Day 9 - windows & glazing
It should come as no surprise that windows are the weakest point in the building envelope when it comes to maintaining internal thermal comfort and preventing air leakages. We’ve all experienced (and possibly still do) single pane glass, rattling in old timber sash windows, the condensation and drafts in winter. Not ideal when you’d like to maintain a constant indoor temperature. It’s safe to assume that at least 50% of energy loss in your average home is through the doors and windows, making it a critical design factor. Pause for a second…. yes 50%.
We’ve got a lot to learn in Australia and I feel this is going to be one of our biggest challenges to overcome - performance over aesthetic. Most clients come to me with inspiration images of expansive glass, small profile steel frames or commercial aluminium frames. None of which is going to stack up in a Passive House. I think this will be a difficult task for Australians to overcome and quite confrontational.
Further on I’ll explain in more detail about the performance measurement of windows and glazing, but first I think it's important to point out that we expect a lot from windows. Here are four properties that we ask;
Airtightness; good seals and no gaps, preventing unwanted drafts and heat loss.
Thermal break; this is where double and triple glazing come into effect, creating air pockets that resist the outdoor temperature infiltrating indoors. Additionally the frame is calculated upon its ability to resist temperature change.
Solar radiation; winter sun is likely welcomed into the home for its warming effects, however the solar radiation in summer and shoulder seasons will cause an overheating effect.
I’ll try and keep this really simple. There are two main measurements for assessing windows and their performance characteristics. This is essential in passive house calculations and assessing the suitability of window materials and brands.
U-value explained in 5 easy steps:
U-Value is a conductive heat value.
The lower the U-value the better resistance to conductivity of heat (and thus reducing the heating or cooling loads).
The overall U-value of the window must be calculated (Uw).
Uw comprises of the U-value for the frame (Uf) and glass (Ug).
For Passive House certification the Uw must be 0.8 W/m2K or less.
SHGC stands for Solar Heat Gain Coefficient
This is the ability of the glazing to resist solar radiation
Is valued between 0-1, the lower the value the higher the resistance
The design of a home must consider the strategic placement of glazing to suit its climate, and also take into consideration the best suited U-Value and SHGC for each glazed window or door to attain the best year round performance.
There are not many options for Australian made windows and doors that will comply with Passive House performance requirements at this point in time. At the top of the food chain in Australia is Paarhammer - producing fantastic high performance windows and doors.
Importing from Europe seems to be a common approach, and the most experienced manufacturer at this is Laros. For first timers like me I’ll be seeking out an experienced crew like this to ensure peace of mind when considering optimised design (sizing), ordering, shipping and delivery. Laros have a great library of video content too.
Comparing european windows vs australian products when considering performance is no easy feat! In Australia we follow the American method for testing standards of glazed elements in buildings. Unfortunately this means European and Australian results vary for a variety of differences in testing standards. We feel that if you’re considering a European product over an Australian Product its vital to get the Australian Test results from the manufacturer to ensure your comparing apples with apples!
Below is a comparison of some of the U-values from common brands we specify. To simplify it, let’s compare sliding doors and assume the glazing is at least double glazed. I’ll let you come to your own conclusions;
AWS Thermal Heart
Windows to avoid
-Timber Aluminium composite)
(Timber Aluminium composite)
Aluminium thermally broken
Aluminium thermally broken
(Solar ban 70XL glass)
As low as 1.0
As low as 1.9
As low as 2.2
As low as 2.2
As low as 1.2
Air Infiltration l/sm2
0.05 tilt and turn
0.31 sliding door
0.53 sliding door 0.05 awning
0.25 sliding door
0.27 tilt and turn
*all values are for openable windows / doors
When considering materials for your project (including windows) we feel a holistic approach to sustainability is crucial. This includes but is not limited to; embodied energy of materials, distance travelled to arrive on site, recyclability or reusability of a material.