Air Barriers and Insulation: What's the Biggest Energy Saving Bang for Your Buck?
Citing a case study of a fluid-applied air barrier, the author argues that air barriers may be underrated in terms of their energy saving impact.
Historically, state energy codes have focused on conductive heat transfer through prescriptive R-value and U-values for building enclosure systems. Lesser focus has been place on air leakage control and air barrier requirements.
But codes requiring air barriers are starting to be adopted on a state by state basis, although they are not currently required in most states. With recent changes in energy codes, continuous insulation is now required for frame construction in at least six out of eight U.S. climate zones—while there are continuous air barrier requirements in five out of eight climate zones. But does this make sense from a building science perspective. Let's look at the energy conservation benefits of continuous air barrier systems, relative to the benefits of increased insulation.
Rising energy costs and environmental concerns have driven changes in energy codes and in construction practices, which are aimed at reducing energy consumption. Rising energy costs have also resulted in an increased focus on retrofit of existing buildings to reduce energy consumption.
Controlling conductive heat transfer through exterior walls and controlling air leakage through the building enclosure are two key aspects to reducing energy consumption.
High-Performance Beauty. Blackwood Builders used Sto's spray-on air and moisture barrier in the construction of this custom home.
Current U.S. energy codes require continuous insulation for frame construction in at least six of the eight climate zones, and there are continuous air barrier requirements in five of the eight climate zones.
The first modern North American quantitative code requirement for air barrier materials was implemented in the National Building Code of Canada in the mid 1980’s. In 2001, Massachusetts became the first state to incorporate a quantitative air barrier code requirement, Wisconsin, Michigan, Rhode Island, Georgia, Minnesota and Florida have since included air barrier requirements in their state codes.
Air Barriers Rising
The specification of air barrier materials for exterior building enclosures is becoming increasingly common. The U.S. Army Corp of Engineers now requires that all new buildings, and all renovated buildings incorporate an exterior air barrier with maximum allowable air leakage of 0.25 CFM @ 1.57 psf.
There are several approaches to air barriers for exterior walls, of which combined air/water barrier materials are one of the more common approaches. Mechanically fastened house wraps, self-adhered membranes, and fluid applied membranes can all be used as an exterior air/water barrier for an exterior wall.
Fluid applied air barriers are often preferred by designers and installers for their relative ease of detailing and installation as compared to sheet goods. Fluid applied air/water barriers have long been used in drainable EIFS systems. The use of these fluid applied air/water barriers is now becoming increasingly common with other exterior cladding types. StoGuard, manufactured by Sto Corp. is an exterior fluid applied air/water barrier, and was part of an energy modeling analysis conducted by Morrison Hershfield.
Case In Point
The study evaluated StoGuard performance for both new construction and for energy retrofit applications, compared to typical construction without a defined air barrier. The system provides a continuous fluid applied air barrier for exterior wall applications.
A prototype medium three-story office building was modeled for Dallas, Seattle, and Toronto climates. Various scenarios for retrofit of existing buildings and for design upgrades for new construction were considered. The baseline case buildings were modeled as meeting the minimum requirements of ASHRAE 90.1-2004 for existing buildings and ASHRAE 90.1-2007 for new buildings. A baseline air leakage rate of 1.55 cfm/sf of above grade envelope surface normalized to 1.57 psf (75 Pa) was used.1
Existing Building Baseline
The baseline existing building was as follows:
- 53,600 sf three-story building meeting minimum ASHRAE 90.1-2004 requirements
- Batt insulation in stud space for Seattle and Dallas, continuous insulation for Toronto climate.
- 33% glazing (minimum code compliance for U-values, and SHGC values) o Air leakage rate: 1.55 cfm/ft2 envelope area at 1.57 psf (75 Pa) pressure
difference normalized to 0.08 psf (4 Pa)
- Steel stud framed exterior walls with brick veneer
Existing Building – Air Barrier Retrofit:
For retrofit case 1: A continuous 2” layer of rigid insulation was added to the baseline model without changing the air leakage rate. For retrofit case 2: a continuous air barrier was added to the baseline model to achieve a reduction in air leakage rate to 0.4 cfm/ft2 at 1.57 psf (75 Pa) pressure difference.
In short, adding a continuous air barrier appears to significantly reduce air leakage in both new and existing buildings, even in those were rigid insulation is installed. While it's not clear how much of the difference results from inaccurate installation of the insulation, that may be simply an academic point--assuming this installation was fairly typical, whereas the real-world results make the strongest case for addition of air barriers.