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Where to Spend: Air-Sealing vs. Insulation

wiscoguy | Posted in General Questions on

I have finally got quotes back for insulation lots of the sizes I’m after I can’t even get or prices went up a bit. So my question is I have to choose between 1” of halo foam which is vapor open and does breathe do to the way it’s made and also using a bunch of good tapes on plywood seams and windows a quality house wrap not tyvek or do I splurge and go beyond by budget for 2 1/2 foam which would then require a different approach with using smart vapor retarders inside the house and adds about 5000 plus time to the project all the way around.

essentially how much more savings is there from r5 to r10 is it worth the extra 5k or am I better off investing in better tapes amd sealing the house better and keeping the 1” foam and poly on the inside wall.

climate is zone 6 close to Chicago I would be above code and still vapor open to the outside with 1” foam using halo on the exterior so I don’t think I’d need to do the intello inside.

appreciate any thoughts this is my last hurdle on the house I just didn’t expect some of these prices

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  1. Expert Member
    BILL WICHERS | | #1

    What is the rest of the insulation in that wall like? Thicker foam will make a bigger overall difference with a 2x4 wall than a 2x6 wall, for example. You don't need the wall to be able to dry in both directions, either (although it certainly doesn't hurt) -- allowing drying to the interior only is fine.

    Note that if you use thick enough rigid foam on the exterior, you don't need a vapor retarder on the interior, so you could skip the intello and use the savings to pay for the extra rigid foam. Personally I like to use a vapor retarder AND thick exterior rigid foam, but if you have enough exterior rigid foam the interior side vapor retarder isn't really required.

    If I was tight on budget on one of my own projects, I would skip the interior side vapor retarder and put the savings towards the thicker exterior rigid foam. The thicker exterior rigid foam makes condensation inside the wall much less likely, and also greatly improves whole-wall effective R values since it isn't effected by the thermal bridging of the studs.

    I would absolutely still do the best job I possibly could in terms of air sealing the house regardless of how I built the walls. Standard "caulk and canned foam" air sealing isn't a big cost, and if you're doing the work yourself it's actually pretty cheap.


  2. wiscoguy | | #2

    Yes I’m definitely doing the caulk air sealing and tapes.

    My wall is 2x6 with rockwool. I can get 1” halo that does have some vapor permeability or if I get to 2 1/2 it’s negligible.

    I agree about using drywall as my vapor barrier but I’m not hanging my own drywall and I’m not really sure I could get the crew to do all the caulking necessary to do have a good drywall vapor barrier. This puts me back at intello which is a big cost and added time. The insulation is also imore but I do agree about the thermal bridging aspect of all this.

    I’m very new to all the building science stuff I read articles regularly and learn as much possible. I know that 2 1/2 inches of GPS is r12.5 which puts me over the 11.25 for code and then I have a foam back siding product that will add a little more say r2 at most.

    The air sealing from the inside in my case would be more of the problem. I’d love to just use drywall but like I previously said not sure it would be done correctly. And the intello even if I did use it since I don’t have a service cavity would have many holes in it for outlets and lights etc.

    I’m trying to do the best I can on my budget. If just doing drywall without all the caulking was good I’d be in there but I’m afraid that doesn’t really work.

    My concern at this point is if I’m unable to have a really good interior vapor retarder will this cause issues? I’d do the best I can and also have a hull house dehumidifier so the humidity shouldn’t ever go above 35%. Also an air exchanger and try to keep temps in an acceptable range and not exact opposite of the current outdoor temps either.

    Appreciate your thoughts and opinions.

  3. Expert Member
    Akos | | #3

    Your 2x6+R5 rigid works out to around an R24 assembly. The 2.5" foam bumps that up to R32.5.

    Chicago is somewhere around 5500 heating degree days base 65.

    So 2000 sqft of wall (2000 sqft of house has about 2000 sqft of wall) looses over a heating season about:

    R24 wall: 5500*24*2000/R24=110 Therms
    R32.5 wall 5500*24*2000/R32.5=81 Therms

    A decently built 2000sqft house would have a leakage around 3ACH@50PA, which is somewhere around 50CFM of infiltration flow. A very well sealed house that would drop to around 0.8ACH, so that drops to 13CFM.

    Your 3ACH house, you loose 50*5500*24*1.08=71Therms.
    The 0.8ACH, that drops to 18Therms.

    So overall, you save more energy by paying attention to air sealing. If you design the place right, air sealing adds very little extra cost, so generally this is your low hanging fruit.

