Insulation

Key points

  • Insulation is a material that slows or prevents the flow of heat.
  • Insulation is a key part of any passive designed home, helping to keep heat inside the home in winter and outside the home in summer.
  • The performance of any insulation product – how well it resists heat flow – is know as its R value. The higher the R value, the higher the level of insulation.
  • Good orientation can significantly improve your comfort and reduce your heating and cooling needs. The best orientation for your home is the one that suits your climate zone.
  • The ‘total R value’ adds together the R value of the various components of a roof, ceiling, wall or floor, including the insulation.
  • The type and R value of insulation that is best suited to your home will depend on your climate and construction type.
  • There are a wide range of insulation products. Bulk insulation uses air pockets within a thick material to slow the flow of heat. Reflective insulation reflects heat back to where it came from, and if double sided does not re-radiate heat on the opposite side. Composite insulation combines bulk insulation with a reflective surface.
  • All insulation should be installed carefully following the product specifications, to minimise the risk of condensation or fire.
  • ‘Thermal bridges’ are pathways for heat transfer through components of the floor, walls or roof. They need to be identified and insulated to prevent heat flow and condensation risk.
  • Some types of insulation should be installed by a professional, while some you can do yourself. Always follow the manufacturer’s instructions.

Understanding insulation

What is insulation?

Insulation is a material that resists or blocks the flow of heat energy. Insulation is used to stop heat inside the home from escaping in winter, and to stop heat outside the home from entering in summer.

The best type and location of insulation will depend on your local climate, and whether the insulation is mainly needed to keep heat out or in (or both). The first step towards getting a good result from your insulation is to understand how your climate will affect the building.

For insulation to be effective, it should work in conjunction with good passive design. For example, if insulation is installed but the house is not properly shaded in summer, built-up heat can be kept inside by the insulation, creating an ‘oven’ effect.

Why is insulation important?

Insulation acts as a barrier to heat flow and is essential for keeping your home warm in winter and cool in summer. A well-insulated and well-designed home provides year-round comfort, cutting cooling and heating bills, and reducing greenhouse gas emissions.

Two diagrams of the same house show the percentages for winter heat loss and summer heat gain.

Typical heat losses and gains without insulation in a temperate climate

Source: SEAV (2002), updated in Energy Smart Housing Manual (2018)

Insulation R values

How well an insulation product resists heat flow is know as its R value. The higher the R value, the higher the level of insulation. The appropriate degree of insulation depends on your climate, building construction type, and whether auxiliary heating and/or cooling is to be used.

Material R values refer to the insulating value of the product alone. The NCC and BASIX (in New South Wales) set out minimum requirements for the R values of materials used in the construction of buildings. It is generally advisable to exceed these for greater comfort and energy savings.

Total R value

‘Total R value’ describe the total resistance to heat flow provided by a roof and ceiling assembly, a wall or a floor. Each of the material components has its own heat resistance (R value), and the total R value is calculated by adding the R value of each component, including the insulation.

Total R values are the best indicator of performance because they show how insulation performs within the building envelope. Total R values are used when calculating energy ratings to measure thermal efficiency.

Total R values for roofs, ceilings and floors that use reflective insulation are expressed as up and down values, depending on the direction of heat flows through the product:

  • ‘Up’ R values describe resistance to heat flow in an upwards direction (sometimes known as ‘winter’ R values).
  • ‘Down’ R values describe resistance to heat flow in a downwards direction (sometimes known as ‘summer’ R values).

Both up and down R values should be considered when installing roof, ceiling and floor insulation. Total R values for walls are expressed as a single figure, without ‘up’ and ‘down’ distinctions.

Many factors can reduce the total R value, including thermal bridging, compression of bulk insulation, dust settling on reflective insulation and the lack of a suitable air gap for reflective surfaces. Careful installation according to specifications is needed to ensure your insulation performs as it should.

Note

R values as used in Australia, New Zealand and Europe are metric and are different from R values used in the United States. American products and publications quote R values that will appear much higher than the values seen in Australian products and discussed in Your Home. There is no simple conversion factor between American and Australian units, so it is best not to use those values – seek out the metric values instead.

Types of insulation

Insulation products come in 2 main categories — bulk and reflective — which are sometimes combined into a composite material.

All insulation materials that are sold in Australia must meet Australian Standard AS/NZS 4859, Materials for the thermal insulation of buildings.

