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Ready to insulate a new home

easyrider470 | Posted in Energy Efficiency and Durability on

I am ready to insulate a new home, and need to make a final decision. I need help.
I have been the recipient of many helpful tips and guidance from thes pages as well as other green forums and it’s all kind of coming to a head in our new home build as I attempt to make a decision about the insulation we will use. Here is the specs on my houses wall stack up

LOCATION: 47460 On the line of upper climate zone 4 and lower 5

2×6 exterior walls, 1/2 inch sheathing with tyvek, 1.5″ polyiso foam with taped seams, vinyl siding

I am considering the following insulation approaches and need some help makign the decision

OPTION 1: 1″ closed cell flash in stud voids then wet sprayed cellulose

OPTION 2: 1″ closed cell flash in stud voids then ruxol or fiberglass batts

OPTION 3: 2″ OPEN CELL in stud voids and then batt or wet sprayed cellulose

OPTION 4: Full Cavity Fill with Wet Sprayed Cellulose

Please give me pros and cons for all the options, I am mostly concerned about the moisture control in the walls. I do not want to cause problems by allowing moisture to get trapped in the wrong place in the wall stack up.

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  1. GBA Editor
    Martin Holladay | | #1

    It's usually not a good idea to sandwich OSB wall sheathing between two vapor-impermeable layers. Since you are planning to install a layer of exterior polyisocyanurate, it would be logical to design your wall to dry to the interior. That's why you shouldn't install any closed-cell spray foam between your studs. So Options #1 and #2 are ruled out.

    Open-cell foam isn't really used for the flash-and-batt or flash-and-fill method, because thin layers of open-cell foam don't stop air leaks very well. So Option #3 is ruled out.

    It looks like you are left with Option #4.

  2. easyrider470 | | #2

    Well that is the first I have thought about the poly as a vapor barrier. I must have totally missed the perm levels of that foam and the fiberglass paper on both sides of the sheets. I had no idea it was considered a vapor barrier.
    Am I going to have an air seal issue without some type of foam or sealer in the stud cavities? I am ready to insulate the house but have delayed due to my confusion about the options.
    Also, would it still not be advisable to use closed cell even in the rim board areas between my TGI joists due to this vapor barrier issue?
    I guess I wasn't planning for this news so I am hoping you can help me make a better plan that what I had

  3. Airithol | | #3


    Air sealing: At this stage of the game, with sheathing and rigid foam installed, you don't have a lot of options.

    You basically need to seal each stud bay as you noted, as well as any plate intersections, sheathing joints, headers and built up, or ganged studs. Collectively, you need to air seal the "framing".

    To do this, you can use caulk and/or tape. Likely a combination of the two, as taping actual stud to sheathing joints would be a pain. Tape would work for ganged studs and sheathing joints though.

    Or you can try to get a good seal with rolled goods, as long as it is vapour permeable (Membrain, Intello Plus, etc).

    Finally, you could try the airtight drywall approach.

    Of the three, caulk & tape is probably the most durable, simply because occupants cannot disturb them once drywall is installed.

    Rim board: You would still be creating a "sandwich" if you install closed cell foam, unless the exterior rigid foam does not cover the rim board area. This area needs to be treated the same as stud bays, air sealed and then install insulation that is not a vapour barrier.

    Anthony, before you worry about any of this, have you checked with your building inspector about a vapour barrier (aka 6 mil polyethylene)? If they force you to install one, your strategy has already been somewhat decided.


  4. dankolbert | | #4

    Have you done a blower door test yet?

  5. easyrider470 | | #5

    TO: Jason
    Indiana just recently adopted the 2012 energy code, the inspectors in my county are not exactly savy on any of the requirements and I have exceeded most of them. On that note, I do not think there is a requirement for a vapor barrier and in the discussions I have had with the local insulation contractors they are not using one.
    I think this is bringing me back to my Knauf Eco-Seal idea for the airsealing. OR i'm just going to push ahead with the dense pack wet sprayed cellulose and not worry about the little improvement i will gain from the stud cavitied being sealed. I already plan to caulk the top plates and the cripple studs for airsealing so I am once again faced with a challenge of cellulose or airseal then cellulose.

