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Community and Q&A

Attaching 2″ polyiso to undersides of i-joists with LVL flanges

etting | Posted in General Questions on
After buying a huge box of 3″ cap nails, I’ve learned that hammering them through 2″ polyiso into the LVL flange of an i-joist without them bending badly is nearly impossible.  I bought the heavier, 10-gauge 3″ cap nails, but they’re ring-shanked, so the solid part of the shank isn’t very thick, and almost all of them bend at least 30 degrees inside the poliso, tearing it up a bit and probably penetrating less than half of the intended inch.  I assume the LVL flange is just too hard for these nails.
I could use 3″ screws that I’ll have to poke through plastic washers one by one.  I have the 500 or so I’ll need of the Ideal Security Inc. SKPHC Plastic Cap Washers for Nails or Screws, but I would love to find screws with the washers/caps already in place.  I haven’t been able to find any, but it seems as if there would be a market for them.   Does such a thing exist?
Would exterior-grade screws be required for attaching polyiso to the undersides of floor joists in a vented crawl space in a desert climate?  

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  1. Expert Member
    NICK KEENAN | | #1

    I would go with screws. Hammering or running a nail gun in a crawl space is just misery.

    They make a cap for insulation called a "prong washer." As the name suggests they have prongs that grip the foam. Push it into place and then drive the screw, no trying to hold several things overhead at the same time. When starting a new sheet you can push washers onto all four corners before lifting it up.

    I would use deck screws with Torx heads. You want something that will easily drive with one hand. It's about $45 worth of screws.

    Donate the cap nail you already bought to Habitat for Humanity and take a tax deduction.

  2. Expert Member
    BILL WICHERS | | #2

    +1 for screws. Less chance of smashing the insulation that way too, since you have more control with a screwdriver than you do with a hammer. Use the large plastic "insulation" washers around the screw head. I'm going to check out those "prong washers" DC mentioned, I haven't seen those but they sound handy -- especially for overhead work.


    1. Expert Member
      NICK KEENAN | | #3

      Here's a link:

      You can also push the prong washer into place and then shoot it with a nail gun. I don't like to use a nail gun in a crawl space because of the close quarters.

      1. Expert Member
        BILL WICHERS | | #9

        Thanks for the link!

        I hear you on the nail gun in tight spaces. I don't use my nail gun with foam anyway, there is too much chance of massive overdrive and that "where did my nail go?" look when you find a hole with the nail buried way down inside. I also always think my nail gun will get me someday, although I think it was Malcolm on here who mentioned he'd rather shoot a nail through his finger than smash it with his framing hammer. He probably has a point on that one...

        Those "prong washers" look just like what I'm using to using, but they add some spikes apparently to hold them in place. I haven't seen those before, but I'll be trying them soon!


        1. Expert Member
          NICK KEENAN | | #11

          I have shot myself in the hand with a framing nailer, and hit my thumb with a framing hammer. While the nail required a trip to the emergency room to be removed, it was less painful and healed faster.

  3. etting | | #4

    Thank you for the suggestions. Yes, screws are definitely better. I tried a 10d box nail that I poked through one of my caps, and it stayed straight going in, but it was a lot of unpleasant work, and the polyiso would take a ton of dents before I'd get 500 of them nailed in. Those prong washers are a good idea for many, but I like to shove the screw through the polyiso before I start using the screwdriver, in part to make sure I'm hitting the joist, and it looks as if the center hole is too small to slip over the shank of a screw. It's strange that their spec sheet doesn't give the center hole diameter. The washers I already have are about the same, minus the prongs, and I have to thread the screw through the hole to get them flush with the head, which will drive me crazy x500. I found some inexpensive plastic washers that will slip over a screw, but their outer diameter is only 1/2". Do you think that's enough for foil-faced polyiso?

    I like the star-drive heads. Do I need exterior grade for the ceiling of a crawlspace in a dry climate?

