Air Barrier Systems
Control of air movement through the building enclosure is key to controlling heat, moisture an indoor air quality. Unwanted air leakage is controlled by an air barrier system - a continuous air-impermeable control layer srrounding the entire building enclosure.
Air barriers are necessary to meet current energy efficiency codes. For residential buildings, a whole-house air leakage is typically required (ASTM E779), with limits on allowable air leakage based on climate zone. For commercial buildings where whole-building air leakage measurements may not be practical, an air barrier is prescribed by the energy codes.
According to the Air Barrier Association of America (ABAA), air barriers consist of:
- Air Barrier Materials - materials with an air permeability not to exceed 0.004 cfm/ft2 under a pressure differential of 0.3 in. water (1.57psf) (0.02 L/s.m2 @ 75 Pa) when tested in accordance with ASTM E 2178.
- Air Barrier Assemblies - A building assembly of air barrier materials and air barrier accessories (e.g., tapes and sealants) with an air permeance not to exceed 0.04 cfm/ft2 under a pressure differential of 0.3 in. water (1.57psf) (0.2 L/s.m2 @ 75 Pa) when tested in accordance with ASTM E 2357.
- Air Barrier System - A building enclosure where the air leakage of the entire building shall not exceed 0.4 cfm/ft2 under a pressure differential of 0.3 in. water (1.57psf) (2.0 L/s.m2 @ 75 Pa) when tested according to ASTM E 779
Air barriers come in several forms, including:
- Mechanically-attached membranes, also known as housewraps, usually a polyethylene-fiber or spun-bonded polyolefin, such as Tyvek is a generally accepted moisture barrier and an air barrier (ASTM E2178).
- Self-adhered membranes, which are typically also a water-resistant barrier and a vapor barrier
- Fluid-applied membranes, such as heavy-bodied paints or coatings including polymeric based and asphaltic based materials
- Closed-cell medium density spray-applied polyurethane foam, which typically provides insulation as well
- Some open-cell spray-applied polyurethane foam that are of high density
- Boardstock, which includes 12 mm plywood or OSB, 25 mm extruded polystyrene, etc.
SPF as an Air Barrier
SPF insulations meet the air barrier material requirements at typically-installed thicknesses of 3.5-5.5 inches for open-cell and 1.0 inches for closed-cell. This data is typically posted by SPF manufacturers in technical data sheets or code-compliance reports. Many SPF products have been tested and meet the assembly air leakage requirements. An NAHB study measuring air leakage for 56 homes using SPF insulation consistently yeild whole-house air leakage values of less than 1.5 ACH50.