. Here, I could blow in R100+ cellulose for what it would cost to spray 3" (~R18) closed cell spray foam. Just be aware that loose fill cellulose at anything >R50 will require consideration of the weight and possible modifications to the ceiling / roof assembly to accommodate the extra weight.

Another thing. 3" of ccSPF on your roof deck won't provide much if any insulation over the wood truss/rafter members. The rafters then provide a thermal short circuit from the hot roof deck directly to the conditioned space, lowering your overall effective R value to something less than R18 (probably closer to R15-R16).

Also note that the temperature of your roof deck can far exceed the ambient temperature on a hot sunny day, depending on the color of roof material you choose. A darker roof can easily exceed 160 deg F on a hot sunny day. With spray foam at the rafter level, you might have an 80 - 90 deg delta T across that insulation. Even a lighter colored roof material will likely be 20 - 30 F higher temperature than ambient in full sun on a hot day which would result in a delta T across of the insulation of 50 - 60F, so the theory that a Florida home doesn't need as much R value in the roof/attic as a northern home because of lower delta T (a theory likely advocated by spray foam installers) doesn't really pan out.

If you are on a budget, why are you considering the highest $/R type of insulation (ccSPF)? Is your HVAC air handler and duct work in the attic? That is the only beneficial reason I can find for spending way more $ to insulate the roof deck. If you move your air handler and duct work into the conditioned space and air seal penetrations at the ceiling plane with canned foam, you don't really gain any benefit by insulating the roof deck and can save ~75% or more on the cost of your roof insulation by using cellulose rather than ccSPF.

Arkie,

Thanks for the rapid response. I'm considering CCSPF for a few reasons. One, the air seal to assist in comfort and lower utility bills. Two, yes, my HVAC unit will be in the attic..along with all ductwork. Lastly, to strengthen the roof for hurricanes. I am in SW Florida, near the Gulf.

I did contact Oak Ridge, a gentleman there did advise me that R value is R value..no matter what auditors or installers have advised me. If the code is R38...then R38 is what I should attempt to achieve. I do not understand how builders are then allowing 6.5 inches of OCSPF or 3 inches of CCSPF if it will not meet code?? Based on my research that seems to be a coke vs Pepsi ... Air seal outweighs R Value in importance?

So, would it be less expensive..and approximate cost reduction to go in and place 4 inches (two sheets of XPS blue foam board) against the underside of roofdeck, then have SprayFoam installer spray the 3 inches of CCSPF ?. Based off my research....CCSPF should be valued at R5.6 per inch....since the gases that are in the bubbles overtime may escape and be replaced by air...slightly diminishing the R-value. This would give me about 7 inches at 5.6 per inch or R39.

Thanks

Rigid foam makes more sense to me. You don't need CCSPF to get a great air seal.

In areas where R38 is code for attics that is usually relaxed to R30 when the insulation is at the roof deck, sometimes referred to as "compact roof" in codes. Adding only 3" of cellulose to 3" of ccSPF is still only about R30.

For it to be structural the ccSPF needs to be 3" which gives you at ~R18-R19. To bring that up to R38 you could put 3-1/4" of rigid iso on the exterior of your peel & stick underlayment, or you could use fiber on the interior (which can get more complicated). Or if you only need R30 you can get there with 2" of exterior iso.

Alternatively if you can forego the additional structural rigidity of 3" ccSPF, you could do a flash-foam/spot-foam air sealing on the interior, a membrane over the structural deck, and put 6.5" of iso above that (R38) or 5" (R30), in two taped-seam layers with the seams staggered. If you want to do it on the cheap, use reclaimed roofing iso from commercial re-roofing. (Try insulationdepot.com if you can't find a local source.) Material cost-wise reclaimed foam can be cheaper than batts in $/R-ft. but the installation is a bit more complicated. With all of the R outside the structural deck an R30 roof will outperform R38 between rafters or joists by a good margin, since the thermal bridging is minimized.

To minimize thermal bridging with an over-foam job strap it down with 2x purlins through-screwed to the structural roof at the trusses with pancake head timber screws (eg FastenMaster Headlok, which is carried by box stores at least in a few lengths) 24" o.c. (or 16" o.c.) if required for hurricane-resistance), and mount the metal roofing to the purlins. You'll need at least 3/4" penetration into something structural, and 1.5" isn't too much in hurricane country. If the roof deck meets hurricane fastener spacing & type code the thickness of the roof deck counts for part of that structural depth on the timber screws.

It's not true that "three inches is all you need"- it would in fact be a code violation. But it IS true that at over $1/board-foot three inches is at or beyond the limit of any rational economic rationale in that climate from strictly an energy savings point of view. If you're willing to pay for the structural rigidity that's something else, but taking it up R30 or more may still be economic using cheap-stuff for the rest of it, even if it isn't with closed-cell foam.

