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Community and Q&A

Air barrier in middle of double wall in mixed climate?

Jeff Cooper | Posted in Green Building Techniques on

I’m in central Arizona, at the cold end of the Climate Zone 2B range, where I have around four months of moderate heating and five of moderate to heavy cooling. In the midst of the complexity of planning for building a 20×48 single-story house entirely by myself, simplicity tempted me briefly away from my plan to use a double wall, but after thinking in more detail about insulating around all of the wires and electrical boxes in a single exterior wall and creating airtight drywall, I’ve concluded that, in addition to the obvious benefit of greater energy efficiency, a double wall will be much easier to build–unless my plan to put an air barrier in the middle of the wall has some major flaw I’m not seeing.

From the exterior in, here’s my wall:
SmartSide panel siding/sheathing
Vapor-permeable WRB
2×4 exterior, load-bearing wall insulated with Roxul batts
Vapor-permeable air barrier
Plywood gussets to keep the 1×3 studs of the interior wall straight and 1-1/2″ wide strips of 1″ polyiso to fill the 1″ gaps between the parallel 2×4 and 2×3 studs
2×3 interior, non-load-bearing wall insulated with 3-1/2″ Roxul batts (extending into the 1″ gap)
Vapor-permeable latex paint

My wires will all run through the gap between the two parallel walls, and I can fit standard electrical boxes in the interior wall plus gap.

I understand that dense-packed cellulose would have made insulating around wires and boxes much easier, but it’s not compatible with SmartSide, I can’t do it myself, and it’s more expensive.

Putting the air barrier on the inner plane of the exterior wall would seem to offer all of the advantages of simplifying and protecting it that Martin notes in his “Service Cavities for Wiring and Plumbing,”, the double wall would leave the insulation in the exterior wall almost entirely uninterrupted, and the inner wall would be well insulated, albeit less perfectly. The nominal R-value of the wall with two layers of R-15 Roxul would be R-30, ample for this climate, and thermal bridging would be minimized by the gap. Martin’s article and the discussion that follows are oriented mostly toward a cold climate. Is there anything about a mixed climate that would make my plan unsuitable? If not, what would be a good, economical choice for the air barrier in the middle of the wall?

Also, if I use 3/4″ plywood for the gussets, what’s the narrowest the strips of gusset could be?

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  1. Expert Member
    Malcolm Taylor | | #1

    Can I suggest a few things to consider.
    - I'm not sure from your desciption if you mean to use your SmartSide panels as structural sheathing. Is it rated for that use?
    - Using 2"x4s on the interior wall would allow you to forgo the gussets at a very small penalty to the width of your assembly. Getting rid of the gussets makes maintaining the continuity of your mid-wall air barrier simpler.
    - As the outer wall is load-bearing, you aren't going to end up with "almost entirely uninterrupted" insulation. That wall will contain posts, headers etc. You might consider making the interior wall load-bearing and having all the service and structural interruptions in one place.
    - I don't know if you are building a floor system or on a slab, but when deciding on exterior wall it's a good idea to do a section through the wall at the roof and foundation before deciding on the assembly.
    Good luck with your build!


  2. Jeff Cooper | | #2

    Thank you, Malcolm.

    The SmartSide is rated as structural sheathing.

    Given that the midwall air barrier would be attached to the 2x4 studs and the gussets would be attached to the 2x3 studs, would they interfere with each other?

    I thought about a 2x4 inner wall, but I would give up an extra inch of floor space unless I removed the 1" gap between the inner and outer walls, which would increase thermal bridging. Also, I wondered whether 2x4s might also twist and bend if not attached to sheathing, even though they're more stable than 2x3s.

    Yes, "almost entirely uninterrupted" insulation was an overstatement; I meant by wires and such. How would putting the interruptions that would occur in framing the outer wall into the inner wall improve overall efficiency?

    Do I surmise from your reply that the midwall air barrier doesn't raise any special concerns in my mixed climate, per se?

    You have given me some good advice previously on housewrap. Do you have any recommendations for my midwall air barrier?

  3. Expert Member
    Malcolm Taylor | | #3

    I'm slowly getting a good idea as to what works here in my climate, but I'd be well out of my depth commenting on your situation there. I can't see why a mid-wall air barrier/vapour retarder wouldn't work, but I'll defer to others with more experience.
    I guess I'm not clear on the gussets. Don't they have to be attached to both walls to be effective? The inner wall shouldn't twist any more than similar interior walls do with drywall on one side.
    I asked about the floor because how you plan to detail it can have a big influence on what you use as the air barrier and how practical it is to install. If you are planning a floor system, it's worth keeping it in at the same plane as the inner wall so it is protected by the continuous insulation of the outer wall. And if you plan to do this, it's very difficult to organize a sequence of construction that would allow you to build the outer wall first and install the air barrier onto it. That's partly why I'd be tempted to make the inner wall load-bearing, use 2"x4"s. attach the air barrier to the outside of it. i guess if you are using a slab this doesn't matter anyway.
    Unlike fiberglass,mineral wool batts than are much harder to work with around plumbing and other obstructions. You can't cram them into voids the same way. That's why I though it might be a good idea to have one wall with a minimum of structure and obstructions where you could minimize the losses and use full batts everywhere.

