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Community and Q&A

Existing 2 x 4 framed exterior wall

MarcoD | Posted in Energy Efficiency and Durability on

I am renovating a condo.  Condo Board regs prohibit any exterior changes that vary from original design.

The exterior walls are 2 x 4 framed construction in climate zone 5.  
No upgrades since 1976.

What is the best way to insulate?  I am considering filling the entire cavity with closed cell spray foam.

Any recommendations are welcome.  Please advise.

Thank you in advance.

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  1. Patrick_OSullivan | | #1

    > What is the best way to insulate?

    Given your constraints, I would consider what has affectionately become known as the "Bonfig wall":

    Basically, you can increase the depth of that exterior wall to an insulation level of your choosing by a combination of rigid foam strips and furring, and then use normal "fluffy" insulation in the wall.

    > I am considering filling the entire cavity with closed cell spray foam.

    Probably not worth it. Read this:

    1. MarcoD | | #5

      Just read the Bonfig link you sent. Awesome!
      I'm on board with this detail. Thank you so much!

  2. Expert Member
    Michael Maines | | #2

    Closed cell foam will give you the best thermal performance in a 2x4 wall, but not by much--this article explains why:

    Can you expand the walls to the interior? The Bonfiglioli approach is a good option in a situation like yours. Or you could simply cross-strap with 2xs on the interior to get a bit more insulation and a partial thermal break.

    Edit: I now see that Patrick responded with similar information.

    1. MarcoD | | #6

      Thank you for your reply. I had not heard of the Bonfigioli appraoch before. What an awesome concept. I'll use it in this project. Thank you so much!

  3. Expert Member
    DCcontrarian | | #3

    Here, if you do a studs-out renovation you have to bring the wall up to the current energy code. We're on 2015, which for Zone 5 means either R20 in a framed wall, or R13 in a framed wall with R5 of continuous insulation.

    R13 is a 2x4 wall with fiberglass insulation, so one way would be add R5 of continuous on the interior. The simplest way would be to add an inch of polyiso.

    There are two ways to get to R20 if you have a 2x4 wall. One is to make the wall thicker and use batt or blown insulation. If you add 2 inches to the thickness of the wall you can use batts made for a 2x6 wall. A regular 2x6 fiberglass batt is only R19, but high density fiberglass or rock wool will get you over 20.

    The other way is to keep the wall thickness the same and use a higher R-value per inch insulation, which means closed cell foam. Spray foam and polyiso board are both about R6 per inch, so a 2x4 gives R21. The challenge with both is getting the full thickness.

    While you have the walls open something to consider is air sealing. I had Aerobarrier done on my house, and the guy who did it said that one of their most frequent jobs is condominiums where the owner gets tired of smelling their neighbors' smells. That sort of air sealing is a lot easier when the walls are open. You may also want to consider soundproofing between units.

    (the 2021 code requires R30 which means you would definitely have to go thicker).

    1. MarcoD | | #4

      Much Appreciated!

  4. Expert Member
    BILL WICHERS | | #7

    I'm going to have to disagree with Michael here a little in his post #2. I think about the best you can expect from closed cell spray foam in a 2x4 wall assembly is right around what mineral wool can provide (R15), assuming you also do a decent job of air sealing the wall prior to insulating with mineral wool (which really isn't all that difficult to do). I find that closed cell spray foam when applied in shallow depth wall cavities (like what you have in a 2x4 stud wall), tends to form a layer of not more than 3" thick, and usually less, and it's usually concave too, so it's thinner in the middle and thicker near the edges. The R value isn't a simple average, so the "average depth" is actually weighted a little more towards the thin spots, which means the overall R value of the insulation layer itself is always less than you'd expect it to be. After you add in the thermal bridging of the studs (which also affect walls insulted with batts), you end up with a layer of spray foam that was almost surely the most expensive way to insulate, but will rarely be the best performing due to the realities of the installation of the material.

    The only time I would use spray foam in a wall is when you either have a very different assembly to air seal (slat type exterior sheathing, for example), or if you already have the spray foam contractor on site so the labor savings makes the overall installation cost lower. In all other cases, I would use more conventional insulating materials in a wall, with my own preference being mineral wool batts. The important part is to air seal that wall prior to insulating, which can be done with some caulk and canned foam.

    You can use interior rigid foam too, as DC mentioned, but if you have services (electrical boxes, water pipes, etc.) in that wall, you have to be careful about where your primary air barrier is going to go. If you have panel type exterior sheathing (plywood or OSB), you're probably best off using that, but you still want to detail the interior drywall as best you can to help keep moisture out of the wall assembly. If you put foil faced polyiso on the interior behind the drywall, there is no need for a vapor retarder, since the foil facer is already a vapor barrier. You then need to be careful with your details at any penetrations to ensure a good air barrier for the wall. I usually seal the electrical boxes in to accomplish this, and I like to use the white fiberglass "hard boxes" that don't have lots of holes, which makes them easier to air seal.


    1. Expert Member
      Michael Maines | | #8

      Bill, hopefully you know that I never promote spray foam unless it's the only product that will reasonably do the job, and I have only used it in 2x4 walls on rare occasions, to meet building codes when we couldn't change the wall thickness. I agree that the average depth using closed cell foam will be less than 3.5". The R/in varies with product and age, but I usually figure around R-5.5/in for an aged value. 3" of that is R-16.5. (I have had 3.5" studs fully filled with closed cell foam and cut off with a hot knife but it's a PIA.)

      In a wall with a 25% framing fraction, assuming R-1.2/in for the framing and R-5.5/in for the foam, the spray foamed wall will perform at R-13.425.

      Mineral wool is R-4.2/in at best; a 3.5" batt is about R-14.7. (Insulation marketers love to round up to whole numbers!) With the same framing fraction as above, the whole-wall performance will be about R-12.075.

      The difference is only R-1.35, or less than 10% of the total; with closed cell foam's significantly higher up-front carbon emissions, the risk of the foam not curing properly, health concerns for the installers and the difficulty foam makes for future renovations, I would use the mineral wool every time. Or, preferably, I'd make the wall thicker and use an insulation product with low up-front carbon emissions such as cellulose.

  5. DennisWood | | #9

    With an older home, unreliable inner vapour barrier (or none), 2x4 limitations, and inside access only, I pretty much always use closed cell spray foam, building a 2x3 staggered stud wall (which is often needed anyway for plumb/levelling old construction) and address window openings with jam extensions.

    This completely encapsulates the old studs in foam, adds about 2" to wall depth, and resolves any air issues in that section of wall where you cannot access exterior sheathing. If space is tight, you can reduce the intrusion by making use of the existing top/bottom plate but ensuring the new staggered studs sit 1/2" to 1" into the room. Once spray foamed, this assembly is more than ok for drywall. Exterior ship lap may have literally thousands of air leaks in an eight foot wall. The difference between an old/existing plaster/lathe wall with blown cellulose vs a new wall section in spray foam and staggered stud is rather startling in thermal images.

    I'm not a spray foam fan necessarily, but for cold climate basement walls (with no external insulation), 2x4 renovation work, or air sealing an older attic, it really is the best solution in my opinion. The other "positive" is the tremendous increase in racking strength of a wall assembly with closed cell. It's also the worst choice if you are renovating again! If you've pulled apart a close cell spray foamed wall, you know what I'm talking about :-)

    When you factor in that air sealing and insulation are addressed in one step the extra cost of closed cell likely works out in the labour equation.

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