In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface and boundary layer resistance is the resistance to heat flow results from convection currents at the surface of a material. In building science, heat flow through a building envelope depends upon the exterior-interior temperature gradient, thickness and conductivity of the building materials and the thickness of the laminar boundary layer of air film adhering to the interior and exterior surface.

As the surface heats up or cools down, it affects the temperature of the air immediately adjacent. This then starts to rise or fall depending on whether it is hotter or colder. This has the same effect as increasing the resistance of the material to the flow of heat. Boundary layer resistance or air film resistance varies depending on the wind speed adjacent to the surface of a material.

## Units and Measures

In the case of multiple layers of a composite wall system, it is reasonable to sum up all the R-values including outside and inside air film to determine total R value of the wall system. For an example:

R-value(outside air film) + R-value(brick) + R-value(sheathing) + R-value(fibreglass batt) + R-value(plasterboard) + R-value(inside air film) = R-value(total).

Units for measuring thermal resistance of a boundary layer is the same as thermal resistance of any other material:

• SI Units: square-metre kelvins per watt (m²•K/W) or square-metre degree Celsius per watt (m²•°C/W)
• Imperial Units: Square feet degree Fahrenheit hour per British thermal unit (ft²•°F•h/Btu)

### Example Units

Outdoor surface, omni direction, moving air 6.7 m/s (winter) - 0.17 (ft²•°F•h/Btu) - 0.030 (m²•K/W)