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Community and Q&A

Perlite vs. rigid foam insulation

Boris Rubinstein | Posted in General Questions on

Hello have a question that will probably generate many opinions, i am building a new house zone 5 , was planing on using rigid foam under slab , but some one recommended that i use Perlite in bags instead , says
1) easier to work with
2) easier to pour slab over as you can level things out with sand
3) cost about the same or cheaper then ridged Foam
4) has better R value

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  1. GBA Editor
    Martin Holladay | | #1

    The use of perlite under concrete slabs is discussed in my article, Building a Foam-Free House. This method is experimental.

    It is untrue that using perlite under a slab is "easier" than using rigid foam. It is also untrue that perlite "has better R-value."

    If you use rigid foam -- a method that is common and easy -- you can choose any R-value you want.

  2. Ethan ; Climate Zone 5A ; ~6000HDD | | #2

    Well, we understand that foam performs better, but there are other environmental reasons not to use foam. Having laid foam sheets, I could see an argiment that arranging perlite bags could be "easier," at lease as shown in this document:

    Are there any potential advantages to perlite? Cost?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Are there any potential advantages to perlite?"

    A. No.

  4. Jerry Liebler | | #4

    I''m surprised by Martin's flat denial of Perlite's advantages. Yes the use of Perlite remains, unfortunately, experimental because it doesn't have the fossil fuel industry pushing it but there are real advantages to a "green builder". The potential advantages of Perlite, some may be a bit controversial, include:
    1. Greener, substantially lower embodied energy.
    2. No global warming due to "blowing agents".
    3. lower cost per r, square foot of insulation.
    4. Stronger.
    5. No possible termite damage.
    6. Can be made without use of ANY fossil fuel.
    7. No degradation due to moisture.
    8. Much longer life.

  5. Jerry Liebler | | #5

    With regard to under slab perlite: The available bag sizes severely limit the available r value choice. You will find that if you use the "leave it in the bags" method, it will result in a layer of perlite that is 7" to 8" thick with 21 > r

  6. GBA Editor
    Martin Holladay | | #6

    I'll admit that my one-word answer wasn't very nuanced. Among the questions that you would need to answer before using the "bags of perlite" approach would be, "How long do I think the bags will last?" and "Can I expect any settling if the bags degrade or decompose?"

    The "bags of perlite" approach is worth considering if the idea of rigid foam really bothers you. If you want to avoid the use of rigid foam, another option for sub-slab insulation is mineral wool.

    If you have studied the issues surrounding rigid foam, and have concluded that some rigid foam products are useful and time-tested, you're probably not going to try the "bags of perlite" approach.

  7. Charlie Sullivan | | #7

    Just for some balance, on the list of advantages, I'd note:

    "2. No global warming due to "blowing agents"."

    It's possible to choose EPS foam and avoid this problem with foam as well.

    "6. Can be made without use of ANY fossil fuel."

    Can be, but isn't. I doubt the machines in these photos of perlite mining are all run on biodiesel.

    "7. No degradation due to moisture."

    If the perlite bed gets saturated with water, the R-value will drop to nearly nothing. The perlite won't be damaged--it will be fine after it dries out--but it doesn't function well when it's below the water level.

    Note that I only had bones to pick with three of the 8, and I'm only adding caveats, not completely disagreeing.

  8. Expert Member
    Malcolm Taylor | | #8

    Does anyone know how you manage sub-grade services with bagged perlite? For instance, how do you compact and maintain slope for drains?

    1. Expert Member
      Zephyr7 | | #12

      If I go into engineer mode for a bit, I’d try putting a poly sheet on top of a contoured perlite layer, and then use rerod to weight the poly so that the contour stays put. It might be necassary to use some ground anchors to hold the rerod solidly enough to stay put during the pour. Admittedly I’ve never tried this, but I don’t see why it wouldn’t work.

      It might be possible to mix the perlite with a binding agent too, which would allow it to setup and hold the contour.

      Any other piping for electrical or mechanical services should be the same as for a “regular” build since those pipes are usually run in the crushed stone under the slab anyway. In some ways perlite would be easier here since there would be no need to cut the insulation around any protruding pipes.


  9. David Evans | | #9

    I've used perlite in bags sub-slab on grade in our house. The foundation cut was overlaid with a compacted, leveled bed of gravel. We placed 6 mil PE with taped seams and turned it up the sidewalls on the gravel. We placed paper-bagged perlite units of 8" thickness edge to edge atop the drains already in place. The material only settled a little from walking on them- none broke open. (This is all best to do in the dry.) Loose perlite and foam scraps filled in the voids
    I would recommend not to compact the assembly at all as this fine grained material is an expanded stone with more bearing capacity than foam. Drains could go below the poly and not be in the way and help place the bags.
    On top of the bags we ran a heavy 25 mil. fiber-reinforced scrim to shield from traffic and the steel bars to come. After supply plumbing and electrical conduits were run, the rebar was placed and the hydronic tubing was tied to the steel. All was then ready to pour and finish.
    This has been both a reliable base for the slab to be stable with only hairline cracks in places. We have no complaints with the insulating value obtained which per mfr. data would be R 23+. Floors heat relatively quickly in the 5" slab and hold heat well in our Zone 5 climate. The cost of the perlite was less than $1.50 persf. I am planning to use this again on our upcoming addition which will be also 'near net-zero' energy.

  10. David Evans | | #10
  11. Expert Member
    Dana Dorsett | | #11

    Perlite is a non-renewable mineral resource.

    Perlite theoretically CAN be expanded without use of fossil fuels, but is that in fact the normal process? How is the high temperatures required for expanding perlite actually achieved in practice?

    Like other silicates, perlite dust presents at least some occupational hazard to those handling it.

    Nothing is perfect, but the advantages (if any) aren't obvious here.

    1. Expert Member
      Zephyr7 | | #13

      > “Perlite is a non-renewable mineral resource.”

      Dana, depending on how you look at it, this applies to any mined material. Crushed stone would be non-renewable, etc. mineral wool too. I wouldn’t use this as either a pro or a con for the use of perlite.

      If I remember correctly, somewhere around 70% of the planet is silica. I know the telecom industry likes to talk about how we’ll never run out of raw material to make fiber optic cable because of that (we’ll just pretend the cable jacket isn’t made out of polyethylene :-)


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