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Insulating rim board with rigid foam: best practice?

CTSNicholas | Posted in General Questions on

Zone 5B, main level + full basement. First floor I built with 2×6 walls, and used mineral wool insulation with a smart membrane. Not that that should effect this question, but just giving a bit of info about the construction.

Scenario : I have 12” thick i-joists, spaced 16” O.C., and want to insulate the rim-board void for two reasons: Noise reduction, and R-Value. Noise transfers surprisingly well from the outside, especially rain/hail and wind making the siding rattle.

I am leaning towards using XPS or EPS foam, 2″ thick, potentially multiple layers. Any better suggestions aside from a professional grade spray foam which I will not be using? I know my rigid foam method will take a lot of time on the two sides that the I joists are perpendicular to the rim-board…and I am literally cutting puzzle pieces for every 16” gap, notching each side of the I joist flanges. The other two sides of my house where joists and rim board are parallel should be much quicker.

One serious concern, if I use the XPS or EPS foam, do I need to worry about moisture issues? The foam would be an air barrier to my knowledge, especially if multiple layers. I intend on using a construction adhesive to just “glue” it to the rim-board, and then using canned spray foam to fill all gaps around the perimeter of each foam piece. Will condensation form on the rim-board, or at the point the I joist touches the rim board since it’s impossible to insulate where that joint is?

Thanks guys. Have a good weekend.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, insulating your rim joists is not optional. It's required by code.

    Condensation problems occur when areas of your thermal envelope aren't well insulated. Adding insulation reduces rather than increases the chance of condensation, moisture accumulation, and rot.

    How much R-value do you need? The code isn't clear on R-value requirements for rim joists, but these areas are part of your wall assembly, so my advice has always been to insulate rim joists to the same R-value required for walls. In your climate zone, that means R-20 (about 4 or 5 inches of rigid foam).

    If you use rigid foam rather than spray foam, you need to pay attention to sealing the perimeter of each piece of rigid foam with canned spray foam. For more information, see Insulating rim joists.

  2. CTSNicholas | | #2

    Indeed, thanks Martin. I have a second follow up question regarding my foundation walls. I may repost this later as a new topic, but I'll give the details here.

    I used 7" thick mineral wool batts (16" x 48") - was going to fir out walls to hold full 7" batts but ended up not doing so, as a result I had to shave each batt down 1.5" to install it on my main floor walls.

    I have I believe 30 bundles left. Each bundle held 3 batts. So I have about 90 7" thick, 16" wide, 48" long batts of mineral wool that ran me about $30 a bundle or so if I remember right. So not just something I want to throw out or give away. Should I use this stuff up in the basement...cutting it in half for a 2x4 framed wall, or use it out in the garage and use rigid foam in the basement? I guess my concern would be using mineral wool up against the foundation wall...with a capillary break in the foundation wall framing, the mineral wool would be very hard to not have touching the concrete. I've never insulated a basement before, so should I treat it like the main floor above ground and go with a smart membrane and all that jazz or try to have the concrete breathe?

  3. GBA Editor
    Martin Holladay | | #3

    First you need to install a layer of rigid foam or closed-cell spray foam on the interior side of your basement wall. Once you do that, you can add additional insulation -- for example, mineral wool -- on the interior side of the foam insulation if you want.

    Without the foam insulation, you'll get condensation on the foundation wall. (Why? Because mineral wool insulation is air-permeable.)

    For more information, see How to Insulate a Basement Wall.

  4. CTSNicholas | | #4

    How much XPS foam would be sufficient for the air barrier to prevent condesation on the foundation wall if I were to use mineral wool and *no* additional air barrier or smart membrane? I am thinking at least 1" of XPS, with the joints glued together including the bottom edge that touches the basement floor.

  5. user-2310254 | | #5

    Nicholas. Do you have a full basement or walkout basement? Are any walls above grade?

  6. CTSNicholas | | #6

    It's walkout. 1 wall fully exposed, the other is half exposed (Triangle shape) which is why I was concerned about condensation issues.

