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Q&A Spotlight

What’s More Important, Air-Sealing or Insulation?

A reader planning a new house wonders how to best use a limited construction budget

If you had to choose one, pick airtightness over high R-values. A blower-door test showing anything greater than 3 air changes per hour at 50 pascals of pressure is unimpressive for new construction.
Image Credit: Dennis Schroeder/National Renewable Energy Laboratory

Green Building Advisor reader Ani Brown is getting ready to build a new house, and like most people in her position Brown will have to make some important choices on how to make the most of a limited construction budget.

Her immediate concern is insulation and air-sealing, two related details that will have a lot to do with how comfortable and durable the new house will be.

Brown has talked to many builders, most of whom are offering a “standard” insulation package consisting of fiberglass batts in the wall, and no separate air-control layer other than drywall or plugging holes discovered in blower-door testing.

Brown recognizes batt insulation can be difficult to install correctly, suggesting that spray-in insulation might be a better choice.

“But, one can argue that no matter what the product is (batt or spray); if it is installed incorrectly it will be a problem,” she writes in a Q&A post at GBA. “One can also argue that it is not the type of insulation that is being used that is of importance, but the ability to control air flow in an assembly. Meaning that if proper controls are put in place to control air flow, it does not matter as much what insulation material is used or if it is installed properly as long as it meets the required R-values of the code.”

In a perfect world, a new house would include insulation that is installed correctly as well as effective air-sealing. “But,” Brown adds, “if additional money is not available for both, what would be best to spend money on? Better installed insulation like a spray product or better air flow controls?”

The same concern was the focus of a Q&A Spotlight in 2010 and makes a return appearance here.

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  1. wicketsss | | #1

    thanks for this info
    Very timely as like Ani Brown finding contractors who 'care' about doing energy the right way is not easy and neither is the constant fight between dollars and benefits :) The recommendation to have testing done and levels met is brilliant...thanks again

  2. Foamer | | #2

    Air sealing or insulation?
    Finding the right contracting team to work with is crucial. With the right people everything will fall into place smoothly, without them it will be a struggle. Try coming at the problem from the side and find an experienced energy rater to assist you. He or she will know who is who in your area and can offer guidance. You can get a list of raters in your area from RESNET. Since you are already considering spray foam, you may also start by talking to foam professionals in your area. Look for an understanding of building science and years in business.

    Don't despair. We routinely achieve below 1.5 ACH50 on standard construction with spray foam and attention to proper caulking and sealing. That said, I am certainly not advocating that you rely on your insulation contractor to do it all. It should be a team effort.

  3. Richard Beyer | | #3

    What’s More Important
    Your health and your wallet is most important in any design. It does not matter much if your contractor is not insured properly either as is the case with many spray foam installers. ie; Contractors Pollution Liability and Workman's Compensation coverage

    It looks like SPF has the lead in this design. A written warranty from the insulation contractor spelling out who pays for what if the consumer feels the job is not performed to standards or if they have a health reaction to the product which is sold as relatively inert and safe for asthmatics.

    Itemized contract including third party inspections after installation, testing will be completed by the SPFI contractor during installation and a indoor air quality test will be performed to verify the job was completed in accordance with it's industry manufacturers standards. I would also suggest the chemical manufacturer inspect the final job and sign off that it was installed per their written guidelines.

    Ask anyone who has experienced a bad job what they would have done different and I think the above is their regret for not dotting all I's and crossing all T's. Better safe today than sorry tomorrow.

    I personally do not advocate using SPFI because of all the variables and unknowns involving the chemistry and unknown health effects. Air sealing with modern waterproofing techniques (spray or sheet applied membranes) and proven insulation techniques such as mineral wool and cellulose will help minimize the concern for manufactured on site products which can result in negative health effects and can potentially cause financial hardship trying to prove who's at fault. Just my opinion..

  4. user-1041981 | | #4

    IRC 2012
    In the article, it says:

    Echoing Holladay's comments that an airtightness result of 3 ach50 is "not very impressive," Dana Dorsett says that result isn't even good enough to meet the 2012 International Residential Code. ..... "In a flash-and-batt job in Zone 5 you must used closed-cell foam to get away from interior side vapor retarders, and for the assembly to meet code the foam needs be at least 40% of the total center cavity R-value for roofs, or 35% for walls," replies Dorsett.

