QA-spotlightheader image
Helpful? 1

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

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

Posted on Aug 25 2014 by Scott Gibson

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 insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. 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 GBAGreenBuildingAdvisor.com. "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.

Find the right builder

In addition to suggesting a few insulation specifics, David Meiland recommends that Brown find the right builder, for without that much the project won't go smoothly.

"Really, it shouldn't cost much more to do a much better job, but you have to find the people who have already done it at least once," Meiland writes. "If your builder is saying 'What?' about this stuff then you have an uphill battle."

Judging by what Brown has learned so far, that's not going to be easy.

"Most [contractors] I talk with," Brown says, "when I say ‘air sealing’ just refer me to the drywall, or say that is what the Tyvek (building wrap) or vapor barrier is for."

She's talked with ten builders (and has yet to sign a contract), and all of them say they'll do whatever it is Brown wants, but they "seem to find many excuses and reasons to talk me out of any sort of insulation other than fiberglass batts.

"I just get more depressed at this," she continues. "They all say that they can pass a blower door test."

If you have to choose one, go with air-sealing

To GBA senior editor Martin Holladay, Brown's query about air sealing or insulation is almost like asking which of the two she should do incorrectly.

"But if we boil your question down to its essence — ‘What's more important, air sealing or R-value?’ — the answer is clear," he adds. "Air sealing always comes first. So find a contractor who understands the need to pay attention to airtightness.

"Ideally, you need to keep looking for a contractor who understands the issues we're talking about, and who cares enough to do a good job," Holladay says. "If you can't find that person, then none of these discussions matters."

As to Brown's comment that builders she's spoken with all say their houses "can pass a blower door test," Holladay suggests asking what they typically get for a result. A result of 1 or less air changes per hour at 50 pascals of pressure (ach50) is very good, he says, while 1 to 2 is good, and more than 3 is "not very impressive."

Don't wait until the house is finished before testing with a blower door, says Lucy Foxworth, and consider requiring the builder meet a specific air-tightness level. If the house doesn't reach that standard, require the builder to go back and plug the holes.

Two tests — one after framing when doors and windows have been installed, and another after the drywall is up — would give Brown a chance to do something if the results were below par.

"You could require a meeting with the builder and the major subs to emphasize your energy goals in building the house. Emphasize the need for air sealing and your concern that it be done right. Offer them a free meal at good restaurant and give them a small stipend to make your point," Foxworth says. "Put signs up for the subcontractors at the building site: ‘If you put a hole in the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., you must seal around the hole.’"

Foxworth recommends two companies that sell high-quality air-sealing supplies, The Small Planet Workshop and 475 High Performance Building Supply.

Building a tight house shouldn't be too hard

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.

"Unless the design is some junior architect's framing nightmare of a gazillion bump-outs, set-backs, and dormers, if they're paying any attention at all to air sealing issues, 3 ach50 would be a cakewalk— more of a stripe painted on the floor than a hurdle to clear," Dorsett says "Over 3 is an indication that they're either clueless or careless (which is unfortunately not uncommon)."

In fact, he adds, with 4x8 sheet goodsMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. on both sides of a framed wall, "There's no excuse for performing worse than 3. It's almost idiot-proof."

"And yet," he adds, "the more idiot-proof you make something, the more creative the idiots become."

Dorsett suggests air-sealing the structural sheathing to the framing as best for a primary air barrier, because the sheathing is less likely to be compromised over time than an interior barrier, and it has fewer penetrations to seal in the first place. Still, he says, when using fiber cavity insulation, don't forget to air seal the gypsum board as well, and make sure to add a bead of caulk under the bottom plates of exterior walls.

What about a flash-and-batt approach?

Brown wonders also about a technique called "flash and batt," which is the application of a thin layer of polyurethane foam on the back side of the exterior sheathing followed by more conventional cavity insulation. The foam provides the air seal, and cavity insulation such as cellulose or fiberglass isn't as expensive as filling the bays with foam.

But she's unsure whether open- or closed-cell polyurethane should be used.

"In a flash-and-batt job in Zone 5 you must use 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 28% for walls," replies Dorsett.

If Brown used less foam than that, a "smart" vapor retarder, such as CertainTeed's MemBrain, would be a good idea, he adds, and "far safer" than using polyethylene as a vapor retarder.

