Rainscreen Gaps and Igloos
A report from the 2013 Westford Symposium on Building Science
For the past 17 years, Joe Lstiburek and Betsy Pettit have hosted an annual conference, the Westford Symposium on Building Science, near their home in Massachusetts. Informally known as “summer camp,” the invitation-only gathering attracts hundreds of builders, engineers, architects, professors, and building science researchers.
The attendees listen to presentations at a conference center during the day and relax in Joe and Betsy’s backyard during the evening.
This year’s conference was held on August 5-7. Speakers included Michael Blasnik, Mike Steffan, Gary Klein, Kent Browning, Achilles Karagiozis, Chris Schumacher, Dave Ober, Joerg Birkelback, Marcy Tyler, Ed Retzbach, Andreas Holm, and Joe Lstiburek.
Covering familiar territory
If there were any devoted readers of GBAGreenBuildingAdvisor.com at the conference, they probably recognized the information provided at several of the summer camp presentations this year. At the Westford conference, Blasnik discussed the limitations of energy modeling software, a topic covered in a GBA blog published in March 2012. Blasnik also discussed bad energy retrofit advice, a topic which GBA has covered in at least three articles: A Plague of Bad Energy-Saving Tips, More Energy Myths, and How Not to Save Energy.
(The PowerPoint slides that Blasnik used in his presentation can be viewed by clicking one of the links at this Web page. The page also includes links to some of the other presentations from this year's Symposium.)
Gary Klein, a renowned expert on domestic hot water systems, discussed several ways to reduce the time required for hot water to travel from a water heater to a faucet. GBA has reported on Klein’s calculations and advice in several articles, including Inefficient Hot Water Piping Layouts Waste Hot Water and All About Water Heaters.
Joe Lstiburek’s presentation focused on his proposal that ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. be replaced with a new residential ventilation standard. His arguments were reported in a June 2013 GBA article, How Much Fresh Air Does Your Home Need? GBA’s follow-up articles on this topic include Resistance May NOT Be Futile in the Residential Ventilation Wars and An Interview with Dr. Joseph Lstiburek.
Lstiburek’s proposed new ventilation standard was published this week, and is available online: Ventilation for New Low-Rise Residential Buildings.
R-mageddon: The thermal metric project
Chris Schumacher and Dave Ober reported on research that is leading to the development of a new metric to replace R-value. (This year, Schumacher told the crowd that he wanted to call his presentation “R-mageddon.”) Much of the material presented by Schumacher and Ober was a reprise of presentations made at previous conferences. (To read GBA’s reports on these earlier presentations, see A Bold Attempt to Slay R-Value and Air Leakage Degrades the Thermal Performance of Walls.)
As he did last year, Schumacher shared some bar graphs showing the effects of high temperatures and infiltration on the thermal performance of wall insulation. Schumacher’s colleague, Dave Ober, proposed several fairly complicated ways to replace R-value with a new metric that takes these factors into account. One proposal called for using a line graph to report the new metric. Another proposal called for reporting the new metric in a very large table listing different R-values for different cities; according to this proposal, the R-value (or perhaps the “Joe-value” or “Schumacher-value”) for a fiberglass batt would be different in Omaha than in Boston.
A new use for WUFI
Achilles Karagiozis is the director of building science at Owens Corning in Ohio. His presentation was a delight. He started out with an amusing scenario for an imaginary TV reality show, involving a helicopter crash in Alaska with only one survivor, Joe Lstiburek. The only tool that Joe had to help keep him alive at -51°F was a laptop loaded with WUFI software. Karagiozis explained how Joe could use WUFI to design an igloo, assuring his survival — and incidentally proving the usefulness of the oft-derided hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. modeling software that Karagiozis helped develop.
Once Karagiozis had completed his Alaskan fantasy, he discussed his efforts to model air flows through rainscreen gaps on walls. If there were any math-averse listeners present, they were probably having an anxiety attack. Karagiozis explained why it is difficult to model turbulent 3-D air flows; why air flow equations need a friction factor to account for roughness in laminar flow; how to calculate buoyancy pressures; when you need to consult Darcy-Weisbach equations; and when you need to perform a multivariate regression analysis.
If (like most of us) you have no interest in studying the Darcy-Weisbach equations, here’s all you need to know:
- The main driving factor for air movement through ventilated rainscreen gaps in walls is solar gain.
- Rainscreen gaps in walls are very effective at drying damp siding and sheathingMaterial, 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. .
- Rainscreen gaps provide an important safety factor to handle the higher moisture loads in today’s high-R walls.
