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Community and Q&A

Air Sealing in a Hot-Humid Climate

Kman91 | Posted in Energy Efficiency and Durability on

I’m about to start having my new house built and I’m working with a friend who is very flexible, but not very knowledgeable about building science.

I could use zip system and tape, but for $32 a sheet, this is pretty expensive. I could also use regular osb ($17 sheet) and 1″ of spray foam.  I could also maybe use regular osb and external used iso foam sheets, but I’m not sure how these are normally sealed. These seem to be good sealing options, but maybe there are better ones.

I know the spray foam will likely be more expensive than zip, but I wouldn’t mind the extra r value of this or the iso.

What’s the most cost effective sealing for a hot humid zone?

Thanks so much for the wealth of info here!

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Kman91,

    Any of the sheet goods used as sheathing (plywood, Zip, regular OSB) can be taped and used as an air barrier. You can also use a variable-perm membrane as an interior one instead. Both approaches work if done diligently. The key is to make sure the air-sealing is continuous, and to think through how that occurs at the transitions between the foundation, floors, walls and roof.

    1. Kman91 | | #3

      Thanks Malcolm, I was thinking of taping using taped OSB or plywood once, thanks for the confirmation. Part of my problem with Zip (and I love the concept) is installation. Local builders, at least the ones I'm using, aren't experienced in Zip, and so overdriven nails, not rolling tape, etc. may be an issue where as spray foam at least will likely seal it, but I agree that it may be cost prohibitive and maybe not the best solution. I'll probably just have to be onsite and make sure they do it right (at least where/when I can). I also completely agree about the continuous sealing part. Thanks!

  2. Walter Ahlgrim | | #2

    “I could use zip system and tape, but for $32 a sheet, this is pretty expensive.”

    When I drive by all the new apartment/ condo projects I never see OSB and house wrap any more. When they build that many square feet the pencils get very sharp looking for even the smallest costs. Small saving turn out to be what I would call big money and all I see is ZIP.

    My guess is buy the time they add the house wrap and the labor to install they come out ahead.

    Personally I see spray foam in new construction plans as a red flag for a poor design being covered up with unnecessary expensive materials. Seems to me laziness combined with contempt for the budget equals spray foam.

    Almost every problem in new construction can be done better at a lower cost while avoiding spray foam.

    Since you are in Texas can I suggest you not be a lemming and spend the time to find a way to keep the HVAC equipment and duct out of the attic and avoid applying spray foam to your roof. This will allow you to fill the attic with R60 of cheap fluffy insulation and cut the electric bills by 40%.

    Walta

  3. Kman91 | | #4

    I agree that Zip can/may be less costly in installation when pricing is more normal (pre covid). I wonder if it's still true now? I guess I could do a hybrid approach like Malcolm suggested with taped regular OSB and house wrap. I'll have to get some cost estimates.

    My original plan did involve having a vented attic and sealing the ceiling to avoid the spray foam costs. Thanks for the vote on this idea. My problem with this was figuring out how to keep the AC inside. My idea then was 2 AC units (2800ft2) located in 2 closets and running ducting in the lowered ceiling rooms of this high ceiling house (bathrooms, etc.), so it can be done. I wasn't sure how to seal above that, but I guess standard plywood or OSB could be used, with caulked edges? I like the mini split idea, but not sure how I can get it to all the rooms. Once mini splits get ducted, it seems some of the benefit is lost.

    1. James Howison | | #6

      When thinking about ducts inside and sealing the attic floor (for a vented attic), I think this is a strong option, building a flat roofed box (with a great service cavity for ducts and electrical) then adding the roof trusses. I think this reuses skills and practices familiar to the most trades. https://www.finehomebuilding.com/project-guides/insulation/simple-air-sealed-ceiling-for-a-high-performance-home

      Completely agree on avoiding the "head in every room" mini-split approach. Ducted mini-splits vs small multi-position air-handlers probably come down to installation location and whether you have good, clear, access from below and a filter grilles approach (horizontal ducted) or whether you have access from the side and an accessible central filter location.

