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Foam insulation PSI loads and rooftop workers

bluesolar | Posted in Energy Efficiency and Durability on

Hi all — I’m a bit confused by the choices of different types of EPS and XPS foam, where the variable is PSI rating. If we want foam on the roof I assume that it needs to withstand workers — or myself — occasionally walking on the roof. What PSI rating is adequate for, say, a 250 lb worker walking or standing?

What I mean is how many square inches do you use to estimate this? I’m having visions of heel strikes and video-recorded running shoe fittings… 🙂 If we assume a 250 lb worker I assume that they can create a load much greater than 125 lbs per shoe area, when their weight is on one leg, heel strikes, etc.

I see very low PSI ratings for EPS (e.g. 20 or 30 PSI at the most) compared to XPS (up to 100 PSI). Are the EPS numbers good enough for heavy workers?

And what if a load exceeds the PSI rating? What are we talking about as far as damage? Dents in the foam? Erosion of R-value?

Thanks for your help.

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  1. Expert Member
    BILL WICHERS | | #1

    I’m going to estimate my shoe footprint is about 30 square inches. Since my weight is likely not to be evenly distributed over that area, let’s assume 20 square inches of actual load-carrying area. With only 10 PSI rated foam, that’s 200 pounds of ALLOWABLE loading. If you exceed that by a little bit, probably no big deal. Bigger overloads might make some small dents in the foam. Dents will have some VERY minimal impact on R value, but aren’t worth worrying about unless it gets really bad. Obviously huge overloads will punch through the foam, so you can’t pile 5 guys onto a scaffold with four little wheels, for example.

    On commercial roofs, it’s common to have some thick rubber mats or pavers placed to form a walkway from the roof access hatch to the rooftop equipment that might need to be serviced. This helps to protect the roof from damage due to contractors walking around.


    1. bluesolar | | #2

      Interesting, thanks. I didn't know how the commercial people did it.

      I wonder why XPS has much higher PSI ratings.

      1. Expert Member
        MALCOLM TAYLOR | | #3

        Flat roof?

  2. Expert Member
    Dana Dorsett | | #4

    The PSI rating has nothing to do with the elastic limits of the material, or what's "allowable". It is only the minimum pressure at which it hits a specified 10% thickness deformation (whether it springs back or not.)

    Commercial membrane roofs insualted with Type VIII EPS or 2lbs roofing polyiso typically rated 12-15 psi are considered "walkable", with very limited PERMANENT deformation issues. Protective measures such as stiffer padding sometimes used on specific paths that might be FREQUENTLY walked on (say, service access to rooftop package HVAC equipment, etc.). But the above-roof padding has more to do with protecting the roofing from wear than with protecting the foam.

    XPS has higher PSI ratings than EPS of similar density primarily because it doesn't have the interstitial spaces of macroscopic beads, and a more uniform cell structure. The interstitial spaces of EPS have weaker shear strength than the beads them selves, allowing beads to separate and move up or down a bit under pressure.

    In practice anything rated under 10 psi is likely to show permanent footprints, but is also much easier do damage in handling. Type-I sheathing EPS (rated 10 psi) often comes with facers to keep corners from getting knocked off or even broken/folded in handling, but is pretty easy to dent with boots or dropping tools, requiring a bit more care. But even Type-I EPS is "walkable" if you're careful.

    Even with permanent compressed footprints the effect on R-value is quite modest. The total fractional area is limited, and dents are visible even at 1% permanent deformation (which would be slightly less than a 1% hit in performance at the dented area, since the higher density yields a higher R/inch.) Even with Type-I EPS permanent footprints that are fully 10% of the thickness would be rare.

    1. bluesolar | | #5

      Thanks Dana, that helps.

      I thought dented or compressed foam would have a *lower* R-value per inch. So it's actually higher per inch? Is there a type of insulation where compressing it reduces its R-value? I must be thinking of something else. Is it mineral wool or fiberglass or something?

      Something that has confused me lately is that I see many references on GBA to Type-this or that EPS and XPS, but I rarely see any reference to Type-n on the products at Lowe's or Home Depot. Is this something that requires ordering from direct wholesalers or something?

      (And the type system is weird in that the PSIs aren't in ascending order with type number – they just flop all over the place.)

      1. Expert Member
        BILL WICHERS | | #6

        You get reduced R value per inch when the insulation is compressed. You just won’t see much overall performance reduction from something like a heel-size squish of a few percent deformation on a 4x8 foot sheet of polyiso. The deformed spot is only a small reduction of R value, and only a small reduction also in a small percentage of the total area of the sheet.

        The “type” numbers for EPS are related to the different densities of the foam. Most box store stuff is type-I, which is low density and has that “crumbly styrofoam” feeling. This stuff is faced too, primarily to protect it during transport. Type-II is denser; and while it’s readily available too, menards is the only box store I’m aware of that actually stocks it — and it costs more, close to XPS pricing. Type II is NOTICEABLY denser and more durable, there is no confusing it with type I. There are other types too, but I haven’t worked with them.

        XPS is usually found in 10 and 25 PSI variants. Both look and feel about the same. There are some higher rated versions out there, but they’re much less common.

        Polyiso basically comes in foil faced and “roofing” varieties. The roofing type is slightly lower density, slightly lower R value per inch (about R5.7 for some that I have, compared with R6 for the usual denser foil-faced stuff). The roofing stuff usually has a kraft paper or fiberglass mat facer, where the denser stuff is usually a shiny aluminum foil facer. The non-foil facers are more vapor permeable than the foil, which is sometimes important although it becomes less of a concern as the foam itself gets thicker.


  3. user-2988384 | | #7

    On commercial roofs, we use cover board to provide better durability against damage and provide a good substrate for adhering the EPDM. The cover board helps distribute the load more evenly to minimize point loading on the foam. I can't imagine adhering EPDM directly to the insulation but I guess that's done sometimes.
    Also, Type II EPS (15 psi) is typically what we use for flat and tapered insul on low-slope roofs. It's R-value doesn't degrade over time, it performs well at lower temperatures, and it's lower density makes installation go quicker.
    The "Type" is per ASTM C578, but compressive strength isn't the only property that varies by type. Density, thermal resistance, and vapor permeance are also defined by type. They do make stronger EPS insulation (up to 60 psi - Type XV), but that's only needed for heavy loaded areas, such as beneath footings and driving surfaces (mechanical bays, garages, roadways on permafrost, etc.)

  4. Expert Member
    PETER Engle | | #8

    I don't think I saw it mentioned above, but once the foam is covered with sheathing, the sheathing will spread any loads over a much larger area. Pretty much zero chance of damage short of falling trees.

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