  7. GBA Editor
    Martin Holladay | | #7

    Q. "How much XPS foam would be sufficient for the air barrier to prevent condensation on the foundation wall if I were to use mineral wool and *no* additional air barrier or smart membrane?"

    A. First of all, the purpose of the rigid foam is not to provide an air barrier. (A poured concrete wall is already an air barrier.) The purpose of the rigid foam is to provide a layer of insulation that is thick enough to ensure that the "first condensing surface" of the wall assembly (in this case, the interior face of the rigid foam) is warm enough in winter to prevent condensation.

    This type of wall assembly does not need an air barrier or a smart membrane on the interior.

    So, to rephrase your question: How thick does the rigid foam need to be? The answer depends in part on how much fluffy insulation you intend to install on the interior side of the rigid foam. To learn more about the principles behind this question, you should read Calculating the Minimum Thickness of Rigid Foam.

    1. If you plan to install rigid foam with no fluffy insulation on the interior side of the rigid foam, you can choose any thickness of rigid foam (without worrying about moisture problems). That said, it's always a good idea to meet minimum code requirements. In your climate zone (Zone 5), that means that you need at least R-15 of continuous insulation for your basement wall.

    2. If you plan to install about R-13 of fluffy insulation on the interior side of the rigid foam, I would advise you to install at least R-5 of rigid foam first. That's a conservative recommendation, based on the advice in Calculating the Minimum Thickness of Rigid Foam.

    3. The thicker the fluffy insulation, the thicker the foam layer has to be. (For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.)

    4. If your walk-out basement has some walls that are above grade and framed with studs -- that is, walls above the concrete foundation walls -- then these above-grade walls can be insulated just like all the other above-grade stud walls in your house.

  8. CTSNicholas | | #8

    Martin, on point 3, I am surpirsed that more fluffy means more rigid. Figured I would get away with say R-5 rigid then R13 to R 15 in 'fluffy'... with R20 in fluffy as an option without upping the rigid thickness.

    On point 4, the basement is all poured. I really hate tornadoes. =) So I assume doing the walls all the same in terms of rigid foam will be okay. Rigid foam is superior in space saving abilities, just not in wallet savings. Perhaps I will do a hybrid. Flat out rigid only insulation in the two bedrooms, then the hybrid after those rooms where space is not so crowded. What article was it that discussed install rigid foam on basement walls, talking about using Poly-iso with foil if it will be exposed (unfinished) for a while? I don't remember if it was the same article talking about how to install rigid foam or a different one.

  9. GBA Editor
    Martin Holladay | | #9

    More fluffy insulation makes the first condensing surface -- the inner face of the rigid foam -- colder in winter, since the fluffy insulation separates the rigid foam from the inner warmth of the basement. That raises the condensation risk.

  10. CTSNicholas | | #10

    So it's a non-issue in the rim joist area & concrete foundation wall, when using two layers of foam glued together - because they are basically one layer of foam then. While it can be an issue while working with two types of rigid, and one mineral wool in my situation.

    I'd really like to not waste my mineral wool insulation, but I also hate working with it since I will be essentially bisecting it down the middle with bread knives to fill 3.5" deep stud cavities. Decisions decisions...

  11. jasonn1234 | | #11

    one thing about the rigid foam, rather than create puzzle pieces around the i joists, i cut pieces the height of the inside portion of the i joist, and 3.5" wide. I put these "inside" the ijoist as i noticed the inside width of the ijoist from the vertical inside piece to the outside flange was approx 1". I then cut rectangular pieces according to the instrucitons on the link provided. I found this simpler than cutting 3 puzzle pieces (1" think each) for everey single ijoist which was perpendicular to the rim board

  12. CTSNicholas | | #12

    Jason, what thickness of foam were you using, and how many layers?

  13. CTSNicholas | | #13

    Martin, do you have an article on detail of insulation rim board, sill (sole) plate, and concrete foundation wall? Foam butting up to foam is how I envision it.