  4. Jeff Cooper | | #4

    Thank you for the further input, Malcolm.

    I picture the gussets running across the outside edges of the inner wall's 2x3s and keeping them from twisting and bowing by joining them to one another, essentially as if they were attached to sheathing, but just a narrow strip of it. Did you picture them running from each 2x3 to its 2x4 neighbor, or did you picture gussets similar to what I described, but attached to the inner edges of the outer wall's 2x4s as well? If the latter, would it compromise the air barrier much to have a gusset attached on its inner side? I could predrill holes, squirt some caulk into each, and screw the gussets onto the 2x4s if necessary.

    I do plan to build a raised, wooden floor. At the moment, I don't understand why my midwall air barrier would be any more difficult to extend to a seal at the floor than airtight drywall would be if my outer wall were the only wall; perhaps I'll get it tomorrow when I'm fresher, but any explanation you offer would be welcome.

    My impression is that although Roxul is harder to cram into spaces, it's much easier to cut so that it doesn't have to be crammed. I understand your concept of putting all of the obstructions in one wall, but I figure whatever loss of performance working around those obstructions would cause will occur in one half of the wall or the other so that the whole wall's performance will come out the same either way.

  5. Nate G | | #5

    I can confirm that Roxul batts are easy to cut. I have found them to be very easy to shape around obstructions in stud cavities with nothing more than a common kitchen bread knife. Because the material is so firm, you really can carve it in a very precise manner. It's nothing like stuffing and manhandling a typical squishy fiberglass batt.

  6. GBA Editor
    Martin Holladay | | #6

    It doesn't sound very buildable to me -- like Malcolm, I think that 2x4s for the inner wall make more sense, and I'm having a hard time visualizing how you are installing the 1 inch polyiso. (Is this a continuous layer of 1 inch polyiso, assembled from strips that are later taped together?)

    But for some reason, your biggest concern is the location of your air barrier. I'm happy to answer that one: there are no concerns arising from locating an air barrier in the middle of the wall. Like all air barriers, you'll want to verify its performance with a blower door.

  7. Expert Member
    Malcolm Taylor | | #7

    What we usually call gussets in a double wall are small strips of material tying inner and outer walls together. Enough of them and they become Larson Trusses. You are suggesting furring strips horizontally on the 2"x3"s. There is really no need. Your framing lumber should be dry enough that it doesn't twist or bow. The interior drywall will help spread the loads and provide stiffness.

    Double walls can be framed basically two ways:
    - You frame the floor system then put up the two walls. As both walls sit on the sub-floor, you still have all the attendant problems of air-sealing and insulating the rim joists, but installing a mid-wall air barrier on the outside wall is simple.
    - You frame the floor and inner-wall conventionally, but set in on your foundation. You then frame the outer wall, extending it past the rim joist to sit on the foundation sill plate, providing a continuous air seal and insulation at the floor. The trade-off in your situation is that you don't have access to the base of the outer-wall below the floor level to air-seal.

    Like Martin I'm not clear on what happens in the gap between your two walls. If the rigid insulation is not continuous, don't you end up with an air space? If it is continuous, where do the wires and boxes go? That's another reason I'd be inclined to use 2"x4"s and drill for electrical.

  8. Jeff Cooper | | #8

    Thank you, Nate, Martin, and Malcolm.

    In the plan I described above, the strips of polyiso just extend the width of each 2x3 another inch so that it looks like a 2x4 with the last inch made of polyiso. The purpose is to interrupt thermal bridging while creating two 3-1/2" spaces to fill with Roxul batts. I will leave a small gap in the polyiso so that I can run wires between the studs. I could use two 2x4 walls to create the same amount of space, but they would have to touch each other, and it would require me to notch each 2x4 to make room for wires to run between the two walls. If I left a gap between two 2x4 walls, I would have too much space for two 3-1/2" layers of Roxul, and that would create a convection channel and waste floor space. More than 7" of Roxul in a wall isn't cost-effective in my climate.

