Building Design, Drafting & Energy Efficiency

Window air infiltration

I have had a few questions lately regarding window air infiltration. This is an important component of energy efficiency but one that is often overlooked. Window manufacturers have their windows pressure tested at 75Pa. The result of the air infiltration test is listed on their window data as AI. The AI figure represents Litres per Second per square metre (l/s/m2).

As an example, a typical awning window and a typical double hung window might have the same thermal performance figures, (Uw and SHGC) but have wildly different air infiltration rates. Awning window AI = 0.3 l/s/m2 Double Hung AI = 4.2 l/s/m2 I have known of people who diligently selected windows with excellent thermal values only to have their curtains flapping (with the window shut) due to the high air infiltration…….not an energy efficient outcome.

LESSON! Make sure you consider air infiltration when selecting your next windows.


International Builders’ Show

As part of my continuing path of learning in the field of building science, I will be attending the International Builders’ Show in Las Vegas in February 2019.

In Australia we are still in our infancy when it comes to energy efficiency, so much so that we have not yet adequately addressed the effects of moisture management and indoor air quality. Moisture and air quality problems are both unintended consequences of insulating our buildings.

In Europe and North America, energy efficiency measures have been developed over decades, and over that time problems such as moisture and air quality have arisen and been dealt with. I an attending the International Builders’ show with an intent to learn more, to speak to industry experts and come home  more knowledgeable and capable of directing my clients adequately in the pursuit of building  energy efficient, durable and healthy buildings.

The show in Las Vegas is huge. It runs for 3 days and expects over 60,000 visitors. Click on the link below to check it out.




Window heat loss:

Windows are great, they serve to provide us with natural light and natural heating via solar heat gain. Windows also allow us to see outside and they  facilitate natural ventilation when operable.

As good as windows are they are really bad at one thing – ‘insulating.’

When we consider our thermal fabric, (floor, walls, ceiling) the weak point is where this fabric is compromised. Windows represent a wound in the thermal fabric.

The NCC minimum thermal requirement, (R-value) for external walls is *R2.8. The R-value of a typical aluminium framed, single glazed window is R0.15, (Uw6.5). This 18.7 times less thermally efficient than the wall! Even a typical aluminium framed, double glazed window at R0.25, (Uw3.95) is 11.2 times less thermally efficient than the wall.

Now that we recognise that windows are a major source of heat loss, what do we do about it?

When designing your house consider where the major heating load is, (typically in the living areas/ kitchen etc.) and ensure that the windows in these areas are thermally efficient in order to minimise heat loss. Double glazing is a starting point. Good thermally broken, double glazed units are the best choice to minimise heat flow between inside and outside. Drapes with pelmets also add insulation to windows, (makes a big difference on cold nights).

How do you know what a good thermally efficient window is? The important thing to look for when choosing a window is the U-value of the whole window. This figure appears as Uw-.-. The lower the U-value the better. Typical Values:

  • Uw6.50 = aluminium, single glazed.
  • Uw3.95 = aluminium double glazed.
  • Uw.2.3 = aluminium framed, thermally broken, double glazed.

Futher explanation:

The U-value is measured as W/m2K. Lets look at the aluminium framed single glazed unit as noted above.

Uw6.50 = 6.5Watts per 1m2 of window area per 1 degree of temperature difference, (temperature difference between inside and outside).

Now lets plug this U-value figure into a house with 30m2 of window area with a temperature difference of 12 degrees, (thermostat set to 21 degrees inside with an outside temperature of 9 degrees).

6.5 x 30m2 x 12 = 2730Watts of energy loss.

Putting this figure into perspective, 2730Watts or 2.73kW is equivalent to running 27 **Television sets. That’s a lot of energy.

Running the calculation again with the thermally broken window gets us down to 8 TV’s.

2.3 x 30m2 x 12 = 828 Watts of energy loss.

It’s not all bad news as of course windows also act to provide solar heat gain. This is heavily dependant on careful consideration when placing the windows. It’s important to get the orientation and shading right to maximize winter heat gain and summer shading to ensure a warm house in the winter and a cool house in the summer.