As well as assessing the insulation performance, you can compare the environmental benefits of different products. Ask about recycled content and how easily the product can be recycled after use. For example, some brands of glass wool, polyester and cellulose fibre insulation contain significant amounts of recycled material. Contact the manufacturer or industry association to find out more. Environmental comparisons of insulation products can be found on ecolabel websites such as Ecospecifier Global, Global GreenTag, Good Environmental Choice Australia, Australian National Life Cycle Inventory Database, Environmental Product Declaration Australasia and Building Products Information Rating.

Bulk insulation

Bulk insulation uses pockets of trapped air within its structure to resist the transfer of conducted and convected heat. Its thermal resistance is essentially the same regardless of the direction of heat flow through it.

Bulk insulation products come with one R value for a given thickness, and include materials such as:

  • glass wool, batts and rolls (often made from recycled materials)
  • wool, batts and loose fill
  • cellulose fibre loose fill (often made from recycled paper fibres)
  • polyester, batts and rolls (often made from recycled materials)
  • polystyrene, expanded (EPS) or extruded (XPS), as rigid boards
  • polyisocyanurate (PIR) as rigid boards
  • polyurethane (PUR) as rigid boards.

A diagram shows that bulk insulation traps air in pockets, reducing heat transference through the insulated material

Bulk insulation traps air in still layers

Source: SEAV 2002

Reflective insulation

Reflective insulation mainly resists radiant heat flow because of its high reflectivity and low emissivity (ability to re-radiate heat). Its insulation ability relies on the presence of an air layer of at least 25mm next to the shiny surface. The thermal resistance of reflective insulation varies with the direction of heat flow through it.

Reflective insulation is usually shiny aluminium foil laminated onto paper or plastic and is available as sheets (sarking), concertina-type batts and multi-cell batts. These products are known as reflective foil laminates (RFL).

Because any foil insulation is electrically conductive, the risk of contact with electrical cables and equipment must be considered with all installations, and measures to eliminate the risk should be followed. Refer to Installing insulation on this page.

A diagram shows double-sided reflective foil used as an insulator reflects 95 per cent of radiant heat, with the remaining 5 per cent being emitted.

Reflective insulation and heat flow

Source: SEAV 2002

Composite insulation

Composite insulation combines bulk and reflective insulation. Examples include foil-faced boards, reflective foil-faced blankets and foil-backed batts. The orientation of the foil needs careful consideration to ensure it is most effective and does not add to condensation risk. Be aware that reflective foil insulation should be on the warm side of any building system. Generally, in cooler climates, this means placing the foil on the inner side of the bulk insulation (foil facing inwards), with an air gap betweenthe foil and the ceiling material (for example plasterboard). In hot humid climates (for example, Darwin) in air-conditioned buildings, the opposite is a better solution (foil facing outwards). Advice should be sought from the insulation manufacturer.

Foil-faced blanket use in roof spaces

Traditionally, a foil-faced fibre blanket has been used in an attempt to prevent the underside of the roof from falling below dew point. But even if the overall R value is adequate, wherever the blanket is compressed over purlins or roof battens or it is not in continuous contact with the roofing material, its effective R value diminishes towards zero and condensation can form on the underside of the roof. As building practices have improved, and houses have become more airtight, roof spaces have also been over sealed and the risk of condensation has been exacerbated. The solution to this is a well-ventilated roof space, to remove excess water vapour from the roof space to avoid mould.

Achieving good insulation

Good insulation that works effectively for your home requires the selection of the correct product for your climate. The right product will make a significant difference to the comfort and energy performance of the home.

However, the right product is often not enough. It is vital that it is specified and installed correctly to perform well and avoid condensation problems.

Insulation should be incorporated when a home is built. Installing high-performance products at the time of construction is a good investment, resulting in lower energy bills over the lifespan of your home.

For existing homes, adding insulation to your ceiling, walls and floor can be an effective part of renovation at any time. However, some insulation can be hard to retrofit in later renovations.

Insulation levels for your climate

The NCC requires minimum insulation levels (total R value) for roofs, walls and floors, according to your home’s location and other building features. The NCC Volume 2 provides detailed descriptions of the insulation requirements for each climate zone.

