Calling all structural engineers
Martin Holladay
| Posted in General Questions on
I’m like the opinion of a structural engineer.
I’m reviewing a book that includes this statement: “Rigid insulation is made from plastic foam (polyurethane, isocyanurate, or polystyrene) or fiberglass, pressed or extruded into panels. It … may have some structural value when used as a continuous layer on exterior structural walls.”
My first response to this statement is: Really? I’m doubtful.
So here’s my question: Do engineers ever take into account the contribution of rigid foam insulation when making structural calculations?
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I believe closed cell foam under the roof deck can be counted on for it's hurricane roof-peeling resistance factor if the foam is at least 3" thick under Florida code (could be just scuttlebutt, but the source seemed competent, though I can't remember who made that statement to me more than 5 years ago.) That has some cred since it's adhesive characteristics to wood are similar to Gorilla Glue (tm), and it would bind the roof deck to the rafters/trusses while stiffening the roof deck, distributing the forces more evenly among the fasteners.
I can't think of a similar case to be made for sheet goods on walls (unless accompanied with the fastener spec and spacing protocol.) Sure, exterior insulating sheathing stiffens the wall up some and has a non-zero (if small) additional resistance to racking forces, but it's probably an order of magnitude behind adding another 1/4" of thickness to the structural sheathing (WAG.)
Of course by predicating it with "...may have some..." makes it inherently true. Whether the magnitude of "some" is of any significance is the only question, and it also allows the possibility of "may not".
Dana,
Yes, you're right. Just as it's true to say that insulating paint may lower your energy bills.
Yup! EXACTLY!
(And the Tooth Fairy may be getting around on a unicorn rather than using wings!)
I'll see if I can't dig up a better reference on the 3" ccSPF hurricane resilience. BASF seems to be making that case (see the statements regarding testing, p.9):
http://www.spf.basf.com/DOCS/BASF_Storm_Resistant_Construction_PPT.pdf
As does the foridadisaster.org website, under the heading "Enhancing Roof Sheathing Attachment with Closed Cell Spray Foam Insulation:" about 2/3 of the way down the page:
http://www.floridadisaster.org/hrg/content/roofs/cant_reroof.asp
But closed cell polyurethane is a foamy glue-all approach, not rigid sheets applied to studs or wood sheathing with fasteners, where the fasteners are a critical factor for any structural benefit.
Thanks a lot. I broached the subject when talking to my structural engineer this afternoon and he sounded like he now thinks I'm a complete idiot.
The structural contribution of foam insulation is VERY important in structural insulated panels AKA SIPs.
SIPs are composites where the foam is "adhered" to skins of other materials resulting in a system that has better properties ( is stronger) than either the foam or skins by them selves.
According to my engineer polyurethane (closed cell) foam does provide structural capabilities. This really comes into play with SIPs. The injected PU foam becomes the actual glue and structural part that binds the two OSB skin layers.
A polyurethane SIP is much stronger than a polystyrene SIP. In the latter they use polyurethane glue to glue the EPS core to the skins and then use a machine press to make sure the panels glue adhered properly. So in essence a very thin layer of PU glue holds the EPS core to the skins. With a PU core SIP the entire injected foam becomes the glue and structural and it runs continuous from skin to skin.
No doubt that PU SIPS are stronger than EPS SIPs. In addition, EPS will melt at 190F while PU will only char at 900F and it will not melt. That is why PU core SIPs attain a Class A Fire Rating while EPS are a Class B. In parts of Europe it is illegal to use EPS on roofs due to the hazard it poses to occupants and fire fighters when it melts.
I did sort of backyard test on an EPS SIP and I was able to pull apart the skins fairly easily once I exerted enough pressure by pulling the skins apart from the EPS glue. Once the thin layer of glue gave way the skins just peeled back like a sardine lid. Mind you I did this with my own two hands, no equipment. I tried the same thing with a PU SIP and it was impossible to break the bond since the PU foam is the glue agent and it runs from skin to skin.
Why are EPS core SIPs more popular? Simple: They are cheaper to make and require less technical & sophisticated equipment. Some people can make EPS core SIPs in their "backyard", while that would be impossible to do with PU core SIPs.
**WE KNOW ABOUT THE BLOWING AGENT ISSUE WITH PU FOAM**
From the website Dana referenced:
Spraying a layer of closed cell polyurethane based foam insulation (about 3 to 4 inches thick) to the bottom side of the roof sheathing.... the expectation is that it will provide a substantial increase, perhaps doubling, the initial uplift resistance of the roof sheathing connection.
OK -- we're straying from the topic.
The question was not about SIPs.
The question was not about spray polyurethane foam.
The question was about rigid foam panels installed in a continuous layer on the exterior side of a framed wall.
I'm not an engineer, but wouldn't the method of installation play into the conversation? If you were to glue the panels with a 100% coverage of a high strength adhesive wouldn't the 'structural' element of the foam would be different to if it were held in place by 8 screws?
Yes, with rigid foam the fastening method makes a HUGE difference whether and how much structure it adds. If you used the appropriate full coverage roller applied adhesive it effectively becomes a half-SIP, but nobody actually does that.
But I'm still betting that the Tooth Fairy DOESN'T ride a unicorn... Care to prove me wrong? :-)