When you are looking for windows for your new home, or more importantly when you are at the design stage of your new home, have a careful look at where the windows are, which way they’re facing and address them appropriately. As a rule of thumb, north facing windows to living areas with well designed shading is optimal. Large expanses of south facing windows are primarily a heat loss issue.

Remember, the U-value is the measure of how much heat is transferred through the window. The lower the U-value the better the insulation properties of the  window – the better it is at keeping the heat or cold out. In all cases regardless of climate zone a window with good insulation properties will help to improve the comfort of your home.

Note: The latest version of our glazing calculation includes window heat loss data, (measured in Watts) for the total window area of the house. This data is on the Window Analysis page and has scenarios for temperature differences from 6 – 21 degrees.

*R-value based on NCC Climate Zones 6 & 7

**Television sets assumed at 100W.

Example data from spread sheet below. Figures based on 30m2 total window area and Uw2.3 window units.


Is your new house insulated?

Is your new house insulated? The answer should be, “of course it is.” Insulation is something that is well and truly covered in the design and documentation stage of your new house, so when you look at your building permit drawings and of course, your energy report there is reference to the amount and location of the insulation. Also, when you see the house being built there is evidence of the insulation arriving and being installed.

So, what’s the problem here?

The problem is not the specification of insulation but the ‘correct’ installation of it. The correct installation of insulation involves installing it without gaps between batts and ensuring that the entire thermal envelope is covered.

You would think this would be simple enough, especially considering it is a Building Code requirement. However, after observing a number of houses being constructed I have found evidence that leads me to believe that most houses are not insulated properly.

Here is a list of identified problems:

  • Wall insulation is squashed into place, (squashing the insulation reduces its potential insulation value).
  • Wall insulation is carelessly placed leaving large gaps between.
  • Wall insulation is NOT installed over windows where there are lintels.
  • Wall insulation is NOT installed at external wall corners and external junctions where internal walls meet the external wall. This problem is due to the wall wrap being placed prior to the insulation contractor appearing on site. Once the wall wrap is placed, these external junctions cannot be accesses for the installation of insulation.





Seal gaps & cracks?

One of the throw-away notes you will sometimes see on construction drawings is, “seal gaps & cracks.”

This simple statement is meant to cover a range of air infiltration/leakage issues in the building fabric. It has been condensed to this simple statement because it is assumed that these air infiltration issues are dealt with as part of ‘good construction’ practice. For this reason, the sealing of gaps and cracks is not part of house energy ratings, (thermal performance assessments).

The assumption that the sealing of gaps and cracks is dealt with as a matter of course is a major problem. It is a problem because most of the time it is NOT dealt with and most building practitioners and energy assessors are unaware of the basic requirements.

Ceiling insulation installed with 5% gaps can cause a 50% reduction in thermal effectiveness. Ineffective sealing of windows, doors, roof lights exhaust fans etc. can also lead to excessive air leakage. This  drastically reduces the efficacy of the building’s thermal envelope and causes a greater heating/cooling load than technically necessary, hence higher power bills.

As a part of our energy reports we include National Construction Code Part 3.12.3 – Building Sealing. This part of the code addresses all items of potential air leakage. By addressing air leakage and providing recommendations in report form there is less likelihood that this important component of energy efficiency is overlooked.  Properly dealing with ‘gaps & cracks’  during construction will ensure that all insulation and energy efficient measures perform to their full potential.

Can it be built?

Last week I had a work experience student in the office. On his last day he brought in a  floor plan of a house he designed for a school project when he was in grade 6. With a mix of pride and apprehension he showed me his design and asked, “Can it be built”?

As I looked over the design I considered his question. I thought about the use of the space, the bushfire requirements, the energy efficiency implications, the required head height over stairs, the rise and run of steps, the width of passages, the swimming pool fence requirements, the difficulty of the build, the practicality of curved glass and the expense of the build.

After considering all of these practicalities and rules I stopped…………. I stopped because I was suddenly reminded of a house I designed when I was 10 years old, (it was an underground house with periscope windows). Just like the design I was currently looking over with a critical eye, it was a house that was designed with pure imagination and uninhibited by any form of constraint. It was pure design, pure joy and a refreshing jolt back to the reason why we do this job in the first place.

So, the question was, “Can it be built’? My answer, “YES.!”




