More questions about EPS vs XPS

[Crunky944]

I know XPS is generally preferred over EPS, but I live in Arizona. I can only find 1" Foamular(XPS) locally, about $22 per 4x8 sheet. I can also find 2" R-tech(EPS) for a few cents less per 4x8 sheet.

From my understanding, the XPS has a higher compressive strength, but if I used the thicker EPS stuff, I would end up with more tensile strength due to the greater spacing between the canvas skins. Is this correct?

I know I could double up the 1" foam, but I don't want to double the cost. In this situation, which foam wouldn't be considered superior?

For some background information- I am in the planning stages of a camper shell build for my pickup. I haven't drawn up any plans yet or even measured my bed. Not planning on going much taller, if at all, than the cab of the truck. I will have overhangs of 4-6" beyond the width of the bedrails. I want to do this frameless, other than where it sits on the bedrails, and will be doing PMF interior and exterior.

[pchast]

I think you need to look around a little more. Check out
commercial roofing suppliers and builders of flat roofs too.
I know. I live in the north. However, Air conditioning in your
area needs roofing insulation to reduce load.

[KTM_Guy]

Im in the east valley and have called every building supplier and roofing supply in PHX and Tucson and no luck on XPS. Home Depot to Lowers would order it in but would have to take a full unit which if I remember was 24 sheets. Most places have no idea what it even is. You might have better luck to make a few calls to Flagstaff.



I kinds gsv up on the thought of building a foamie.

Todd

[Crunky944]

Yikes, that doesn't sound promising for me. I'm in Tucson, guess I'll have to call around some and hope someone knows what I'm talking about.
I might buy a single sheet of the 2" EPS to make a test panel. Maybe I'll come up with some sort of fixture to test highway wimdloads on it

[GPW]

Cru’, we used the EPS ( bead foam ) on our Foamstream ( 3.5” thick ) and it’s been working fine for 5 years now living outside , no shelter… And yes, we’ve been down the highway … no problems at all .. Use what you can find ( for the best price ) , and when in doubt about the strength of foam , just make it thicker … Twice as thick is 8 times as strong ...

[QueticoBill]

Because EPS is used for stressed skin home construction, I am inclined - not engineering data - to believe its should be perfectly adequate for foam with PMF skins.

GPW - are you sure you don't mean 4 times as strong, at least for bending? proportional to the square of the depth, not the cube?

Having worked with both, I feel like there could be a little more waste with EPS. It seems to break a little. Would depend on edges/corners I guess, but I could imagine a few more pieces needing to be recut.

[Tigris99]

EPS would do ok but is much harder to work with because it's a bunch of foam beads basically. Same stuff as you find inside boxes when you buy new electronics and such. Really messy too. Cutting with a saw or anything but a hot blade/hot wire is going to make rough edges and have crap everywhere.

Truly 2" xps can be total overkill depending on the size your building. Like a 4x4x8 box, 1.5" xps is plenty if you doing solely foam walls with no wood involved in structural support.

Now 1" XPS alone isnt going to cut it though. Would need structural support or a second one glued to it to double the thickness. Sadly that costs more.

As for xps price. dont be shocked. it's only a few dollars more than the cost here in the north. a 2" xps board is $24-25 here. 1" is 20.

I was wanting to get my hands on 3/4 but that is proving impossible unless I want to custom order. I was considering doing a sandwiched wall design with the thinnest underlayment ply I can get my hands on then PMF over that but probably just going to go 1.5 xps with PMF (put some thin underlayment inside to protect the walls).

Sent from my SM-G950U using Tapatalk

[GPW]

Different foams require different methods …The reason both were considered and tested was sometimes you have to take what you can get , especially if you’re applying the Foamling principle of “Salvaged materials” … Our local XPS 2” is running from $28-32 /sheet depending where you get it … The EPS we used on the FoamStream was cut from huge blocks a fellow ( Boudreaux) was selling on the bayou in Lafitte… We sliced it up to 3.5” X 24” X 10’ slabs , making a well insulated and strong trailer cabin …( Quiet too ) A “Croaker” ( bayou talk for a $100.00 bill) for the whole trailer’s worth of foam , and then some ...
Note : both sand and shape well, but making different kinds of “dust” EPS makes beads which are pretty easly to clean up , light enough to get blown away . The XPS makes a sharper smaller sanding dust that works it’s way into everywhere the beads could not … Much finer dust and a dust mask is suggested… Either way is messy , one just looks worse initially …

Bill, RE: That 8 times as strong thing … Apologies if my accuracy has suffered materials drift . I was using statements made about Wood , gleened from my years as a Primitive bowyer … Foam figures may vary , but it’s really only a core for the skins which give the whole structure great strength … for little weight … After owning a basically useless RV trailer that weighed 4000 + pounds , and had almost 0 cargo weight … A < 1000 # foamie was a Joy to tow … ( not to mention < one tenth the price … and those are accurate figures … )

[QueticoBill]

I meant that strength in bending is the square of the depth whether steel, wood, or foam, that's all, not the cube.

