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Most cost-effective method for achieving 3 ACH 50?

Nick Sisler | Posted in Green Building Techniques on

What are your strategies?

Here’s our scenario: climate zone 5, trying to get away using fiberglass batts. One colleague is recommending the air-tight drywall approach. Approaches would be either use a spray on gasket material like Owens Corning EnergyComplete or manually caulking of drywall (which will significantly slow down the drywallers). Both of these seem like they will be a lot more expensive than a traditional exterior housewrap (i.e. Zip-wall, BlueSkin or just Tyvek) but he thinks it will be tougher to achieve 3 ACH 50 with that scenario and there will be moisture risks if we do get there. I understand the moisture risks if we have exterior rigid insulation, but the exterior housewraps mentioned above should allow the wall assembly to dry to the outside, correct?

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  1. GBA Editor
    Martin Holladay | | #1

    Housewrap like Tyvek is vapor-permeable. Installing a water-resistive barrier like Tyvek is not optional; it is a code requirement. Installing Tyvek does not "increase the moisture risk."

    If you are worried about the cold OSB problem, you may also know that installing a layer of rigid foam on the exterior side of the OSB wall sheathing (assuming it is thick enough) lowers the chance that the wall will have moisture problems -- it doesn't increase the risk.

    There are lots of ways to reduce air leaks in your thermal envelope. You can't just point to one element (for example, housewrap or EnergyComplete) and assume that you have addressed all air leaks. Instead, everyone who works on your job site has to be aware of the need to make your thermal envelope as airtight as possible.

    The Airtight Drywall Approach is a perfectly reasonable way to proceed, but even that approach leaves many other areas of your thermal envelope that need to be addressed at the framing stage.

  2. Expert Member
    Dana Dorsett | | #2

    The issue with using air-tight drywall as the PRIMARY air barrier is that it is so susceptible to damage over time. Every picture hanging nail adds another tiny leak, and as the house settles a bit (which never really happens, right? :-) ) drywall tape gives up, seams open, etc.

    Detailing the housewrap as the primary air barrier has similar issues, though done right it's somewhat less susceptible to incidental damage over time than the drywall layer.

    Detailing the structural sheathing as the primary air barrier is a bit more robust, and has fewer electrical & plumbing penetrations to seal than the drywall layer, and is more rugged than housewrap. A bead of caulk between the bottom stud plates to the subfloor, and between any doubled up top plates then makes caulking the 4x8 sheets to the framing pretty tight. In most places you can even add structure to the assembly if you glued the sheathing to the framing at every stud, but that makes it too rigid for safety in seismic zones. (Using acoustic sealant caulk would be fine though.)

    While the 3ACH 50 hurdle is higher than a stripe on the floor, isn't a very high hurdle at all. If the shape of the house is reasonably simple, without a gazillion bump-outs and dormers to detail it can even be dead-easy. A bead of can-foam between the foundation sill and foundation fixes one large leak, and as long as the sheathing & wallboard are all there (including behind tub-surrounds, etc.) and you didn't make the "sea of stars" recessed lights on the upper floor ceiling it's pretty straightforward if paying ANY attention at all to air tightness as you go.

    Now if you're talking about hitting the PassiveHouse spec of 0.6 ACH/50, that's another can o' worms, demanding quite a bit more attention.

  3. Expert Member
    ARMANDO COBO | | #3

    Pretty much all of my client builders achieve between .8-1.5 ACH50 on every job. To achieve the "1.5 goal", we seal the building enclosure with calking and foam to small cracks and holes, and/or eco-seal, and use blown-in FG or Cellulose. To achieve the coveted "1", we add sealed and taped outsulation. In both cases, no big holes allowed!
    We live in CZ3 with nasty soil movements and termites, therefore most foundations are pier & beam w/o insulation, unless a few of the clients want to installed conditioned crawl spaces.
    If we build unconditioned attics, the attic floor is sealed with eco-seal or energy complete type products, and then we blow full R38 min. insulation on top. For conditioned attics we install outsulation above sheathing and O.C. foam under the roof decking to a full R38 min insulation.
    It really is not that hard, it's just about commitment by all trades.

