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Community and Q&A

Cavity Insulation for an Upgraded Standard Wall

AHTTAR | Posted in General Questions on

The last several houses I’ve built had 2-3″ of polyISO outside of zip sheathing. I was planning the same this go around but I’m thinking the money/time can be spend better elsewhere. That brings me to potential upgrades on a standard wall. Is there any in cavity insulation that will perform noticeably better than the standard pink stuff? The envelope will be air sealed very well, the roof will have 6″ of ISO on the deck and air sealed zip. In my experience of living in foam cladded homes, I’d say if anything it helps more in the heat than it does in the cold and I don’t know if its worth the time an expense in my climate. If I can get rid of the exterior foam and allocate some of the budget to a better cavity system, I’m all ears. Thoughts?

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  1. Expert Member
    Akos | | #1

    This a good resource for wall assemblies:

    Because of the thermal bridging of the studs, even if you insulate with magical unicorn fur, you can't get much more than R19 assembly out of a 2x6 wall.

    If you want more R value, you can go up to 2x8 24"OC. With either high density fiberglass batts or mineral wool you end up with an R24 assembly. About the same as a 2x6 R24 + 1" polyiso wall.

    Unlike cold weather, even on the hottest of days, there is very little temperature difference between indoors and outdoors, so very little of your heat gain is from walls. Almost all of it is from glazing and roof. When it comes to cooling, wall R value doesn't matter much.

    1. AHTTAR | | #2

      We started to have this conversation in the other thread. I guess minimum exterior foam thickness that will actually be a noticeable comfort level is the question. I just don't know that 1" iso is going to be any better in a very tight home.

  2. kyle_r | | #3

    With a single story slab on grade home would you consider interior foam? Possibly easier to detail.

    Have you considered a roof that doesn’t need 6 inches of polyiso? A vented attic with blown in cellulose?

  3. AHTTAR | | #4

    I don't find the exterior foam details particularly difficult. I also think its the best way to insulate if you do a lot of foam, but its hard to know if its really worth the time/money when only doing an inch on a very well detailed air seal.

    I much prefer unvented single pitch roofs for lots of reasons. 6" on the roof is easy and very effective.

  4. maine_tyler | | #5

    What would you like to put the money saved towards? I don't see how this question can be answered without a larger budget approach / modeling. It sort of sounds like you just don't want to build a wall with exterior foam and are reverse engineering the question from there. Are you hoping to put the savings towards nicer counters/cabinets/finishes, or towards improving other weak thermal points? If the latter, energy modeling is probably the surest way to go. At the least, the question cannot be answered without knowing what other weaknesses exist (i.e. what weak points exist in your typical builds where spending that money there would save MORE energy than the ext. insul.).

    There are certainly diminishing returns with insulation, but it's also a given that if you remove an entire continuous layer of insulation from your assembly, it will perform worse (thermally speaking). I think the goal needs to be made clearer.

    1. AHTTAR | | #6

      I don't think it matters at all what the money would go to. For example, if I build a sex dungeon with the savings, is that going to help me figure out if 1" of continuous insulation is really making a difference in comfort in an air tight building? Assuming all the low hanging fruit has been picked and licked in the new dungeon, exterior foam shows up to the party. My point is, there must be a minimum thickness of foam in an extremely air tight structure where the comfort level goes up and I don't think the minimum allowable amount of 1" is really going to do that. For example, you'd probably be better off putting the money toward fatter windows to feel less radiating cold/drafts as opposed to 1" of foam. I've already done that so I guess I'll buy a second swing for the dungeon? Maybe there are some test out there that show these progressions but my good ol gut tells me that you'd need at least double that to start noticing the performance outside of a few pennies of savings on your electric bill. So the question starts to be, why even do 1"? Go big or go home may apply when it comes to continuous insulation.......but don't go too big because that's a waste too. I believe there may be a sweet spot in each climate zone. Too little, not a quantifiable gain. Too much, better for your ego than your comfort level.

      I'm not completely trying to be a smart ass here but it is easy to get caught up in the science of all this, I know and I have. But like I've said, I've lived in houses I've built with 3" of foam and others with no foam and the difference in comfort was minimal.......I think. All you can do is watch how the building performs at different seasons. Both buildings had heavy roof insulation, air tight details and were sited correctly with intentional landscape. I think those are the things that matter most.

