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Community and Q&A

Fiberglass insulation hangers for under roof deck

David Johnson | Posted in Green Building Techniques on

I’m having trouble finding fasteners for hanging fiberglass insulation from sprayed-on polyurethane foam in a conditioned attic conversion application.

The stack will look like:
roof sheathing
sprayed polyurethane foam
——–|—-|—-|—-|—-|—-|—-|—-|—- <- fasteners pushed into foam fiberglass insulation to cover foam to meet code and increase total insulation value ————————————————– Attic conditioned air space The hangers need to be: -Thermally insulating so the R40 insulation used here at 7000′ in Colorado is not shorted out -Able to anchor into the sprayed polyurethane foam at one end, and hold low density fiberglass batts at the other It looks like: Might work, but they are not available in the US. It looks like the pin on the following product is too smooth to hold low density fiberglass if the head is glued to the polyurethane foam, and if inserted the other way, the pin might fall out of the polyurethane: d/ihangers_nylon.php Most of the hangers I find have some sort of metal in them, and if my calculations are correct, this is a really bad idea for this application. For example, it looks like 10d nails on 1′ centers will lower an R40 ceiling to R27! The best thing I have found for this application so far is a screw type drywall anchor without the internal screw: I would screw them into the polyurethane through the fiberglass, then pull on the outer layers of the fiberglass to hide the anchor. The biggest problem I see with these is that they are so short, I’m concerned I won’t get enough bite into the polyurethane. A related question: does any know how much I can shorten the nails coming through the roof sheathing without reducing the integrity of these fasteners used for a tile roof? after the 10d nail calculation I referred to above, It looks like these nails will have a significant affect in system R value. Dave

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  1. GBA Editor
    Martin Holladay | | #1

    If I understand your plan, you want to attach fiberglass batts to the underside of a layer of cured spray polyurethane foam.

    There are two problems with your plan:

    1. Fiberglass batts are never held in place with fasteners through the fiberglass batts. Kraft-faced batts are sometimes temporarily held in place by stapling the paper flanges at the edges of the batts, but that is only temporary, until the batts are held in place by a layer of drywall (or, occasionally, by a layer of chicken-wire -- but that's not a method I recommend). Most batts are held in place by friction (assuming that there are studs, joists, or rafters on either side of the batts) and solid sheet goods (OSB, plywood, or drywall) on both faces of the enclosure. You can't nail or screw up fiberglass batts.

    2. Cured spray polyurethane foam cannot be used as a material for anchoring fasteners. It is soft and spongy. Fasteners can be anchored in wood, plywood, concrete, or masonry -- but not cured foam.

    In other words, it's time for Plan B.

  2. David Johnson | | #2

    "Fiberglass batts are never held in place with fasteners through the fiberglass batts"
    "Never" is a pretty big word. I think I can hold the fiberglass in place by impaling it in the same way semi-rigid fiberglass (703 board and 705 board) is held in place, but the pin needs to have more friction than this case. I have found examples of pins like this, just not in the US. Here is one:
    "Cured spray polyurethane foam cannot be used as a material for anchoring fasteners."
    I"m thinking I might be able to do this with coarse enough threads, but in the worst case, I can attach the pins to the roof sheathing(deck) .


  3. GBA Editor
    Martin Holladay | | #3

    Here in the U.S., similar fasteners are available from Hilti. They are called Hilti IDP fasteners. But as far as I know, these fasteners are used to attach rigid foam to a concrete wall -- not to attach a fiberglass batt to cured spray foam.

  4. David Johnson | | #4

    Thanks, I'll get some for testing.

  5. David Johnson | | #5

    Hilti USA said the IDP was a europe-only product. The US version (X-IE):

    look too smooth to even test.


  6. Bob Manninen | | #6

    I just did this last year. I used 3M High Tack adhesive (the spray can stuff) to hold at least a portion of the batt (meaning, I sometimes just peeled a portion of the batt away so I could tuck it into the uneven foam layer). It's nasty stuff, though, so make sure your area is well ventilated. Anyway, the High Tack adhesive did nothing to the foam, and I was able to just "stick" up the fiberglass long enough to sandwich it with Roxul. Sometimes the Roxul would fall down (did I mention how much I enjoyed this project?). Then I just used either plastic shipping straps screwed across the rafters, or, when I ran out of that, I used plastic plumbers tape to hold up the batts. Good Luck!

  7. David Meiland | | #7

    Why not go with a standard application and use blown-in fiberglass behind insulation netting under your SPF?

  8. David Johnson | | #8

    This might be useful. But I still need a method of attaching the netting to either the foam or the truss member below it. Do you know of a good specification somewhere on the installation of the netting?

  9. David Meiland | | #9

    What is the roof framing, trusses or rafters?

    If it's trusses, there probably isn't a very good way to add batts to SPF, because the framing will probably be buried, or close to it, and as you are finding there isn't anywhere to attach more insulation. In that case, I would just have the SPF sprayed to the total R value you want, and forget about trying to add anything.

    If it's rafters, there are more options. Spray SPF then net the rafter faces and blow the balance.

    From what you've said, I suspect you have trusses and are looking for a way to add more insulation to the bottom of SPF after the framing is buried. It's not going to be easy.

  10. David Johnson | | #10

    Yes, trusses. The last time I checked, my local code did not allow any exposed foam in the attic. So the reason for the fiberglass is to meet code, and for cost savings.

    There is always a way, for example, I could mount 1/4" x7" PVC Pipes on a stud or cheap flange fastened to the sheathing. These pipes would work as both a foam thickness gauge and a method for mounting the fiberglass. There are a variety of fasteners I could drive into the pipes and hold the fiberglass. Since it is always easier to use a preexisting solution, I was hoping to find the European style insulation mounts in the US.


  11. David Meiland | | #11

    Doesn't the foam guy have a coating he can finish with that will satisfy the inspector?

    I think you'd have to do an impeccable job of "hanging" batts in order for them to stay there permanently. All the usual methods I see fail after a while and the attic and crawl space have insulation falling out.

    The other issue you'll have with batts against SPF is a lot of air pockets between the two materials... not good.

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