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Community and Q&A

Estimating air tightness in design phase

JeffHewitt | Posted in General Questions on

How do I estimate air tightness (ACH 50) at the beginning of  a project?  This is an important input for BEOpt or other heat load calculators.

We’re designing a home for Climate Zone 7 and will be insulating everthing very tightly.  Lots of foam will be used – walls (R24 OCF+R6 board), floor joists (R38 OCF) and roof (R49 SIPs on rafters).  Everything will be sealed and taped.  Triple pane windows with modest fenestration.

Is this likely an ACH <1, 1-2, 2+?  Are there any rules of thumb for the early stage?

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  1. STEPHEN SHEEHY | | #1

    I think the most important thing is to clearly define your air barrier on the drawings so the contractor/ subs know exactly what is expected. Your air barrier needs to be continuous and maintained throughout construction. If you are meticulous, you'll end up below 1 ach50. If not, you won't.

  2. BrianPontolilo | | #2


    I've not heard of anyone modeling air-tightness, but most energy-smart homes have a target that they are shooting for and, as Stephen said, a really good plan. The plan has to include not just large surface areas like walls and roofs, but all of the transitions too. There are lot's of articles and videos on this site and on Fine Hombuilding that will be helpful for those details. Let us know if there is anything specific that you can't find with a search.

    1. JeffHewitt | | #3

      Yes, there is great information here and I have reviewed much of it. What I am trying to understand, maybe from people who have done many blower tests, is what a reasonable range of air exchange is for assumptions in a heat loss model (BEOpt). It makes a big difference if I can expect to achieve 1.o vs 3.0, with a high level of diligence during construction and lots of spray foam as mentioned above. So, does anyone have thoughts on whether 1.0 is a reas onable assumptions or is it too aggressive?

  3. lance_p | | #4

    Jeff, I'm not a builder but I'm in the final planning stages of our new house project, which is to start this coming spring. I am building it myself. I've read and watched pretty much everything I can get my eyes on for the past 1.5 years and I've mentally built this house over and over in my head.

    After all that, I've come to this conclusion: this build will be as airtight as I'm willing to make it, and the level of effort to get to where I want to be will be inversely proportional to the amount of time and effort I invest in planning it out ahead of time. By this I mean, I can achieve a very tight house with poor planning, but it might take an unreasonable amount of effort to get there. I'm going for the better planning = less work approach.

    My approach is separating the three layers: Primary (outer) air barrier, insulation, and vapor retarder. My air-tightness will mainly come from my primary air barrier; the wall sheathing. This house has been designed with a minimum of exterior design details in order to make it easier to build tight (and just plain easier to build altogether).

    My sheathing and attic floor will be my primary air barrier. For this I will need a sheathing product that doesn't leak on its own (OSB has been known to leak), and this sheathing will need to be sealed and taped meticulously from the outside. Window and door penetrations will be sheathed over and sealed completely prior to initial blower door testing, and that initial blower door test will be required to come in at HALF the leakage of my final assembly target because I know adding windows, doors and other penetrations will only add leakage afterwards.

    I'm targeting 0.6 ACH50 on a standard depressurization blower door test. I'm not planning a goal for a pressurization (reverse) test like PH standards require, though I might test it just out of curiosity if time allows. With range hood and dryer vents, as well as non PH certified windows, I'd expect the reverse test score to be less impressive.

    This means my bare shell needs to hit 0.3 ACH50. That's tight. Real tight. But with the right planning ahead of time as well as some real perseverance in tracking down and sealing every possible air path, I don't see it being impossible. Especially when I'm in control of the process.

    I used 0.6 ACH50 for my BEOpt models. I'm honestly hoping with a 0.3 ACH50 shell to begin with, that I can beat that target. I will have a DIY blower door setup installed during the entire construction process and plan to test after every window and every door is installed. I will keep a list of any and all penetrations, from the jack posts through the basement slab to the plumbing vents in the attic. Everything will be accounted for and everything will be checked over and over throughout the build with a smoke pencil.