  6. easyrider470 | | #6

    No blower door yet Dan

  7. rob kohaus | | #7

    Your questions about air barrier reminded me of this article of Martin's.

    Ask yourself where your air barrier is and stick to it. Don't just half ass a few different air barriers. If you want it to be at the drywall than do ADA. If you want it to be at your sheathing (too late) than do it at your sheathing. If you want it to be your 1.5" foam than do that. Your sheathing, taped tyvek, 1.5" foam and drywall (even if not done to ADA standards) are still going to result in a pretty tight house compared to standard construction.

    As far as cavity insulation. After using it, I'm a big fan of Roxul. Martin is right about options #1,2,3 being off the table. My first though with #1 was, "Exactly what size Roxul batt is he planning on using?" You'd have to flash 2" and then go with a 2x4 batt.

    If you like cellulose than just go cellulose. Don't worry about sealing every darn stud. Have your drywaller make a best effort at ADA and keep your project moving forward.

  8. easyrider470 | | #8

    So my air barrier is my sheathing because of the taped poly-iso outside the taped tyvek and sheathing correct?
    INSULATION....I get the no foam scenario I have created, and I like the sound of using roxul....however, I can get the wet sprayed cellulose in the wall for .80cents a ft. I'm pretty sure that is way cheaper than Roxul and a lot less work....BUT I have not priced Roxul yet.
    Care to share a best way to get the drywallers to attempt ADA? Haven't researched that at all.
    Also, I am going to have to be careful with paint type still on my walls right?

  9. Richard Beyer | | #9

    Blower door test the house, seal the leaks found and Roxul the structure. In my opinion this is the best fire resistance a home can get from insulation and still maintain r-value. After all tests are showing air leakage is the real culprit to energy loss. With the use of all this engineered lumber you should also seriously consider a sprinkler system.

    "In three to four minutes, houses are flashing over because of the heavy fuel load," said James Burns, president of the Firemen's Association of the State of New York. "They are collapsing on the ground before the fire service arrives."

  10. easyrider470 | | #10

    I think it's abundantly clear that Foam is not an option for me in the house except for the wall between the garage and the main part of the house and some walls upstairs in the bonus room over the garage.
    RUXOL is one of the 2 options for my exterior walls but it's considerably more expensive that the blown in cellulose.

  11. easyrider470 | | #11

    Ok after much debate and research I am back to the foam discussion..DANA seems to think that 2" of closed cell will not limit the wall drying to the inside so I am considering 2" closed cell in the stud voids again

    Also what about a full cavity fill of open cell. The new foam from Icynene is R-4 so I could get an R-22 of foam plus my R-9 exterior foam if I did the full cavity fill in open cell. Can someone tell me the thermal bridging loss of a full cavity fill???

    Also, the R-12 of the closed cell flash plus an R-15 Roxul batt would greatly outperform ANYTHING else. I wonder if the cost is worth it though in my climate. With that stack up I would be at R-36 before ANY deductions for thermal bridging. In 47460 zip code is that kind if R-value worth the effort and money?
    I like the whole stud void fill with open cell option but I have heard that after 3 " the performance of the last 2.5" dropps way off....AHHHH this is a tough decision.

  12. GBA Editor
    Martin Holladay | | #12

    Q. "Can someone tell me the thermal bridging loss of a full cavity fill?"

    A. The answer depends on your framing factor (the percentage of your wall area that is taken up by studs, plates, and headers). Many people assume a 25% framing factor.

    Regardless of what type of insulation you use, 25% of your 2x6 wall will have an R-value of about R-6.8.

    However, I wouldn't worry too much about thermal bridging if I were you, because it sounds like you plan to install a continuous layer of R-9 exterior foam. The R-9 foam will adequately address the thermal bridging problem.