"Air seal outweighs R Value in importance?"
You don't need CCSF to get a good air seal, but it helps. Adding a good tape helps even more.
The envelope needs to be essentially airtight. If you go with sheet foam, be sure to seal it well around the edges. I tried using sheet foam between rafters, sealed it carefully with one part spray foam, (no tape) removed a section it a few years later and found condensation, meaning that air had gotten past the foam to the roof deck. Then I pulled it all down & had the area sprayed with CCSF.

Posted By Dana1 on 04 Oct 2012 05:56 PM

In areas where R38 is code for attics that is usually relaxed to R30 when the insulation is at the roof deck, sometimes referred to as "compact roof" in codes. Adding only 3" of cellulose to 3" of ccSPF is still only about R30.

>> Can cellulose be added directly to the underside of the 3" is ccSPF?...will it stick and not fall down?

For it to be structural the ccSPF needs to be 3" which gives you at ~R18-R19. To bring that up to R38 you could put 3-1/4" of rigid iso on the exterior of your peel & stick underlayment, or you could use fiber on the interior (which can get more complicated). Or if you only need R30 you can get there with 2" of exterior iso.

>> What about what was stated earlier..about if I use ccSPF under the 3/4 " plywood roof desk and then put a peel and stick membrane on top..then put another 2-3" of rigid foam on top...the plywood could rot..since it cannot dry to inside or outside?...essentially that configuration would be a built on site SIP roof would it not?  Would SIP panels be more economical to purchase and use for the roof?  If so..could I STILL spray the 3" of ccSPF on the underside to aid in R value and more importantly increase racking strength?

Alternatively if you can forego the additional structural rigidity of 3" ccSPF, you could do a flash-foam/spot-foam air sealing on the interior, a membrane over the structural deck, and put 6.5" of iso above that (R38) or 5" (R30), in two taped-seam layers with the seams staggered. If you want to do it on the cheap, use reclaimed roofing iso from commercial re-roofing. (Try insulationdepot.com if you can't find a local source.) Material cost-wise reclaimed foam can be cheaper than batts in $/R-ft. but the installation is a bit more complicated. With all of the R outside the structural deck an R30 roof will outperform R38 between rafters or joists by a good margin, since the thermal bridging is minimized.

>>Thanks for the link..never knew of that site.  Drying concerns for decking?

>>What about using a Zip system roof decking..that would not need to dry...then put rigid foam on top and ccSPF under decking?..then I assume I would need 1/2 plywood on top of >>rigid foam to attache cool metal roofing to?

It's not true that "three inches is all you need"- it would in fact be a code violation. But it IS true that at over $1/board-foot three inches is at or beyond the limit of any rational economic rationale in that climate from strictly an energy savings point of view. If you're willing to pay for the structural rigidity that's something else, but taking it up R30 or more may still be economic using cheap-stuff for the rest of it, even if it isn't with closed-cell foam.

>>I read an article (http://www.fsec.ucf.edu/en/publications/html/fsec-cr-1220-00/) , where R19 (3' of ccSPF or 5.5" of ocSPF) under roof decking was still more efficient than R19 >>fiberglass in an vented configuration (on attic floor).  Would assume that if you were to combine a cool white reflective roof WITH R19 ccSPF under decking..the efficiency would even >>have been better in that test.

Cellulose won't stick to the foam, it has to be blown in netting which makes it more labor intensive. At 3"+ the cellulose itself becomes a code-compliant ignition barrier for the foam though, eliminating the need for intumescent paints or gypsum, etc.

At 3" generic closed cell foam is ~0.3=0.4 perms, only slightly more vapor retardent than a kraft facer on a batt- the roof deck can still dry toward the interior.  If you want even more drying capacity, use 4" of Icynene MD-R-200, which is a 2lb closed cell foam that would be about 1-perm @ 4", and about R20.  (At 3" it's 1.3perms, R15.)  Another aspect of MD-R-200 is that it's blowing agent is water, not HFC245fa.  In your climate the greenhouse gas potential of the HFC blowing agents use exceeds the greenhouse potential that of the energy savings it offsets over it's lifecycle at any thickness over 1 to 1.5", making the water-blown goods far more environmentally benign.  (Most open cell foam is also water-blown.)

For exterior foam you can make it somewhat like a site-built-SIP, but rather than exterior skin, use 2x furring.  That way the foam will always be able to dry toward the exterior. (Polyiso is somewhat hygroscopic.)

