There is some confusion amongst assessors when it comes to understanding the role of an air gap in the thermal performance of a wall, roof, ceiling or floor construction.

Air gaps exist in many building elements from the way we build and may provide a small improvement in a building’s thermal performance, a modest improvement or no improvement at all.

For an air gap to improve the thermal performance of a building element, it requires the addition of a low emittance surface (shiny aluminium foil) to one or both sides of the air gap. Without the addition of the foil surfaces, the R-Value of the non reflective air gap is small (R0.16). The addition of foil to the building element may also provide a vapour barrier to control condensation if installed correctly.

However, if foil is installed into a building element without a corresponding air gap, it will be of no benefit or only provide a small added benefit if the foil facing is part of a foam or bubble sandwich product.

Firstly, let’s have a look at the science behind an air gap.

The following extract is from the Short Course in Building Thermal Performance Assessment (Residential) training notes for assessors by Dr Holger Willrath of Solar Logic.

“The radiation component of heat transfer across an air gap depends on the emittance of the surfaces on either side, but otherwise does not depend on gap width.  When either one or both surfaces have a low emittance, the heat flow across the gap is reduced significantly.

Still air conduction / convection resistance increases as the gap width increases to about 30mm, after which it remains nearly constant.  When air spaces are vented, or air is forced to circulate in, or move through the space, the heat transfer by convection becomes the dominant factor.  So the combined heat transfer across airspaces varies in a non linear way with distance.

Both conduction and radiation heat transfer are independent of the tilt angle of the surfaces. Because of the buoyancy effect in convection heat transfer in fluids, heat flow up will always be greater than heat flow down. So apart from horizontal heat flow across vertical cavities, air gaps are given both an R_up and an R_down value”.

The principles and underlying relationships used in NatHERS software tools are derived from experimental data by Robinson and Powlich.

But what does this mean for the assessor or building designer?

Air gaps have a thermal resistance to heat flow that is represented by an R-Value with the optimum or best R-Value achieved for a gap of 30mm. Wider air gaps do not  achieve higher R-Values. To achieve higher Total R-Values, multiple air gaps must be incorporated into the building element which is not practical in many instances. Generally, vertical air gaps in walls have the same heat flow inwards and outwards. Heat flow through air gaps associated with floors, ceilings or roof elements will have R-Values that are greater downwards than upwards.

The R-Value achieved by an air gap is dependent on the emittance of the surfaces on either side of the gap. The type of surface determines the emittance value used to calculate the air gap R-Value.

High emittance surfaces have little or no resistance to heat flow and as a result the air gap has a low R-Value.

Low emittance surfaces on one or both sides of an air gap result in air gaps with higher R-Values. Foil surfaces have a low emittance and are modelled in NatHERS tools with an emittance of 0.05. Antiglare foil has a medium emittance and is modelled in NatHERS tools with an  emittance of 0.4. All other surfaces are considered under NatHERS to have a high emittance of 0.9.

Let’s look at some examples of the R-Values of a FC sheeted wall with and without sarking and added bulk insulation.

FC sheeted wall - no sarking - Total R-Value = R0.40

C sheeted wall - added sarking - Total R-Value = R0.84

FC sheeted wall - added sarking & R1.5 bulk insulation - Total R-Value = R1.74

Designing air gaps into a building element provides some improvement to the thermal performance of a building but they are often insufficient on their own to provide the required performance. Adding bulk insulation to the cavity combined with vapour barriers will often provide a better design solution that meets today’s higher thermal performance standards.

Michael Plunkett is Principal of SmartRate based in Cairns and a practising ABSA accredited assessor.