Our expert's opinion

Here's how GBA technical director Peter Yost sees it:

Hands down, air sealing, in all climates in all buildings, is priority one, given its impact on indoor air quality and energy performance. It also is really hard to correct air sealing defects after the insulation goes in, so sequencing is another reason for air sealing priority (unless of course your primary air barrier employs the airtight drywall approach — ADA).

Others have suggested a combination of “carrot-and-stick” to get the air barrier done right; I would add that requiring an open cavity blower-door test with a performance threshold connected to final payment is a good way to get what you expect and when correcting deficiencies is still possible. Take a look at this GBA blog: High Performance Scopes of Work.

Be careful about assumptions regarding the inherent airtightness of spray foam installations; while quality issues rear their ugly heads with any type of insulation, spray foam has its own special and critical issues. Take a look at this blog on foam-in-place insulation that I just wrote for BuildingGreen.

Finally, a great industry reference document on hybrid insulation systems (sometimes called flash-and-batt or flash-and-fill) can be downloaded from the Spray Polyurethane Foam Alliance website: “Spray Polyurethane Foam for Hybrid Insulation Systems.” (AY-146 is Part 1 for Climate Zones 1 – 3, and AY-147 is Part 2 for Climate Zones 4 – 7.)


Tags: ,

Image Credits:

  1. Dennis Schroeder/National Renewable Energy Laboratory

1.
Mon, 08/25/2014 - 10:00

thanks for this info
by gideon brontë

Helpful? 1

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.
Mon, 08/25/2014 - 10:30

Air sealing or insulation?
by Torsten Hansen

Helpful? 1

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.
Mon, 08/25/2014 - 14:15

What’s More Important
by Richard Beyer

Helpful? 0

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.
Mon, 08/25/2014 - 14:28

IRC 2012
by C. B.

Helpful? 0

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.
Mon, 08/25/2014 - 15:52

Edited Mon, 08/25/2014 - 15:52.

The prescriptions in chapter 7 are based on code minimum Rs
by Dana Dorsett

Helpful? 0

Chapter 11 prescribes a minimum of R13+ R5 continuous insulation 2x4 framing or R20 for 2x6 framing wall stackups in US climate zone 5. http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

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!)

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_p...

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 (http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm ), 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.
Mon, 08/25/2014 - 16:41

Edited Tue, 08/26/2014 - 10:00.

Follow-up
by C. B.

Helpful? 0

(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
  • HardiePlank

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.


7.
Mon, 08/25/2014 - 22:02

I'm with Peter
by Steve Baczek

Helpful? 0

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.


9.
Tue, 08/26/2014 - 11:55

C.B.- I'm getting sloppy :-( (air-impermeable only if interior)
by Dana Dorsett

Helpful? 0

See comment "a" at the bottom:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_p...

and R806.5 here:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

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.


10.
Wed, 08/27/2014 - 16:22

I agree airsealing is of
by Donald Jahnke

Helpful? 0

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


11.
Wed, 08/27/2014 - 18:26

Insulation without air sealing is like...
by Curt Kinder

Helpful? 1

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.


12.
Wed, 08/27/2014 - 20:57

Insulation can't work in a wind tunnel
by Jay Walsh

Helpful? 0

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. http://www.greenbuildingadvisor.com/green-basics/insulation-overview#AIR AND MOISTURE ARE PART OF THE PICTURE


13.
Wed, 08/27/2014 - 21:54

The Modular/Prefab CURE
by Harris Woodward

Helpful? 0

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.

Period.


14.
Thu, 08/28/2014 - 16:40

Air sealing or insulation?
by Michael Ginsburg

Helpful? 0

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.


15.
Sat, 08/30/2014 - 10:17

my suggestion to Ms Brown
by Gregory La Vardera

Helpful? 0

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.
http://blog.lamidesign.com/2012/01/what-you-don-know-about-mineral-wool....
(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:
http://blog.lamidesign.com/p/usa-new-wall-info.html

And the overall approach to framing is very important for ease of air sealing and improving thermal performance. Use this model:
http://blog.lamidesign.com/p/swedish-platform-framing-info.html

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


16.
Mon, 09/01/2014 - 06:36

INSOFAST
by CG Gillard

Helpful? 0

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.
Review:
http://www.finehomebuilding.com/toolguide/product-finder/insofast-insula...


17.
Mon, 09/01/2014 - 14:41

And sound
by Kelly Craig

Helpful? 0

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.


18.
Fri, 11/14/2014 - 12:43

Nicely written Scott
by Ted Kidd

Helpful? 0

Nicely written Scott


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!