R19 fiberglass without a top side air barrier is the 3-legged mangy dog with fleas imitation of a straw-man of the insulation industry.  Being highly air permeable and translucent to infra-red radiation, just about anything can beat it on performance, particularly in a no-topside-air-barrier configuration as is commonly done.  I don't get all gooey inside when someone points out that an air-impermeable  R19 insulation beats it- even higher density fiber beats it (and soundly!).  Foam also passes some amount of IR in the relevant spectrum, but not nearly as much as low density fiberglass and it isn't compromised by convection & infiltration currents.   The other important aspect of the FSEC article is that the ducts are in the attic, which puts them outside the thermal & pressure boundary of the building with R19 on the floor, but fully inside with the R19 (any type) at the roof deck, which has a HUGE impact on system efficiency.

But yes, a cool roof exterior + R19 ccSPF under the roof deck will outperform R19ccSPF with a more absorptive roof during the cooling season, but slightly underperform it during the (less consequential in your climate) heating season.

If you want the structural benefits of the ccSPF, go for it, but adding cheaper insulation beyond the ~R20 can still be cost effective.  Air tightness can be acheived more cheaply by other methods, as can total R, and taking it to R40 or even R50 with cheap stuff isn't necessarily a stupid investment even if the financial payoff is very long-term.  Even with a cool-roof you'd be able to measure and likely feel the difference in your peak ceiling temperatures going from R20 between the rafter elements to R40, especially with R20 outside the rafters as a thermal break.  I've been able to buy reclaimed 3"  or 3.25" roofing iso for anywhere from $12-20/sheet for 4x8s.  That's about R18-20 at 38-63 cents a square foot, compared to $3/ft for ccSPF. Yes there are other material and labor costs for the sheet iso as well as some amount of scrappage, but you probably won't hit more than a buck-fifty/ft -  about half the cost of ccSPF for "the rest of it" taking it up to R30 and beyond.  (On the low end of the price range reclaimed iso has been R/R cost competitive with R19 batts.)

Posted By ArmyChief on 04 Oct 2012 10:24 AM

So, would it be less expensive..and approximate cost reduction to go in and place 4 inches (two sheets of XPS blue foam board) against the underside of roofdeck, then have SprayFoam installer spray the 3 inches of CCSPF ?.

If you install the rigid foam against the underside of the roof deck and then spray cc foam over that, you loose the benefit of the added structural strength provided by the cc SPF.

If you want additional structural strength for your roof deck, have you considered having the roof decking screwed and glued down to the trusses with polyurethane construction adhesive similar to the way floor decking is installed?  If not screws then at least ring shank or spiral shank nails?  The additional cost for having the roofing installed that way is probably on the order of several hundred dollars to maybe $1000 more than standard nailing.  That would be ~1/10 the cost of having ccSPF sprayed on the roof deck and it would be stronger.

Posted By Dana1 on 05 Oct 2012 02:25 PM
Cellulose won't stick to the foam, it has to be blown in netting which makes it more labor intensive. At 3"+ the cellulose itself becomes a code-compliant ignition barrier for the foam though, eliminating the need for intumescent paints or gypsum, etc.

>> Still might be an option..depending on how much it cost versus the other few alternatives.

At 3" generic closed cell foam is ~0.3=0.4 perms, only slightly more vapor retardent than a kraft facer on a batt- the roof deck can still dry toward the interior.  If you want even more drying capacity, use 4" of Icynene MD-R-200, which is a 2lb closed cell foam that would be about 1-perm @ 4", and about R20.  (At 3" it's 1.3perms, R15.)  Another aspect of MD-R-200 is that it's blowing agent is water, not HFC245fa.  In your climate the greenhouse gas potential of the HFC blowing agents use exceeds the greenhouse potential that of the energy savings it offsets over it's lifecycle at any thickness over 1 to 1.5", making the water-blown goods far more environmentally benign.  (Most open cell foam is also water-blown.)

>> Not sure of the two insulation contractors my GC likes to use for these type projects uses...I will find out when the final plans are submitted for bids.

For exterior foam you can make it somewhat like a site-built-SIP, but rather than exterior skin, use 2x furring.  That way the foam will always be able to dry toward the exterior. (Polyiso is somewhat hygroscopic.)

>>If I use XPS or EPS..do I need to use furring strips?  Can I screw the cool metal roof directly on top the foam into the joists?  Also, I might use Zip Systems roof decking..which I >>believe does not need to dry like OSB or plywood might.  If I should use furring strip, instead of penetrating the roof decking by nailing the furring strips...could I glue the strips to the >>foam..then screw the metal roof directly through the roof, furring strips to joist?

