Passive Design

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

Options for floor insulation

The type of floor insulation used will depend on whether the floor is concrete slab or a suspended timber-frame floor.

On this page:

  • Insulating timber-framed floors
  • Insulating concrete slab floors

Timber-frame floors are typically insulated with polystyrene boards or sheet insulation made from glasswool (fibreglass), wool, polyester, wool/polyester mix, and mineral wool.

Concrete slab floors are typically insulated with polystyrene board.

For information about performance, durability and environmental properties of each material, see our insulation materials factsheet(PDF) and the materials section of this site.

Insulating suspended timber floors

Insulate suspended timber ground floors using:

  • glasswool (fibreglass), wool or polyester sheets fitted between the floor joists and securely fixed or strapped in place. For very exposed subfloors, protect the insulation by fixing a sheet lining material to the underside of the joists. Check that the specific insulation products are recommended for use under floors by the manufacturer
  • polystyrene panels inserted between the floor joists.
Polystyrene insulation between joists 
Polystyrene insulation between joist

Polystyrene boards between joists give a moderate cost insulation option. The polystyrene must be fitted hard against the underside of the floor and should be a tight fit between joists without gaps.

Suspended timber floor with bulk insulation and lining 
Suspended timber floor with bulk insulation and lining

For open subfloors, a sheet lining material such as plywood, tempered hardboard or fibre-cement must be used to protect the insulation.


Composite floor insulation option 
Composite floor insulation option

Composite construction will give higher performance values than the individual materials and will generally more than meet minimum Code requirements.

Note: Perforated aluminium foil draped over the floor joists without any additional insulation currently meets the minimum permitted R-value according to the Acceptable Solution H1/AS1 Table 1, Note 4, if the subfloor has a continuous, closed perimeter wall. However, reflective insulation is often poorly installed and tends to lose performance over time. BRANZ therefore recommends that other types of underfloor insulation are installed under suspended timber floors.

Insulating concrete slab-on-ground floors

Insulate under a concrete slab-on-ground by placing a continuous layer of 50 mm minimum, S grade expanded polystyrene (EPS) board over the damp-proof membrane before the slab is poured. However, unless a thermal break or perimeter insulation is used, this will only raise the R-value by around R0.2. Slab perimeter insulation is more essential than the underside of the slab as most of the heat loss from the slab occurs at the edges between the air and the ground.

BRANZ research has looked at perimeter insulation for both conventional slabs and waffle slab foundations. Expanded polystyrene (XPS) was chosen for the insulation as it has a history of successful use in this application. The polystyrene was protected with 3 mm grey uPVC sheet on the outside. 

Depending on the circumstances, combining underslab with slab edge insulation can result in thermal performance of the slab improving by 100% or more. Perimeter insulation can bring significant gains in energy efficiency.
Much of the thermal performance improvement can be achieved with a perimeter insulation R-value of less than 1.0. Even an R-value of 0.8 (achievable with 25 mm XPS) still provides a reasonable thermal performance improvement. See BRANZ Study Report SR352 for more details.

A thermal break to the perimeter of the floor slab, between the slab edge and the foundation, greatly increases R-value. In older details a timber strip was used, but BRANZ Bulletin 576 Edge insulation of concrete floor slabs shows a new detail that incorporates a 10 mm thick strip of XPS with an R-value of R0.25. The reason for the change is to minimise the potential for differential movement at the junction between the slab and the foundation wall under earthquake loads. This is achieved by limiting the thermal break thickness to 10 mm (rather than 45 mm when timber was used).

Insulating a concrete slab-on-ground 
Insulating a concrete slab-on-ground

XPS sliver used as a thermal break at slab edge. The sliver will need to be forced over the reinforcing between slab and foundation wall. Fill any gaps after installation with expanding foam.

Determining under-slab insulation requirements

According to the schedule method of calculation R-values in NZS 4218, the floor R-value minimum requirement for all climate zones and wall types is R1.3. For passive design, achieving a higher R-value is recommended - using R1.9 (this is the minimum R-value required for a heated floor) as the minimum is recommended.

The under-slab R-value calculation is complex due to the R-value’s dependence on the thermal conductivity of the soil under different parts of the slab i.e. thermal resistance is greatest at the centre of the slab and least at the perimeter due to the different lengths of the heat flow paths to the exterior of the slab. The calculation depends on the:

  • area/perimeter ratio of the floor
  • thermal conductivity of the soil under the slab
  • thickness of the external walls.

For example, minimum under-slab insulation requirements may be met by the following:

  • If the slab area/perimeter ratio is greater than 1.9, 1.2 m x 50 mm perimeter expanded polystyrene (EPS) insulation and no thermal break with a 90 mm thick wall will give an R-value of R1.3 (a 140 mm thick wall will give a higher R-value of R1.4).
  • If the slab area/perimeter ratio is 1.3 and has a thermal break, a 90 mm thick wall will give an R-value of R1.3.
  • Where full under-slab insulation is installed using 50 mm or 100 mm thick EPS with a thermal break incorporated, the R-value will be well above the minimum requirement.

Embedded floor heating

If embedded floor heating is incorporated in a concrete slab-on-ground, the slab must be insulated so that heat from the slab is delivered up into the space above and not lost to the exterior and ground below. NZS 4218 Table 3 sets out minimum R-values for concrete floor slabs with embedded floor heating.


Updated: 23 May 2016