  13. easyrider470 | | #13

    That's correct Martin but you told me in another post that if I did the closed cell or open cell in the stud voids that the thermal bridging of the foam in the studs would reduce the effectiveness of the exterior foam. That's why I am asking. I am trying to get the most bang for my buck here and not waste any money

  14. GBA Editor
    Martin Holladay | | #14

    Studs are always thermal bridges. Adding a continuous layer of insulation on the exterior side of the wall sheathing is an effective way to address the problem of thermal bridging.

    If you have both types of insulation -- some insulation between your studs, and some insulation installed as a continuous layer on the exterior side of your sheathing -- and you want to upgrade your R-value, you will always get more bang for your R by thickening the continuous insulation. Improving the R-value per inch of the insulation between the studs is a poor strategy in comparison.

  15. Expert Member
    Dana Dorsett | | #15

    A flash-inch of 2lb polyurethane at ~0.8-1.5 perms is still 2-3x as vapor permeable than a kraft facer, and is in now way comparable to a true vapor barrier, and does not constitute a moisture trap. At 2" it's getting pretty tight, but still not a show-stopper.

    At 1.5" the polyiso itself is sufficient for dew point control on 2x6 framing in your climate, even when derated it for wintertime temperatures.

    But Martin has it totally right. Spending the foam budget for thicker exterior foam is the right solution. Any R6/inch foam thermally bridged by the studs is pretty much wasted due to the R1.2/inch of the framing. Spending the foam budget on more exterior foam in the form of EPS has these benefits:

    * Adding 1.5" of EPS over your 1.5" polyiso costs LESS than a flash-inch of 2lb polyurethane between the studs. (about 60 cents/ square foot + installation, vs. $1/square foot for an inch 2lb polyurethane, or $2 / foot for 2" of 2lb polyurethane, installed.)

    ** Adding 1.5" of EPS would also keeps the average temp of the polyiso layer high enough that it performs at it's labeled R value of R6-6.5/inch instead of ~R5-ish.

    ***Adding 1.5" of EPS over the polyiso brings the temperature of the sheathing up to where you have HUGE dew point margin, making it far more moisture resilient than just 1.5" of polyiso.

    With 1.5" continuous polyiso + 1.5" continuous EPS + 5.5" cellulose cavity fill your "whole-wall" R is going to be about R30.

    With 1.5" continuous polyiso and 2" ccSPF+ 3.5" of cellulose between the studs your whole-wall R is going to be about R25.

    Closed cell foam between the studs would be paying substantially more money, for substantially less performance.

    If you're trying to max the bang/buck, skip the closed cell spray foam (open cell would be fine if the quote is cheaper than cellulose, which sometimes happens), and spend the difference on the outside where it does the most good. R30 whole-wall done this way is just about where you want to be from a long term financial analysis point of view in your climate.

    Any more than that would have lower return on investment than grid-tied roof top solar, and it is sufficiently high performance that you may even be able to hit Net-Zero-Energy with an array of 20% efficiency PV that actually fits on the roof, provided the rest of the house has similarly higher-than-code-min performance. Read at least the first chapter of this, and note the whole-assembly R recommended starting points in Table 2 on p. 10:

  16. easyrider470 | | #16

    Thanks for the info. I reviewed the building science document you attached, thanks.
    I have one more scenario, since adding more rigid foam is not an option at this point as the house already has the siding installed.
    What am I looking at whole wall R-value if I just spray the wet sprayed cellulose in the wall with the 1.5" polyiso on the outside?
    I would eliminate some of the thermal bridging the foam causes and would greatly reduce air infiltration due to the density of the wet sprayed cellulose.
    The installer is telling me that the cellulose has an R-value of 3.75...but I havew always heard 3.5
    What kind of whole wall R am I looking at this way? Is it going to be my best bet?

  17. GBA Editor
    Martin Holladay | | #17

    When you discuss switching from spray foam insulation between the studs to cellulose between the studs, you justify the change by noting that "I would eliminate some of the thermal bridging the foam causes..."

    But the foam doesn't cause any thermal bridging. The studs do -- in either case.

