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Community and Q&A

Insulating between studs with polyiso scrap?

sheeschen | Posted in Building Code Questions on

Wasn’t sure what category to put this in, but my situation is that I had most of the exterior of my house covered with foil-faced polyiso insulation (Rmax Thermasheath-3). The walls between the living space (bathroom, laundry, pantry, hallway) and garage did not get this treatment, but they still need insulation. I ended up with some scrap due to normal wastage, as well as some extra sheets (since we had to buy some of it by the pallet). My plan has been to cut it to fit in the stud bays and foam around to hold it in place/air-seal. With the smaller scraps, I was thinking of just piecing it together like a jigsaw and foaming it together.

The work area is relatively small and I’ve got time on my hands, but before I went down this path, I wanted to check with the community if this would be a good idea, if there were some things to watch out for, or any other information people might have. None of this will be exposed, it will all have sheetrock and/or OSB on one side or the other.

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  1. GBA Editor
    Martin Holladay | | #1

    Your plan will work fine. Plenty of other people have been down the same road. You'll find the work time-consuming.

    Here's a hint: It's better to cut your pieces sloppy and loose rather than carefully fitted and tight. That way you'll have room for your foam nozzle.

  2. sheeschen | | #2

    Thanks, Martin, for the quick response. I felt it would be ok, but wasn't sure about moisture travel. The little bit of test work I've done confirms your statements regarding time-consuming and sloppy fit.

    A followup question - this is in 2x6 framing, and I'm using either 4 inch pieces or 3 pieces of 1.5 inch, so I will have a gap (also gives me the radiant barrier bonus). Is it better to have the gap on the interior or exterior side? I'm in the San Francisco bay area if that matters.

  3. GBA Editor
    Martin Holladay | | #3

    I'm not sure that it matters, but my gut says that the gap should be on the interior.

    In any case, it would be hard to install if you tried to create a gap at the wall sheathing.

  4. Riversong | | #4

    Leave the radiant gap on the interior. If it were exterior walls exposed to direct sun, it might make sense to keep the radiant barrier on the exterior, but since these walls face an enclosed garage and not the sun, that will not help.

  5. sheeschen | | #5

    Having gone through the process now, I would not recommend it. Very time-consuming and messy. Since I had the scrap laying around and would have to pay to get it taken away, this seemed like a decent plan to keep the material out of the landfill. No way I would pay someone to do this - too costly, but as I said, I had some time on my hands.

    Also, I hit a glitch with the inspector - he insisted that the stud cavities be full. I don't know where that idea came from, but no amount of arguing would persuade him - the fact that the 4 inches of polyiso exceed the original specs for 5.5 inches of fiberglass, the fact that filling the gap eliminates the radiant barrier advantage, no need for fireblocking in 8 foot cavities. Not sure if any other inspector would be so picky, but I figured I would update this to let others know.

  6. GBA Editor
    Martin Holladay | | #6

    Too bad you encountered an ignorant inspector. How did you satisfy him?

  7. sheeschen | | #7

    He is "allowing" me to staple on an inch or so of fiberglass to the polyiso to fill the gap. From other experience on this project, It's easier and quicker than fighting it at this point.

  8. Bob Alsop | | #8

    I used the same approach except filled the stud bays with XPS (two layers of 2 " and one layer of 1 1/2"). I'll agree that its a lot of work, but hopefully it's worth it. Because of the fact that it was built during the wintertime (in Vermont) it gave me the ability to insert the first 2 inch layer of foam and at least get some initial heat into the building. Then it was a matter of running wires,cables,etc and then finishing up the job (all seams were taped.)
    Cutting XPS on your tablesaw is not for the makes me wonder if you were utilizing one of the new "safe" tablesaws whether or not the saw would interpret the foam as being a "finger" and kill the process!!!!!

  9. Riversong | | #9

    If you cut rigid foam board on a tablesaw, be sure to connect a shop vac onto the sawdust outlet.

  10. Jim Merrithew | | #10

    If you are cutting the foam with a table saw, you should wear a filter mask.