Did you know that Services are part of energy efficiency compliance? Most people don’t.

Click here to find out more.

Lighting allowances

A bit of confusion exists in the understanding of the requirement for artificial lighting. As part of the current energy efficiency regulations, artificial lighting needs to be calculated to ensure a reasonable level of power usage. The allowances are measured in Watts per square metre as per the following:

  • Class 1 (house): 5w/m2
  • Class 10 (garage/carport): 3W/m2
  • Verandah/Balcony: 4W/m2

There is also an allowance for exterior perimeter lighting. I won’t confuse things by addressing this right now, (it’s all about efficacy, motion sensors, daylight sensors and switching arrangements).

The main confusion is that many people regard the allowable Watts as an indication of the amount of light produced. This is incorrect.

A ‘Watt’ is a measurement of power. Light is measured in ‘Lumens’.

Still confused? Think of an old car and a new car. The old car uses a lot of fuel to travel from A to B. The new car uses less fuel to travel the same distance.

Now think of an old incandescent light globe and a new LED globe. The old incandescent globe uses 100 Watts to light a small room. The new LED globe uses 22 Watts to light the same room.

  • 100W incandescent = 1600 Lumens
  • 22W LED = 1600 Lumens

So in summary, just as buying more petrol to run an old car requires more money; more Watts to run an old light globe requires more money to be spent on your electricity bill.

When you choose light fittings make sure you choose energy efficient fittings such as LED (Light Emitting Diode) or CFL (Compact Fluorescent). Look for the Lumens to gauge how many you need for the task.

So, how many Lumens do I need?

This depends on your individual requirements, so the below is a general guide, representing the amount of light needed in typical areas. All you need to do is multiply the area of the room by the number of lumens required.

  •  In dining rooms and corridors you will need around 10 – 20 lumens per square metre.
  •  Kitchens and rooms where you read casually will require more lighting, around 20 – 55 lumens per square metre.
  • Rooms for more intensive reading or study need to be better lit, with a lumens requirement of around 55 – 110 per square metre.

As an added extra we can provide a room by room lumen calculation with our energy compliance reports.





How does double glazing work?

Is double glazing worth having?  Won’t it be cheaper to have single glazing?

These are two questions that I am asked quite frequently. The answer to both is yes.

Although both answers are ‘yes,’ the first question is the important one, so let’s deal with that one first.

Is double glazing worth having?

Double glazing works by trapping air between two panes of glass. This trapped air creates an insulating barrier which serves three functions:

  1. First of all it prevents heat loss or gain through the window because the barrier insulates the room from differing air temperatures at either side of the glazing unit.
  2. Secondly, double glazing helps keep noise pollution down.
  3. The third benefit of double glazing is that it helps reduce condensation. Condensation commonly occurs on the inside of single glazed windows due to the warm air inside the house being cooled by the cold outside air. With a double glazed window the insulating barrier prevents the transmission of air temperature and therefore helps prevent condensation occurring.

Although double glazing insulates against differing air temperatures, it does not insulate against radiant heat. This makes a double glazed unit a good choice for permitting winter sun to warm the inside of the house whilst at the same time insulating from the cold outside temperature.

So, back to the second question.

Won’t it be cheaper to have single glazing?

Of course it will be cheaper but it will also have a poor thermal performance and you will end up endlessly wiping condensation off the inside face of the window during the winter months. Also, just remember, if you’re not motivated to wipe the condensation away you will end up with mould growth which in turn will cause health problems for your house and its occupants.

This may seem like a pretty strong point of view on glazing but just consider the following points:

  1. Current thermal requirements for external walls states a minimum insulation value of R2.8. We achieve this with wall insulation. A typical aluminium framed, single glazed window has an R-value of R0.15. That’s over 18 times less than the wall next to it. So, if you’re going to insulate your walls, it makes sense to insulate your windows with double glazing.
  2. A consequence of highly insulating and draft sealing our homes is the increased risk of condensation. condensation will occur on the thermally inefficient windows. Double glazing goes a long way towards mitigating this risk.

If you are trying to cut costs on your building project, be careful when choosing your windows. A budget based decision might be good in the short term but you will certainly pay in the long run.

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