  18. easyrider470 | | #18

    Right, but based of of some of the comments I was under the impressiosn that the spray foam tied everything together bridging even worse. I'm a little confused I guess.

  19. wjrobinson | | #19

    The long time designer and builder of externally insulated homes

    Google; Bruce Brownell Adirondack Alternative Energy

  20. Expert Member
    Dana Dorsett | | #20

    Nothing that you fill the stud bays with increases the thermal bridging, as long as it has an R-value greater than the wood. The thermal bridge is the low R/inch of the stud.

    The problem some folks run into is when they go for code-min using closed cell foam and don't completely fill the stud bay. Any part of the stud that doesn't have insulation on both sides of it is at room temperature, and the path through the wood that IS fully insulated on both sides is now much shorter, with a lower total R-value due to the fewer inches of wood between the warm & cool side of the layer.

    As long as the cavity is completely filled, you've minimized the thermal bridging. But when you bridge R6/inch foam with R1.2/inch wood, at a 25% framing fraction the majority of the heat flow at that layer is going through the wood- you don't reap the full benefit of the high-R/inch foam.

    As long as the cavity is air-sealed to the sheathing, there isn't sufficient additional benefit to the closed cell foam. With 1.5' of polyiso you have more than sufficient exterior R to limit wintertime moisture accumulation in the sheathing (with thinner exterior foam there would have been a rationale for a flash of cc foam for it's vapor retardency). The fact that the high R-/inch foam is thermally bridged by low R/inch wood, the additional thermal performance you get out of a flash-inch or two of closed cell foam is pretty tiny.

    Best bang/buck would be to fill it completely with either:

    A: cellulose , which has a bit of thermal mass benefit, and moisture buffering protection for the sheathing, but would require you to caulk the sheathing to the framing before insulating for maximum air-tightness


    B: open cell foam, which does a decent job of air-sealing the cavity on it's own, without pre-caulking.

    Splitting the cavity fill with an open cell foam layer and a fiber layer doesn't buy you anything, and a closed cell flash-foam doesn't buy you enough performance to make it worthwhile. The ~R9 polyiso (even if derated to R7.5 for winter time design purposes) on the exterior already gives you sufficient dew-point margin at the sheathing layer, and that R6/inch flash foam improves the whole-wall R performance by less than R1.

    If the quotes for o.c. foam and caulking + cellulose are equal, I'd probably opt for the cellulose- it's lower risk and offers more moisture protection, but it's not worth paying a lot more for. Sometimes open cell foam is substantially cheaper than cellulose, sometimes it's the other way around. It depends on how hungry & competitive the local market is.

  21. easyrider470 | | #21

    Thank You very much for the thorough post. That made it a ton easier for me to really compare price with benefit. If you could now post the same thorough help post on my bonus room thread I would be STOKED!!! lol....
    I like the Cellulose option because of the price point, however the guys installing aren't doing any caulk in the stud bays, they are offering the caulk on the king studs, cripples and top/bottom plates but nothing for the bays. I will see if they can get a decent price together for that. So far the cellulose with the caulk package I mentioned above it .85 cents a ft. The price for the open cell spray foam at 5.5 inches thick is right at 1.75 per ft.

  22. BobHr | | #22

    Eco seal seals on the inside of the sheathing. There is no reason the inside of the sheathing could not be your air barrier. I would use that as your air barrier. Then I would go with cellulose. It has reasonably good air sealing properties and would back up your primary air barrier. Cellulose is also good for filling odd sized bays and around wiring, outlets, etc

    As far as using closed cell. Have you ever seen it installed or after the install. It is no way a uniform thickness. Also it can be very bumpy, a good path for air movement, I would think you would have some problems with the Roxul bulging out against the drywall.

  23. BobHr | | #23


    To understand thermal bridging. You have studs with a low insulation value of about 1 per inch or an R6 for a 2x6, In between the studs you put insulation. The R value of fiberglass would be about R16. So you have strips of R6 and bays of R16, The R6 week points are the bridge. Changing to close cell you still have the strips of R6 with better insulation in between.

