Double glazed windows help to reduce heating and cooling costs
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Double glazed windows ordered

Double glazed windows were a non-negotiable item right from the start.  Expert advice from organisations such as the Alternative Technology Association (ATA) clearly spelled out the insulation advantages.  From a heating and cooling perspective, single glazing is about as good as leaving a hole in the wall.

Renovation included installation of double glazed windows

At our last house, we experienced the improvements that come from double glazing.  After renovating the kitchen and family rooms, double glazed windows and doors kept the rooms at a more comfortable temperature during the year.  An added bonus was stopping most of the outside noise so indoors was quieter.

Started by selecting an appropriate frame material.

We began investigating double glazed windows about 2 years ago.  The starting point was visits to suppliers of the different frame materials – uPVC, aluminium, aluminium/wood combination and wood.  What are first impressions of the different materials?  Our thinking being it’s the frames, not the glass, that catches the eye. There’s no point is having a frame material that clashes with the style of the rest of the house.

Windows can be supplied in a variety of frame materials

Seeing different frames in the flesh meant reducing the options down to either aluminium/wood or wood.  We think wood fits best with the feel of our design. Recycled timber floorboards are used in the bedrooms and as cladding. Also, oiled wood frames look great next to the Timbercrete block walls. I wonder if there’s something karmic about timber frames going into walls made mostly out of waste from mills that could have produced the sawn timber?

Reviewed energy performance of local manufacturers.

Apart from looking good, double glazed windows must also keep heat inside the house during winter and keep heat out during summer.

A recent article in the ATA Sanctuary magazine said the key indicator of performance is the whole window U-value (Uw).  It’s a measure of how well the window and frame assembly conducts heat. Low numbers are what’s needed as that shows the assembly is a better insulator.  The air or inert gas in the gap between the two panes of glass slows down heat conducted through the window.

Double glazed windows reduce heat transfer

If that’s a bit confusing, inverting the Uw changes the number to an R value.  Most people are familiar with R values because that’s the number stamped on bales of ceiling insulation batts.  This number is a measure of resistance to heat flow.  High numbers are good as that shows the material is a good insulator.

It’s easy to check the performance of a potential supplier’s windows using the Australian Window Association Window Energy Rating Scheme (WERS) website.  All reputable suppliers provide performance specifications on their products.

Windows are an insulation weak point.

Double glazing does reduce heat transfer through windows but this has to be put into context.

The R value for single glazed windows is typically 0.125 (with a Uw of 8).  That’s not good at all!  Well-constructed double glazed windows achieve an R value of around 0.5 (with a Uw of 2).  That’s better.

Insulation batts that go into the ceiling or walls have R values ranging from 1.5 to as high as 7.0, much better than double glazing.  A well insulated wall will always do a better job than a window.

All windows contribute to heat loss or gain in a home

Energy loss or gain in a well-insulated house

Windows are by far the largest contributor to heat loss or gain.  That’s not to say they should be banned from sustainable homes.  Who wants to live in a dark box?  It’s a matter of carefully considering the purpose, size and location for all windows.

Most of the glazing in our home consists of large, north facing windows (and a sliding door).  In winter, sunlight shines through the glass to warm the thermal mass in the concrete floor.  Eaves prevent sunlight reaching the glass during summer.  Windows on other walls are much smaller.  They allow natural light into rooms and help with ventilation after warm days.

Price was another consideration.

After checking the specifications of windows from potential suppliers, we asked for quotes.  Every company we contacted was helpful with providing a detailed, informative quote. They were happy to answer follow-up questions.

When we combined appearance, performance and cost, the successful bidder was Aspect Windows. Their oiled timber frames will look dramatic nestled into the the Timbercrete walls.

Using hopper windows up high.

Natural ventilation is an important component in passive cooling systems.  Warm air has to vent easily from the house after a hot day.  Clerestory windows located at the highest end of the raked ceilings are ideal for this purpose.

Clerestory windows from John Robinson Decor

Clerestory windows as defined by John Robinson Decor

Aspect Windows are making an outward opening hopper style window for these locations. Opening a hopper window even a slight amount vents air from just under the ceiling.  Awning windows can’t achieve the same effect as their opening is some distance down the wall.

High windows are outward opening hopper windowsUsing clerestory windows does introduce the issue of opening and closing something that’s 3 metres off the floor.  We specified a rod and special winding mechanism for the hoppers. Remote controlled electric motors are an option but we couldn’t justify the cost.

Sashless window fits a tall, narrow opening.

The main bedroom has a tall, narrow window running from floor to ceiling.  It’s there for two reasons.

Firstly, it allows light into the room and frames the view of the driveway garden.

Secondly, the window is part of an evaporative cooling system using a water feature just outside the window.  Warm air passing over water cools as the water absorbs energy and evaporates.  A cool sea breeze is a good example of this natural process.  The trick is to create the conditions needed to encourage air to flow over the water feature and into the room.  Once again, nature comes to the rescue.  Venting warm air that has risen to the ceiling of the room means cooler air is drawn in to replace it.  

Sashless window (image provided by Grange Joinery)

Example of sashless window (image provided by Grange Joinery)

The bedroom’s clerestory window vents the warm air.  The best window style we found for the incoming air is a sashless window.  Having the two panes of glass slide over each other means the lower opening is as close as possible to the floor level.  This makes it easier for the cooling air flow to enter the room.

Moving on.

Deciding on the windows felt like quite an achievement.  However, there’s no time to rest.

The next big decision is lighting – what type of lights, how many, where are they located, …

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