Additional insulation above minimum levels can further improve building performance. The optimal level should be determined by your local climate, construction type and budget. Your architect, designer or building energy assessor can help you to identify your insulation needs.

Roof and ceiling insulation

Installing roof and ceiling insulation can save up to 45% (or more) on heating and cooling costs.

Roof construction types

Most roof constructions will be ventilated and should include air gaps in their design to allow condensation to be carried away or to dry out.

For roofs that are unventilated, hygrothermal analysis must be completed by an appropriately trained consultant to demonstrate compliance with the National Construction Code.

In principle, condensation needs air spaces to form. If there are no air gaps in a roof construction (for example for some flat roofs), then there is no opportunity for vapour to fall into liquid form. However, this does not prevent the risk of mould developing and it is therefore very important that roof construction materials be considered carefully and installed correctly.

Ceilings and roofs are not considered part of a building’s breathable envelope for controlling internal humidity, which must be done through fully breathable walls or a mechanical heat recovery ventilation system.

Pitched roofs with flat ceilings

This is the most common type of construction and the easiest to insulate.

Roof

In Climate zone 1 (high humidity summer, warm winter), a layer of reflective insulation (either sarking or foil batts) beneath the roof increases resistance to radiant heat. In air-conditioned buildings in warm tropical climates, reflective foil should be used on the outside (or warm side) of bulk insulation.

In other climate zones, reflective insulation can be used on the inside of bulk insulation to keep heat inside the home in winter. There must be an appropriately specified vapour-permeable moisture barrier (sarking) below the roof to carry away any condensation.

Generally, ensure that there is an effective air gap between reflective surfaces and other materials – depending upon what the material and construction system is. If assembling non-rigid materials on site, it is wise to allow at least 25mm between layers to ensure the air gap is maintained. Rigid board materials can be installed with air gaps of as little as 10mm, and some pre-manufactured products may have 5mm gaps. Some products form their own air gap, such as a concertina profile. Always refer to the product manufacturer regarding installation.

" " A diagram of a section view of inside a pitched roof showing insulation, timber battens and roofing material.

A pitched roof with a flat ceiling, showing 2 options for using reflective foil on the inside of bulk insulation; this is useful in all but warm tropical climates

Source: Envirotecture

Ceiling

In most climates, it is appropriate to place ceiling insulation between the joists. Suitable bulk insulation includes batts, loose-fill and rigid foam boards such as XPS, PUR or PIR (but preferably not EPS, because it can break into small particles that escape into the external environment).

In alpine climates, it may be necessary to use multiple layers of insulation to achieve the very high R values needed. This may require innovative detailing in the roof and ceiling design.

Install insulation in accordance with manufacturer’s instructions. Failure to do so can significantly reduce insulation values.

If ceiling joists are covered with insulation, safe places to walk cannot be seen when accessing the roof space, and platforms or access planks should be installed. For this reason, bulk insulation is usually installed so that the top of ceiling joists or roof trusses remain exposed, even though this diminishes the insulation somewhat. If insulation is removed or moved when the roof space is accessed, it must be reinstalled in accordance with the Australian Standard.

" " A detailed drawing of a section view of inside a pitched roof showing insulation, timber battens and roofing material.

Typical roof and flat ceiling insulation construction detail.

Source: Envirotecture

Raked or cathedral ceilings

Raked or cathedral ceilings include sloping ceilings, vaulted ceilings, and flat or skillion roofs where there is no accessible roof space. Design and construct ceilings with enough space to accommodate adequate insulation, including any necessary air gaps. Ceilings with exposed rafters are generally difficult to insulate without using expensive materials. This is because space limitations within the ceiling require products with a higher R value per unit thickness.

Consult the insulation manufacturer about installation clearances. As a rough guide, minimum clearance heights for ceilings that are parallel with the roof are:

  • R3.0 bulk batts – 190mm
  • R3.0 PIR foam boards – 60mm.

Use an appropriately specified vapour-permeable moisture barrier (sarking) under roofing, with longitudinal battens installed over the membrane on top of each rafter, to create a drainage gap for condensation to trickle down to the gutter or outside of the wall. Drainage battens can be as thin as 9.5mm, made from any resilient material – some manufacturers have products specifically designed for the purpose. Roofing battens are installed in the usual way across the top of the drainage battens.

Suitable bulk insulation may include polyester or fibreglass batts, or rigid foam boards such as PIR or XPS boards.

