In February I travelled to Canberra for a seminar on the changes to the BCA for 2013. A couple of interesting but noteworthy points…
1) New definitions for U and SHGC in BCA/NCC 2013.
Previously, many architects and builders were confusing the meaning of U value and SHGC value (applied to glass (glazing) performance only) and applying these values to whole
windows when complying with Section J2 of the BCA. The terms ‘Solar Heat Gain Coefficient’ (SHGC) and ‘U-Value’, have now been amended to include ‘total system’ to reflect the
correct method for calculating glazing performance values when complying with the Code.
This was a important change as many projects have been caught out using the U & SHGC value for the glass only. The total system Uw or SHGCw value is the value to use and applies
to the “total system” or “whole” window unit. (Note the subscript “w” which should designate a whole window value.
A basic example of Uw and SHGCw values and what they mean….
U-Value (Uw) measures how readily a ‘total system’ window conducts heat. It is a measure of the rate of non-solar heat loss or gain through the assembly.
The rate of heat flow is indicated in the terms of the Uw value of a window assembly which includes the effect of the frame, glass, seals and any spacers.
The lower the Uw value, the greater a window’s resistance to heat flow and the better its insulating value. A low Uw value is ideal for all climates as it stops unwanted heat gain in
summer and unwanted heat loss in winter.
Solar Heat Gain Coefficient (SHGCw) measures how readily heat caused by sunlight flows through a ‘total system’ window. The SHGCw is the fraction of incident solar radiation
admitted through a window system, both directly transmitted, and absorbed and subsequently released inward.
SHGCw is expressed as a decimal between 0 and 1. The lower a window’s SHGCw, the less solar heat it transmits. In cooler climates, a high SHGCw is beneficial for north facing
windows during winter; however shading will be required in summer to prevent unwanted solar heat gain. In hot climates, a low SHGCw is always ideal.
2) New BCA Rules for 1st floor Windows**
The 2013 BCA/NCC defines new rules to help prevent the situation where young children have fallen out of 1 st floor windows, sometimes through a flyscreen.
Windows over 2 metres above ground and where the window opening is less than 1.7m above the floor will be governed by these changes.
This requires all openings falling into this category to include some type of barrier. These barriers must have a child resistant opening mechanism and include elements no more than
125mm apart. They must withstand an outward force of 250 newtons.
Different rules apply to different window types. For example, awning windows will now be restricted to an opening of no more than 125mm. Casement windows will need some type of
external barrier.
From a energy efficiency / saving point of view, this external barrier could be used to incorporate a shading device. Just a though for designers to think about….??
(**Note: the above comments are for information only, they are taken from my notes listening to a speaker talk about the 2013 BCA. I may err in this interpretation and you should check the 2013 BCA when it is available).
TECH CORNER: The effect of recessed downlights on ceiling insulation.
NatHERS protocol and the BCA both require that penetrations in ceilings be compensated for by extra insulation in the ceiling. In NSW, BASIX is predominate over the NatHERS
protocols but not to the BCA. If you do not specify lighting, your NatHERS assessment should allow loss of insulation due to recessed downlights as per the BCA Table J1.3b.
For example, In a ceiling with R2.5 insulation, if your downlights (including the safe area around the light where no insulation can be placed) result in a 5% uninsulated ceiling area then
R5.0 insulation will be required to compensate for this loss.
Solutions could include unventilated downlight covers, unventilated light fittings or unventilated downlight covers with normal fittings. There are now products that will allow you to insulate
completely over the light.
