Insulation Basics – Double Glazing

Large single glazed windows are one of the biggest reasons for heat loss in a modern house.

It is also a major source of heat coming into the house in summer.

So what can you do?……………here is a comparison between single and double glazing.

Single Glazing
A single glazed window with an aluminium frame has a U value of around 7watts/degree C/m2 (an R value of 0.14 )

So if your house has got 30m2 of windows and its 5 degrees C outside.

You will be losing the following amount of heat through the windows if you keep the house at 20 degree C

30 x (20-5) x 7 = 3,150watts = 3.15kW/hour

For refrigerated cooling from 300C to 200C you will need the following amount of cooling to balance heat gain through windows:

30 x (30-20) x 7 = 2,100watts = 2.1kW/hour

Double Glazing

If you have double glazing with timber or uPVC frames you will reduce the U value to around 3 watts/degree C/m2 (An R value of 0.33 ).

For the same conditions as the above example the heat loss through the windows will be reduced to:

30 x (20-5) x 3 = 1350watts = 1.35kW/hour

For refrigerated cooling from 300C to 200C your heat gain through windows will be reduced to:

30 x (30-20) x 3 = 900watts = 0.9kW/hour

Other ways to reduce heat loss are

  • Reduce window size. As walls are better insulation than windows this can offer significant reductions in heat loss
  • Curtains or Blinds. Will provide similar performance to double glazing. . . but only during the time when they are closed.

Extra Benefits of Double Gazing

Improved Security: Its much more difficult, and noisy, to break in through a double glazed window.

External Noise Reduction The bigger the gap between glass the better the performance.

See Insulation for similar Posts

For Posts about Green Building see Sustainability


West, or East, Facing Windows

Well the first rule is to avoid West, or East, facing windows . . . or at least keep them as small as possible….and here is why:


West windows get direct sunlight from mid-afternoon to late in the evening in summer causing overheating.

East facing windows get sun from sunrise to midday.

This isn’t normally as bad because the house is cooler in the morning, but the heat built up during the morning does stay making the house hotter later in the day.

Because of the low angle of morning, afternoon and evening sun the over window shading that works well with North Facing windows will be inadequate for West and East facing windows.


In the winter there is minimal heating benefit with little or no direct sunlight getting into West or East facing windows.

In the last house we built the master bedroom full length windows all face west.

I would have preferred to decrease the size of the windows but the builder would not alter the size of the windows on the front. (They were quite happy to change all the other windows, just not on the front)


To shade, our west facing windows, we adopted the following three stage strategy.

  1. A highly reflective tint was applied to the window, by Tint-a-car. This had the immediate effect of lowering the temperature in the room by about five degrees on a sunny afternoon. Even the installer was amazed at the difference it made. Expect to pay around $50-60/m2,
  2. We planted shrubs in front of the windows to shade the bottom third of the windows and also provide a micro-climate to reduce heat loss in winter.
  3. We have fertilised and trimmed a couple of trees on the nature strip, in front of the house, to encourage dense foliage which now shades the window from mid afternoon to sunset.(see photo below)

For similar posts see Passive Solar

Choosing a House . . . A new E-book for only $4 to help plan your new house


Window Size

Building Code Rules on Size

The Building Code of Australia requires you to have at least 10% of the floor area as windows with at least 5% of the floor area being able to be opened.

If you go in many show houses you will see windows that are much bigger, typically 40 – 50% of the floor area.

They do this to give a light airy feel to the house.

In reality, for most modern house on a typical size blocks you could finish up with floor to ceiling windows looking directly at a fence!

Why Have Smaller Windows?

  • Well glazing is an expensive building element. You may save money on a custom build by having smaller windows. Even with project homes builders will often reduce the size of windows as a no cost option.
  • Single glazing transfers over 20 times as much heat as a modern brick veneer wall. With the step up in cost to double glazing you will still transfer more than 10 times the heat through windows compared with a brick wall.
  • Windows that go down to the floor restricts where you can put furniture.

If you stick to sensible glazing ratios and get away from floor to ceiling glazing by raising sill heights, you can reduce the area of glazing considerably.

By considering the positioning of glazing and room layout relative to external shading elements you do not really need to compromise on natural light.

The surprising thing is that if you follow these principles you can get a more sustainable house and save on heating and cooling.

If you also want the added benefits of double glazing you will also save money as the smaller windows will be cheaper. (See the following link for more information: Smaller Windows or Double Glazing?)

For myself I have tended to aim for glazing around 20-25% of floor area in South facing rooms and around 40% in North facing rooms.

West or East facing windows are avoided, or if there is no alternative, minimised.


Did you change window sizes on your new house?