  9. Expert Member
    Malcolm Taylor | | #9

    I get it now. If you are doing the work yourself that sounds like a good idea.
    To get back to your original question: I don't think it makes a great deal of difference what material you use for air sealing. Even (blasphemy!) poly wouldn't do much harm as the wall can dry both ways. Probably the best bet would be an intelligent vapour barrier, rather than all the work of OSB and tape when you already have exterior sheathing in the SmartSide.

  10. Jeff Cooper | | #10

    Thank you, Malcolm. You once suggested the commercial Tyvek. If it, or another housewrap, has a sufficiently low air permeability and high vapor permeability, would it make sense? I've read that housewrap isn't a great air barrier in normal application, but that seems to be mostly because it gets so abused on the exterior wall and has all of the windows, doors, etc. to seal around.

  11. Expert Member
    Malcolm Taylor | | #11

    I don't see why not. Commercial Tyvek is only about half as permeable as the regular stuff, but you do want some vapour resistance in the assembly. I'd compare the price to one of the smart air barriers and see where you stand. Tape sticks to it very well and seeing as how it may end up being installed while you are still performing other tasks nearby, the robustness will be very handy.

  12. Jeff Cooper | | #12

    Thanks again, Malcolm.

  13. Expert Member
    Dana Dorsett | | #13

    In a central AZ zone 2B climate you can make the assemblies as vapor-open as you like. The 99% outdoor design temps in 2B aren't much lower than the interior air dew point, let alone the winter average outdoor temp. The air conditioned interior temps in summer are never going to be anywhere near the outdoor dew points either.

    In Phoenix or Tucson outdoor dew point peaks rarely hit 70F, with a summertime average of about 60F. You'd have to air condition to the mid-60s or lower to accumulate enough moisture in the wallboard to cause a problem, even with a 100-perm wall from 5 perm interior paint outward.

    The 99th percentile temperature bins are ~30-32F, and the average mid-winter temps are north of 50F. The wintertime outdoor air dew points averages are about 32F as well- you'd have to actively humidify most homes to keep the indoor dew points much above 35F when it's actually cold enough out to accumulate moisture in the outer layers. Any short term moisture accumulations on the exterior layers will be gone by noon the same day.

    Central AZ is pretty different from 2A climate locations along the gulf coast, with summertime outdoor dew point's averaging in the 70s. In that climate some vapor retardency to the exterior layers is useful for protecting the interior wall finishes in air conditioned houses, but 5-perm fiberboard is enough. The primer finish used on SmartSide panels tests at 6 perms @ 5 mils thickness, which is good enough even for the muggy southeast, and not even needed in zone 2B. And SmartSide would not be damaged from condensation events even if they had longer duration.

    But does a double studwall really make any sense in a zone 2B climate? Do any of the stackups in this discussion have a rationale? What are the actual design goals here?

    In that climate it's possible to hit Net Zero Energy with a 2x6 /R23 type wall (R15-ish whole-wall, after thermal bridging) and even in zone 3 you can get there with a 2x6 / R23 + R5 continuous insulation, which is R20 whole-wall. Download a copy of BA-1005, and read the first chapter, taking note of Table 2, on p.10.

    A 2x4 /R15 wall with an inch of polyiso on the interior comes in at about R16-R17 whole-wall. With 2" you'd be over R20. Tape the seams with foil tape and you're done. Build an interior side 2x3 wall as a convenient wiring chase to protect the air barrier you can insulate it with split R23s if you like, but it's really getting on toward overkill at that point unless you're going for PassiveHouse or something.

    A 2x4 / R15 wall with a 2x3 MooneyWall configuration with split R23s stuffed in the 2x3 after running the wires also comes in around R16-R17. If you want a continuous interior-side air barrier, any housewrap on the interior side of the 2x4s would get you there with this approach.

    Making the wall more complex than it needs to be makes it more likely to suffer from construction defects and air leaks, and it costs more. Since you don't need even an R20 whole-wall to make it "Net Zero Ready", it probably makes ,more sense to build a simple & robust R15-R20-ish wall, and focus the design time & materials budgets elsewhere.

  14. Jeff Cooper | | #14

    Thank you, Dana, for lots of useful numbers to think about. I read the reference you cited and interpolated a bit for my colder end of the 2B range, around 12 degrees colder than Phoenix on average. I have debated whether the wall I describe insulates more than I need, but I concluded that it will make the air barrier and the wiring considerably easier than a standard wall, and my energy savings will eventually cover my small increase in initial cost. Attaching the strips of polyiso to the 2x3s is the most unusual part, but it should take less than a couple of hours altogether.

    Where you do you find the climate data you cite?

    Are "split R23s" 5-1/2" Roxul batts cut into 2-3/4" thicknesses?