    IRC 2012 Section R702.7.1 says that one does not need an interior vapor retarder and can use a Type III vapor retarder (aka. just use regular latex paint on walls) if you have insulation on the exterior of your sheathing meeting Table R702.7.1. In Zone 5, it is R5 on the outside of a 2x4 wall and R7.5 on the outside of a 2x6 wall.

  5. Expert Member
    Dana Dorsett | | #5

    The prescriptions in chapter 7 are based on code minimum Rs
    Chapter 11 prescribes a minimum of R13+ R5 continuous insulation 2x4 framing or R20 for 2x6 framing wall stackups in US climate zone 5.

    Chapter 7 specifies a minimum of R5 of air-impermeable on the exterior in 2x4 construction which is presumed to be R13 cavity fill. The total center cavity is then R5 + R13= R18, the foam fraction of the total is then R5/R18= 28% (and not 35%- mea culpa- I was probably looking at the prescriptive minimum for zone 6!)

    In 2x6 framing the presumption is R20 cavity + R7.5 exterior, for R27.5 total, and R7.5/R27.5- 27 %, about the same.

    For roofs the prescriptive for continuous air-impermeable insulation is even more stringent, since unlike walls, there is zero drying toward the exterior in unvented assemblies. The prescriptive minimum-R on the exterior for zone 5 is R20 ( ), but based on a code-minimum total R of R49 (found in chapter 11). The minimum foam-R to total-R fraction is then R20/R49= 41%

    The average temperature at the sheathing is what determines the risk of accumulated wintertime adsorption in the sheathing. That temperature is largely a function of the ratio of exterior-R to the overall R. It's important to increase the amount of exterior R whenever increasing the total R value to maintain (or even increase) the ratio. R20 exterior foam on an R100 roof (R80 of fiber) is woefully inadequate dew point control, being only 20% of the total, but R40 foam (R60 fiber) works.

    The IRC prescriptives do not discriminate between foam on the interior side of the sheathing or roof decking, but there is a strong argument that they should. As little as R12 of most closed cell foam on the under side of the roof deck has sufficiently low vapor permeance to be protective of the roof deck in climate zone 5, since at ~0.4-0.6 perms it's a class-II vapor retarder at that thickness. Unlike wood sheathing, closed cell foam insulation is not particularly susceptible to damage or loss of function from moisture cycling, nor is most fiber insulation (within limits.)

    It's important to remember that these are minimums, not optimums, and if the wintertime indoor humidity is not well controlled going with the prescriptive minimums still carries some risk. There's a pretty good long term energy cost economic rationale for considerably more than R13 +R5c.i. 2x4 for construction (~R15 whole-wall) code minimum in a US zone 5 climate, even R20 +R12 c.i. (~R25 whole-wall) wouldn't be pushing any rationality limits. But when bumping performance spending it on more exterior-R has both better economics (due to lower thermal bridging) than any amount of cavity fill, and adds dew point margin at the sheathing layer above the prescriptive minimums.

  6. user-417066 | | #6

    I'm with Peter
    I'm with t. he Yost man. Air sealing is hands down the priority. We have many houses in America with OK insulation and no air sealing that perform absolutely horribly. Many air sealing practices are buried in the assemblies when constructed. There is very little after the fact solving for. A good air sealing job with marginal insulation is far better than a good insulation job with poor air sealing. The problem with good air sealing is that most architects and contractors see it as an application to their work rather than an integrated part of the assembly. Good air sealing is hard to find, believe me. As for insulation material, I'll let others hash that out with their opinions. For me, workmanship outweighs material selection hands down. Getting it right, is far more important than getting it done cheap.

  7. user-1041981 | | #7

    (reply to Dana, who has greatly educated me over the past 2 years while I've tried to learn as much as possible to apply to my own, and first, house)

    The quote in the original article read that one had to use closed-cell foam in wall flash-and-batts in order to avoid interior side vapor barriers. I was just noting that isn't quite accurate for WALLS.

