More often than not, energy reports are produced at the end of the design process in order to obtain a building permit. If your priority is to produce an energy efficient home this is NOT the way to go about it. Have a good talk to your building designer at the start of the process regarding energy efficiency. A good energy efficient design is one that is well considered. Don’t just tack things on at the end.

Energy efficiency is more than just insulation and windows. If your priority is to build an energy efficient home you need to start with the design. The size, form, layout, location and orientation of the building will determine how the insulation and windows come into play and therefore how effective they will be.

The other consideration that goes hand in hand with energy efficiency is moisture management and subsequently, ventilation. As soon as you insulate a building, you reduce the energy flow, (which is good for thermal efficiency). However, when you decrease the energy flow through a building you also reduce the drying capacity of the building structure. With a decreased drying capacity comes a higher likelihood of moisture related problems such as condensation, structural decay, mould growth and health issues.

Energy efficiency, moisture management and ventilation need to be considered together in order to produce a safe, healthy, durable, comfortable & energy efficient building.


The following points are all components to be considered at design stage. An understanding of these considerations will lead to a better performing building. There will of course be design considerations such as cost, aesthetics, views, lifestyle, functionality etc. as there will also possibly be other constraints due to Town Planning regulations and building code regulations. All of these design considerations need to come together in order to build your house, and compromises will need to be made.

  • Size of the building. Small buildings require less energy.
  • Layout of building. The less external surface area the less heat transfer. This relates to the point above and is called the Surface/Volume ratio. The greater the surface area the more the heat gain/ loss through it. So small Surface/Volume ratios imply minimum heat gain and minimum heat loss.
  • Location of the building. Is the site windswept? Is it shaded? Is it surrounded by other buildings? Is it in a paddock? Is it in a cool or warm region? Is it in a mixed climate that experiences cold winters and hot summers?
  • Orientation. Think about how the sun is going to impact the building in both winter and summer. Also consider the dominant wind load directions.
  • Cost. This is where reality hits us. Super energy efficient triple glazed windows with thermally broken frames might be what we want, but the budget might say otherwise. All energy efficient measures come at a cost and it is important at the design stage to see how they fit within the budget and if the design can be altered to suit. Spending a bit more on windows and insulation may result in spending less on heating equipment and power bills but unless money is not an issue the decision should be carefully considered using a cost/benefit analysis.
  • Aesthetics. Everyone has their own idea of what a house should look like, and for many this is the main design consideration. Depending on your creative design aspirations, there may be a compromise with the energy efficiency of the building. The form, colour, materials and glazing that are designed for aesthetic purposes may not marry up with optimum energy efficiency outcomes. By no means are we saying to not design for aesthetics. We want beautiful buildings. Just be aware of the consequences, and compromise one way or the other in accordance with your priorities.
  • Views. Quite often the desired views are not in a direction that will be advantageous for thermal efficiency. Windows are the main weak point in the thermal envelope of a building. Consider the size of windows, the shading, the window’s insulation properties (U-value) and their Solar Heat Gain Coefficient (SHGC).
  • Lifestyle & functionality. This is quite justifiably the main design consideration for most people. You want your house to suit the way you live. This is another area where a potential compromise with energy efficiency comes into play. A large sprawling house with a lot of jut-outs and indents in the external walls will result in a large surface area and more heat gain/loss as mentioned above.
  • Town Planning and Building Regulations. Even if your priority is to build a super energy efficient house by following all of the energy efficiency principles and by employing best practices, you may be hampered by rules and regulations. Siting requirements relating to offsets, setbacks, heights, building size, material selections, material colours, site coverage, and even heritage requirements may force you into a compromise. These are things to discuss with your designer in order to reach the best outcome given the restraints.


As soon as we began to insulate buildings we also caused a moisture management problem. As mentioned previously, when you decrease the energy flow through a building you also reduce the drying capacity of the building structure. Being aware of this fact is critical in being able to prevent problems. When we design the building envelope we need to consider the following items in the order as listed.

  1. Water
  2. Air
  3. Vapour
  4. Thermal
  1. If Water gets into the structure via rain, flood or leaks, air and vapour will also certainly get in. Wet insulation doesn’t work. A wet structure causes health issues and structural damage. We obviously want our roof system to be waterproof and for the most part this is generally the case. We also want our walls to be waterproof but this is a bit harder. Most rainwater is prevented from entering the building fabric by correctly installing and flashing our external cladding system. However, even a well installed external cladding system will let in a small amount of water. This is where we have a second line of defence in our wall wrap. It is important for the wall wrap to be waterproof and to be installed so there are no holes in it where water can enter, especially around window and door penetrations. The wall wrap should also have a drainage plane to allow any water that enters through the cladding to drain out. It is also important for the wall wrap to be vapour permeable. If any water does get into the wall assembly the vapour permeability will allow it to dry out.
  2. If Air gets into the structure it carries dust, toxins, pollen, and MOISTURE which in turn causes health issues and structural damage. Air moving through insulation also dramatically decreases the performance of the insulation. Air leakage from the building can also equate to up to 60% of your heating/cooling bill. Aside from liquid water intrusion as discussed above, 97% of the moisture ingress into the structural assembly is via air intrusion. Warm air can carry a lot of water, and when this warm air reaches a cool surface it can condense. The best way to stop this from happening in a structural assembly is to stop the air getting in in the first place. Airtightness of the building envelope can be detailed, implemented then verified with a blower door test. In doing this 97% of your moisture issue will be resolved and your insulation will perform to its full potential.
  3. If Vapour gets trapped in a structural assembly it can condense on an internal surface. Again, this MOISTURE causes health issues and structural damage. As mentioned above, air caries water in the form of vapour. Vapour is not all bad as we need a certain amount of it in the air for our health and comfort. However, vapour travels through surfaces such as plasterboard and can cause trouble if it gets trapped by an internal condensing surface inside the building assembly. The installation of vapour permeable wall wrap and roof sarking prevents vapour from getting trapped by allowing it to pass through and condense on the outside. Exterior insulation can also help with vapour issues by moving the dew point location to the outside of the assembly. Air tightness, vapour permeability and dew point locations need to be considered to ensure an adequate assembly.
  4. Thermal is listed at number 4 because it won’t work unless we address items 1-3. We need to get our priorities right. Increasing insulation values can actually make our moisture problems worse if we treat ‘Thermal’ in isolation and don’t address moisture management. Apart from liquid water intrusion, air infiltration is the biggest enemy. Air moving through insulation dramatically decreases its effectiveness. Poor installation of insulation also dramatically decreases its effectiveness. Insulation that is installed properly with a good airtightness and moisture management strategy will work to its full potential.


Insulating the building requires air tightness of the building envelope. Airtightness of the building envelope requires ventilation of the building.

We have established that for insulation to work effectively and for the building fabric to remain dry we need to prevent water from getting into the building assembly. The 97% of moisture that we have prevented from entering the building fabric by making the assembly airtight now remains within the rooms of the house. We remove this moisture laden air by the way of ventilation.

A ventilation strategy will depend on the level of airtightness that is achieved and the heating or cooling system that is installed. A careful consideration of the heating and cooling needs of the house will determine the best outcome. A ventilation strategy can be as simple as opening windows on a daily basis or as involved as introducing a mechanically balanced fresh air system.

This is another decision that should be made early in the design process rather than as an afterthought.


This document is an overview of design considerations. For a good outcome a strategy based on these considerations should by implemented as a collaboration with owner, designer and builder. As informed as we are, and as much as we can detail things, we need to make sure all of our recommendations are understood and successfully implemented on site.

Contact this office to arrange a consultation to go through these considerations in more detail, specific to your project.


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