    Since lumber prices are now back to normal, if you are looking at a budget high R value wall, your best bet is 2x8 24"OC wall with HD batts and no exterior insulation. This gets you the same R value assembly (R24) as your 2x6+R5 wall. This does require a warm side vapor barrier (craft faced batts or 6 mil ploy is fine) and a rain screen for your siding. I would also spend the extra money and go for CDX instead of OSB for your sheathing.

    1. wiscoguy | | #4

      ThNks for the reply. The 1” foam in this case is halo which is vapor open so I could still use poly inside in this construction and no matter what route I take I will be sealing the house with tapes foams and caulks to the best of my ability.

      I like the exterior insulation to help prevent the thermal bridging the air ceiling from the outside is the easiest part for me honestly. It’s the interior sealing well that is the bigger challenge without a service cavity. So many outlets etc with potential small air leaks.

      I plan on doing everything possible to prevent air leaks I’m very confident I can get the outside very very tight.

      I appreciate any insights on interior air sealing or is the outside being tight enough going to be enough to prevent air leaks and heat loss. Also prevent condensation issues.

      Appreciate any more thoughts you may have.

      1. Expert Member
        BILL WICHERS | | #5

        It's usually inadvisable to use interior polyethylene. That's a vapor BARRIER. You really want a vapor RETARDER (semipermeable) on the interior in heating dominated climates.

        Note that poly, and things like MemBrain too, are fairly easy to damage when installing things like drywall. They also take about the same amount of effort to detail airtight when compared to drywall. I'd really try doing airtight drywall and not trying to detail something like a vapor retarder as your air barrier here (although I have done both). If you don't think your drywall guys can apply sealant reliably, try a drywall gasket GreatStuff Pro Drywall Gasket can be pre-applied to the framing so that the gasket material is already in place when the drywall guys start 'rockin. This would let YOU install the gasket material, and put it all the places you want prior to the drywall going up. The drywall crew would just need to be told to try not to scrape off or damage the gasket material while they hang the drywall.


        1. wiscoguy | | #6

          Thanks I’m going to check this out I wasnt aware of this

      2. Expert Member
        Akos | | #7

        Walls only need to be air sealed on one side, the easiest as you point out is the sheathing. No matter what type of construction, you generally still want a decent warm side air barrier, but you don't need to go crazy on this.

        What you want is to caulk the perimeter of your drywall, make sure the drywall covers all the bottom plate (sometimes drywallers are not careful and cut sheets too short which is a big problem) and use vapor tight device boxes in exterior walls. Provided you have a solid exterior air barrier, there is no need for service cavity or fancy air barriers.

        When it comes to heat loss, what matters is the effective assembly R value. The effect of thermal bridging is captured in the assembly R value, so if two walls have the same effective R value, they will have identical heat loss.

        Chasing zero thermal bridging is an passive house thing, while it is good to eliminate it, there are much better spots to focus your effort on.

        Generally walls with exterior rigid insulation are more robust as they keep the sheathing warmer. How much this matters in the real world is up for debate as we have been building thick double stud walls in cold climate without issues for a long time now.

        1. wiscoguy | | #8

          Thanks for posting. I’m starting to learn this as well I can seal the outside super tight. Inside with gaskets and caulk plus drywall I can do pretty well. What kind of boxes do you recommend for keeping a tight air seal I have been struggling to find outlet and switch baskets although I also plan on the gasketed covers. More so though led lights in the ceiling like insulted cans or I prefer the led puck lights that are more of a wafer but that’s just a wire hanging then they cut the light in later with a hole saw it’s a pretty snug fit but deffinetely not what I would call air tight.

          Any suggestions on boxes and cans or led lights that are easy to seal for exterior walls would be greatly appreciated.

          Thanks, Tom

          1. Expert Member
            Akos | | #9

            The search term is "vapor tight device box", might be a special order at your local box store:


            The best for ceiling lights it install a vapor tight ceiling device box then mount a slim low profile LED light on top. Properly installed, these are tight enough that you won't see much if any leak during a blower door test.

          2. Expert Member
            BILL WICHERS | | #10

            If you can't get the sealed boxes, or don't want to use them (they have a few downsides), try the white fiberglass "hard boxes". The hard boxes have no holes, only thin spots in a few places that you can punch through with a screwdriver to get wires in. Since the hard boxes have no holes, they don't leak which makes them easier to seal on exterior walls. You still need caulk and canned foam with the hard boxes though -- they don't have gaskets the way the factory-made sealed boxes do.


          3. Jason_K | | #21

            "they have a few downsides"

            Bill, can you elaborate on these?

    2. Jon_R | | #13

      > how much more savings
      > 71 Therms ... {maybe} drops to 18 Therms {per year}

      To add perspective, while this varies widely, you might be looking at around $1 per therm or $55/year.