      1. tim_dilletante | | #7

        I'm glad you brought this up, because I am in CZ6 and trying to plan a backyard studio with the minimum effort/comfort ratio, and I'm not a big fan of foam for Earth reasons.

        Also you should make sure the humidity is under control in the dungeon, maybe put the money there.

        1. AHTTAR | | #8

          Ha! Money already allocated to dungeon humidity control.

      2. maine_tyler | | #13

        I suppose I was considering energy use and not solely comfort. I guess you are talking the latter so you are right, it doesn't totally matter what the savings go to... But, as Akos says, it's hard to quantify comfort. Maybe you should not put in any windows and get the deluxe double duty dungeon swing ;)

        Much easier to quantify energy use and ROI. I still think clear goals and a budget to manipulate would help with any decision, but I see your point.

        1. AHTTAR | | #14

          Double duty swing? Can you send me a link to that please? I already ordered windows and doors unfortunately!

          True, comfort is very hard to quantify but I think we can all just hope for even heat in the winter and even cool in the summer. Obviously the colder it is outside the more heat you need to offset cold from glass and walls. I do think air sealing takes care of drafts though which I believe is the biggest culprit of discomfort with glass being second.

      3. Expert Member
        MALCOLM TAYLOR | | #17


        I agree. The only two areas in a code minimum build where I have experienced comfort issues are the air-sealing and windows. Except perhaps for floors, I think you can achieve comparable levels of comfort by spending a bit more conditioning the house as you can by improving the R-value of the ceiling and wall assemblies.

  5. jwolfe1 | | #9

    I struggle with this quite a bit. I am building at 11,000 ft in climate zone 7 of Colorado in an extremely in-demand area with a crazy tight labor market as an owner builder with extensive DIY. Even here the default wall is just 2x6 with r21 fiberglass batts (2012 IECC code amendments downward allow for just r19 walls and r38 ceilings. I am planning to do what is outlined in this article with very good air sealing and will put money towards other building envelope items. The extra all-in cost of enough foam or doing double stud walls right now is tough to stomach.

    Consider this approach:

    1. AHTTAR | | #11

      Labor or learning curve is not a problem for me. I am the labor and I could easily wrap this house with 1" in a day. Thing is, my county is now requiring R22 walls which means either source the silly stuff labeled R22 that you stuff and compress into the wall or do 1" of ISO on the exterior to get to somewhere between R24 and R27 depending on who you ask.

      I've seen the method you linked to in the past. I don't think it would be that much less work than continuous though I suppose it would use a lot less foam. I guess its a question of how warm exterior foam is really keeping your walls.

      To be clear, I still think continuous exterior makes a lot of sense but I'm not building passive houses. I would also think a thicker wall assembly makes even more sense if you're really going all in. Design sacrifices are made for northern glass, extra sf, not a perfect rectangular layout, etc, etc. So much of my walls are glass anyway, especially on the south, are you really going to feel the 1 degree cooler drywall when there is a 9' wall of glass 3 feet away? A tight house facing south with proper glazing will be 87F inside with windows open on any day above 10F. I will never live in a house without a full slab for this reason, in my opinion its a comfort game changer. Most winter nights in a house like this, my wife and I find ourselves saying "should we start a fire tonight"? Ive studied my thermostat data when going out of town and my radiant heat is set to 50F. Dead of winter late January, if the sun comes out for even 3 hours a day the heat won't run unless it gets into the negatives for at least a few hours at night which is actually rare in CZ6. Now that's with 3" of exterior foam but a whole lot of dual pane glass tuned for solar gain. I guess what I'm feeling more and more over the years is just air seal like crazy, insulate the lid like crazy, face south, do a whole house slab, spend some dough on triple pane if you can, pay attention to ventilation and you'll have a pretty darn comfortable house in CZ6. I have no doubt you could build a house in my climate that wouldn't need more heat than the oven, but then you sacrifice other comforts. Its all trade offs and until you build and live in different levels of performance, its hard to know if the science is practical.

  6. Expert Member
    Akos | | #10

    Comfort is hard to quantify. The closest you can come is the interior wall surface temperature. Interior air film has a small R value (R0.67) so the drywall is always slightly colder than the room. This is the ratio of RairFilm/Rwall value and temperature difference between inside and outside.

    If you are looking at 0F outdoors, this generally means an R10ish assembly is good enough as it puts the wall at 5F bellow room. If you bump that up to R25, that drops down to 2F. You probably feel that as well. Much above that, I don't think you'll notice a change in comfort.