  14. GBA Editor
    Martin Holladay | | #14

    Below are some illustrations to give you the basic idea. In some jurisdictions, local code inspectors prefer rigid foam to be protected with 1/2-inch drywall for fire safety. When in doubt about local code requirements, call your building department.


  15. CTSNicholas | | #15

    That's what I had in mind, thanks, but potentially not the 'fluffy stuff' in the rim joist as indicated...more XPS probably since it could be glued to the first layer.

    In terms of keeping it tight and up against the rim board material, construction adhesive (the general purpose contractor grade, brown color) sufficient, or do you suggest a certain brand/material? Manufacture websites don't seem to be specific on what works best per cost.

  16. GBA Editor
    Martin Holladay | | #16

    Q. "In terms of keeping it tight and up against the rim board material, is construction adhesive (the general purpose contractor grade, brown color) sufficient?"

    A. You can use any foam-compatible adhesive, canned spray foam (which is a type of adhesive), or cap nails for this purpose. For more information, see Insulating rim joists.

  17. CTSNicholas | | #17

    Curious, what happens if Poly-Iso with foil facing is used in layers for the rim joist? Would the foil on the first layer cause moisture to sit there and not escape, or would the top layer with foil prevent moisture from ever getting to that point? Then same question all over again, but applied to foundation walls..which we know are porous and moisture can migrate through them.

  18. GBA Editor
    Martin Holladay | | #18

    The main moisture transport mechanism for interior moisture that reaches a rim joist is via air. If the rigid foam is sealed at the edges, so that interior air can't reach the rim joist, you've basically stopped the moisture flow.

    What about vapor diffusion? A tiny amount is possible with EPS or XPS, but if you install foil-faced polyiso, the vapor diffusion through the foam is stopped by the aluminum foil.

    The permeance rating of foil-faced polyiso is 0.05 perm. Stack up polyiso all you want -- it's still vapor-impermeable, and having multiple layers of foil doesn't cause problems.

  19. Robert Opaluch | | #19

    Since you also asked about sound insulation...
    Mineral wool batts do a good job of reducing sound transmission. Roxul SafeNSound gives STC specs and Roxul ComfortBatt gives R-value specs, but not vice-versa. But I suspect they have similar specs for both STC and R-value per inch. See:
    It seems that mineral wool does better than fiberglass at attenuating lower frequencies, but there are various densities of each that might be more important than glass wool vs. rockwool material differences.

    Unfortunately most of the sound transmission will be through the wood members (joists, studs, subfloor) once you install batts in voids. Carpet and pad, or floating wood flooring over pad, will help reduce band joist noise transmitted through the joists to the living area above. Some pads specifically mention their sound-deadening properties.

    Double layer or 5/8" drywall will help reduce sound transmission through walls. Caulking the space between drywall and subfloor helps reduce sound transmission through a wall or partition bottom plate. For non-load-bearing partitions, I split the studs except the top and bottom 18" (where most of the stress occurs), and stuffed voids with cheap fiberglass batts. Avoid putting electrical boxes or other openings on both sides of a wall in the same stud cavity. These are all relatively cheap modifications that are quite effective at soundproofing, when used together. Solid core doors do better than hollow core, and weatherstripping or reducing air movement below the bottom of doors helps too.

    There are specialty products that cost more, like acoustical foams, acoustical sealants, and drywall mounting clips or resilient channel.

    I haven't seen STC numbers for foam but in my experience, foam is not as great at sound insulation. But anything that stops air leaks will help. Adding layers with an air space or changing materials, will reduce the transmission of some frequencies through layers of a wall or floor system.

    Concrete walls transmit little noise, including low frequencies. Better quality windows and doors reduce noise levels as well as provide better insulation and air-sealing.

    I'd like to hear others ideas for soundproofing inexpensively. Would be nice to develop a checklist with cost and STC numbers for various options.

  20. user-2310254 | | #20

    Nicholas. Certainteed makes a sound attenuation drywall with a Green Glue layer. I have quite a bit of it in my house but would advise pricing it against two layers of regular 5/8-inch drywall. It seems to be much easier to reduce noise between rooms than between floors.

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