    Now if you add insulation on the outside you have covered the strips- bridges- of R6, Or you can do a double stud wall as another method of eliminating the bridge.

    I think at this point the best bang for the buck it to create the best air barrier that you can. I would look into the eco seal method and follow with a blower door before insulating.

  24. wjrobinson | | #24

    I second Robert and call for a vote and all have voted in favor. The Eco is the way.

  25. Richard Beyer | | #25

    Grace Vycor has a spray applied vapor barrier used in new construction which will air seal the entire structure from the outside eliminating the need to find all those holes from the inside. Then you can insulate your home with anything. After all it is the air leakage causing the problem, not the insulation you decide to use. May I suggest cellulose, fiberglass or Roxul for fire protection and insulating value. You can not achieve the same fire protection from spray foam or rigid foam.

  26. BobHr | | #26


    I think that would have been a preferable way to go but the siding has been installed. His options at this point are to seal the inside of the sheathing or to ADA or both.

  27. easyrider470 | | #27

    Right Richard, I am wqay past an exterior solution because I am already weathered in and ready to insulate.

    Any thoughts on just moving forward with some roxul batts in the stud cavities and closing it up? The exterior Polyisio is going to do a good job of air sealing since I taped the seams and detailed the coreners well. It will be leaps and bounds ahead of a regular home built with tyvek and sheathing. Not giving up but I am lookingat some seriour money to have the darn foam sprayed in. I am still considering the ecoseal product then using ruxol.

  28. GBA Editor
    Martin Holladay | | #28

    In my first response (Comment #1), I suggested that you adopt Option #4 (cellulose between the studs).

    You are now leaning toward mineral wool batts between the studs -- another similar option, since both of these types of insulation (cellulose or mineral wool) are air-permeable and vapor-open. So, yes -- either cellulose or mineral wool between the studs will work.

  29. Richard Beyer | | #29

    I agree 100% with Martin on this one.

    Did you perform a blower door test after you closed in the house? (before insulating) I'm seeing a lot more people advising test before and after. You can not seal air leaks easily after the walls are up.

  30. Expert Member
    Dana Dorsett | | #30

    It's still worth caulking the studs to the sheathing before installing batts, even if you've done a pretty good job taping the facers on the iso. With a powered caulking gun an a couple cases of acoustic sealant this it does not take a huge amount of time to air-seal the stud bays.

    With rock wool you get slightly higher thermal performance than with cellulose or most open cell foam, but only if it's installed perfectly- no compressions, gaps or voids. Since batt are not installed under pressure, they have no ability to air seal the stud bays the way open cell foam or cellulose does, which bumps the priority of caulking it.

  31. easyrider470 | | #31

    Ok, thinking more and more about this caulk and batt approach and I think it could save me a TON of money and greatly improve my timeline. That being said the Eco seal product comes into play here. Imply because I was quoted .85 cents a foot to do that install. That's cripples, doubled top plates. Bottom plates the whole works. Only thing left for me to do is the foam around the windows and doors then install the batts. Can someone give me an idea of cost per foot of the caulk method? Honestly the Eco seal product is documented effective stuff and 85 cents a foot is cheap especially since I would have to buy the expensive caulk gun and cases of caulk. Please advise on your cost ideas.

  32. easyrider470 | | #32

    No blower door yet as the ceiling drywall is not installed and the flow of things with my subs won't allow for total drywall until after insulation
    As for the whole wall r value can anyone speak to the whole wall r value of 1.5 polyiso outside and 4" of open cell in the 2x6 studs?
    What about the Eco seal or caulk with an r23 ruxol batt

  33. GBA Editor
    Martin Holladay | | #33

    You're going around in circles a bit, aren't you? At some point you need to make a decision and move forward.

    Accurate whole-wall R-value calculations require sophisticated software or a large guarded hot box. Roughly speaking, though, "1.5 inches of polyiso outside and 4 inches of open-cell spray foam in the 2x6 studs" will provide about R-10 + R-12 = R-22, and "Eco Seal or caulk with an R-23 Roxul batt" will provide about R-18.

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