Letters: January/February 2013
Reader Questions Cost-Effectiveness of Weather Stripping
Energy savings based on engineering models do not have a good track record for accuracy, tending to overestimate savings in real buildings. While the authors don’t explain how their savings numbers were derived, other information in the article suggests these savings are not measured, but are based on some sort of engineering model (see “Weather-Stripping Windows with V-Strip,” Sept/Oct ’12, p. 38).
One might question some of the assumptions used in this study. The authors acknowledge that their estimate of HDD (heating degree-days) for Ithaca might be on the high side and I agree, but the more important assumption, which I suspect many will take issue with, is their cost of natural gas—$1.50 per therm. Gas prices were headed in that direction a few years back, but now, with markets being flooded with gas from shale fracking, that assumed cost is less defensible. While these factors are important and raise serious questions as to the cost-effectiveness of this weatherization measure, they miss the main point—the big lesson learned over the last 30 years that prompts me to write this letter.
The “natural infiltration rate” is based on blower door readings; the authors report an average infiltration reduction of 314 CFM50 per house. While I do not doubt the accuracy of their blower door numbers, it’s a huge leap to go from CFM50 to actual infiltration. Whether one divides by 20 or by some factor gleaned from the Lawrence Berkeley Laboratory infiltration model is not the issue. The issue, and the reason why a reduction in CFM50 does not correlate terribly well with energy savings, is much more basic.
A blower door provides an excellent measurement of the sum total of the air leaks in a house when the house is subject to a driving force—a pressure difference between inside and out—of 50 Pa. This blower door reading, however, provides no information about the location of this infiltration. Exactly where are these air leaks the blower door is measuring? It matters. Why? Because holes and cracks in a building’s envelope don’t leak (infiltrate or exfiltrate) any air unless these holes are exposed to a driving force—an air pressure difference that causes air to move across the holes. No air pressure difference across the holes means no air leakage. No air leakage means no savings when these holes are sealed.
So what are the air pressure driving forces of concern in a one- or two-story single-family home? There’s wind, which gets folks’ attention when it blows, but it doesn’t blow that hard or for long in most locations. There are air pressure differences resulting from unbalanced forced-air systems—very important for health and safety and moisture reasons, but not a major factor in most houses. Finally, and most importantly, there’s stack effect, the air pressure differences resulting from the density difference between heated air inside a house and cold air outside. In a typical house, stack effect results in air pressures at the top of the house that are positive with reference to (wrt) the outside and air pressures that are negative wrt the outside at the bottom.
Approximately halfway between the top and bottom of the house, we find the neutral pressure plane (NPP), where there is no air pressure difference between inside and out. How much air will be lost through infiltration or exfiltration across holes and cracks at or near the NPP? Not much at all. No pressure difference, no driving force, no air leakage. In a single-story house with double-hung windows, where’s the crack that is being carefully sealed by weather-stripping? Pretty close to the NPP. How much energy does sealing cracks that aren’t leaking much air save? Not much at all.
Another important piece of the puzzle is that these cracks around windows are tiny compared to the ones near the top of the house that are out of sight to the untrained eye—the very large holes that are typically found up in attics, at the intersections between walls and ceilings, under knee walls separating conditioned spaces from supposedly unconditioned side attics, and the myriad of other locations that those in the low-income weatherization program have learned to find and seal.
Two energy evaluation studies performed more than 20 years ago that I authored or coauthored back up the above assertions with actual measured savings in large samples of houses. The first was an evaluation of energy savings in homes receiving energy conservation measures (ECMs) under a program in Indiana (Hill, 1990). Forty of the homes in that study with reliable pre- and postconsumption data had installed replacement windows and no other ECM. Average savings in those 40 homes? Only 1.4%, worth about $8 per year at the then-prevailing natural-gas cost of $0.50 per therm. Why were the savings so small? First, most of windows being replaced also had wood storm windows, a system that provides a pretty good R-value. Second, and most relevant to the topic at hand, while the replacement windows were certainly more airtight than the old windows and storms they replaced, the air leakage reduction wasn’t that important because it was near or at the NPP.
The second evaluation study, done with colleagues at Virginia Tech (Randolph, Greely, and Hill, 1991), looked at homes in Virginia’s weatherization program in 1988–89, prior to the training we provided that would fundamentally change their approach to weatherization. In 1988 and ’89, Virginia crews were using blower doors and achieving large reductions in whole-house air leakage, mostly with weather strip and caulk—lots and lots of caulk! The median savings being achieved for the 188 houses in the final reliable sample? Only 6.5 site MBtu/yr (6%) for gas-heated homes and 1.5 site MBtu/yr (4%) for electrically heated homes. There are two data points from this study particularly relevant to the topic at hand. First, houses that received infiltration reduction work only (weather stripping, caulking, window and door repair) showed no energy savings. Second, the median infiltration reduction, as measured by blower door CFM50 readings, was 39% for the sample of 62 site-built homes for which we had data. That is, lots of caulking and weather-stripping resulted in large CFM50 reductions but little or no energy savings. Why? Because the air leaks being sealed were small leaks at or near the NPP, not the large holes near the top of bottom of the house that mattered.
