Like many other owner/builders planning an energy-efficient home, RedFalcon6 is well aware that careful air-sealing is considered a cornerstone of high-performance building. But do the numbers really add up?
“I have to confess,” RedFalcon writes in a recent Q&A post, “I’ve never sat down and done ‘the math’ on the payback for increasing airtightness from ‘OK’ to ‘Pretty Good’ to ‘Awesome.'”
All else being equal, RedFalcon asks, what’s the financial value of reducing air leakage by two thirds—from 3 air changes per hour at 50 pascals of pressure (3ACH50) to 1ACH50?
RedFalcon points to a 2013 California study by the Lawrence Berkeley National Laboratory that suggests the financial return isn’t dramatic. Researchers found that the average value for a California home where leaks had been reduced to Passive House levels (0.6ACH50) amounted to about 2000 kwh of electricity per year.
“So going all the way to passive standards only saves an incremental $200 bucks on an average home?” RedFalcon asks. “Seems not worth it!”
“Prove me wrong!” he adds. “Am I an idiot or just misreading this study?”
Is airtightness worth as much as it’s cracked up to be? Or, as the title of RedFalcon’s query asks, is this revered benchmark “…mostly BS”? That’s the topic for this Q&A Spotlight.
It depends on where you live
Several GBA readers say the answer depends in part on where you live—not only from a climate perspective, but also taking the local cost of energy into account.
Russell Miller, for example, says he is a builder in a poor state where energy costs are not a burden, at least not now.
“I build for a living,” he says, “and I can state for a fact, it’s hard to justify extreme insulation and airtightness to a customer because our electricity rates are so low.”
Joel Cheely points out that the study on the financial benefits of careful air-sealing came from California, where “you can leave your doors open!”
“Here in the real world (zone 5), I place airsealing ahead of R-value in home comfort and energy efficiency,” Cheely adds.
John Clark adds that the Passive House standard comes from Germany, a heat-dominated climate.
“Air-sealing is much more important in climates with high humidity and/or which are heating/cooling dominated,” he says. “As for PH standards? Ya, the economics don’t work for PH housing in much of California with its temperate, dry climate.”
A factor, yes, but is it overkill?
There’s no doubt that climate certainly plays a role in energy consumption, RedFalcon replies. Citing statistics from the U.S. Energy Information Administration, RedFalcon says energy use in a very cold climate can be 80% higher than a more temperate marine climate. But, he adds, does that really justify going to “extreme” air-sealing measures?
“I’m not saying do old school ACH 10 numbers,” RedFalcon says. “But pushing really hard for anything beyond 3 seems like total overkill to me? You probably experience the max ROI [return on investment] on comfort and energy savings then?”
Air-leakage rate not the whole story
A blower-door test measures air leakage of the entire house, but the result doesn’t pinpoint where leaks might be. And Rick Evans says that can be important.
“A specific air-leakage number doesn’t tell the full story,” Evans writes. “If the air leaks are all in the center of a wall (around a window for instance), then the heat loss won’t be that dramatic. It might be a comfort issue, however.”
But if the leaks are at both the ceiling and at the foundation, he adds, heat losses can be significant because of the stack effect. Leaky houses also can be more prone to long-term problems, Evans adds.
“Holes in a superinsulated wall can lead to big trouble as there is little thermal energy finding its way through all that insulation and warming up the exterior surface,” Evans says. “If warm air does hit that super cold surface (due to hole) then it will condense. Over time, your walls may develop mold or rot.
“Old, leaky homes never had to worry about this,” he adds. “With little or no insulation the sheathing [and/or] siding always remained warm from the heated interior space.”
Hitting an airtightness level of 1ACH50 really isn’t that difficult, he adds, and it makes it possible to reduce an annual heating bill to $400 or $500, even in a cold spot like New Hampshire (where RedFalcon is building his house).
Retrofits are a different story
Reducing air leakage to 3 ACH50 isn’t a big deal in new construction, Dana Dorsett writes, but getting those results in a retrofit can be more difficult.