Suitable composite insulation includes foil-faced polystyrene boards. If rafters are exposed, the batten height must allow a minimum of 20mm for reflective air space adjacent to the foil face – this allows for some deflection over time. In cooler and hotter climates, high R values are required and larger batten heights will be required to accommodate thicker insulation.

" " A top down view of two options for bulk roof insulation. In both options, the insulation is placed between the rafters. A waterproof vapour permeable membrane wraps the insulation. Additional roof battens run atop of the membrane. Insulation in the first option is placed directly on the ceiling. In the second option, a foam/foil board is placed between the insulation and the ceiling.

Concealed rafters with a hybrid of bulk insulation between rafters and an option of continuous foam/foil sheet below, foil face down; this is useful in all but warm tropical climates

 

" "

A top down view of a tiled roof. Roof tiles sit atop tile battens. The tile battens are fixed to rafters below. High R-value dense rigid foam insulation board is covered in roof wrap and sits underneath the tile battens. The ceiling sits on the exposed rafters.

 

Exposed rafters with rigid foam board insulation.

Note: The roof battens must be secured through all intermediate components and into the rafters with appropriate fasteners to prevent roof failure in storms or high winds. The detail is fundamentally the same for metal roofing.

Flat roofs

Roofs with less than 5° pitch cannot be relied upon to drain the condensation that will gather under cold roofing sheets, and so the condensation must be prevented from forming in the first place). This means a different approach to pitched roofs is needed. For all roofs that are unventilated, hygrothermal analysis must be completed by an appropriately trained consultant to demonstrate compliance with the National Construction Code.

Structural insulated panels

These roofs have a structural skin (usually precoloured metal) on both sides, and dense closed cell foam core made of PIR, PUR or XPS foam. The tight assembly of the panel leaves no space for air and thus no condensation risk, if the R value is adequate. The required R value of the panel, and its structural capacity will need to be calculated for your climate zone and site.

A cross-section diagram shows structural skin of metal on both sides of a foam core.

Structural insulated panel.

 

Composite roof built up from conventional materials

Precoloured steel roofing laid on roof battens and rafters with a ceiling below, requires bulk insulation installed in full and direct contact with the metal roofing, leaving no air gaps. Thus the thickness of the insulation batts must be coordinated with the depth of the battens and rafters. The uppermost layer in contact with the roofing should be slightly thicker than the batten depth, so that they are compressed by about 10% of their thickness when the roofing is fixed down. While this reduces their effective R value by about the same proportion, it will remove the air gaps. The required R value of the batts will depend on your climate zone and site.

 

A cross-section diagram of a composite built-up roof with metal roofing at less than 5 degrees pitch.

Composite roof built up from conventional materials.

 

Flat membrane roof on lightweight structure

A membrane of either heat-welded or bonded poly sheet is adhered to a dense substrate such as structural ply or compressed cement sheet, or a liquid is applied over the substrate sheet. Thick bulk insulation batts are installed between rafters, such that there is very slight compression when installed (less than 5% of total width). While this reduces their effective R value by about the same proportion, it will remove the air gaps. It is best practice to hold the batts up with string or tape stapled to the underside of the rafters.

The required R value of the batts will depend on your climate zone and site and the structural material. Some additional thermal breaks may be required to prevent thermal bridging under structural members.

A cross-section diagram of a flat membrane roof on lightweight structure.

Flat membrane roof on lightweight structure.

Note: the rafters will act as thermal bridges, which may cause problems in some climate zones with cold winters.

Flat membrane on suspended concrete slab

A membrane of either heat-welded bonded poly sheet is adhered to a layer of dense closed cell rigid foam boards which are also adhered to the concrete roof slab. Because all the components are adhered to each other, and the closed cell nature of the insulation, there is no air gap for condensation to form. However, it is essential that the insulation R value is climate appropriate to prevent the slab temperature from falling below the dew point, or else condensation will form on the ceiling inside.

A cross-section diagram  of a flat membrane on suspended concrete slab.

Flat membrane on suspended concrete slab.

 

Wall insulation

Insulating your walls can typically save around 15% on heating and cooling costs. The R value of many common wall types is insufficient for building code compliance or energy efficiency requirements and needs to be supplemented with additional insulation. The required R value of the insulation will vary according to design and climate zone.