  15. Expert Member
    Dana Dorsett | | #15 has massive temperature, precipitation, and dew point histories from 10s of thousands of local weather stations, and displayed graphically it's easy to estimate the mid-season averages using the cursors. See:!dashboard;a=USA/AZ/Prescott

    (Punch in your zip code or city name.) There's a pull down menu for which graphs you want to display in the upper right, just above the graph. Use the sliders at the bottom to zoom in/out.

    If your temps average 12F cooler than Phoenix and your 99% outside design temp is 20F rather than 30-32F odds are your very local climate is a zone 3B (or maybe even 4B) climate, probably due to higher elevation (?).

    Yes, splitting an R23 batt yields a thickness that is easily compressible into a 2x3 cavity. But if you're adding the 1" polyiso edge strips (which I wasn't recommending), you'd use standard R15s.

    A 2x5/R15 wall with SmartSide and no other sheathing will come in about R10 whole wall. Adding continous 1"/ R6 polyiso to the interior brings that to R15. Add a 2x3 wall on the interior side of that, but not Mooney style, just a self supporting wall so that you don't have to penetrate the polyiso to support the interior framing, and detail the iso as an air barrier. Stagger the 2x3 studs with those of the 2x4 wall for enhanced thermal break. Insulating the 2x3 wall with split R23s (and no edge strips) would put it reliably over R20. If you're really going that thick, a staggered stud 2x4/R15 interior wall with the continuous 1" polyiso thermal break between the walls would put it in the R25 whole-wall range.

    According to the BSC cheat-sheet (Table 2 in BA-1005), in a zone 4B climate an R25 wall should be both cost effective in most energy markets over it's lifecycle, and sufficient to enable Net Zero Energy, provided, the house was similarly up to snuff.

    BTW: If you're going to build an interior wall with 2x3 studs, order up some finger-jointed 2x3s, which will be far straighter than milled 2x3s, making for a much flatter wall.

  16. Jeff Cooper | | #16

    Thank you, Dana. Weatherspark is very cool. Odd as it may be, my town officially considers itself to be in Climate Zone 2B despite a winter design temp of 20. The surrounding area is 4B; we're at a slightly lower elevation, but I imagine diverse other factors were involved.

    I really appreciate the alternative suggestions for a wall, but a 2x3 wall with continuous polyiso behind it would require nonstandard electrical boxes, retard vapor more, and put more flammable and pest-navigable material inside the wall. The polyiso might also shrink and pull away from its tape. The strips of polyiso I have in mind are just small space-fillers that provide a thermal break between the studs and allow for an exceptionally easy way to create space for wiring; I don't see their downside, other than an extra hour or two installing them that will be made up by the ease of wiring.

    If I take the time to find kiln-dried 2x3s that are straight to begin with, do you think they will bend and twist after they're installed? If so, would, say, 6" of 3/4" plywood across the middle of the exterior side of the 2x3s add enough to the support the drywall provides to keep them straight?

  17. D Dorsett | | #17

    Isn't it simpler to just build a pair of 2x4 walls with staggered studs then?

    With foil facers if you tape the cut edges it's very hard for insects to use polyiso as a transportation highway. And why would they, when there is much easier digging in the rock wool or wood studs? Polyiso doesn't have the same shrinkage with age issues as polystyrene, but it's coefficient of expansion is somewhat different than the wood framing. I doubt temperature rated foil tapes would give up over many decades, but there is potential wear at the fastener points.

    In your climate the wall can dry to either side during any season, so having a vapor barrier in the middle of the wall doesn't put it at any greater risk. If you wanted something more vapor permeable, 3/4" fiberboard detailed as an air barrier is over 5 perms when bone dry 15+ perms in high humidity, has roughly the same coefficient of expansion as framing timbers, and adds about R2 to the stackup. It changes dimensions more with humidity, but it won't be humidity cycling enough to come undone over time. Leave 1/8" between sheets, tape the seams with housewrap tape, then apply fiber reinforced duct mastic over the edges of the tape by 1" to guarantee a very long term flexible bond.

    If a 2x3 stud is straight when dry, it's not going to change much over time. I takes a lot of sorting to find the straight ones though, compared to just ordering up some finger jointed stock. Finger jointed comes in multiple strucutral & non-structural grades, but they're all much straighter than the average milled 2x3. Since it's not a structural wall, you don't need to go with structural 2x3s fully rated for exterior use.

  18. Jeff Cooper | | #18

    Finger-jointed 2x3s are a great idea, Dana. I couldn't find them on the US Home Depot or Lowe's sites, but the Canadian Home Depot has 2"x3"x8' for $2.44 each, which is way more reasonable than I would have expected, so I'll try to order them. I considered staggered 2x4s too, but I won't be able to stagger them at windows and doors, and, less significantly, where my wiring runs around the studs, it would press into my air barrier and the insulation behind it.

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