ASHRAE Mulls Continuous Air Barriers

In a major study, NIST researchers conclude that tight building envelopes reduce energy costs.
August 11, 2010

A study from the National Institute of Standards and Technology (NIST) reports that continuous air barrier systems can reduce air infiltration by up to 60% and energy consumption by almost 40% in commercial buildings.

Researchers Steven Emmerich, Timothy P. McDowell, and Wagdy Annis completed "Investigation of the Impact of Commercial Building Envelope Air Tightness on HVAC Use" last fall. They used a combination of building airflow infiltration computer models and building energy models to study the impact of continuous air barriers on commercial buildings in five cities (Miami, Phoenix, St. Louis, Minneapolis, and Bismarck, N.D.) in different climate zones.

Using baseline energy, climate, and building data from each city, they modeled an apartment building, an office structure, and a retail facility. The research team used air-barrier systems with components that met minimum material air tightness levels.

The results showed a wide range of gas-electric cost savings across the board. Improved air tightness in frame construction buildings in Bismarck would be 33% for an office building, 22% for retail, and 31% for an apartment building. In Minneapolis, the cost reductions were even higher, while more modest gains were observed in Miami and Phoenix.

"We used these methods because air tightness comes from a lot of different things working together," said Emmerich, research engineer for NIST's Building and Fire Research Laboratory. "We wanted to show how air affects a building differently if it's wood or masonry. Air tightness isn't just like installing insulation. Everything has to be accounted for."

The study, sponsored by the U.S. Energy Department's Office of Building Technology, tested building wraps and coated air barrier models. Air leakage from other parts of the envelope was also a factor. The study recommends further testing of continuous air barriers on more building types, analysis of costs and potential savings from tightening of existing buildings, development of more refined air tightness targets, testing of air tightness of buildings in states with state tightness standards (such as Massachusetts) to determine if standards are met in practice, and examination of interaction between air tightness and other building parameters.

Emmerich said the study was done in part to assist the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) in making changes to its Standard 90.1, which includes building air leakage. Proposed Addendum Z to 90.1-2004 contains requirements for a continuous air barrier. ASHRAE's Standard Project Committee is currently considering public comments on the proposed change.

A continuous air barrier requirement from ASHRAE would cause a surge in sales for suppliers, but many engineers are still skeptical of requiring it as a best practice. Many have already said the NIST study's results will cause designers to simply plan more commercial buildings without operable windows, rather than invest in a continuous air barrier.

"That's not the message I would take from this research," Emmerich said. "It's certainly possible to build to this level of tightness with operable windows. Although it's not specifically evaluated, improving building envelope air tightness also reduces the potential for problems caused by leakage, such as poor indoor air quality, thermal comfort, and degradation of building materials due to moisture damage. It's a whole building issue."

Energy cost savings

City Gas savings Electrical savings Total savings
The NIST study looked at the effects of building envelope tightening on three building types in five different climates. Results show cost reductions in each region and building type modeled. Apartment buildings saw an increase in electrical costs from losing the "free cooling" effect, but still posted significant savings overall.

Office building
Bismarck $1,854 42% $1,340 26% $3,195
Minneapolis $1,872 43% $1,811 33% $3,683
St. Louis $1,460 57% $1,555 28% $3,016
Phoenix $124 77% $620 9% $745
Miami $0 0% $769 10% $769
Retail building
Bismarck $1,835 26% $33 2% $1,869
Minneapolis $1,908 28% $364 18% $2,272
St. Louis $1,450 38% $298 9% $1,748
Phoenix $176 64% $992 14% $1,169
Miami $6 98% $1,224 14% $1,231
Apartment building
Bismarck $2,187 40% -$116 -9% $2,071
Minneapolis $2,421 43% -$165 -14% $2,256
St. Louis $1,794 57% -$232 -12% $1,562
Phoenix $133 65% $0 0% $133
Miami $31 63% $380 9% $411