Air Permeability

Introduction

The air tightness of a structure has a direct impact on the natural ventilation rate. The unnecessary air leakage of buildings in arbitrary locations leads to occupant discomfort and higher energy consumption. Buildings should be designed and constructed to provide minimal air infiltration except in locations which are part of the building design.

What Constitutes an Effective Air Barrier System?

Control of airflow has to be designed into the building envelope right from the conceptual stage. We often refer to the use of an 'air barrier' to perform this function in a wall, but it is most important to note that an air barrier is not a single material. We cannot buy an 'air barrier' off the shelf. An air barrier is a system, made up of materials, joints and assemblies. We must consider a line, or plane, of air tightness. This line of air tightness must be continuous, so that in the design stage we can see impermeable materials, held rigidly or rigid in themselves, sealed at any joints, and joined and sealed to other assemblies.

Test Methods for Determining Air Leakage Rates

The tightness of an air barrier system depends on the use of impermeable materials. One of the easiest test for the effectiveness of the air barrier systems in assemblies is inspection. If you go on the building site after the air barrier is installed and can see through it, then you dont need an expert to tell you that it should be fixed.

IRC has developed a test method to determine the airflow resistance of exterior membranes and sheathing. A range of pressure differences is applied across a sample of the material and the airflow produced by the pressure difference is measured. From the data produced, a graph of airflow versus pressure difference is generated and the airflow rate ata given pressure can be calculated.

The procedure has been used to measure the air leakage of a number of common building materials, as shown in Table 1.

Table 1 Measured air leakage for selected building materials

Independent Test Results Available from our Tech. Dept.

References:

CSA Standard CAN 3-A440-M84, Windows. Canadian Standards Association, Rexdale, Ontario, 1984.

BSRIA Specification 10/98.

IRC Institute for Research in Construction.