Efficient Windows

Windows can really enhance the comfort and efficiency of your home, or they can severely compromise it.

Apart from letting daylight in and allowing you to see outside, windows are very important for ventilation. They are also a major gap in the thermal infrastructure of your house, with poor windows allowing draughts, excessive solar heat gain and major heat transfer through the glass and frame. But there are solutions to these issues and we can help you find the best ones.

The Problems with Windows
Windows can severely compromise the energy efficiency of your home by being responsible for major heat gain in Summer and heat loss in Winter. According to Your Home Technical Manual, windows can be account for up to 40% of a home’s heat loss and 87% of it’s heat gain.


Solar Gain
Sunshine through a window is a wonderful thing most of the time, but not on a hot Summer’s day when the energy contained in the light is absorbed inside your home and converted to heat. The midday Summer sun typically has a power of around 1000W/m2. Not all of this will make it though the glass, but a good portion of it can, meaning a large window can be like running a heater on full in the room. North facing windows can be effectively shaded with appropriate eaves, but East and West facing windows are more difficult.
Draughts
Many windows have significant gaps around the openings, but they can also have gaps between the frames and the walls. Uncontrolled air movement through a window can lead to significant heat losses and gains. There are effective draught proofing strategies for most types of windows.


Thermal Transfer
The roof, floor and walls of your house typically consist of several layers of structure, like tiles, timber, plaster etc, hopefully with layers of reflective and bulk insulation as well. Windows on the other hand are a significant area of a single layer of highly thermally conductive glass. The frames can also be good heat conductors, especially steel and aluminium frames, This means that outside temperatures are easily transferred through to inside. In the evenings you can close heavy curtains to seal them off, but during the day you want the light and view. Double glazing adds significant extra insulation value to a window.

 

Solutions for Windows

Solar Gain
The hot Summer sun coming through a window can really heat up your whole house. In a two storey house heat from an exposed window downstairs will rise up and contribute to upstairs becoming an oven.
The best solution is to physically block the sun before it hits the glass, using eaves, trees, external blinds, roller shutters or shadecloth. Internal blinds or curtains will cut down on glare, but they will usually absorb a lot of the sun, get hot and heat up the room.You have to be careful with external shading that is close to the glass, like roller shutters or some blinds, as they will heat up the air behind them which is against the glass, allowing heat to transfer inside. Make sure you allow some ventilation, e.g. by keeping the roller shutters cracked open a little.


There are many situations where external shading is not possible or practical, like rentals, upstairs windows, skylights etc.
The solution is Renshade, a perforated foil-coated paper sheet. Simply cut to size and stick into the window using the provided velcro dots or other mounting method of choice, or make them into roller blinds. Around 80% of the light (and heat) is reflected back out the window, but some light still gets into the room and you can still see outside, getting the benefits of a window without most of the heat gain. Very simple and very effective.

Draught Proofing
Depending on the type of window, there are several draught proofing strategies. Awning windows usually work well with stick-on stripping, while sliding, casement and double-hung windows usually require something more specialised like Draught Dodgers. Don’t forget to check for gaps around the framing as well, which is usually solved easily with flexible gap sealant.

Thermal Transfer
Window Frames – The glazing is a bigger issue, because it’s a much larger area, but the window frame can also conduct quite a bit of heat. Timber and PVC window frames have decent insulation value, but aluminium and steel are very thermally conductive. Modern aluminium windows are ‘thermally broken’, meaning there is a layer of insulation between the inside and outside frame.

Double glazing – The insulation value of double glazing comes from the layer of air trapped against the glass. Moving air is a very efficient way of transferring heat (which is why draughts are bad news) but if you can keep the air still it is actually an excellent insulator. There are fancier versions of double glazing that use Argon or other inert gases in the gap, or triple glazing with 3 layers of glazing, but they are generally overkill in our mild temperate climate zone.
Double glazing also helps with condensation, which happens when the warm, moist inside air hits the cold window glass. By having a warmer internal layer separated from the cold outer glass, condensation is prevented. Double glazing will not help with hot sun coming through a window, although it will help with heat transfer through the window in Summer.

DIY insulating films – $10s /window
Because the insulation value primarily comes from the air, anything that effectively traps the air will work. Common in colder climates but only just appearing here are DIY double glazing films. They are a clear plastic film, similar to cling wrap, which is applied across the window frames using double sided tape. The depth of the frame leaves an air gap, which provides the insulation. If they are applied well they can last several years, but they can be easily damaged and because the film is flexible it provides none of the soundproofing benefits of rigid double glazing.
They are very cheap to buy, apply and remove yourself, so replacing damaged films is not a big deal. Ideal for rentals and people on a budget, we also recommend them to people considering the more expensive options to see how much of a difference double glazing will make.

You can download our info sheet on how to apply the window insulating film here.

You can download a study on retrofit double glazing done by Sustainability Vic here Window Film Secondary Glazing Retrofit Trial February 2017

Aluminium framed windows can be very difficult to insulate well with these films, but there is a clever method you can use to make frames to hold the film and help insulated the aluminium frame as well. You can download a DIY instruction sheet here. Aluminium Window Retrofit Glazing

Retrofit Double Glazing – $100s /window
The next level up is to retrofit a custom made internal frame with some acrylic glazing to your windows. This is a much more robust and permanent solution with some soundproofing benefits as well. They are usually made to be removable in case you need to clean the glass, repaint frames etc. Talk to us about reputable companies who can provide this service.

Replacing Windows – $1000s /window
The best but most expensive solution, especially with conductive frames like aluminium, is to replace the entire window and frame with a new double glazed unit. This can be very expensive and we don’t generally recommend it unless the windows need replacing anyway due to age or damage. Speak to a reputable builder for pricing.