Weatherboard walls

The total thermal resistance of typical uninsulated weatherboard wall construction is approximately R0.45. This needs to be supplemented with additional insulation.

Use an appropriate vapour control layer over the outside of the frame. Ensure bulk insulation batts fit within the cavity without compression or gaps.

A cross-section diagram of a casement window in a timber frame.

Waterproof vapour permeable wall membrane and bulk insulation under weatherboard

Source: Envirotecture

Brick veneer walls

The total thermal resistance of typical brick veneer wall construction is approximately R0.45. This is the same R value as weatherboard walls, but brick veneer walls will have different thermal lag times (the rate at which heat is absorbed and released). For example, in summer the bricks will reach peak temperature in the late afternoon, and slowly radiate that heat into the evening – just when you need the house to be coolest.

This R value needs to be supplemented with additional insulation. Some wall-wrap products come in wide rolls that will cover the wall frame of a whole storey, but wherever joints are required, ensure at least 100mm overlap and tape the entire joint with the manufacturer’s approved adhesive tape.

Add insulation batts between the studs, ensuring they are fit for the wall-frame thickness to avoid compression, and ensure no gaps are left.

For better insulation, a rigid foam board can be installed into the cavity between brick and wall frame, with optional foil face to the interior (for cool climates). This can be installed with or without conventional bulk batts in the wall frame (if installed with bulk batts, ensure there is no foil face on the foam board). The wall cavity and brick wall ties may need to be increased to compensate for the extra wall thickness.

Fixing insulation to the outside of the studs helps reduce thermal bridging in cold climates. Suitable materials include PIR and PUR or XPS boards, or foil-faced boards with a reflective surface and air space of at least 25mm. Placing the insulation on the outside of the wall frame gives a higher total R value than placing the insulation between the studs. Leave sufficient space for bricklayers to lay the outside skin (about 40mm), and keep in mind that brick cavity ties need to be installed, typically through sheet joints.

A cross-section diagram shows foam board insulation that has been installed as a layer between the veneer and the wall studs and nogging. The foam board forms a single layer.

Brick veneer with foam board and/or bulk insulation

 

Cavity brick walls

The total thermal resistance of typical cavity brick wall construction is approximately R0.45. This needs to be supplemented with additional insulation.

Use foam boards or cavity fill (loose-fill or injected foams). Foam boards with reflective surfaces do not perform properly if air gaps are not large enough or the reflective surfaces get dirty during construction. Refer to the manufacturer’s installation requirements for your climate.

Cavity fill insulation is mainly used to insulate existing cavity brick walls. Using cavity fill in double brick walls provides a total R value of around R1.3 (dependent on cavity width). Check that local building regulations allow use of cavity fill. It must be treated to be water repellent.

A cross-section diagram shows foam board insulation has been installed as a layer between inner and outer brick veneer walls. The foam board forms a single layer and there is a cavity between the foam and the outer leaf of brickwork.

Cavity brick wall with extruded foam.

 

Solid walls

Solid walls include concrete block, concrete panel, stone, mud brick, rammed earth (pisé) and solid brick construction without a cavity.

The total thermal resistance of solid wall construction without a cavity is approximately R0.3 to R0.4. This should be supplemented with additional insulation in most climates.

Solid walls can be insulated on the inside or the outside. However, do not insulate the inside of walls used for thermal mass. For more information refer to reverse brick veneer walls. Insulation isolates the thermal mass from the interior, wasting its beneficial passive heating potential.

Suitable and climate appropriate materials include rigid foam boards, bulk batts between battens, and foil-faced foam boards with an air gap of at least 15mm with the foil facing inwards (these products could be vapour impermeable or vapour permeable). For internal walls of the home, plasterboard bonded to rigid foam is also suitable. The R value of the insulation will vary according to design and climate zone.

On the outside of external walls, polystyrene cladding with an external finish such as render can be installed according to the manufacturer’s specifications. Fix bulk batts between battens and cover with a climate appropriate water and vapour control layer. The required R value of the insulation will vary according to design and climate zone.

A cross-section diagram shows the layers of insulation in a solid wall. Rigid foam board is attached to cavity battens which sits behind external renderings. Sitting behind the battens is the solid wall which is wrapped in a waterproof membrane. The internal wall lining is the final layer

Solid wall with external polystyrene and render.

