Reduce house heat loss

Hydrosol image showing a timber covered slab absorbing sunshine radiation in winter, which will improve house heat loss.
Timber covered slab absorbing sunshine radiation in winter, which will improve house heat loss.

Heat Loss Causes & Choices

This BLOG discusses the causes and suggests steps to reduce house heat loss in winter and heat gain in summer.

The causes come from these 3 areas, broadly one third each:

  • Ceilings
  • Windows
  • Walls, floors and general air leaks

Your choices to reduce unwanted heat loss  and heat gain depend on whether you are:

  • Building a new house
  • Renovating an existing house
  • Improving an existing house
  • Your local climate
  • Your budget
Drawing showing the areas to improve house heat loss in winter.
Areas to improve house heat loss in winter
Drawing showing the typical sources of house heat gain in summer.
Typical sources of house heat gain in summer


Insulate Ceilings

For a new build, renovation or existing house, properly insulate your ceilings. This is the most cost-effective step you can take to reduce house heat loss in winter and heat gain in summer.

Install or add ceiling insulation to achieve a thermal resistance (R Rating) up to R6. For existing houses, add ceiling insulation to bring the R Rating up to R6.

Thermal Bridges

  • Make sure your insulation is fitted snugly around framework because a bare ceiling makes a thermal bridge for heat transfer to/from the outside.
  • Reduce the number of down lights because they require a bare ceiling around them. Therefore, this will reduce thermal bridging.
  • A well-insulated ceiling significantly reduces rising heat loss in winter. It also reduces heat gain from the roof space in summer.
Thermal image showing heat loss from slab-sides during winter, which need to be insulated to improve house heat loss.
Thermal image showing slab-sides that need to be insulated to improve house heat loss
Schematic showing the where air leaks from convection, which needs to be sealed to improve house heat loss.
Air leaks need to be sealed to improve house heat loss

Insulate Floors

Concrete Slab

  • Insulate the sides of your slab or this will cause a thermal bridge to the outside air. Consider a foam high-density closed-cell type with R rating of 2.
  • Insulate underneath your slab with an R Rating of at least 1. 
  • Slab insulation is particularly important if you are hydronically heating your slab because the greater heat in your slab will cause greater heat flow to the colder air outside.
  • If you choose a decorative screed layer over the structural slab, then this should be applied over insulating panels with an R Rating of at least 1.
  • A screed layer is often used when installing underfloor hydronic heating and cooling circuits embedded in the screed layer. It can also be used as a decorative, polished floor layer.

Suspended Floors

Air is a good insulator but only if it doesn’t move by convection.

  • Apply insulating material under your suspended floor. This is easier for new builds. However, it may not be feasible for some existing suspended floors. Consider carpets if underfloor insulation is not feasible, particularly in bedrooms.

Insulate Walls

Insulate your walls and block air leaks:

  • Insulate your walls with an R rating of at least 2 to reduce heat transfer from radiation.
  • Include a foil lining to reduce radiation.

Reduce Air Gaps

Blocking up air gaps around external doors and windows is one of the most cost-effective things you can do to reduce house heat loss and heat gain.

Here are some things you can do easily:

  • Install weather strips on external doors and windows
  • Install self-closing exhaust fans
  • Make sure fire flues are closed when not in use
  • Close windows when outside temperatures are significantly higher or lower than your indoor set temperatures, open them when more temperate

Read More

Schematic showing the ability of eaves to provide shading which reduces heat gain in summer but allows in winter heat as the sun tracks lower.
Eaves provide shading to reduce heat gain in summer but allows in winter heat as the sun tracks lower.
Schematic showing how thermal mass works by storing heat energy during the day then re-radiating it into the evening which improves house heat loss.
Thermal mass stores heat energy during the day then re-radiates at night, which improves house heat loss.


Reduce Heat loss & Heat Gain

Install double-glazed windows to reduce house heat loss in winter.

Install blinds or drapes with pelmets as a less expensive option or if you have lead light windows. This relies on maintaining an air gap between the window and its covering, hence the need for pelmets.

Shade your windows to reduce unwanted radiant heat gain in summer. Consider trees, vines, external awnings, glazing type or reflective film.

Apply window films to existing windows or choose high performance windows designed to reduce heat gain from direct radiation in summer.

Short wave radiation from the sun is high in energy and can pass through glass, even double and triple glazed glass. This is a good thing in winter but may be a problem in summer. Therefore, shade windows in summer but not in winter.

Radiation enters your house via windows in three ways:

  1. Beam – sunshine directly trained on windows.
  2. Diffuse – radiation scattered by molecules in the atmosphere, for example clouds.
  3. Reflected – radiation that bounces off an object, for example walls and paths.

Make pathways near windows less reflective. For example, choose gravel over concrete or make concrete less reflective. Increase the amount of grass and foliage near your windows.

Window Frames

Choose wooden window frames for aesthetics and also because they are weak conductors of heat. Therefore, they do not need to be thermal broken to prevent thermal bridging of heat between inside and outside.

However, keep in mind the issue of conductivity if you choose aluminium window frames. Make sure they are thermally broken to prevent heat bridging from inside to outside in winter and vice versa in summer.


The thermal mass inside your house will re-radiate energy in the evening. This is less powerful than direct sunshine because it is long-wave energy. Install double-glazed windows to effectively insulate against inside heat re-radiating outside in winter.

Feel this energy when you walk past a brick wall in the evening that has been exposed to the sun. That’s what you need in winter but not in summer.

You know that direct sunshine can burn you in summer. It can also burn you through a sunny window. And it makes no difference whether it is single or double -glazed.

This is the power of short-wave radiation. In fact, double-glazed windows may be a problem in summer because they don’t let your house cool down in the evening unless you open them. Therefore, external window shading is the best option for summer.