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EPS characteristics

MALCOLM TAYLOR | Posted in General Questions on

I have a simple question about EPS that I’m sure one of you can answer for me. Outside of buildings I encounter EPS in two places where it seems to act in very different ways. I see foam anchor buoys and fish floats off our coast and washed up on the beach that seem to maintain their buoyancy and lightness or years. But those of you with hot tubs will have noticed that the foam inserts gain weight over time and within a couple of years the initially light cover can easily triple its weight. If you remove the foam and leave it in a warm environment to dry it is reluctant to give up the water and it can take months to return to its original weight.

So my first question concerns what the difference is between the two? My second one would be what implications does this have for using EPS foam in damp environments? We generally talk about the perm rating of various types of EPS with regard to their ability to dry, but how much moisture do they hold if they can’t dry and what is the effect of the encapsulated water on their insulating performance?

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  1. user-1137156 | | #1

    Great question, and one that I asked in a different way some time ago. Hopefully someone who really knows will answer but my understanding is that the foam is indeed vapor permeable yet not permeable by liquid water. Over a hot tub the vapor pressure is very high due to both high temperature and 100% humidity, the vapor penetrates but then collects, condenses and is trapped. The vapor pressure floating on a lake is far lower mostly because the temperature is 30 to 40f cooler. Likewise, the vapor pressure underground is even lower. In both the float and underground situation the vapor pressure on the "back side" is close to that on the wet side so the rate of accumulation is far slower. In short the hot tub cover behaves quite differently.

  2. Expert Member

    Jerry, So in most cases where it couldn't dry, like under a slab, it shouldn't take on any water in the first place as it isn't exposed to water vapour?

  3. kevin_in_denver | | #3

    I've had the polyethylene covered EPS from HD soak up water just sitting out on a slab over one winter. Tried to dry it out for 2 years without luck

  4. jackofalltrades777 | | #4

    The biggest factor at play is the TYPE of EPS. It comes in different densities:

    Absorption (vol.) % as per ASTM C 272:
    Type 1 = <4.0 Water Absorption
    Type 2 = <3.0 Water Absorption
    Type 1X = <2.0 Water Absorption

    As per ASTM E96 - WVT perm. in =
    Type 1 = 2.0 - 5.0
    Type 2 = 3.5 - 1.0
    Type 1X = 0.6 - 2.0

  5. Expert Member

    So Peter, is the situation simply the result of the low density of the foam used in hot tub covers?

  6. jackofalltrades777 | | #6


    The short answer; yes. I am pretty sure that hot tub manufacturers are using the cheapest EPS they can find, which would be Type 1, maybe Type 0.5? I say that jokingly but I've heard of some junk EPS that isn't even worthy to be called Type 1. So that type of EPS will take on water. You don't see ICF quality Type 2 EPS taking on water and bloating out.

  7. Expert Member

    That's reassuring, thanks.
    Also nice to know the manufacturers don't skimp on the quality of materials they put into the $600 lids :)

  8. [email protected] | | #8

    Malcolm, Try replacing the hot tub cheap foam with high density foam and see what happens. It could be that high density foam takes longer to load up with moisture but also takes longer to dry out.

  9. user-1137156 | | #9

    The data you showed only says that the best foam may accumulate water at a rate 1/3 as fast as the best samples of the type 1 and the best (lowest vapor permeability) is 1/12 that of the worst. As a former hot tub owner I've seen the cover get water logged in 6 months or less. I sure hope there is something different happening to below grade foam if not none of them will last 6 years!
    I believe if you try a hot tub cover made of type IX foam it will last longer but it won't last over 6 years.
    I'd try it but I no longer own a hot tub. While I did I made one cover out of XPS, it was saturated in a bit over 1 year.

  10. Expert Member
    MALCOLM TAYLOR | | #10

    I've done a lot of work for a nearby resort which as 22 hot tubs, which is where my observations come from, and they spend a huge amount on hot tub lids. All the manufacturers seem to use the same EPS, or at least it appears to become waterlogged at the same rate. The cynical part of me thinks they may make a lot of money off the after-sales replacement parts and not be too interested in improving the quality of their foam.
    I'm not really worried about the hot tubs, they just got me thinking about whether the same water uptake mechanism might be operating in the EPS we use in our buildings. If as Jerry says there is something unique to the conditions hot tub foam faces that doesn't occur elsewhere then I guess I can stop worrying.
    I'd imagine down where you it wouldn't be much of an issue either way, but up here in the PNW the soil is saturated by November and stays that way for a good five months.

  11. bobhol | | #11

    Good Discussion ...I have a hot tub and replace the lids every two to three years ...except the last one ..It is over 3 years now and is still light..but..when I ordered from the company that takes custom orders I had them wrap it in 3 layers of plastic before the foam was placed in the vinyl case...maybe thats the key ,to isolate the foam from the vapour for me ,regards Bob

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