 

Internal walls

Internal walls only need to be insulated if they adjoin an uninsulated or unconditioned space (for example, garages, laundry, bathrooms, storerooms). Insulate internal walls between the home and uninsulated spaces to the same standard as other external walls.

Floor insulation

Suspended floors

The NCC specifies that a suspended floor, other than an intermediate floor in a building with more than one storey, must achieve a certain R value for the downwards direction of heat flow for the relevant climate zone. In addition, such a suspended floor with an in-slab heating or cooling system is required to be insulated around the vertical edge of its perimeter and underneath the slab, with insulation having an R value of not less than 1.0. Higher R values will deliver better thermal performance.

In cool climates and climates that require heating in winter and cooling in summer:

  • ensure sufficient subfloor ventilation as specified in the National Construction Code
  • where appropriate install underlay and carpet, or lay insulation board under floor finishes
  • insulate the underside of timber floors or suspended slabs exposed to outside air
  • insulate the underside and edges of suspended slabs
  • if using foil-faced boards to insulate the floor, care must be taken to manage condensation risks – consult the manufacturer’s technical information and installation guide.

In Climate zone 1 (high humidity summer, warm winter), in air-conditioned buildings, insulate with cyclone-rated products, with foil facing outwards on the building envelope (for example, down when under floors).

Timber floors

The total thermal resistance of typical timber floor construction must be appropriate for your climate zone and topographical location. The thermal resistance of timber is approximately R0.25, so insulation is required.

Bulk insulation can be added under the floor, supported by nylon cord or wire, if you can be confident that pests will never be a problem. Otherwise, install an impervious sheet below the joists, such as a thin fibre cement sheet or foam boards such as extruded polystyrene (XPS) or polyisocyanurate (PIR).

Consider insulating the underside of raised timber floors or suspended concrete slabs with expanding spray foam (most commonly Polyurethane (PUR)). This type of foam has the advantage of providing good R values and adheres well to most overhead surfaces without additional fixings. Concrete slabs with a smooth soffit (such as after good quality formwork is removed) may need either a primer or some mechanical fixings installed first to give the expanding foam something positive to cling to. Speak to the installer about what is required for your situation.

In a hot climate, if you can be confident that the building will never be air-conditioned, use perforated foil or concertina-type batts, stapled to the side of the joists with nonconductive staples.

Concertina-style reflective foil is installed underneath the timber floor boards between the floor joists.

Timber floor with perforated concertina foil.

Note: Alternatively, a flexible foil-foam sheet can be installed from a roll continuously under the joists. Care must be taken to manage condensation risks – consult the manufacturer’s technical information and installation guide to prevent pest entry, and ensure that all termite barriers remain fully visible.

A cross-section diagram shows a timber floor that has bulk insulation fitted between floor joists; the insulation is held up by nylon supporting wire.

Timber floor with bulk insulation and no solid protection sheet.

 

Suspended concrete slabs

The total thermal resistance of typical suspended concrete floor slab construction is climate dependent, and should be thermally modelled to obtain the best result. The R value of suspended concrete slab floors is approximately R0.30.

Add rigid foam boards or foil-faced rigid foam boards. Special fixings should be used with foil-faced boards. Care must be taken to manage condensation risks between the insulation and the slab – consult the manufacturer’s technical information and installation guide.

An underside view of a concrete suspended slab with insulation installed below.

Suspended slab with rigid foam board installed to the underside

 

Slab-on-ground

The NCC specifies that vertical edges of a slab-on-ground must be insulated if located in Climate zone 8 (cold climate) or when in-slab heating or cooling in installed within the slab. The thermal resistance of slab-on-ground is approximately R.026.

However, slab edge insulation is nearly always advisable, even though it is not mandated in the NCC. Thermal modelling suggests that slab edges are likely to leak heat into and out of houses in all but Climate zone 1 and some sites near the northern extremity of Climate zone 2.

Slab edge insulation is often sufficient on its own, as approximately 80% of the heat loss occurs through the edge. Install edge insulation before the slab is poured. Do not install insulation under concrete edge footing beams. Follow the manufacturer’s directions, especially the placement of insulation in relation to the vapour barrier membrane.

Insulate the underside of ground slabs where groundwater is present, and always obtain expert geotechnical advice. Insulation under slabs must have a high compressive strength and be resistant to moisture penetration and rotting. If the material is compressed, it no longer acts as an insulator and can even lead to structural failure. Some waffle pods can be used for under-slab insulation, as long as they meet these criteria.