  4. RZR | | #4

    Ecoseal The MSDS states in the "reactivity section",

    "Hazardous Decomposition Products: Thermal degradation can generate Carbon oxides, Nitrogen oxides, Phosphorous oxides, Sulfur oxides, Hydrochloric acid, Chloride gas, Hydrogen Bromide, irritating gases and vapors. "

    Thermal degradation is what happens to SPFs to cause it to out-gas and explode after cure, this elastomer or rubber made of carbon, hydrogen, oxygen, silicon, is no different when it reaches it's glass transition or passes it's melting temp, neither are listed. You read the data sheets and no where do you find they performed in a lab an accelerated fatigue temp and pressure cycle test showing they took it to this state or failure, and measured carbon monoxide, dioxide, and the acids, etc, listed by an ASTM. They did some cure test up to 7 days that's it.

  5. claumergiddens76 | | #5


    We've found the simplest way to achieve tight envelope is to do it on the outside. ZIP System and Outsulation (as recommended by Martin and Armondo) are both methods we've used, and they are very effective when done correctly.

    Either way you go, you will be taping every seam on the outside, theoretically stopping the air on the outside of the structure. A very good approach. We also install tape BEFORE the ZIP or insulation goes up at top and bottom plates. After the ZIP or insulation is in place and we've taped all seams as required, we install continuous tape at the top and bottom of the sheathing (ZIP or insulation), where it meets the wall. The top and bottom of exterior walls are among the most vulnerable areas for air leakage. Where feasible, we also continue the sheathing to the underside of the roof decking, trim around the roof structure and air seal between sheathing and structure. This gives us one less continuous seam to tape and worry about. An EPDM continuous gasket under the bottom sill and between the top plates is also a way to address that weak point.

    No matter what we do, we always think "continuous". There should be no interruption in the air barrier, or any other control layer (water, vapor, thermal)

  6. RZR | | #6

    The most efficient sustainable method to air sealing is to use homogenous, continuous, monolith mass around the entire envelope reducing or eliminating gaps.

    Speaking of temperature and moist-pressure cycles and toxic vocs outgassing of SPF’s, elastomer sealants and gaskets, has anyone seen an accelerated fatigue test on ZIPs website taking their peel and stick tape to failure, or for those that use it against cellulose sill plates, etc, against their recommendations and limited testing? If I remember right their tape is designed to adhere to their bonded WRB (ZIP), fiberglass, not wood. I could not find their lap tensile and shear strengths tested, qualified, to a test standard and third party either? Those values are usually generated by coupon testing in a clean room that is not the case at a job anyway, but are a useful guide.

    I can tell you this for a fact, having first-hand experience with such lab testing of these sealants and seals, the higher the moist- pressure and temperature cycles, the more fatigue, the less live cycle, less is more when it comes to gaps. Some other things to consider are, room temperature sealants in general are low in lap tensile and shear and the amount of sanding dust, debri, moisture, that gets in the bond line can have a large impact on mechanical and thermal properties, degradation.

    A blower door test is a snap shot in time, immediately after installation the least effect in measuring sustainability. Has anyone seen a field blower door test series of a ZIP system installed after say 5-10-20-30 years? Again, a lab can accelerate a cycle test to these years based on a test standard. How about the same for commonly used elastomers, SPFs, seals and sealants?

    I'm not interested in opinions here only data I asked for above.

  7. Nick Sisler | | #7

    Folks, thank you very much! This is super valuable info for me.

  8. BillDietze | | #8
  9. RZR | | #9

    Bill, nice, I like Joe'c comment, lol!

    "I usually go have a bourbon because it tends to be more satisfying than getting a gun."

    He failed to address sustaining any seal or the toxins. I like this comment, so true....

    "What I have been more or less able to figure out is that the 0.6 ach@50 Pa doesn’t come from any energy conservation rationale directly; it seems to be based on the need to prevent moisture problems in highly insulated building enclosures. That is the argument for the number 0.6 ach@50 Pa as I understand it. Never mind that that the number, in itself, makes no sense as you can easily design highly insulated building closures without moisture problems that are not anywhere that tight."

    I like the way he advertises thermo-ply in is write-ups at the same time criticizing others for it. Everyone has to pay the bills some how including him. He could have used a no-label product or shown a natural moisture resistant design he refers to you don't need a lumber yard or manufacture to find. Don't ever think these guys don't have some hidden money making agenda to filter through, most do.

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