If you want the structural benefits of the ccSPF, go for it, but adding cheaper insulation beyond the ~R20 can still be cost effective.  Air tightness can be acheived more cheaply by other methods, as can total R, and taking it to R40 or even R50 with cheap stuff isn't necessarily a stupid investment even if the financial payoff is very long-term.  Even with a cool-roof you'd be able to measure and likely feel the difference in your peak ceiling temperatures going from R20 between the rafter elements to R40, especially with R20 outside the rafters as a thermal break.  I've been able to buy reclaimed 3"  or 3.25" roofing iso for anywhere from $12-20/sheet for 4x8s.  That's about R18-20 at 38-63 cents a square foot, compared to $3/ft for ccSPF. Yes there are other material and labor costs for the sheet iso as well as some amount of scrappage, but you probably won't hit more than a buck-fifty/ft -  about half the cost of ccSPF for "the rest of it" taking it up to R30 and beyond.  (On the low end of the price range reclaimed iso has been R/R cost competitive with R19 batts.)

>>I priced 3.5" EPS from that site you posted..it's about $2800 + freight.  I'm sure that truckload/pallets is more than I need also (have to wait for final plans to prepare roof area estimate).  Lastly, I talked to my CG about foam on exterior of decking.  He says it will allow the metal roof to dent to easy and anything hitting the roof will cause denting.  Also, if you need to walk on the roof, it makes it very hard.  Any truth to that?

arkie,

Yea, thought of the loss of racking strength.  I will ask metal roofing contractor what they typically use.

Thanks.

What if I foamed in place 3.5 inch sheets of EPS/XPS to the 3" of ccSPF after they do the underside of the roof.  As funds or used/cheaper foam becomes available?

I wouldn't be surprised if someone, somewhere has come up with a way to add enough glue to cellulose to get it to stick as well as spray foam or shotcrete.

Won't wet sprayed fiberglass stick to spray foam?

Posted By jonr on 06 Oct 2012 05:03 PM
I wouldn't be surprised if someone, somewhere has come up with a way to add enough glue to cellulose to get it to stuck as well as spray foam or shotcrete.

Maybe someone here knows..or I can try calling insulator contractors.

Just another possibility to throw out. What about getting rid of the metal roof and doing a cool spray foam roof?

"If I use XPS or EPS..do I need to use furring strips?  Can I screw the cool metal roof directly on top the foam into the joists?"

That would be 10x as many fasteners and the hurricane would STILL blow the foam and metal roof away.  By using panCAKE head timber screws and furring on a wider spacing you get less thermal bridging.  If you don't want to use furring you could use a OSB/ply screwed down on 24" o.c. spacing with pancake head timber screws, but that's more expensive than furring. (Stagger the nailer-deck seams with that of the foam for better air tightness if you go that route.)  The thermal conductivity of steel is more than 100x that of foam insulation, and putting in lots of fasteners degrades performance significantly.

"I might use Zip Systems roof decking..which I >>believe does not need to dry like OSB or plywood might.  If I should use furring strip, instead of penetrating the roof decking by nailing the furring strips...could I glue the strips to the foam..then screw the metal roof directly through the roof, furring strips to joist?"

Rigid foam glued to the roof deck is not structural- no way would that meet code (and it would disappear fast in a hurricane!)

" He says it will allow the metal roof to dent to easy and anything hitting the roof will cause denting.  Also, if you need to walk on the roof, it makes it very hard.  Any truth to that?"

Depends on the roofing, but in general, yes, it can be an issue if mounted on purlins.  If you use the OSB or plywood nailer deck option that problem goes away. Use a good well-draining/aerating underlayment between the metal and nailer deck though.

I can't say I followed why screwing a metal roof through foam and into joists requires more fasteners or is less strong that adding furring.

Posted By jonr on 09 Oct 2012 08:25 PM
I can't say I followed why screwing a metal roof through foam and into joists requires more fasteners or is less strong that adding furring.

Pancake head timber screws have a huge head and can take a lot more load than any fastener appropriate for most metal roofing. 

If you had metal roofing of sufficiently heavy gauge to not need the tighter spacing you'd have zero concerns about denting while walking on it.

AFAIK, A screwed to B screwed to C can't be stronger than A screwed to C. But OK, lots of smaller fasteners on the metal and fewer larger ones for furring to joists has some logic.

Example of cellulose being sprayed on a ceiling.

http://www.youtube.com/watch?v=jUqlMsBFo34
http://www.youtube.com/watch?v=SndVcTXUXP4

The tearing strength of the metal roofing is very low compared to wood at any given fastener head size- the manufacturers usually spec both a fastener and a fastener spacing.

Pancake head screws through wood materials have the same or better retention at much wider spacing than any thin metal roofing, leading much reduced thermal bridging of the foam.

I've not seen cellulose sprayed directly on ceilings/rafters like that before- even wet-sprayed I've only seen it installed in netting when the slope is anything but vertical. It sticks to wood & paper facers pretty well, but I've never done any stress-testing on the stuff after the fact. I'd be a bit leery about how well it would stick to ccSPF without some assurances from at least an experienced installer.