Passive Design

Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.

How insulation works

Insulation works by providing resistance to heat flow.

On this page:

  • Where heat is lost
  • How bulk insulation works
  • How reflective insulation works
  • R-values
  • Thermal bridges

Insulation slows heat losses from a building, either by using bulky, lightweight materials such as glass-fibre or wool between framing elements, and sometimes by using reflective materials to reflect heat back into the building.

Insulation is a very significant element in a building’s thermal performance, but it is not the only one. Even if a home is well insulated, heat can still escape through air gaps, windows, gaps in the insulation, and building elements such as framing, as explained in thermal bridges below. A building’s thermal performance depends on all elements of the building working together.

Where heat is lost

Typical heat losses from an uninsulated house 
Typical heat losses from an uninsulated house

In an uninsulated timber-framed house, 30–35% of heat is lost through the roof, 21–31% through the windows and 18–25% through the walls. The floor and air leakage account for the remaining heat loss.

Heat loss from a house insulated to pre-2007 levels 
Heat loss from a house insulated to pre-2007 levels

In a house insulated to pre-2007 requirements, the windows account for the largest single proportion of heat loss.

R-values

Insulation performance is measured in R-values, which quantify the thermal resistance of a building material, or any part of a building such as the roof, wall, or floor.

Commercially available insulation materials are labelled with R-values. However, the R-value of any part of the building depends not only on the insulation but also on the thermal performance of other elements such as the framing and cladding.

High density materials such as concrete, brick or stone provide excellent thermal mass but have low R-values and so are poor insulators. Thin metals such as profiled steel claddings and fibre-cement sheets also have low R-values and are therefore also poor insulators.

To determine insulation requirements, it is necessary to calculate R-values for each part of the building. See determining insulation requirements for more detail.

How bulk insulation works

Bulk insulation works by trapping dry air in lightweight, bulky materials. Still air is a poor conductor of heat, so bulky materials that can trap large amounts of air can reduce the ability for heat to be transferred by conduction. If a material consists of many small pockets of trapped air rather than a large, contiguous volume of air, the ability to transfer heat by convection is also reduced. An everyday example is a feather or fibre duvet.

Bulk insulation 
Bulk insulation

Bulk insulating materials such as wool, polyester, glass wool and foam boards work by trapping air and reducing the speed of heat transfer.

How reflective insulation works

Reflective insulation, often called foil insulation, has typically been installed under timber suspended floors where it is draped over the joists before the flooring is installed to create still air zones between joists. It gets its insulation performance by reducing radiant heat transfer through the building envelope. It has no inherent R-value and is not an insulating material in its own right.

The quality of the installation has meant that a still air space is seldom achieved between the foil and the underside of the flooring. The foil also loses reflectivity (as a result of dirt accumulation and tarnishing) and therefore efficiency.

Therefore, although it can be used in some circumstances to achieve Building Code minimum floor insulation requirements, BRANZ does not recommend using foil on its own for floor insulation. See insulation options for floors for more detail.

Thermal bridges

Thermal bridges, also called ‘cold bridges’, are parts of the building envelope where heat can escape more readily because the building material connects – or bridges – both sides of the building envelope. Examples of thermal bridges include:

  • timber or steel framing in external walls that connect to both the interior and exterior faces of the wall
  • aluminium window frames that do not have a thermal break
  • gaps in (poorly) installed insulation.

If insulation has simply been installed between joists or studs, the R-value of the building element is likely to be less than the R-value of the insulation used because of the thermal bridging. Thermal bridging can be reduced through correct installation of insulation, and by using insulating features such as sheathing on the outside of studs or using thermal breaks in aluminium glazing. More detail is provided in the pages on roof insulation, wall insulation, floor insulation and windows.