    Also, you just wrote:

    Chapter 7 specifies a minimum of R5 of air-impermeable on the exterior

    Does it say the R5 is supposed to be air-impermeable in IRC? I can't find that in my copy of the code with ICC commentary.

    I am building my house in Zone 5 with wall consisting of:
    drywall with typical latex paint
    studs/blown-in fiberglass (R23 cavity fill) and Owens-Corning EnergyComplete sealing -- since air-sealing is the most important thing after water/moisture management
    Zip Wall sheathing
    1-1/2" Roxul ComfortBoard IS (R6)
    1-1/4" Roxul ComfortBoard CIS (R5)
    3/4" airgap/furring-strips
    which will allow my wall to dry in both directions and avoid the risk of accumulated wintertime adsorption in the sheathing by keeping it warm. Don't need closed-cell foam (or other chemical foams with their environmental issues) or a vapor retarder membrane.

  8. Expert Member
    Dana Dorsett | | #8

    C.B.- I'm getting sloppy :-( (air-impermeable only if interior)
    See comment "a" at the bottom:

    and R806.5 here:

    On the exterior-R it's generally better to go higher permeance, (as you have rightly done.) Your stackup looks well thought out and highly resilient.

  9. donjahnke | | #9

    I agree airsealing is of
    I agree airsealing is of utmost importance. We do all SIP panels so doing rough-in blower door tests are real easy as soon as windows are in and under 1ACH 50 is a very doable number. I always like foam in walls with no airspace makes the science side easy to work with and takes the majority of the lumber out of the walls

  10. watercop | | #10

    Insulation without air sealing is like...
    France fortifying only its border with Germany with the grand Maginot Line. The Germans simply went around it through Belgium.

    Similarly, R-Gazillion will fail miserably if hot humid (or cold dry) air can simply sashay around it via a myriad of gaps and holes.

  11. Jay Walsh | | #11

    Insulation can't work in a wind tunnel
    Taken from GBA article"AIR AND MOISTURE ARE PART OF THE PICTURE": No matter what type of insulation you choose, it will perform poorly if installed in a house that is riddled with air leaks. Because many types of insulation (like loose fill and batts) work by trapping air, leaky walls, roofs, and floors mean poor thermal performance. For this reason, building scientists are fanatical about air-sealing. To get the most out of batts and blown insulation, every house needs an air barrier adjacent to or contiguous with the insulation layer. AND MOISTURE ARE PART OF THE PICTURE

  12. Harris_FinishWerks | | #12

    The Modular/Prefab CURE
    Between the martini sipping prefabbers and the Bud swilling modular dudes (it's always fun to paint the opposing stake holders in ridiculous relief), scrapping over a measly 3% market share, there's no time to discuss the merits of our trade, and how they fit this subject. As evidenced by our contributions to this blog (next to nil).

    So I will. My homes are tighter than my fellow stick builders without much trying.

    Modular construction has come a very, very long way. In 2014, on the East Coast, you’ll find modular factories ready and willing to seal up envelopes. In fact, a decade or two ago they realized that in order to keep modular home sections from breaking apart drywall during transport, and because it facilitated framing and drywall assembly in their jigs, they began liberally spraying poly-u spray foam for rigidity and speed of production. What they didn’t realize is that they were building inherently tighter building sections in the process. How?

    For the uninitiated, prefab (wood framed, IRC code, not HUD mobile) homes are typically built inside out. Drywall is hung first on the exterior wall assemblies. Sheathing 2nd. In this way we have the rare opportunity to address the dew points and vapor drive FROM THE OTHER SIDE – the interior where the action (occupant behavior) is. We can control for climate zones, but adjusting for human behavior is like grabbing a greased watermelon, with WD-40 on it for good measure.

    In addition, we have the luxury of having all the trades inside a large conditioned building at the same time (with no weather events, no scheduling nonsense, etc.) consulting with each other to make sure the details are respected. Guess what happens: the details are cared for!... whoops, “missed this exterior GFI outlet in the Prints”… now walk 80ft down the production line and tell the window flashing guy to come up and “hit” the junction box that just got cut into the sheathing. Voila: air infiltration solved.