See Insulation for similar Posts

For Posts about Green Building see Sustainability


Shading Northern Windows

In a post about Passive Solar I talked about shading North facing windows.

That doesn’t mean installing roller shutters, or blinds that pull down over the window.

Shutters and blinds tend to make the rooms very gloomy and depressing, and with good design are unnecessary.

The best methods of shading North facing windows are:

• Overhanging eaves
• A well designed veranda
• A pergola

How Much Shade?

For areas North of Brisbane its better to have  enough shade to keep the sun out of the house as much as possible.

For areas from Brisbane and South the keys are:

  • Keep direct sun off the window during the summer months of; December January and February.
  • Allow full sun to shine on all of the window in June July and August.


Keeping the summer sun out for areas South of Brisbane can generally be achieved by making sure the eaves overhang is 45% of the height (H) the shade is above the bottom of the glass.

The eaves, or other method of achieving shade, should also extend horizontally along the wall window 45% of H to either side of the window.

To minimise shading of the window in winter the shading needs to be around 20% of H above the top of the glass.

West and East Facing Windows

West and East facing windows are almost impossible to effectively shade in summer and get minimal benefit from winter sun.

Information on dealing with these windows can be found at: West and East Windows


For more Green Ideas see Sustainability


Passive Solar – What Does It Mean?

Do you want a house that is filled with natural light in the summer without overheating, and minimises your heating bills in winter? ……….If you do, then a passive solar house is what you need.

Here are some of the things you consider.

Get The Winter Sun Into The House

From Wikimedia Commons

This means having most of the main living rooms facing North with lots of windows.

It also means making sure the Windows aren’t shaded by fences, trees etc.

Maximising Thermal Mass

This means having dense absorbent materials like concrete  and brick inside the house.

  • In winter the thermal mass heats up during the day and releases the heat during the evening.
  • During summer if you open the house in the evening/night the thermal mass cools and helps keep the house cool the next sunny day.

Ideas for increasing thermal mass include:

  1. Have the house on a concrete slab rather than stumps.
  2. Have tiles or a slate floor especially in front of the north facing windows.
  3. Brick feature walls and or brick fireplaces.

Keeping the Summer Sun Out

  • Minimising West and East facing windows. These windows are the worst for heating up the house with the low morning and evening sun.   (In winter they hardly get any benefit from the sun)
  • Shading North Facing Windows.
    Typically the shading will be in the region of 1 m from the outside wall. It can be either deep eves, a veranda, or a pergola.

Minimise Heat Transfer

That’s transfer of heat from outside to inside in Summer days, and inside to outside in Winter.

To minimise heat transfer:

  • Provide good insulation to walls and ceilings.
  • Have small windows on the South Side.
  • Closing curtains at night in winter.

The first house we built in Australia was built according to these principles. Although we lived in it for 10 years we never felt the need to fit air conditioning. We didn’t require awnings on the windows or wanted to shut the curtains on hot days.

The above advice applies to Australia and other Southern Hemisphere Countries such as New Zealand and South Africa. If you live in the Northern hemisphere you need to have the Large windows on the south side of the house.

What Passive Solar ideas have Worked for you?

For more Green Ideas see Sustainability


Window Fail

I was out walking the other day and saw this unusually shaped window.

So why do I think it’s a FAIL?

  1. As it occupies the full front of a dormer there is no way you can fit a curtain or a roller blind to this window.
  2. It faces North without any external shading.
  3. Consequently the room behind it will be baking hot in summer and, with the exception of a few hours of daylight, freezing cold in winter.

For more Fails and unusual houses go to What the………….?


Is Low-E Glass Worth The Money?

What is Low-E Glass?

It is a normal glass that has an extremely thin, transparent coating applied. (Single glazed low-E glass is peculiar to Australia)

The coating reflects part of the ultraviolet, and infrared light, while allowing most of the visible light through.

In Australia the main advantage is that it helps cut down the amount of summer direct sunlight coming through the windows and overheating rooms.

A secondary benefit is it will reflect some of the  heat back into a room in winter reducing the radiant heat loss through the glass.

A disadvantage is that the coating may be damaged by cleaning.

Suitable For All Orientations?

A Low-E Glass reduces solar gain in winter as well as summer.

North Facing

I definitely wouldn’t recommended Low-E glass or Reflective Films for sun control for windows where you expect to get the advantage of heat from the sun in Winter. That is North facing windows . . . proper shading will be a better solution.

East or West Facing

Low-E glass will be a better performer for East and West Windows although I would be going for a highly reflective film myself.

South Facing

Some practical issues are; is difficult to clean, can spontaneous fracture from thermal shock, and its performance won’t be close to double glazing.

