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Combining Rockwool and Polyiso

mahasandu | Posted in Energy Efficiency and Durability on
I am in zone six and I have a 2 x 4 wall construction, the ceilings I can put our 30 rockwool but with the sidewalls the structure is already built and while I wish it was 2 x 6, I am wondering what I can do to increase the R-value on the walls.

There is a vapor barrier against the outer wooden walls, I’d like to put rockwool in the 4 inch gaps – then I was hoping I could put 2 inch r13 or r10 poly iso-over the studs to get me towards our 30 or our 25 which would make the structure much more usable in this climate. Do you see any problems with this approach I have a ridge vent and soffit mounts on the eaves..

The walls I thought could just be a Louan poplar finish adhered to the poly iso as I don’t need them to be heavily weight-bearing, I’m wondering adhering the poly iso-to the studs would you use liquid nails or some kind of adhesive or could I screw through?

The most important question I have though is where there be any negatives in terms of moisture problems or do you think this situation is the best way I can increase our value in my situation

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  1. Expert Member
    BILL WICHERS | | #1

    You'll have 3.5" deep stud bays in a 2x4 wall, not 4" deep, since 2x4's aren't actually 2 by 4 inches. You can fit R15 rockwool in those walls no problem, it's made to fit.

    You'll be fine putting 2" polyiso on the exterior, which will add R13 to the wall for ~R28 total. As long as you don't have a vapor barrier on the interior too you'll be fine. I would recommend an interior side smart vapor RETARDER though (Membrain or Intello) to make the wall a bit more robust, but with R13 of exterior insulation, it's not a requirement.

    You can glue up the polyiso, but normally it goes up with screws, or vertical wood furring strips to create a rain screen. You can then hang your siding on the furring strips, so the polyiso isn't subject to any stresses.


    1. iwatson | | #2

      If I understand it you’re looking to put the foam on the INSIDE, I.e. make the rooms a little smaller? If so, this would be your vapour barrier. You can attach with cap nails.

  2. mahasandu | | #3

    Thank you yes I am talking about the inside wall, and yes it would make the room a little smaller, but I think the insulation gsin is totally worth it in terms of efficiency. I’ll have to look but against the outer wall on the inside while it’s some kind of plastic vapor barrier I don’t know if it’s a retarder or full barrier I can look at the product name later my only concern is putting the poly iso- between the vapor barrier and the inner room the rockwool will be in between them. I don’t know if this will create any weird moisture issue but I don’t think so as I have the right venting and ridge vent and soffits etc. I’m wondering if a foil faced poly iso-with the foil facing towards the room would help with heat retention?

    1. Expert Member
      BILL WICHERS | | #4

      If the wall would look something like "vapor barrier / rockwool / foil faced polyiso", then you'd have vapor barriers on BOTH sides of the rockwool, which is something you want to avoid.

      The foil facing only helps if you have an air gap, then it gains you about R1 or so. I wouldn't bother trying to get that air gap here -- you'd have a lot of materials and lose more space for very little gain.

      Is there any way you can put the polyiso on the exterior? That's the usual way to do it, and it helps keep the wood framing warmer, which means drier. Drier is better. If you put the polyiso on the interior, you keep the framing cooler. It will work either way, but it's better on the exterior if you can manage it.


  3. mahasandu | | #5

    It has to be inside in my application- so avoid foil ?

    1. Expert Member
      BILL WICHERS | | #6

      If you have an existing vapor barrier that you cannot remove on the exterior, then I would use something other than foil faced polyiso on the interior. You can get "roofing" polyiso that has a kraft or fiberglass mat facer that is more vapor open. I would still make sure to do a good job air sealing though.


  4. mahasandu | | #7

    Thank you so much for your reply, it’s really helpful basically I’m just trying to get the R-value of my walls higher than a standard 2 x 4 rock wool insulation of our 15, since I already purchased that product I am trying to find a way to up the R-value, again without doing external insulation. I don’t mind losing 2 inches on the inside, and if I stay away from foil face I think I should be fine, that said is there any other suggestion or any other ways to increase the R-value on a 2 x 4? I’m trying to do DIY in my budget, so I hadn’t thought of spray foam insulation. But any and all suggestions are welcome, perhaps spray foaming the two by fours along with the Rockwell would achieve something similar for a similar cost? If not I’m happy to go with the poly iso-, I just wanted to make sure I’m not creating a moisture situation that will degrade my structure

    1. Expert Member
      BILL WICHERS | | #8

      I wouldn't waste money on spray foam in these walls. Closed cell won't get you a full depth fill, so you'd probably end up pretty close to the R value of mineral wool. Open cell spray foam would get you a full fill, but again, you'd end up pretty close to the R value of the mineral wool.

      You could potentially furr out the 2x4s with 2x2s and some plywood strips so that you could use batts designed for 2x6 walls, which would get you about R23 if using mineral wool. 2 inches of polyiso instead of 2" worth of furring strips would get you up to a total of about R28 (R15 for the 3.5" mineral wool in the studbays, and R13 for 2" worth of polyiso). The interior side polyiso would also help with thermal bridging, so it would have better whole-wall performance than the thicker mineral wool would.


    2. iwatson | | #9

      Since you already have an exterior vapour barrier another option would be to furr the studs to make them deeper and put 5.5" rockwool batts instead. This will probably be cheaper and will avoid the vapour issues. If you want to avoid thermal bridging at the studs you can cut strips of foam insulation to use as your furring. However, this will complicate mounting of electrical boxes and drywall, so you’ll need to give some thought to that.

  5. mahasandu | | #10

    Thank you Ian

    This is exactly what I think I’m going to do and where my mind has landed. I like this idea the best of all. Thank you again everyone what an awesome helpful forum.

    1. Expert Member
      BILL WICHERS | | #11

      If you use foam strips for furring, there is an easy solution to the electrical box mounting problem Ian mentioned: just put a short piece of wood in place of the foam furring strip at the location of the electrical box. This has minimal overall thermal impact, since it's just small piece, and it lets you use a normal side-mount electrical box tacked to the side of the "stud".

      Polyiso is firm enough that I haven't usually had much issue with drywall screws, you just have to be a little more careful not to overdrive the screws an excessively dimple the drywall. Aside from that, it's easy -- just make sure to mark your stud locations first. Studfinders don't always "see" the foam furring strips reliably.


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