[GPW]

QB, OK, Cool !!! Thanks !!!

[AzAv8r]

I bought Foamular 150, which is XPS, at the Home Depot on Country Club in Mesa and US-60 about 2 years ago, and they also had Foamular 250 at the time. As I recall, they only had the 250 in 2" thickness. My application was not for a foamie - I needed 1/2" thick foam, which was only available in the 150 (15 psi compressive strength). 250 (25 psi) starts at 3/4" in availability. Iirc, they did not stock the 250 sheets thinner than 2".

[Crunky944]

I bought Foamular 150, which is XPS, at the Home Depot on Country Club in Mesa and US-60 about 2 years ago, and they also had Foamular 250 at the time. As I recall, they only had the 250 in 2" thickness. My application was not for a foamie - I needed 1/2" thick foam, which was only available in the 150 (15 psi compressive strength). 250 (25 psi) starts at 3/4" in availability. Iirc, they did not stock the 250 sheets thinner than 2".

Hmm, checking their website doesn't list anything other than the 1" 150 stuff using 85210 as a zipcode. I'll have to call that store directly and see if they stock it. Thanks for the info!

Edit- Actually, googling "Home depot foamular" I can find the 2" 250... Playing around with the zipcode store finder, they stock it in Payson, Prescott Valley and Show Low, I'd imagine they'd have it further north too but I didn't bother checking. $40 per sheet in Show Low, probably similar prices at the other stores that stock it in the state. When I'm ready to build I'll have to ask my wife which town she'd like to daytrip to

[Xanthoman]

As far as the squared vs cubed strength; the equation for moment of inertia, I,which dictates bending max strength,Mc/I, is geometrically: (bh^3)/12, so yes it is cubic. But when you account for shear and other things it doesn’t translate exactly. Also, as far as sandwich structures go, a major point many here seem to forget/not fully understand is that the moment of inertia being larger is only applicable in composites as long as the interfacing layer is intact. The interface between the skins and the core/web/spacer is critical as it is in shear along the face when a moment is applied to the sandwich system. A sandwich failure is likely to fail in shear along this interface before the moment compresses or ruptures the outer skins. This is particularly true with unstable cores that utilize expanded bead foams instead of extruded bead foams. Composites are a bit more complex than the simple fat sandwich equals strong bending resistance but we get away with a lot on these small builds because of the relatively low forces and usual overbuilding that comes with not running the numbers.

I would highly recommend XPS over EPS if given the option. Also because EPS holds water like a sponge in addition to poor surface strength. The compression on it isn’t terrible, they use it under slabs just fine as far as compression ratings go, but that interface would be less than desired.

An idea for bonding panels is to use contact cement, just the cheap $10/gal weldwood stuff, but since it melts the foam you could prime the board first and then use the contact cement to bond the primer to primer. I haven’t tested this concept, but if the primer holds well to foam, and contact cement holds well to primer, it should work.

[GPW]

“ but we get away with a lot on these small builds because of the relatively low forces and usual overbuilding that comes with not running the numbers. “

Xman , Instead of "doing the math" ,we just did a lot of pre-testing , which seemed to prove it would be suitable for our low tech overbuilding … More intuition and TLAR than Science.

We do Much Prefer the Extruded foams any day , just in some areas they are difficult to find … EPS seems to be ubiquitous and cheap ! We used it on our FS build for both reasons , but realizing it’s limitations , we used it over a wooden aircraft style framing . Technically a composite , not a Pure Foamie … “ overbuilding “ as you pointed out !!!

[Aguyfromohio]

Around here the big box stores all carry extruded polystyrene foam sheets, 4 x 8, in a few thicknesses.
Lowes has green house brand; Home Depot has pink Corning Foamular; others have blue Dow StyroFoam brand.
It's really not hard to find XPS around here, although you might not find every thickness or every density in stock every day.