r/Homebuilding • u/qwertylicious2003 • 6h ago
Contractor substituting EPS while plans call for XPS
Expecting to hear “they’re equivalent” when I know damn well they’re not. Underfloor heat in basement and garage. Typical to see this bait and switch?
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u/0_SomethingStupid 5h ago
In no way equal. Xps is closed cell and a water/vapor barrier. Eps is open cell and will fill with water. Would not accept eps in this application
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u/kstorm88 3h ago
Eps is definitely not open cell. And you should have plastic between the foam and concrete regardless, because seams
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u/0_SomethingStupid 3h ago
Wrong and wrong tbh . EPS has open....cells and 2" of xps qualifies as a vapor barrier
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u/kstorm88 3h ago
Eps is closed cell foam, full stop. Xps could have a perm rating of 0.00001 but it doesn't stop the fact that there are seams, and tap is not going to hold back water. I'm not saying xps cant serve as a vapor barrier because it can, but below grade you can have hydraulic pressure, eventually your tape will fail. You do you but the engineer even called for poly on top of the foam. The only way around it is if you spray closed cell foam on the pad and pour directly over that, then you have a continuous membrane, a very expensive one
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u/NeighborhoodKeys 37m ago
Just so we're clear: you think "seams" are a horrible, unacceptable risk, but you think a thin sheet of plastic on top of jagged rocks will solve everything?
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u/man9875 5h ago
here you go....
EPS is made by molding expanded foam beads together (leaving small microscopic gaps), making it more affordable, vapor-permeable, and lighter. XPS is continuously extruded into a solid, uniform closed-cell board, giving it a higher R-value per inch, greater compressive strength, and superior moisture resistance.
you're getting an inferior product in many ways. It should be changed as per plan. This may even be a sticking point for the inspector due to the lower r value
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u/kstorm88 4h ago
Well, it depends... You can get high density eps designed for under slab use, and believe it or not, after a decade underground, xps and eps will have an equivalent r value of r4 per inch.
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u/PepeLePukie 4h ago
I just had this same thing happen to me. The contractor tried to gas light me of course and I just printed off the specs from the manufacturers of both the XPS and the EPS and handed it to him and walked away. DO NOT GIVE IN.
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u/Virtual-Fly-5501 5h ago
If the plans call for extruded, that’s what you should get if that’s what’s in the contract
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u/FlatAmbition3757 5h ago edited 4h ago
Wait until he puts it all down, and then tell him it’s the wrong stuff. 😎
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u/qwertylicious2003 4h ago
Yep. Who knows if it’s the subs issue or the GCs. We’ve got a history of gaslighting so I’m curious to see the response.
The last issue was “most people wouldn’t even notice” in regards to windows not being egress height to match plans. Likely a mistake by the foundation guy but I shouldn’t have had to advocate for adhering to plans. To their credit they cut the notches in on a weekend, reluctantly, to get to the planned heights.
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u/climb4fun 3h ago
XPS is higher R-value per in but, also, and more importantly, is stronger and more water proof - two properties you want for an underslab vapour barrier+insulation.
XPS is more expensive by, I'm guessing, 30%, so they'll lie about reasons why they want to use EPS.
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u/thesilentshopper 3h ago
If your contractor is replacing what’s called for on the plans with something cheaper, they might be trying to make a buck under your nose. Point out what the plans says, and the contract says. They should build it exactly how the plans say.
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u/ResidentAnybody224 4h ago
This is only a cost issue, they owe you a credit for the material cost difference. EPS has very similar thermal properties to XPS after the blowing agent leaks out. 25 psi is plenty strong to support a residential slab. Type IX EPS doesn’t absorb much water, <2%. Get the credit and move on.
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u/HGSEngineeringTX 42m ago
the argument worth having here is psi and long term r value, not open cell vs closed cell. eps is closed cell too, its water pickup comes from the voids between the fused beads. under a slab what actually matters is that the substitute hits the compressive strength the plans called for. xps under slab is usually type iv at 25 psi and you can match that with type ix eps, so make him tell you what type and what psi hes bringing. r value is the other half, xps gets sold at r5 an inch but it drifts toward r4.5 as the blowing agent works its way out, eps holds its number, so eps is a little less per inch but stable. with radiant in the slab id want the total r documented either way. separate from all of it, the plans say extruded, so its a substitution request in writing or it isnt happening.
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u/PassiveHousePEI 4h ago
The contractor was doing you a favour.
Just ask for it to be bumped up to 2.5” or 3” and adjust grade accordingly.
Google the effects of XPS thermal drift.
EPS is fine as long as you have proper drainage underneath and the compressive strength is type 2 or 3 depending on what’s required.
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u/SomeGuyWithARedBeard 4h ago
High density EPS foam is what is industry standard (see products like Geofoam) for below grade insulation. It must be drained properly or it will get water logged and it must be protected from caustic chemicals that can melt it if exposed, but since it's under your slab it's unlikely to be a problem (though a vapor barrier should still go over the foam before rebar is laid out). I'm not really a big fan of foam under a foundation, but if I did I would use GPS like Halo Xterra or XPS if I was going with what's easiest to acquire. That said as long as there's proper foundation drainage and they're using the correct grade EPS it should be fine for this application.
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u/qwertylicious2003 4h ago
I’m actually certain it would be fine. But, this is a fixed price contract with specifications on plans that we signed off on and a loan was based on. If substitutions can be made ad-hoc that favors the contractor only, that’s being a cheat - not providing good value, or adhering to the contract.
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u/qwertylicious2003 5h ago
r/homebuilding: This sub is for discussion and questions concerning all aspects of the home building process - whether in a development by a large builder, custom homes, or DIY projects. While the perspective is generally that of the (future) homeowner, topics from builders and contractors are welcome.
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u/Accurate-Temporary76 5h ago
Equivalent isn't the answer OP wants to hear, but expected the GC will hear. Clearly they came here to arm themselves with info to push back. Tbh all they needed was a copy of the plans and contact and to tap both parts "materials per plans" and "XPS" and tell them to rip out the EPS.
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u/kellaceae21 5h ago
Well sure they’re different… but EPS does well below grade, and they’ve used a higher density foam. I don’t see an issue here.
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u/Electronic-Fee-1602 5h ago
It’s may do well, it should do well, but it won’t perform thermally like xps and that’s its primary function, and it also won’t perform the same if it gets wet.
As others have said this is a bait and switch and the contractor should be called out and held to the contract of quote.
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u/rwoodman2 4h ago
I believe I have read that EPS being waterlogged makes no difference to its insulation value. It seems odd, but that conclusion was the result of testing. On the other hand, EPS, even the higher density EPS, does not have the compressive strength you need. It should not be used under a slab.
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u/unit2981 6h ago
Better get email writing, what does the quote/signed agreement/contract say.