What’s More Important, Air-Sealing or Insulation?
A reader planning a new house wonders how to best use a limited construction budget
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.’"
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.)
- Dennis Schroeder/National Renewable Energy Laboratory
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