  4. wiscoguy | | #11

    Thanks for the reply I’m thinking your talking about like a lessco type box

  5. Jon_R | | #12

    Lots of relevant recommendations here, including "air barrier on interior and exterior sides":

    Or here if you want more explanation of the advantages of an interior side air barrier.

    Good options wrt exterior foam and vapor retarders are here:

  6. qofmiwok | | #14

    Same thing happened to me, and I also bought lumber at the peak :-(
    I'm in 6B which is admittedly dryer than your 6A climate. And also only 2 people in a large house with a full ventilation system, so I am not worried about moisture from the inside. I am doing no internal air sealing and just drywall/paint as an internal vapor barrier. I'm not actually sure why people spend money on an internal air barrier when they've also got an external air barrier. Having 2 is no better than a single good one. I can only assume it's because they are in a more humid climate than mine, or have high occupancy, or for some other reason are worried about moisture from inside the house and so are combining it with a vapor retarder.

    I've got 2x6 with 2" exterior rockwool, and am air sealing the entire outside "box" of the house with self adhered membranes before putting the roof on.
    My priority is air sealing first before more insulation. (Remember in building science the order of priority is 1. bulk water, 2. air, 3. vapor, 4. thermal.) In fact according to the models for my house, even going from 1.5 ACH50 to 1.0 ACH50 made more of a difference than adding more insulation.
    For me the priority was air quality which meant focusing on bulk water first, then condensation second. So that means impeccable air sealing, almost everything is vapor permeable (all except a very small amount of spray foam in a few areas like rim joists,) reduced thermal bridges including high R value windows (which aren't any more expensive if you buy them from the right places), ventilation, and a rainscreen.

    1. Expert Member
      BILL WICHERS | | #15

      The reason for two air barriers is for some redundancy, since seals inevitably begin to leak eventually. The idea is more barriers = longer timespans with high levels of performance. Since it's relatively easy and cheap to detail the interior drywall as a secondary air barrier, I always recommend doing so even if you have a very well detailed exterior side air barrier. The extra iar barrier doesn't hurt anything, might help, and adds very little additional cost to the project.

      I personally prefer to detail exterior rigid foam as a primary air barrier, and then detail the interior drywall on exterior walls as a secondary air barrier. I have on occassion also detailed a vapor retarder as a sort of third vapor barrier, but that was more of an experiment just to see if it was faster or easier than detailing the drywall as the air barrier (it wasn't, drywall is easier/faster to detail as an air barrier).


    2. wiscoguy | | #17

      That’s interesting. I’m using tapes to tape osb seams and then good tapes for any pertrusion window door small amount of spray foam where needed. Then memento 1000 which is a good water and air barrier with all of the tapes caulks and flashing outside. Lastly will be 2 1/2” of gps I choose gps because there’s no thermal loss over time and it actually performs better as the temperature drops. Lastly there’s also the siding I’m using is a composite similar to vinyl but with a class A fire rating and better look made with fiberglass and resigns. It also has a foam back it’s r2 so around r14.5 total on exterior walls plus all the tapes and a quality house wrap.

      I do have a family of four so I’m a little concerned about moisture inside but with bath fans hooked to my erv and a whole home dehumidifier I feel like that will combat any of these issues. The inside of the wall will be 2x6 with rockwool. For a total of about 37.5 roughly. I was shooting for 40 but money just wasn’t there.

      I feel like I can get a pretty good job of sealing up the outside and with the air exchanger running every 15-20 min plus dehumidifier I should t have problems. My sheeting should really never get cold. And moisture in the walls should be minimal if there’s any at all and deffinetely shouldn’t condensate.

      In the end I still may use intello or membrane if I could find it anywhere. If anyone has a supplier they know of where you can buy membrane I’d already have it otherwise I may go with the drywall gasket approach another member pointed out to me seem like a really good compromise to caulk that will do a good job.

      Thanks for your post.

      1. Expert Member
        BILL WICHERS | | #18

        I've been looking for MemBrain too, but it's unobtanium everywhere -- even the salvage places don't have any!

        If you want to save some money here, skipping the Intello will save you a lot. It is extra protection, but if you have enough exterior rigid foam you can safely leave out an interior side vapor retarder. I would still detail the drywall to be an air barrier though, which is extra insurance for the wall regardless.


  7. wiscoguy | | #16

    Lots of good information in all of these posts great responses thank you to everyone that’s been posting. I’m starting to feel more confident about my wall assembly and techniques I will be using. Another thought I had is with most of my trim being white and painted and caulked in place that will be helpful as a sealer on some level as well.

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