    1. AHTTAR | | #12

      Agreed. This seems like logical info to me. For me the slab warmed by the sun probably offsets a lot of the cold radiating off the walls. On super cold days we leave our efficient wood stove simmering which never lets anything get all that cold.

    2. AHTTAR | | #15

      So I talked to the city to clarify what they'd accept as "R22" walls. Any batt R22 or more or R19 + R5. That brings me to the question of best basic assembly.

      R22 or R23 batts don't seem to come paper faced, Rockwool for sure doesn't. What would you do if anything other than painted drywall for vapor in CZ?

      For minimum continuous, would you put R19 plus 1" ISO or spend a little more and put R21? Faced or unfaced? Any ISO or something more vapor permeable? This would be the cheapest option.

      To upgrade to 2" continuous, what batts? Faced, unfaced? ISO type? Two layers of taped 1" or 1 layer of taped 2" on top of taped Zip walls?

      I'm basically trying to put together the cheapest options for in cavity and continuous as well as one upgraded option to 2" continuous.


  7. kyle_r | | #16

    You can buy R22 faced fiber glass batts, but they are 6 1/2 inches thick. By the the time you compress them into a 2x6 wall you get R19. You can however compress an R30 batt into a 2x6 wall and get an effective R22.

    If you go with mineral wool you could add membrain or intello as a smart vapor control layer. In zone 6 you would need the same vapor control layer if you went with less continuous insulation than R11.25 on a 2X6 wall or 7.5 on a 2x4 wall.

    I think the cheapest option would be R30 faced fiberglass batts. If you want continuous insulation, then a 2x4 wall with 1 1/2 inches of polyiso and no vapor control layer.

    1. AHTTAR | | #18

      Whether or not R22 actually is R22 or not really doesn't matter to the fools making these energy codes. All that matters is what it says on the paper. So R21 is actually a better performing wall than R22? That's kinda like putting Argon in windows even though the gas will be gone before 5 years. Gotta love our government.

      On a wall with continuous insulation, wouldn't you want the wall to dry to the inside? So no warm side vapor barrier?

      Honestly, stuffing R30 batts in the wall sounds like it would be a pain in the ass. 2x4 framing is not an option for me, I hate working with them. The quality, especially these days on 12' + are pretty bad.

      1. Expert Member
        MALCOLM TAYLOR | | #19


        "On a wall with continuous insulation, wouldn't you want the wall to dry to the inside? So no warm side vapor barrier?"

        If the foam is thick enough to keep the interior face of the sheathing above the dew point then yes. If the foam is too thin, an interior face vapour-retarder helps reduce the amount of moisture entering the wall, and makes the small amount of drying to the exterior the thin foam allows sufficient to keep it safe.

        That's a wall assembly which has been used pretty widely in Eastern Canada for a few years now - 1" of foam with interior poly - and it appears to work fine. A variable perm vapour-retarder would make it even safer.

        1. AHTTAR | | #21

          Ok thanks. What is considered thick enough to not be too thin? CZ6

          You're talking just plain ol' poly with unfaced batts? R19 or R21 make any difference? 1" ISO?

          1. Expert Member
      2. kyle_r | | #20

        Membrain and intello are “smart” vapor retarders, they will allow the wall to dry to the inside

        1. AHTTAR | | #22

          I'll check these out, thanks. Might be some piece of mind. I'm sure they're spendy.

          1. kyle_r | | #23

            I think you will find membrain cheaper but less robust. This is why I mentioned stuffing larger fiberglass batts. Or you could frame your house 2x8?

  8. AHTTAR | | #24

    We're a bit too far along to switch to 2x8's. What would you achieve by stuffing larger batts in? I thought you were just giving another option to meet R22.

    1. Expert Member
      Akos | | #26

      You can order R22/R23/R24 high density fiberglass batts. They are essentially the R30-ish batt pre-squished to 5.5" thickness. The R22 is about 40% higher density than R19, the R24 is about 2x.

      1. AHTTAR | | #27

        You said the R22 squished in is effectively R19. Do you know if R21 batts are actually more effective than R22?

