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Insulation and the building shell, what are they and why are they important?

Your home may have efficient appliances, but still end up with a costly energy bill if the heat is coming in through an uninsulated ceiling or under doors. This is where the building shell is so important.

The building shell consists of the roof, walls and floor of your home, as well as windows and insulation. The materials used to build your home all have different properties. Some allow heat to move through them easily, others help retain heat. How well your home is sealed is another factor that determines how much it costs to heat and cool and how comfortable the house is to live in.

About your home’s building shell 

There are four parts to look at to improve the building shell. The improvement options on the certificate will highlight the weaker elements of your home.

  • insulation, which acts to slow the rate of heat transfer through a material or building element, helping to keep the inside of your home warm in winter and cool in summer.
  • thermal mass, which affects the rate your home heats up and cools down. Materials like concrete and brick have high thermal mass and help to slow down the rate that a room may heat up or cool down.
  • windows, which have the combined function of letting in light and allowing heat movement in summer and winter.
  • air leakage, which occurs through any gaps between the inside and outside of your home. These include deliberate openings like wall vents and chimneys, as well as gaps and cracks that develop over time.

Impacts of different building shell upgrades on House Energy Rating (HER)

Source: Sustainability Victoria


Ceilings and roofs 
  • Between 25 and 35 per cent of heat loss or gain occurs through the ceiling. Adding ceiling insulation will make a huge difference to the comfort of the home, helping to keep it warm in winter and cool in summer.
  • If the ceiling space is easy to access, you can add or top up insulation so that you have a minimum of R3. 5. If the ceiling space isn’t easy to access, consider adding insulation when the roof needs replacing or install an insulated false ceiling below the current one.

  • Between 15 and 25 per cent of heat loss and 25 to 35 per cent of heat gain occurs through walls. Walls can be difficult to insulate, so it’s best to do it while you are renovating and can replace lining or cladding .
  • Most stud walls have a 90 mm stud that allows insulation up to R2.5 to be installed. Alternatively, wall cladding is now available that can be retrofitted internally or externally that includes an insulated layer.
  • Between 10 and 20 per cent of heat loss or gain occurs through floors. Insulation is most effective on floors that have a large space underneath – and these are also often the most accessible. Raised floors are generally timber floors that can be insulated using batts or insulating boards affixed between the joists.
  • Slab on-ground concrete floors can’t be insulated after construction, so insulating the slab is only an option for new homes.
  • Installing insulation should be done in accordance with the Australian Standard (AS3999:2015) and the wiring rules (AS/NZS 3000:2007) to ensure it is done safely and will give the best possible results .
Thermal mass

Appropriately installed thermal mass inside your home can make it more comfortable. High thermal mass elements, such as brick, rammed earth, tile and concrete, tend to be slower to change temperature than low thermal mass elements. In winter, thermal mass exposed to the sun or your heater will mean your home remains comfortable for longer. In summer, thermal mass connected to the ground or other cooling sources will mean your home remains comfortable for longer. Thermal mass should be positioned so it is exposed to sun in winter but shaded from sun in summer.


Even the best performing windows will not insulate your home as well as an average  wall. This means that windows can be a major weak spot, letting heat out in winter and heat in during summer. Both the frame and the type of glass effect the performance of windows.

It is important to understand how to make your windows work to your benefit. While 10 to 20 per cent of heat is lost through windows in winter, the potential for heat gain in summer is much higher at between 25 and 35 per cent. There are many ways to manage windows to make your home more comfortable, including replacement or improvement.

Impact of different window coverings/protections on heat loss through window in winter

Image of a graph showing a comparison of winter heat losses through windows and impact of the type of window protection. Bare single glazed windows 100% heat loss whereas double glazing with heavy drapes and boxed pelmets Source: Sustainability Victoria

Fixing gaps and cracks

For many homes the best way to improve comfort and save on your energy bills is to deal with gaps, cracks and other points that allow draughts. Between 5 and 25 per cent of heat loss or gain is due to gaps and cracks. Draughts also create airflow over your skin which make you feel colder in winter.

If your home has fixed ventilation, such as ceiling or wall vents, only remove it after seeking expert advice. Wet areas often need ventilation to avoid mould. You may need to install alternative ventilation such as exhaust fans.

Target these areas for sealing:

Exhaust fans

Exhaust fans should have louvres or flaps that close when not in use to reduce unwanted air leakage. If you can’t do this, try to keep doors closed to these rooms. You may even consider draught proofing internal doors if a room is a particular problem.


Chimneys for open fires can cause large amounts of air leakage. If you want to use your fireplace, you can fit a damper that blocks the chimney when it’s not in use.

You can block the chimney permanently if you don’t intend to use the fireplace. Often it is good to block the fireplace from the bottom or within the room. If the blockage is visible someone is less likely to try to use the fireplace in future.

Door and window seals

It is very common that windows and external doors are a source of draughts.  Signs are that windows rattle, dust accumulates around the inside of the frame or you feel cold air movement in winter.  If you can see light around a door this indicates a big problem.

There are many window and door seal products available that are suitable for different situations. If you’re looking for a less-permanent solution, you can stop gaps at the bottom of doors with a door snake.

Gaps can also occur around the outer edges of the frames where they meet the wall. These can be sealed with caulking and painted over.

Wall vents, ceiling vents and vented skylights

Homes built before the mid-1980s often incorporated wall or ceiling vents. You can seal or remove these vents unless you intend to use a portable un-flued gas heater or an open-flued gas heater in the room. Never use a portable gas heater in a sealed space.

Vented skylights incorporate a permanent opening, usually covered by fly screen or mesh. These are generally found in bathrooms and laundries. You can replace the skylight with a new type of sealed roof window or install a sheet of acrylic at the bottom of the shaft that blocks the entire hole .

A cheaper option is to close the door to the area with the venting. You may even consider draught proofing internal doors if it is a big problem.


Older style incandescent downlight fittings and gimballed (swivel) halogen fittings allow significant amounts of air leakage.

You can replace these with high-efficiency, sealed LED fittings that reduce air leakage, as well as energy use .

Floorboard gaps

Older floorboards may develop gaps between them and allow air leakage. Floorboards tend to shrink, expand and bend when walked on, causing fillers to fall out. This is a difficult issue to solve . The simplest approach might be to install carpet or rugs.

If the under-floor space allows it, you can insulate the floor from below with a product that provides an air seal like foil or insulating boards.

Skirting board gaps

Air leakage can occur in gaps between the skirting and the floor, especially in older homes. On a raised timber floor, there is likely to be significant leakage.

These gaps can be addressed by using a flexible caulking material, or you can use a foam noodle or filler on large gaps.

Learn more

For more information about how improving the building shell can help improve your home's energy efficiency and comfort, download the Building shell improvements factsheet.

Please contact the team to request an accessible version of this factsheet: scorecard@delwp.vic.gov.au

Page last updated: 11/12/23