Passive Design
Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.
Insulation Options for New Homes
There is a range of insulation options for new building design.
The option chosen depends on the construction methods being used, the preferred insulation material in terms of sustainability, the location of the insulation within the building envelope, the use of thermal mass, and the desired level of performance.
Ground floors – timber framed
For timber framed suspended ground floors, insulation options include:
- perforated aluminium foil draped over the floor joists (where there is a closed perimeter with 100 mm draped foil) – this is the lowest cost option, and it is unlikely to meet the required insulation level if the floor area is less than 120 sq metres;
- polystyrene inserted between the floor joists – this is a medium cost, higher performance option
- perforated aluminium foil draped over the floor joists protected by a sheet lining material fixed to the underside of the joists (this is required where the underside of the floor is exposed to the elements) – it is a higher cost, medium performance option
- mat or blanket fibreglass, wool or polyester fitted between the floor joists and protected by a sheet lining material fixed to the underside of the joists – this is a higher cost, higher performance option.
- Perforated foil insulation under the flooring
Perforated foil is a low-cost option for underfloor insulation in situations where there is a continuous perimeter wall sheltered from the wind.
- Polystyrene insulation between joists
Polystyrene foam boards between joists give a moderate cost insulation option. The polystyrene can be fitted hard against the underside of the floor, and the polystyrene should be a tight fit between joists with no gaps.
- Floors exposed externally must be closed in
Where a subfloor space is open, such as a pole house, foil insulation should be protected with a durable sheet lining fixed to the underside of the joists
Concrete slab on the ground
Insulation under a concrete slab on ground is usually provided by installing a 25 mm thick layer of polystyrene board over the damp proof membrane before the slab is poured. Insulation – such as a strip of H3.2 treated timber – needs to be incorporated around the edge of the slab to mitigate the thermal bridge effect that occurs under the bottom plate.
- Insulation under a concrete slab on ground
Insulation under a concrete slab on ground can be provided by installing polystyrene board over the damp- proof membrane before the slab is poured. Insulation needs to be incorporated around the edge of the slab to mitigate the thermal bridge effect – in this detail, it is provided by timber.
Where a concrete slab on ground is heated, insulation under the slab is required so that heat from the slab is effectively delivered to the space above and not lost to the ground below.
Exterior walls – framed
In new framed exterior walls, mat or blanket insulation or rigid foam boards can be installed within the framing cavities to provide effective insulation at moderate cost. The performance of the wall will depend on the framing thickness (140 mm framing will allow more insulation to be installed than 90 mm framing), the insulation material chosen, and the amount of framing present.
For steel-framed walls, a thermal break of 20 mm of polystyrene or similar rigid insulating material must be installed on the outside face of the framing before the cladding is installed to reduce the thermal bridging effect at the steel framing locations.
A higher cost option is the installation of an exterior insulation finish system (EIFS) fixed to the outside of the framing. In buildings with a low weathertightness risk score (less than 6), the insulation value of the polystyrene is fully utilised. Where the risk score is 7 or more under NZBC E2/AS1 the cladding must be fixed over a drained and vented cavity which reduces the insulation value provided by the EIFS by 45%.
- Cladding and insulation for steel-framed exterior wall
With steel framing a thermal break must be installed on the outer face of each framing member to restrict the effect of thermal bridging. This applies to all cladding types.
Exterior walls – concrete masonry or in-situ concrete walls
Providing insulation to single skin concrete masonry or cast in-situ concrete walls can be done (in order of increasing performance) by:
- plastering the outside or the inside surface with a proprietary insulating plaster
- plastering both inside and outside faces with a proprietary insulating plaster
- strapping the interior wall surfaces and infilling the void created with polystyrene or matt or segment insulation material and lining
- installing an EIFS cladding system to the outside wall surfaces – adding 40 mm of H grade polystyrene can increase the R-value of the wall by approximately 1.1.
- Externally insulated concrete or concrete masonry wall
Plastered and painted polystyrene sheets are applied here to the external face of a single skin concrete masonry wall. Adding insulation to the outside face of the wall allows the masonry to provide thermal mass that absorbs heat from inside the building.
For walls constructed of precast concrete, insulation can be provided as described for concrete masonry (above) or the panels may be constructed with a core of rigid insulation material.
Roofs
In a timber-framed roof with accessible ceiling space, bulk segment, mat or blanket insulation provides very effective insulation at moderate cost. Insulation may be laid over the framing to reduce the effect of the timber thermal bridges.