The Building Code of Australia mandates suitable insulation be installed in residences including apartments. As a result, many innovative cost-effective solutions have been developed. Being a member of Australian Institute of Refrigeration Air-conditioning and Heating (AIRAH), I am qualified to provide independent assessment and advice on your system to obtain Building Control Commission approval.

"R-value" (symbol R), or insulation rating, is the thermal resistance in the units m².K/W (S.I. units). The higher the value, the better the insulation.

"Resistivity" is the resistance per unit thickness in the units m.K/W.

"Conductivity" (symbol k) in the units W/m.K, is the reciprocal of the resistivity

"Conductance" in  the units W/m².K, is the reciprocal of the resistance, so the lower the value, the better the insulation.

The preceding terms are normally applied to single elements.

"Thermal Short-Circuit" also called "Thermal Bridge" is the path of low thermal resistance that is commonly adjacent insulation. e.g. hardwood joists are thermal short circuits to bulk insulation laid between joists.

"Overall Thermal Resistance" is the net resultant thermal resistance through an insulation system due to the combined effects of insulation components and thermal short circuits. Overall thermal resistance includes the thermal resistance of indoor and outdoor air films.

The conductance of a composite structure is normally called the Overall Coefficient of Heat Transfer (symbol U) with the units W/m².K. It is the reciprocal of Overall Thermal Resistance

The following terms are also in usage and the definitions are mine (for lack of a better reference!)*:

"Added R-value" for a particular heat flow path, is the increase in R-value for that path due to the addition of insulation. e.g. If between joists the original R-value is 0.6 and R2.0 bulk insulation is added between joists, the added R-value for this path would be 2.0 although the Overall Thermal Resistance might only increase by 1.3.

"Equivalent R-value" was a term developed to show the advantages of insulation installed on top of joists to reduce thermal short-circuiting. "Equivalent R-value" was defined as "the R-value required by bulk insulation laid between joists to obtain a specified overall R-value". As the "Equivalent R-value" of an insulator depends on the building system components, "Equivalent R-value" should only be quoted with full description of its context.
e.g. In certain conditions, an R1.8 insulation blanket laid on top of joists might provide the same Overall Thermal Resistance as R2.5 insulation laid only between joists, hence the "Equivalent R-value" of the insulation blanket in this system, would be R2.5.

"Economic Thickness" is that thickness of insulation that provides minimum life-cycle costs. i.e. thicker insulation costs more, but it provides comfort and energy savings. But there comes a point where the addition of thicker insulation costs more than it will ever save. Unfortunately "economic thickness" is very sensitive to the assumptions used in its calculation, e.g. heating and cooling usage patterns, cooling type (air conditioning or evaporative cooling), material and energy costs and inflation rates.
 

Note: * If you know of better definitions, please e-mail me.
 
 

Practically all insulators have R-values that are dependent to some extent on surface boundary conditions. e.g polystyrene is a better insulator for cold items than for hot items.

Reflective insulation is extremely sensitive to boundary conditions as the radiative heat transfer is proportional to the difference in the 4th power of the absolute temperatures of the opposing surfaces enclosing the reflective air space.

In brief, still air is a pretty good insulator so reflective cavities may form good insulation. Resistance is highest for heat flow down through horizontal air spaces (summer), and lowest for heat flow up through horizontal air spaces due to the generation of convection currents (winter). Good reflective insulation requires low emittance aluminium foils (typically e=0.03 to 0.05) and the absence of dust and air currents.

For details, please click here to see my paper "Computational Analysis of Reflective Air Spaces" published in the October 1997 AIRAH Journal.

For text examples and case studies, click here. For pictorial examples, click here.  Please ring, fax or e-mail me to discuss your needs.