- Karagiozis recommends that all walls should include a ventilated rainscreen gap.
Summing up, Karagiozis said, “Increasing levels of insulation are making us rethink moisture transport in walls. We need to think harder. We need to design. To withstand the moisture loads, we need to passively ventilate walls to increase the drying potential.”
Michael Blasnik is an energy consultant from Boston, Mass., who is renowned for his skill at analyzing large datasets. Here are some quotes from his presentation.
Michael Blasnik: “I went to the Home Energy Saver website and entered information on my house. The model said that I will save 24% of my heating energy by insulating my basement ducts. But studies show that measured savings are on the order of 1% to 3%. The model told me that if I seal my basement ducts, insulate the ducts, and insulate the floor above the basement, I will save 41% of my heating bill!”
“A SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. 13 air conditioner uses less energy than a SEERSeasonal Energy Efficiency Ratio (SEER) is the total cooling output (in BTU) of an air conditioner or heat pump during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period. The units of SEER are Btu/W·h. SEER measures how efficiently a residential central cooling system operates over an entire cooling season. The relationship between SEER and EER depends on location, because equipment performance varies with climate factors like air temperature and humidity. 10 air conditioner. But a SEER 17, 19, or 20 air conditioner doesn’t really perform as well as the SEER numbers would lead you to believe.”
“Cooling load is not driven by delta T. Cooling load is driven by solar gain through windows, solar gain through the attic, duct efficiency, and internal gains.”
“Here is a list of things that are hard to model well: Foundation heat loss, infiltration, wall and attic heat loss, window loss and gain, and HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. performance. Everything else we’ve got nailed down.”
“If you have a vented attic, most of the ventilation air isn’t coming from the soffit vents. Most of the ventilation air comes from the house to the attic, and then out.”
After Blasnik explained that annual tune-ups of gas furnaces don’t save any energy, Henry Gifford commented, “You can’t tune up a gas furnace. There is nothing to adjust.” Blasnik responded, “Right. It’s a placebo measure. There is nothing to do — but pay the guy $180 for the visit.”
Tankless water heaters are not instantaneous
Gary Klein is an energy specialist who used to work for the California Energy Commission. He is now leading efforts to change plumbing codes to legalize hot water piping systems that minimize the time it takes for hot water to reach a distant tap. (Here are the two most important principles to remember: small diameter pipes work better than large diameter pipes, and short pipes work better than long pipes.)
Gary Klein: “It does not make sense to discuss efficiency if you are not providing the service people want. Most people want hot water in 3 to 4 seconds. For that to happen, you need a source of hot water that is close to the tap, and it has to be hot before you turn on the tap. A tankless water is not instantaneous. It takes 4 seconds of water flow before the igniter turns on, and it takes 30 to 60 seconds until the water is hot.”
“This study shows that 92% of the time, hot water draws are within one hour of each other. That’s why pipe insulation makes sense.”
Talking to contractors about duct leakage testing
Kent Browning is an HVAC engineer from Austin, Texas, who commissions homes, condominium buildings, and museums. Browning said, “If a contractor can’t show duct leakage testing experience, then the contractor is not qualified.”
He also said, “If you are working in any area where contractors aren't used to duct leakage testing, you will encounter six predictable stages in your conversations with contractors:
1. ‘Our ducts are so good and they don’t need to be tested.’
2. ‘Ducts can’t be leak tested.’
3. ‘This test is wrong.’
4. ‘Ducts can’t possibly be sealed as specified so the current leakage should be accepted as is.’
5. ‘This testing and sealing wasn’t included in our bid; it will cost extra.’
6. ‘See, I told you we would take care of you.’ ”
Electric blues band
Each evening, the Building Science Boogie Band provided live entertainment in Joe and Betsy’s barn. On Sunday night, Steve Sauer was on keyboard, Mike Lubliner was on guitar, Collin Olson was on bass, Rob Moody was on sax, and Randy Martin was on drums. The band was mesmerizing the crowd with an electric blues riff that was slowly building in intensity, eliciting cheers and hoots as the musicians’ power cranked up.
Right behind my chair, I was distracted by some shuffling and scraping. There was Betsy Pettit, bending down to move the musicians’ equipment cases that were on the floor under the stairs. “They’re blocking the HVAC return,” I heard her say.
In a crowd of energy nerds, most of whom are either leaning on the bar or captivated by the band, it’s a pretty safe bet that there will be at least one architect in the room who (thank goodness) is worried about return-system airflow.
Martin Holladay’s previous blog: “If Only Green Homes Could Be Sold Like Breakfast Cereal.”
- Martin Holladay
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