Typical Performance

Total Solar Energy Rejected 49%
(Higher % = less heat in summer)

Glare Reduction 22%
(Higher % = less glare)

Ultraviolet Rejection 99%
(Higher % = less harmful UV rays)

Visible Light Transmittance 70%
(Higher % = more light shining through)

Visible Light Reflectance (Interior) 4%
(Lower % = less reflective/clearer view)

To compare with plain glass windows check this link Energy Through The Windows


I am not convinced that the benefits of Low E Glass are worth the additional cost when other solutions are available such as:

Overall I feel Low E glass is more appropriate at complementing double glazing rather than replacing it.

Window Lighting Levels

A recent question I received concerning my post on Window Size was “What size window will I need to receive reasonable light levels?”

Well this photo shows the window in one of our bedrooms.

This, South facing, window has an area of approximately 0.9mand provides adequate light to easily read a newspaper in the furthest corner of the 12m2 room on an overcast day.

Light Entering The Room

On a bright day, without direct sunlight, 10,000 – 25,000 lumens come through each m2 of Window.

Even on an overcast day you would get 1,000 lumens per m2 of window.

So on an overcast day the incoming light through this window will be around 900 lumens.

Room Lighting Level

With a room area of 12m the average overcast day lighting level will be 900/12 = 75 lumens/mfloor area

This level compares well with these suggested light levels for various tasks.

  • 10-20 lumens/sqm floor area – Conversation, Eating, Watching Television, General Circulation.
  • 20-50 lumens/sqm floor area – Cooking, Casual Reading, Bathrooms.
  • 50- 100 lumens/sqm floor area – Detailed Craft Work, Study.

Additional Considerations

The above calculations apply to my case where the window is plain glass and the walls and ceiling are white.

Using a tint or Reflective Film will reduce the lighting level by between 20 and 80% so this fairly small window may not be adequate.

With a less reflective decor the room lighting level will reduce significantly across the room.

In either of the above cases a 2mwindow (which is smaller than many windows I see in display homes) would provide adequate light in most conditions.


Reducing Noise Through Windows

General Principles

The main factors in window noise reduction in order of importance are:

1. Glass Thickness

Thicker glass will give a better result. (Laminated glass is slightly better than solid glass of similar thickness as the laminated layer provides additional ‘Damping’)

2. Air Gap,

A secondary window system with a 100mm air gap will perform much better acoustically than a Insulated Glass Unit (IGU).

Thermally however the performance will be lower.

Having the glass thickness of the two panes vary by more than 50% improves the accoustic performance.

3. Sealing

Effective compression sealing around opening sashes, and sealing around the frame will prevent noise ‘getting around’ the window.

Noise Levels

Noise is measured in Decibels (dB).

The scale  is logarithmic, which means that each 10dB increase means the sound is twice as loud.

External Noise Levels

Conversation 65dB

Aircraft    65dB

Traffic    70 – 80dB

Construction  75dB (measured at your property boundary)

Recomended Internal Noise levels

Bedroom    30 – 35dB

Living Room    35 – 40dB

Noise Insulation Performance

Sometimes you will see performance quoted as a Sound Transmission Class (STC) others use Weighted Sound Reduction (Rw) values. Both units are essentially the same  and are equivalent to decibels,

Installing windows rated 30 Rw, or STC 30, will reduce an external 75dB noise  by 30dB, to an internal noise level of 45dB.

Approximate performance of various glazing  systems are:

Single – Clear glass 3 30
Single – Clear glass 6 32
Single – Clear glass 10 36
Single – Laminate 6.4 33
Single – Laminate 7.5 34
Single – Laminate 10.3 37
Double – IGU 6  – 8mm air gap  –  4 34
Double – Secondary 6  – 100mm air gap –  4 46

You will need to talk to your proposed window suppliers to establish the exact performance and costs.

Solar Tube Options

Sometimes our homes are a little lacking in the light department.

Even if there are lots of windows, if your property is overlooked at all, or faces the ‘wrong way’, you might find yourself craving that natural light.

You may need to light

  • Bathrooms,
  • Walk in Robes
  • Corridors
  • Pantries
  • Toilets

That’s where Solar Tubes come in handy.

By allowing bright natural light to pour into your home, these economic fittings add a stylish new dimension to your space.


Room Sizes

Domestic units come in two sizes:

A smaller size suitable for rooms 3m x 3m

A larger unit suitable for rooms up to 5m x 5m.

For larger spaces you can use multiple fittings, or go for a commercial unit.

The units come in a wide range of prices from a simple unit with a flexible light duct, through solid duct models, to top of the range models which incorporate LED lighting.


Ceiling Diffusers

Because they have a range of ceiling diffusers they fit perfectly with all styles of interior – from modern contemporary to more traditional older-style properties.