        1. kyle_r | | #29

          The R 22 batts I mentioned were regular density

  9. kyle_r | | #28

    1) You can install regular density fiberglass R30 faced batts in a 2x6 wall and compress them your self to get an effective R22.
    2) Akos mentions you can order pre compressed faced fiberglass batts that are labeled R22/23/24. I was unaware of this, I thought they were R21 for high density (compared to R19 standard density), but worth looking into.
    3) Unfaced R23 mineral wool would require a class 2 or higher vapor retarder. This is where I mentioned membrain or intello, because they will allow drying to the inside as well under the right conditions.
    4) Exterior insulation with an Rvalue of less than 11.25 on a 2x6 wall would still require a class 2 vapor retarder. But if you are using foam which is also a vapor retarder, you really want the smart vapor retarder not poly.
    5) Install R11.25 or greater on a 2x6 wall without an interior vapor retarder.

    1. AHTTAR | | #30

      Thanks for the summary, I think it will be very helpful to people in the future.

      It will all be a "value" proposition for me. Ultimately I think they will all end up being somewhat similar in price and time. I have a hard time justifying 1" of continuous so the front runners for me will probably be two layers of 1" ISO vs Rockwool w/ smart retarder. Something about putting polly in the walls doesn't feel right at all to me which is likely irrational since the Canadians have tested it for a while.

      1. kyle_r | | #31

        If it was me, I would either go with faced high density fiberglass batts (cheapest option) or 2 inches of polyiso and forget the interior vapor retarder.

        If I was going to add exterior insulation, I would add enough to forget the interior vapor retarder.

        1. AHTTAR | | #34

          I'm in the same boat as you. 1" isn't enough but 2" might not be all that different in price from R23 plus vapor.

    2. Expert Member
      Akos | | #32

      Both R21 and R23 are high density, one is higher density. Here is the list of available R values from one manufacturer:

      1. kyle_r | | #33

        Looks like the R23 isn’t available Kraft faced

        1. AHTTAR | | #36

          Yeah, once you pass R21 I think you have to add your own vapor coverage.

      2. AHTTAR | | #35

        I guess nobody knows if R21 is actually R21? Somebody said R22 was effectively R19. So is R21 a marketing ploy just to get you to upgrade from R19?

        1. Expert Member
          Akos | | #37

          Generally anything that is labeled as 5.5" will give you the R value listed on the bag. R21 HD is really R21.

        2. the74impala | | #38

          Have you looked at rockwool instead of foam for the exterior?

          1. AHTTAR | | #39

            Not in much detail yet. I think it's pretty spendy. I like the idea of it though

  10. dennis_vab | | #40

    Im following this thread as I am in a similar situation. In CZ6, NY state. Will start construction this spring and already have my 7/16” zip. I was planning to do 1” of rigid on the outside but I’m struggling to figure out if it’s worth the additional cost and labor of furring out all the windows to accept the 1” of rigid, or if I should just upgrade the interior wall cavity insulation to meet code without exterior foam.

  11. AHTTAR | | #41

    Talked to the building dept. today and it appears there is a spray foam guy in my little town that is apparently reasonable. Probably much cheaper than Rockwool actually. That said, I've never used spray foam for anything other than sealing the blocking between the roof joists. Any thoughts? One of the more appealing points of Rockwool was the "healthier" nature of it vs other insulation. Doesn't spray foam just throw that all out the window or is there a healthy responsible version of it these days?

  12. AHTTAR | | #42

    Ok, final decisions, figured I'd put them here for closure. I'm going to give Rockwool a try, its about $1000 more than R21 glass for my house plus about $600 for Membrain vapor retarder. Still waiting to hear from Intello on which product to use. The main reasons I chose Rockwool over exterior foam are the house has so much glass anyway, I think exterior isn't all that critical to increased comfort levels. The money is better spent on better glass and the roof. Also if the rumors are correct, Rockwool is better for indoor air quality. Lastly cost. To do a double layer of 1" ISO would be about a $5k premium over Rockwool by the time you add extra flashing, tape, long screws, etc and that's not including my labor as a cost.

    So final build:
    - 15mm stego vapor barrier under the entire slab.
    - Zip walls with every air sealing detail possible plus 3/4" rain screen gap.
    - R23 rockwool in walls w/ Membrain vapor.
    - R38 rock wool ceiling batts no vapor barrier
    - 6" Firestone polyiso on the roof deck under EPDM.

    With all the confusion about exterior wall insulation, we never talked about the roof. Do you think 6" is overkill? I've done it before and I personally think a properly faced house with a full slab and a heavily insulated roof are the most important details to a comfortable home.

    Thanks guys

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