At the Tipping Point of Airtightness
Forty years ago, after the first oil shock, people began to look at residential energy efficiency and realized that it was terrible. People vaguely knew that houses were drafty and that they wasted energy, but they didn’t know what to do about it, or even how to measure air infiltration. Not only was there no Internet, there wasn’t even a blower door. The best technology was measuring the length of cracks with a ruler.
I was a physics graduate student at Berkeley, and Art Rosenfeld convinced me to do my dissertation on the building physics of air infiltration; at the time, building scientists knew little about it. By the time I got my PhD, almost 35 years ago, we had invented the blower door; we could measure air infiltration; and we had simplified physical models on how to connect the two.
The international community recognized the importance of the issue in 1979, when it created the Air Infiltration Center (AIC)—part of the International Energy Agency. The professional societies began integrating their research efforts. The American Society for Testing and Materials (ASTM) created a standard method to measure airtightness (ASTM Standard E779). ASHRAE incorporated results into its handbooks. There was even a funky little magazine called Energy Auditor & Retrofitter (now called Home Energy) that cared about such things.
Roughly 30 years ago, $1 million worth of blower doors were being sold in the United States. There was a consensus that air leakage and the infiltration it caused accounted for one-third to one-half of the space-conditioning load. Air sealing could save a lot of energy. Twenty-five years ago, we developed ASHRAE Standard 119 to specify how tight homes should be. Had I known the phrase then, I would have said we had reached the tipping point on airtightness, and that it would be a downhill battle to deploy it. But that would have been naïve.
People began to make buildings tighter; utilities and DOE’s Weatherization Assistance Program (WAP) funded work to do so; but it did not always work out as planned. There was, for example, a huge reluctance on the part of some to use blower doors. Crews thought they already knew what to do, and using a blower door cost time and money that they would rather spend to fix more houses. People running programs did not necessarily want to have before-and-after measurements, which could be used to evaluate their programs.
Another problem was that sometimes indoor air quality (IAQ) was reduced in homes that had been tightened. This led organizations like the Bonneville Power Administration to require expensive air-to-air heat exchangers in homes that were to be air tightened. This in turn made air tightening a prohibitively expensive option. No one wants to make a building “sick.”
The concern about protecting IAQ when doing air sealing was and is a real one, but no one knew the right thing to do. Costs of risk mitigation and the fear of litigation made many people simply shy away from air tightening, despite its long-term cost-effectiveness. Eventually, we found a solution, which became known as the building tightness limit (BTL). The BTL told contractors how tight they could get the envelope in a given climate without having to worry about IAQ. Since that value was well below the typical tightness of existing homes, there was substantial energy to be saved.
The situation in new-home construction was somewhat analogous to that of existing homes. Builders and contractors had no motivation to test their airtightness levels, and might be open to litigation if they did. They rightly claimed that new construction was generally tighter than existing homes, so there was less energy to be saved. New construction was principally driven by codes that sometimes required prescriptive sealing methods, but did not require airtightness testing with a blower door.
Some of us recognized that the BTL was only a stopgap measure. It facilitated air sealing down to the limit, but it set up a functionally impenetrable barrier to doing better. In order to overcome that barrier, it would be necessary to have an operational and practical definition of acceptable IAQ for homes. ASHRAE had a ventilation standard (62-89) that purported to do that, but it was more of a guideline than an actual standard and was not operational.
Twenty years ago, several of us started working with ASHRAE to develop a real residential ventilation standard that would clearly specify what one had to do in terms of tightness and mechanical ventilation (and source-control measures) to meet it, and presumably achieve acceptable IAQ. But a lot of the stakeholders objected to having actual requirements. There was so much objection that the residential part of ASHRAE Standard 62 was split off to fend for itself, and it would be a decade of wrangling that was often more political than technical before it would see the light of day.
About ten years ago, ASHRAE Standard 62.2 was finally published. It was the first national consensus standard on residential ventilation. The reason I am discussing ventilation standards in an article on airtightness is that without the operational definition of IAQ that ventilation standards provide, a lot of potential energy savings from air tightening gets left on the table. Of course, merely having a standard does not mean that anyone will use it, and there was no initial rush to adopt Standard 62.2. During the time that 62.2 was being developed, airtightness in new construction had continued to improve. Some of this improvement was driven by regulation and some of it was driven incidentally by improved construction practices. Several states that had energy codes or used voluntary programs needed to show that homes built to those codes or under those programs would not be so tight as to endanger the occupants. Standard 62.2 provided a solution to that problem, and after a few years states began to adopt it. Maine and California were the first two states to require 62.2 in new construction. Now many state and federal programs require it.
A few leading-edge states, such as Wisconsin, began to experiment with using 62.2 in weatherization soon after it was passed. But many weatherization providers were reluctant to give up the tried-and-true BTL methods. About five years ago, with the impetus of federal stimulus funds, DOE’s WAP adopted 62.2 as a requirement, and much of that resistance has subsided. Over the last five years, federal programs such as Energy Star and Building America have required both air tightening and 62.2. It is clear that momentum is building to Build Tight, Ventilate Right.
Read a history of the blower door—itself written almost 20 years ago.
The AIC evolved into the Air Infiltration and Ventilation Center (AIVC) in the 1980s as it was realized (at least internationally) that airtightness could not be improved without considering ventilation.
Thirty years ago, Sweden coined that phrase and set a standard for residential airtightness that was considered extreme. New homes in the United States were typically five times leakier than the Swedish standard allowed (the Canadians were doing a bit better). That same level of tightness is now the requirement in the International Energy Conservation Code (IECC) model building code. More telling than that is the fact that builders can easily meet that requirement, and typically do so in high-performance homes. I think we have reached the tipping point where air tightening is going to happen without our having to push the battle uphill.
That is not to say that there are no air leakage issues left. We still have to figure out the best ways of tightening; we need to figure out when tighter is not any better; and we need to develop products and methods to reduce costs. Energy codes and calculation methods need to advance accordingly, and improved retrofit methods need to be developed. But it seems to me that this is now a downhill battle with momentum behind it. Those of us who have been pushing uphill for these many years can breathe a sigh of relief and even pat ourselves on the back.
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