Termite protection for slab-on-ground applications is critical in all states except Tasmania (but climate change may expose the island state to termites in the future).

" "A section view of a concrete slab floor including insulation protecting the edges from the ground temperatures.

Slab edge insulation detail.

 

Installing insulation

All products come with manufacturers’ installation requirements – always refer to these first. Check whether the product must be installed professionally or can be installed yourself.

Insulation must be installed correctly to reduce the risk of condensation. Most insulation materials will suffer poor performance and reduced service life if they get wet, so it is also important to ensure that the wall system (cladding, render etc) is robust and resilient to rain and storm events.

Avoiding gaps

Avoid gaps in all types of insulation. Even a small gap can greatly reduce the insulating value. Fit batts snugly and do not leave gaps around ducts and pipes. Tape up holes and the entire lengths of joins in reflective insulation using a high-quality tape with a warranty life corresponding to the insulation product lifespan. Make sure the ends of multicell and concertina foils are well sealed with tape or other material specified by the manufacturer, and ensure that corners of walls, ceilings and floors are properly insulated as these are areas where heat leaks most often occur. Wall insulation must butt into door and window frames to avoid gaps.

For safety reasons, minimum manufacturer’s specified clearances must be left around hot objects, such as flues from fires, recessed halogen downlights and their transformers. Note that LED downlights run much cooler than halogens and many can be rated for being covered by insulation – check before purchasing.

A diagram shows a wall frame with insulation. Insulation batts fit snugly in the frame; they are trimmed to fit narrower or smaller areas, and cover all spaces. Batts are trimmed around service penetration areas, such as pipe or wiring spaces

Avoiding gaps when installing insulation in a wall frame.

 

Installing bulk insulation

Do not compress bulk insulation because this reduces its effectiveness. Ensure there is sufficient space for the insulation to retain its normal thickness.

Keep moisture away from bulk insulation, or its performance will be reduced (unless you are using a water-resistant type). Use a vapour control layer where there is a risk of condensation.

Restrain bulk insulation in cavities so it does not come into contact with the porous outer skin of the wall. This can be done with a rigid sheathing or a building wrap.

Cavity fill insulation (loose-fill or injected foam) is useful for insulating existing cavity walls. Check the manufacturer’s technical information for its suitability to your project. This insulation method carries a high risk of moisture ingress with timber-framed construction systems, but is generally less risky in full cavity masonry constructions.

Potential problems to be aware of include the overheating of electrical cables, dampness (if the insulation is absorbent) and moisture transfer across the cavity by capillary action. Injected foams can also cause bowing of the walls in some cases.

Check that loose-fill insulation does not settle more than a few percent of thickness over time. Ask your contractor for a guaranteed ‘settled R value’.

Installing reflective insulation

Be aware that reflective foil insulation must be on the warm side of any building system. This means it should be inward facing and on the inside of insulation for all but Climate zone 1.

In Climate zone 1, it should face outwards regardless of whether the building is air-conditioned or not. This is because, in the case of the air-conditioned building, the outer surfaces are always above dew point. In a passively cooled building, the whole building envelope is above dew point and the location of reflective foil insulation becomes less important.

Maintain an air space of at least 25mm (45mm is ideal), next to the shiny surface of reflective insulation. Because it only works by radiation and non-emittance, contact with any other building element will reduce its insulative properties to zero.

Dust settling on the reflective surface of insulation greatly reduces its performance. Face reflective surfaces downwards or keep them vertical (except in Climate zone 1).

Reflective foil insulation should be installed by a qualified professional. Because foil insulation is electrically conductive, the risk of contact with electrical cables and equipment must be considered with all installations, and measures to eliminate the risk should be followed in the manufacturer’s installation instructions and the Australian Standard AS 3999-2015 – bulk thermal insulation - installation. Foil insulation is best not installed directly on top of ceiling joists where electrical cables are, or where light fittings penetrate ceilings and may contact the foil sheet. Similarly, installation under floors with electrical cables exposed under floor joists should be avoided. Always check for stray wires – these may be unlikely in new buildings, but are quite common in older homes.

Foil insulation must also be secured with nonconductive (non-metallic) staples.