    Once all penetrations are in and sealed I'm hoping to see less than 0.6 ACH50. If not, I'll be doing some more sealing legwork to make that a reality. Only once I get to where I want to be will I start with the insulation and vapor retarder, which in theory should improve the air-tightness but I'm not planning to depend on it. If the house gets tighter during that phase of construction I'll just consider that a bonus.

    If this level of detail is something your builder doesn't seem up for, well, I doubt you can plan too seriously around a specific air-tightness value unless you want to get your hands dirty and take responsibility for it. I wouldn't sweat it too much though. It's not like a few points on a blower door score are going to send your bills through the roof. If it's well planned and the details aren't too intricate you should expect a careful builder to do a good job.

    Remember that spray foam does not equal an airtight house. Matt Risinger has some good videos on spray foam and air-tightness, you may want to look those up to see the extra detailing that is still required in a spray foam house to get it tight. Your insulation, no matter what it is, should not be relied on for your air-tightness plan.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      Both your post and the OP's bring up a few interesting questions.

      Does it make sense to design the envelope and heating requirements for a house around the air-sealing achieved at a certain p0int during construction?

      Can we rely on that level of air-tightness being maintained over time, especially as buildings are frequently altered or repaired?

      I guess a third would then be: Given the importance of air-barriers to high-performance houses, should greater attention be paid to making them easy to identify, more robust, and easier to repair?

      Whatever the other shortcomings of poly air-barriers, they are a distinguishable layer in building assemblies that people understand have a certain function. Using some other material, or relying on a combination of several (like drywall, and sheathing), that have others functions, makes the air-sealing a lot more vulnerable to inadvertent damage. How would even someone who knew to look for an air-barrier during a renovation know which materials had been tasked to do the job?

      Just to clarify - I'm not advocating for poly, just using it to illustrate the problem.

      1. lance_p | | #7

        Excellent points, Malcolm.

        I would think it prudent to assume the air-tightness of a house would degrade slightly over time. By how much? No idea, but it may be a smart move to ensure enough heating capacity for temps a little lower than the 99% normally used just in case. That furnace or heat pump may lose a few points of efficiency/capacity over the years as well, so cutting it too close might not be the best approach. As far as I know, I will be expected to detail the heating capacity and heat loss calculations of the build in order to get my permit approved, so deciding this after the build is underway could be tricky.

        As far as repairs or alterations go, that could be a real wild card. In a perfect world I would include a detailed envelope schematic with the New House Ownership Package by law. In the real world I guess the best we can hope for is contractors who study the existing structure and maintain its design and integrity throughout their work. Wishful thinking perhaps. I trust that someday when I eventually sell our house I will put together some sort of information package to properly direct the new owners, especially since the envelope will be far from what builders normally see (12" double-stud with cellulose and a fiberglass filled 2.5" service cavity inboard of the vapor retarder).

  4. Reid Baldwin | | #5

    If your builder has had blower door tests on previous houses, you can make an educated guess by adjusting those numbers based on whatever additional steps you are taking to limit leakage relative to his usual practice. Or, you can include an ACH50 standard in your contract which would force your builder to take extra steps after the test if the standard is not satisfied. For example, they can use an aerosol product that is sprayed in a mist in the house during a blower door test to seal gaps. The contractor for that just continues misting until the target is reached.

    1. lance_p | | #8

      Something about that "Aerosol Air-Tightness" approach just doesn't sit well with me. I have no doubt it works, but for how long? If we're questioning the long-term integrity of tapes and caulks, many of which have a proven history if used correctly, how can we place our trust in these gap filling particles that just randomly build up in cracks?

      My worry is that once builders have decided to use this method, they will skimp on (or eliminate entirely) traditional efforts at air sealing, seeing value only in the ACH50 numbers when they hand over the keys. I'd like to see 5 and 10 year re-testing become the norm for air-tightness targets.

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