  11. sheeschen | | #11

    Ok - passed the inspection. He gave me grief about not insulating some kneewalls to the conditioned attic space, but finally relented.

    Yes, yes, yes! Wear a mask if cutting any foam insulation with power tools. I was using my Festool circular saw with the track to make the initial cuts on the 4 inch boards, then finishing off with a Sawzall (the Sawzall blade bent and cuts weren't great if I used that alone). The dust collection on the Festool helped tremendously.

    My understanding of the Sawstop system is that it brakes the blade when it detects an increase in conductivity. Probably not an issue with unfaced foam - unknown issue with foil-faced. Interesting question.

  12. Riversong | | #12

    I believe that vigilance and care are better "insurance" than any high-tech SawStop. Relying on mechanical "fail-safe" devices can actually encourage complacency and inattention - the two most dangerous things to have at a building site.

  13. 2tePuaao2B | | #13

    Used to use 2" Dow board foam to protect flat highrise building roofs when rigging beam and weight swing staging. A good old rip handsaw would easily cut through an 8" stack, clean & quick. Light wax the blade works like a charm. Not alot of stuff in the air either.

  14. rustyjames | | #14

    Cutting XPS on your tablesaw is not for the feint-of-heart

    I just score and snap XPS. For the others, a hand saw works fine.

  15. bobmiller | | #15

    uhh just the opposite for me. I like ripping with the tablesaw and then using jigsaw or box cutter for the smaller crosscuts.

    "Cutting XPS on your tablesaw is not for the feint-of-heart"

  16. gtsawyer | | #16

    Apologies for bringing this thread back from the dead.

    I'm just finishing the walls of a 24x24 garage, insulated with 3.5" of poly iso board cut to fit between 16"OC 2x4 studs.

    Things I've learned:
    * A LOT of labor is involved
    * Cutting pieces sloppy is definitely easier (as noted by MARTIN HOLLADAY)
    * Use something like a long bread knife to cut off the extra foam flush with the studs (after curing)
    * Look for a good deal/sale on the expanding foam - you will use a lot (I used maybe 50 cans)
    * I used a Sawzall with a long pruning blade to cut the pieces to size, but wished I would have tried the waxed hand saw - less dust and straighter cuts
    * I'm convinced that using insulation board sealed with foam provides much better insulating value than fiberglass
    * If I would have had another couple of grand laying around, I would have gone with the 2 part spray on insulation

  17. GBA Editor
    Martin Holladay | | #17

    Thanks for the hints. Since this thread was started, GBA has published an article on this topic. Read it here: Cut-and-Cobble Insulation.

  18. user-2413173 | | #18

    As for Sawstop: you can override the protection mechanism and cut some of it. I'm pretty sure that when you have the protection override on, it will flash a light to indicate that it would have triggered the brake, so you can figure out if the material is problematic or not. I've heard about people doing this when they have green lumber and they aren't sure if it might be too wet. (The answer usually seems to be that lumber sold as "green" is actually dry enough and you really need to cut lumber that's been sitting out in the rain to trigger the brake, but it's still worth testing, especially if you have an expensive blade.)

  19. ohioandy | | #19

    George - FIFTY cans of spray foam? (comment #16) At some point switching to the bulk method (i.e. Dow Froth Pak 200, available at Big Box for $300 on sale) might make more sense. There's a learning curve to these, but then you're flying. The one drawback is you gotta get more done per session--although each kit comes with about a dozen nozzles, a certain amount is wasted each time as you get the mix flowing right. With their little straw nozzles, the cans allow for more precise injection; on the other hand the big sprayer fills big gaps easily so the rigid foam can be placed VERY loosely.

    I've always wondered how many cans of foam equals one of these "200 board-feet" kits.

    3.5" of polyiso is great, but wouldn't you have had better results putting some of that foam in a continuous layer on the exterior side of the studs? A lot less spray foam, less work, and a nice reduction in thermal bridging across studs. Then add cheap fluffy in the stud bays to further increase R-value.

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