    And you don’t have to count on your GC/Builder to manage such annoying details on a stick-built home where the subs desperately try to complete their scope in as few visits as possible. Time is money, which includes calling and beating down insulators to come back out because the HVAC contractor penetrated the wall with a lateral range hood duct, or the plumber breached the enclosure with a vent stack, or the direct-vent chimney guy… and so on and so on….

    I seal up my envelopes in one 3-day modules line move across the factory floor. And my Roxul insulation is hung Class I nearly every time. Why? Because they build in a controlled environment. Indoors, with no scheduling drama. The factory guys, punching a clock (and not with a cost-to-complete subcontractor mentality) with headsets on serving up good music, are meticulously getting the job done. With no disruptions until one occurs. I still have to seal up the marriage lines between modules upon erection, but this is as simple as connecting the dots: see gap? Fill it.

    I took the time to compose this Comment because Ani Brown deserves it. She has gone far beyond the average inquisitive custom home buyer in bldg science (even exceeding most folks that inquire about how we build). She is asking for more and we building science nerds, aka Builders, must deliver.

    But I also wrote this from my home office, late at night, for purely selfish reasons:

    I’ve built stick and modular. Indoor prefab construction is far and away the less stressful, less costly, and fundamentally better way to build custom and low-volume high performance homes.


  13. user-875633 | | #13

    Air sealing or insulation?
    To really understand the relationship between air sealing and insulation-R value one can think about the following real world analogy: (assuming a cold climate context), air sealing is akin to zipping your jacket up on a cold wintery day but, if you are wearing a wind breaker shell like jacket as opposed to a down jacket, you're going to be cold. Conversely, if you are wearing a down jacket and it is NOT zipped up, well you're going to be cold. Bottom line is you can't have one without the other because with either one by themselves you will be paying much more in lack of comfort and higher utility costs. Bottom line, is you 'should' to do BOTH, it's a ONE time expense that absolutely determines your daily comfort and monthly cash flow. It is as important as the structure itself.

  14. Gregory La Vardera | | #14

    my suggestion to Ms Brown
    I suggest you read 3 articles, and share them with your builder as an screening mechanism. If they are enthusiastic you have a wall assembly and an eager partner. If they try to convince you of another approach you can move on.

    Batts are are the lowest common denominator. If you want to be affordable then work with batts, but use the best best.
    (that's also republished here)

    Use a batt based wall system: I suggest the "Better" with interior wiring space and variable perm smart vapor sheet:

    And the overall approach to framing is very important for ease of air sealing and improving thermal performance. Use this model:

    This was all designed to ease status quo builders in to higher performance building. Your project is the perfect profile for this approach.

  15. cgermaine | | #15

    Look at INSOFAST insulation (for the basement walls) which incorporates air sealing in their well-designed engineered panels. There is not much on internet about the product, but it installed quickly on my home basement finishing project. The website contains instruction on how-to-install, how to insulate, cost breakdown, etc.

  16. dejure | | #16

    And sound
    Years ago, I did my last major remodel. On a whim, after the Tyvek and siding were up, but the insulation was not in and the rock was not on, I burned up a lot of caulk sealing joints where studs met sills and plates in the kitchen. I did this for insulation purposes. Until then, I never realized what an effect this had on sound. One day, working in the quiet of the kitchen, I opened the door, to discover a significant storm carrying on outside.

    Since then, I've come to appreciate the importance of correctly installing the vapor barriers and sealing ever point where air could pass.

  17. Tedkidd | | #17

    Nicely written Scott
    Nicely written Scott

  18. mikeolder | | #18

    I'm curious if any research has been done regarding how "cost effective" additional insulation after R13 is, but with a super low <1 ACH? Or rather, starting with a envelope tested below 1 ACH, how "cost effective " is it to continue to insulate?

  19. user-723121 | | #19

    An airtight R-13 wall will not get you very far in MN. Assuming infiltration heat loss is minimal, it is just a straight calculation to evaluate additional insulation. Area x U-value x 24 hours x heating degree days.

    1. mikeolder | | #20

      I have R13 in Iowa and pay $1300 a year for 1700 sq' and I'm going broke lol

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