    1. Expert Member
      NICK KEENAN | | #5

      I'd use deck screws. The only other screws you're likely to find in 5 lb quantity are drywall screws, which aren't really suitable, they don't come in star head and they snap off rather easily.

      1. etting | | #7

        Thank you. Ace has Hillman Power Pro No. 9 x 3 in. L Star Yellow Zinc Wood Screws 5 lb. 450 pk for $27.99, but I just found essentially the same thing in exterior grade at Amazon for less than $5 more, which is worth the certainty, as I can't find anything in the IRC or elsewhere indicating whether exterior grade is needed for a crawlspace ceiling. If anyone knows, I and probably many others who have looked for an answer would be greatly interested.

    2. Patrick_OSullivan | | #6

      GRK cabinet screws. By far my biggest 'utility player' screw. They come in all sorts of lengths, are now stocked in even the small Home Depot closest to me, and have a coating to help with mild exterior/exposed use. The washer head makes them useful for all sorts of things.

      1. etting | | #8

        Thank you. If the washer head is big enough to prevent pull-through on the polyiso, those are a nice solution, although a bit pricey.

  4. Expert Member
    BILL WICHERS | | #10

    I think a 1/2" washer is small, but would probably work if you're careful. You would be better off using fender washers, or just do it right and use a proper insulation washer like this:

    BTW, I see no reason for exterior screws on the ceiling of a crawlspace. Your crawlspace is protected from the weather, so interior screws should be fine. A little bit of elevated humidity is all the screws might be exposed to down there, and that shouldn't be a problem for a standard coated or flash galvanized screw.


    1. etting | | #12

      Thank you. Fender washers have the advantage of slipping over the screw shank easily, and they're less expensive in bulk than the insulation washers, probably because they're so much less specialized.

      Good to know you don't see any problem with interior screws. Nor do I, but lacking any documentation to support using them in the crawlspace, I'll probably get exterior ones, as the difference in price (for star drive) is minor. The really inexpensive interior screws have Phillips heads, which I find amazingly terrible for most purposes.

      1. Expert Member
        BILL WICHERS | | #13

        The insulation washers I've used act like captive washers when put on a screw (i.e. the washer locks onto the screw and won't fall out). This is handy when working since you can preassemble a number of fasteners, throw them in a small bucket, and save a step while doing the installation work.

        I agree phillips is a pain on these things because the driver is always slipping out. Torx/star drive is much, much better. I try to only use torx and square drive screws myself for anything using a power driver. I like hex head screws for manual stuff (like mounting electrical boxes), which I install using my nutdrivers.


        1. etting | | #14

          Thanks again, Bill. Do those insulation washers slip all the way down the shank of the screw easily, or are they captive near the tip of the screw so that they get driven to the head as the screw is installed?

          1. Expert Member
            BILL WICHERS | | #15

            I can usually hold the washer in my hand and push a screw through the washer's center hole with the screw going between my fingers (hopefully that makes sense). It's tight, but not too difficult to do. I've done hundreds of washer/screw installs like that.

            I suppose you could just push the washer on part way and have it stay whereever you want along the shaft of the screw until you're ready to install it. The washer will bite into any part of the threaded area of the screw if you pick the right size screw. I think I used a #9 deck screw before, but it's been a while so don't trust me on that :-) It's easy to test fit a few screws to find the size that holds the way you want. Insulation support isn't a very demanding application for a fastener, so you don't need to use super heavy duty screws here.


  5. etting | | #16

    Thank you, Bill. Yes, I know exactly what you mean about pushing the screw through. It's good to know it can be done easily. For anyone interested in a price comparison, 600 zinc-plated, 1" fender washes with a 3/16" inner diameter that will drop onto a #9 or #10 screw without effort will run around $31 shipped; a search for "bulk fender washers" will turn up a few good options. Fender washers drop quickly in price per piece once you get into the hundreds. From what you've described, the plastic washers made for insulation have the advantage of staying put so that you can pre-position them on the screws, but for screwing into a ceiling, gravity will make fender washers pretty easy to work with too.

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