      Unducted returns can be done with either equipment (Fujitsu ducted minisplits can go vertical, Mitsubishi can't but they have small multi-position air handlers that can). Short jumper ducts through the deep service cavity can give good return paths (without severe door undercuts).

      Oh, and if you think managing the nail depth on Zip is hard, won't you have to do the same on the OSB and then manage/inspect/worry about the house wrap. Zip at least is one system with one set of instructions ...

      1. Kman91 | | #9

        That is a cool idea, thanks for the link! I'm not sure I need that level of access, but it's great to see how someone has done this.

        >Ducted mini-splits vs small multi-position air-handlers probably come down to installation location and whether you have good, clear, access from below..

        Agreed, although I'm not sure how many installers here know about mini splits. I hope there are more, but currently they don't even want to deal with 2 speed systems (my current system). There are so many AC options.

        I'm glad to hear you talk about the idea of in-ceiling or in-wall ducting for return paths. I haven't heard much about this, but I've done it in a few rooms even in my current house (door undercuts are not sufficient). I wonder why there's no one doing small quiet fan systems for this kind of thing. It would greatly enhance a non-ducted mini split installation to expand it to a few rooms.

        >Oh, and if you think managing the nail depth on Zip is hard, won't you have to do the same on the OSB and then manage/inspect/worry about the house wrap.

        Nail depth on OSB isn't important since it doesn't impair the water barrier from doing its job like it does on Zip, but yes, house wrap can definitely be done wrong as well.

  4. Kyle Bentley | | #5

    Zip pricing must vary regionally, but here its at least 2x as expensive as commodity OSB right now. I was discussing this with a friend, and I think there are a lot of factors to consider -

    1. If you're paying someone for their labor, it's probably a wash whether it's zip + tape, or osb + tape + housewrap. If you're doing the labor yourself, the material costs are lower for tape + osb + housewrap.

    2. I'll throw in another observation point to Walter's - there are about 10 apartment complexes / hotels going up on my drive in to work, a few of them are using Zip, some are using Typar, and some are using Tyvek. With the current price swings I'd say those decisions are made only a few weeks at a time.

    3. If space is a concern, there are ways to put the ducts in the attic, you just have to seal them well and bury them pretty deep in the insulation. This isn't my favorite detail, as repairing anything becomes a real chore, but it can be done.

    1. Kman91 | | #7

      Thanks Kyle, I think you're correct on all counts.

      Zip here is $32 vs $18 as of today (it changes often), so pretty similar to your area.

      I have reservations about blown fiberglass in the attic, since fiberglass not enclosed on all sides means you won't get the stated R value, but I guess cheap is a quality overcoming many faults, and it's still pretty good for the money. I may consider blown cellulous. It seems pretty cheap here too.

      I agree, attic ducting can be done, and if I can seal them, then it's a done deal, even if they leak, they only leak down into the house (if I seal them correctly).

      Part of my problem is that it's a T shaped house, and the smaller section has an upstairs room in the attic, so this will have to be an unvented section of attic, which complicates sealing and insulation. I'm in zone 2, so I could do dense pack cellulous and do a semi permiable membrane at the roof ridge, but that makes me a little nervous due to the moisture issue. I could also do the vented idea and seal the inside as Malcolm suggested, or do the June issue idea with dense pack cellulous. I'm not sure what's best.

  5. GBA Editor
    Kiley Jacques | | #8

    For your consideration (regarding OSB as an air barrier):
    Study Finds OSB Not a Reliable Air Barrier .

  6. Kman91 | | #10

    Thanks Kiley. Interesting comments on that one. I'm not looking for passive house quality sealing, but it does raise some good points.

    The major variable in all this is cost. Peal and stick WRBs, spray on WRBs, Zip (with integrated WRB). Ideally Zip costs will go back down again, but I wonder when or if that will be.

    I noticed today that Forcefield is available here now for $23. I'll have to check into this. Looks like some good articles comparing Zip and Forcefield here too.

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