A photo shows reflective foil insulation secured above the timber frame of a house.

Reflective foil insulation

 

Clearances around fittings

It is important to allow insulation clearance around hot flues, exhaust fans, appliances and fittings that penetrate the ceiling to ensure heat does not build up and cause a fire.

For lighting, the approach to insulation depends on the type of lighting you have:

  • Older-style halogen lighting cannot be covered with insulation as it is a fire risk.
  • Some current LED downlights are rated to be covered with insulation (though they may have a reduced warranted life).
  • Some current LED lights cannot be covered with insulation, but can be used in combination with a fire safety barrier tested and classified in compliance with Australian Standard AS/NZS 5110, Recessed lighting barrier.

Take note of manufacturers’ installation instructions for lights that include warnings about covering them with insulation, or display the following symbol meaning ‘Do not cover’.

An illustration of a ceiling downlight with insulation covering the top. An 'x' covers the insulation illustration.

‘Do not cover’ symbol

For recessed light fittings, where the manufacturer’s installation instructions do not provide information on required clearances, the light fitting can be installed using a suitable Australian Standards approved enclosure for electrical and fire safety. Where barriers are not used, allow a minimum clearance of 200mm above and to either side of any structural member, with a 50mm gap for lighting transformers (see Australian Standard AS/NZS 3000 Electrical installation – wiring rules).

A diagram shows the cross-section of a roof with a recessed downlight. A suitable restraint is used to hold it in place in the ceiling. Insulation sits adjacent to the restraint.

Safe installation of ceiling lighting

 

Where the ceiling insulation is loose fill or not fixed in position, or there is the possibility of extraneous combustible material such as leaves and pest debris getting into the roof space, maintain clearances by providing a barrier complying with Australian Standard AS/NZS 5110, or a guard or collar constructed of fire-resistant material.

A diagram shows a recessed ceiling lamp in association with a number of ceiling features.

Default minimum clearance for recessed lights

Source: Adapted from AS/NZS 3000:2007 Figure 4.7 — reproduced with permission from SAI Global

Where recessed lights are installed in an accessible roof space, a permanent and legible warning sign must be installed in the roof space adjacent to the access panel in a position that is visible to a person entering the space. The sign must comply with Australian Standard AS 1319 Safety signs for the occupational environment, and contain the words shown here.

Sign reads: Warning. Recessed lights have been installed in this roof space. To reduce the risk of fire, DO NOT cover the light fittings with thermal insulation or any other material unless in accordance with instructions provided by the light fitting or barrier manufacturer.

Warning sign to be installed in accessible roof spaces containing recessed lights.

 

Thermal bridging

Thermal bridges are pathways for heat and cold to cross from the inside to outside (or vice versa) through floor, walls and roof components. Thermal bridges reduce the effectiveness of insulation and can also lead to condensation problems. The building frame can act as a thermal bridge, particularly in cold climates. Metal framing is a particular problem because of its high conductivity. Windows and doors can also be thermal bridges, particularly aluminium frames that are not thermally broken.

Thermal bridging can be minimised by:

  • installing thermal breaks between metal frames and cladding
  • fixing bulk insulation over frames
  • choosing glazing to suit your climate
  • using thermal breaks in aluminium door and window frames, or less conductive framing materials like timber or uPVC.

Health and safety tips

If you are installing insulation yourself, consult the manufacturer’s Material safety data sheet (MSDS) and installation instructions for the product.

Generally, wear protective clothing, gloves and a face mask when installing glass wool, mineral wool or cellulose fibre insulation. These materials can cause irritation to skin, eyes and the upper respiratory tract. It is good practice to always wear protective equipment when working in dusty roof spaces.

Wear adequate eye protection when installing reflective insulation, as it can give off a painful glare, and be aware of the increased risk of sunburn. Insulation materials containing reflective foil must be kept clear of electrical wiring and fittings, and should be secured using nonconductive staples.

Electrical wiring must be appropriately sized or it may overheat when covered by insulation. Have it inspected by a licensed electrician.

Allow clearance around hot flues, exhaust fans, appliances and fittings that penetrate the ceiling to ensure they meet the manufacturer’s installation instructions.

References and additional reading

Learn more

Authors

Original authors: Max Mosher, Caitlin McGee

Contributing author: Dick Clarke

Updated: Max Mosher 2013, Dick Clarke 2020