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For a design project, we're looking to use XPS foam boards that are 5cm thick (15 cm wide strips) to support the underside of a bath without legs or wall fixtures, but a supportive frame at each end.

The bath is steel enamel with a slightly curved underside and will be supported by 4 layers of XPS.

Will the XPS foam be able to withstand this kind of pressure with regular use?

Intuitively, we were thinking several thicker boards would be superior in terms of pressure resistance rather than many thinner boards, but please advise otherwise if this is incorrect.

  • none that I'm aware of. You need something that isn't going to compress. – noybman Nov 9 '19 at 8:59
  • Isn't XPS foam rated very highly for pressure resistance? I've seen demonstrations where it can bear loads of up to 30 kg per square inch with minimal compression. – Market12 Nov 9 '19 at 10:02
  • What are the dimensions of the bath? – Kevin McKenzie Nov 9 '19 at 14:06
  • The bath is pretty standard for a steel enamel type. About 1750 x 700 cm. The underside is curved, but fairly flat for much (>60%) of the surface area. – Market12 Nov 11 '19 at 18:25
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It depends on what you mean by "support". It will hold the tub up but it will flex. The movement of the flexing may loosen plumbing connections ; not the first time , but movement over an extended time could work things loose. It may even cause leaks at what ever caulk or trim is used on the top edge of the tub.The weigt of 6" water in a 5' tub is roughly 300 lb. plus occupant(s).

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It depends a great deal on the particular foam. XPS is available in many compressive strengths, though probably you only get one option at mass market home stores. That option would appear to be 25 Pounds per Square Inch (PSI), based on some quick product research. So a 500 lb tub+occupant would be held up by 20 square inches of that foam, if the load was applied evenly (i.e. the foam must fit the bottom of the tub well, or you must use an interface material that fills the gap nicely - I'm personally fond of the old-fashioned but highly effective plaster bed for this part.)

If you put a curved bottom on flat foam without fitting the foam or using an interface, expect to have high pressure at the point of contact and potential movement.

From a better class of supplier you can get very high compressive strength versions... 40, 60 and 100 PSI appear to be commonly available, based on more quick research. But the fact that the bottom of a tub is far more than 20 square inches means you probably don't need them - if you spread the load onto the foam evenly.

I (and many other people) have 25PSI XPS foam with a concrete slab floor sitting on top of it - it can take loads just fine if they are within spec and spread so there is not excessive point loading. I've driven a 7000 lb tractor on that slab floor, and nothing moved appreciably/noticeably. The load could be held by just under 2 square feet of foam, and is in fact spread across many more square feet.

Oh - the primary effect of sheet thickness (assuming you have a floor under this design project to which the load is ultimately being transmitted) is the potential for gaps between sheets if they are not assembled with care. If there's no floor and you want to transmit the load to the end frames it's a whole other ballgame and you might want to call in engineering help.

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    It's worth mentioning that the compressive strength values assigned for XPS foam, at least for the two brands commonly found in US home stores, are "at yield or 10% deflection, whichever occurs first." So OP's four layers of 5 cm board could compress at least 2 cm if loaded to the published limit. – Greg Hill Nov 10 '19 at 4:08
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    ...and it's worth paying attention to the fact that the 5-7 square feet that are the bottom of a typical tub would be loaded with 18,000 lbs "at the published limit" so 500 or 600 lbs of tub/water/really heavy people in tub are going to be nowhere near that published limit. – Ecnerwal Nov 10 '19 at 15:17
  • Agreed: a tub-sized piece of foam would carry much more weight than a tub would ever contain. The question mentioned "15 cm wide strips," though, so it isn't clear whether they intend the tub to be fully or just partially supported. My comment was meant only as a footnote to the assertion that the 500 lb tub could be carried on a patch as small as 20 square inches. It's true, but for readers less familiar with foam, I thought it worth mentioning that the foam should be expected to compress in that hypothetical situation. – Greg Hill Nov 11 '19 at 4:25

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