“The additional cost of going from 3 ACH50 to 1 ACH50 could be pretty daunting in a retrofit, and may not be ‘worth’ chasing in [a] new construction house that already tests under 3 ACH50,” Dorsett says. “But in new construction just paying enough attention to details to be sure to beat 3 ACH50 often leads to leakage rates that aren’t too much greater than 1 ACH50, often hitting the 2 ACH50 range at no ‘extra’ cost.”
He says the 3-ACH50 airtightness standard became part of the International Residential Code because it’s pretty easy to get there, and it makes a difference in both comfort and energy use when compared to construction in areas where there is no standard.
Savings potential in Maine
Michael Maines, a residential designer in Maine, offers some specifics on savings derived from a BEopt model for a 3000-sq.-ft. house built to Pretty Good House standards. The all-electric house uses 13,000 kwh of electricity per year, at a cost of 16.3 cents per kwh.
When compared to the Maine code minimum of 7 ACH50, a house with a leakage rate of 3 ACH50 saves $281 per year, while a house with a Passive House leakage rate of 0.6 ACH50 saves $467 a year.
“As others have said, once designers and builders understand how to build relatively airtight, it’s really not very hard to get to 1 or 1.5 ACH50,” Maines writes. “Getting to 0.6 (or lower) ACH50 takes more effort and may or may not be worth it to you.
“In my example above, going from 3 to 1 ACH50 saves $165.28 per year. Even if the associated cost to do so is $1650, that’s a 10% return on investment, which is very good, especially since it’s virtually guaranteed. For an experienced high-performance builder, the cost is probably just a few hundred dollars, resulting in ROIs of 50% or higher. Try getting that in the stock market.”
RedFalcon’s plan and our expert opinion
The discussion has led RedFalcon to these conclusions:
- Aim for an air-leakage rate of 1 ACH50, and don’t worry if it’s a bit over that.
- Focus on air-sealing where air and vapor leaks may expose vulnerable components to damage.
- A blower-door test showing an air-leakage rate of 2 or more ACH50 may point to a bigger problem that should be addressed.
Here’s what GBA technical director Peter Yost has to say:
First, I love how this GBA exchange identified just about all of the issues tucked into airtightness, building performance, and economics. Here’s a summary of the important factors:
- Climate: There’s really a big range of how many Btu are associated with air leakage depending on where the building is located.
- Actual fuel costs: All fuel costs vary across the U.S., but particularly electricity.
- Qualitative and quantitative impacts of air leakage: The size and location of specific air leaks can be important.
- Associated building-performance issues with air leakage: These include moisture and condensation, indoor-air quality, durability associated with interstitial (building-assembly cavity) moisture, and thermal comfort.
- Difficulty in achieving airtightness: In retrofits vs. new construction.
One additional issue is the connection between building airtightness and the importance of whole-building mechanical ventilation. I distinctly remember the first time I taught a two-day advanced green-building science course for the National Association of Home Builders at the International Builders’ Show. One builder said: “OK, let me get this straight: I spend time and money getting better separation of the outside and inside of my buildings (increased airtightness) to the point that I have to then spend more money to bring outside air into the building?”
We all know that the answer to that question is “yes,” because superior indoor-air quality means introducing fresh air. The air needs to be delivered in the right amount, on a schedule, and from a dedicated source.
Even for some informed builders and their clients, all this can sound a bit like an HVAC contractor’s business plan. I firmly believe that trying to justify increased airtightness just based on energy savings—and not on increased comfort, air quality, and building durability—is a losing proposition.
Finally, GBA readers probably know how I feel about simple payback analysis for energy measures such as airtightness (for more, see Capture Green Value Over Time). If both the building and the environmental control associated with airtightness are long-lived, you can capture the benefit at transfer of ownership as well as seeing a payback while you own it. We can’t let a short-term payback analysis drive our decisions.
—Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.