In summary, I doubt that weather-stripping windows in one- and two-story homes is ever going to be cost-effective in terms of energy savings, except perhaps for a house on a very windy and exposed site. There are, however, thermal-comfort benefits to be had from weather-stripping the window next to your favorite chair or kitchen table. Fix those windows, don’t bother with the others, and let’s all let go of the notion that weather-stripping is a good way of saving energy in single-family homes.
Hill, William W. Replacement Windows in the Heartland: Indiana’s Energy Conservation Financial Assistance Program. In Proceedings of the ACEEE 1990 Summer Study on Energy Efficiency in Buildings, 9.103–13. Washington, D.C.: American Council for an Energy-Efficient Economy, 1990.
Randolph, John, Kathy Greely, and William W. Hill. Evaluation of the Virginia Weatherization Program. Final report prepared for the Virginia Association of Community Action Agencies. Blacksburg, Virginia: Virginia Center for Coal and Energy Research, Virginia Polytechnic Institute and State University, 1991. Download at: www.energy.vt.edu/Publications/report9101.html
College of Architecture and Planning
Ball State University
Coauthor Iain Shapiro replies:
Professor Hill’s comments on the V-strip weather stripping article are interesting, especially the findings of his study that show that infiltration reduction through weather-stripping windows and doors does not produce energy savings. However, I do not share his conclusion that weather-stripping is ineffective. Professor Hill dismisses wind as a factor in infiltration; however, authoritative studies show that wind indeed has an effect on infiltration and cannot be neglected. He also dismisses infiltration induced by forced-air system pressure imbalances, but here again, studies have shown that imbalance-induced infiltration is often significant. Finally, he states that we are left with stack effect, which he dismisses as negligible at typical window locations.
Whenever I crack open a first-floor window in winter in just about any house, I can feel stack effect airflow in through all the windows. Further, neither of the two studies that Professor Hill cites focuses specifically on weather-stripping alone. The first is a study of replacement windows; it does not discuss pre- and postreplacement infiltration. The second is a weatherization study in a relatively mild climate, and again, the focus is not limited to weather stripping. (Professor Hill says that the crews in this study used “lots and lots of caulk.” Maybe the study should have concluded that caulking is ineffective?) This study was also conducted on homes in Virginia, which is a fairly mild climate, compared to the northern United States, where infiltration reduction makes more sense. In such a mild climate, it might be possible for analysts to miss heating savings in the noise of utility bills.
Professor Hill closes by saying that there are thermal-comfort benefits to be had by weather-stripping the window next to your favorite chair or kitchen table. The only thermal-comfort benefits derived from weather-stripping are, in fact, reduced infiltration. I doubt that any such improved comfort would be felt if the reduced infiltration were less than 1–2 CFM (natural) per window. Multiply that by the 20 or so windows in a typical house, and you’re right at the infiltration reduction that we measured with blower door tests.
In response to Professor Hill’s other questions: Savings were estimated just as he suggests, with a simple heating degree-day calculation, which I agree may be slightly high. Likewise, I agree that our cost of natural gas at $1.50 per therm is on the high side relative to recent drops in gas prices. But as we note, our estimated hourly labor rate of over $50 per hour is fairly significantly on the high side, and offsets possible overestimation of savings in calculating SIRs.
Preliminary results for energy savings on one of the six houses we treated show that energy savings are indeed delivered. In the spring of 2013 we will have data on the other five houses. So we are left with a conundrum: Professor Hill concludes that we should not weather-strip windows because blower door tests overpredict savings for windows and doors, because they are close to the neutral pressure plane. We conclude that, yes, weather-stripping windows is highly cost-effective, when plastic V-strip is used. As we move into an era when the stakes will be higher to achieve substantial energy reduction, the answers to these questions are too important to draw simple conclusions. At the very least, more information is warranted and welcome.
Transition for a Hero of Weatherization
As many of you are aware, Eunice Herren retired from the Washington State Department of Commerce after 12 years with the agency at the end of October 2012. Eunice became the first tribal liaison for the weatherization program six years ago.
Eunice’s personal experience with Native American culture proved invaluable as we explored ways to increase service delivery to Native American households, especially those living in Indian Country. Eunice brought passion and commitment to the position, as well as a calm determination to guide and facilitate both tribal entities and local agencies for better coordination and collaboration. She developed our first tribal weatherization program, coordinated innovative competitive proposals, built a strong foundation for serving Native American households, and provided technical assistance and quality improvements along the way. (See “Taking Weatherization to Native Americans in Washington State,” p. 40.)
We are pleased to announce that Donn Falconer has been working with Eunice and transitioning into the weatherization tribal liaison position after Eunice’s retirement. Donn has been with the Department of Commerce for three years in our Compliance Monitoring section. Donn was chosen for this role based on his experience working with the tribes, our local weatherization agencies, and his technical expertise.
Please join us in thanking Eunice for her invaluable contributions to statewide weatherization services and welcome Donn to his new role.
Housing Improvements and Preservation
State of Washington Department of Commerce