TECode - The Building Act 2004 governs all building work in New Zealand. The New Zealand Building Code (NZBC) is the compliance framework for which buildings must conform. The key clauses of the NZBC affecting glass and glazing are as follows;
Each building code clause has options to comply using an Acceptable Solutions (AS) or an Alternative Solution – An alternative solution normally requires a Verification Method (VM) or specific engineering design for compliance.
Standards – Building control standards play a key part in glass selection and glazing. The following are the key standards used in the New Zealand glass industry:
The standard may be cited as a compliance document in its entirety or in part.
There is a wide range of international standards that are often used in special applications
There are many factors that drive the selection of glass - below is the common and key terminology often referred to in regards to performance with a brief explanation of what each means, with a few tips for designers;
VLT (Visible Light Transmission) - Measured as a percentage of visible light that passes through the glass in the 380 to 780nm wavelength range perpendicular to the glass surface. Also known as Tv, Tvis, LT and VT.
Tips; A high percentage indicates a glass that may have high clarity but may create glare. Low VLT does not mean you cannot see through as think of sunglasses which can have 10% or lower VLT. Balancing the VLT with other critical factors is important.
VLR (Visible Light Reflectance) - Measured as a percentage of visible light that is directly reflected from the glass in the 380 to 780nm wavelength range perpendicular to the glass surface. The reflection can be from the eternal surface VLRe or internal surface VLRi. Also known as LR, VR and Rvis.
Tips; A low reflectance value is around that of clear float at 8% or clear IGU at 15%. Specialist anti-reflection (AR) glasses are as low as 1%. Higher reflectance glasses help to reduce VLT and solar gain as they tend to reflect both heat and light, but sometimes they are restricted to 20-25% by building by-laws. If the VLR is too low the building can look flat and recent trends are toward more reflectivity for more life in the façade.
UV (Ultraviolet light) – Measured as the percentage of ultraviolet light measure over the wavelength range of 290 to 380nm. It can be expressed as the UV transmitted Tuv or UV eliminated. UV elimination = 100 – Tuv.
Tips; In general the lower the UV transmission the better for fading, but don’t get trapped thinking 99% UV elimination with clear laminated glass will stop fading, as it won’t.
Fading – Fading is a function of the UV, VLT and SHGC, and is expressed as a Fading Reduction Coefficient (FRC) or damaged weighted UV transmission (Tdw-k or Tdw-ISO). The FRC compares any glass with that of 3mm clear float.
Tips; Fading reduction or damage weighted transmission numbers give a better account of what is likely to happen in terms of fading when comparing glass types.
SHGC (Solar Heat Gain Co-efficient) -. The SHGC is the combination of incident solar radiation directly transmitted and absorbed and subsequently released inward. It is normally expressed as a number between 0 and 1 but it is also known as the solar factor (SF) of g value and these number are expressed between 0 and 100. (e.g SHGC 0.5 = g 50%)
Tips: The lower a window's SHGC the less solar heat it transmits. Solar heat gain can provide free heat in the winter but can also lead to overheating in the summer. How to best balance solar heat gain with an appropriate SHGC depends upon the climate, orientation, shading conditions, blinds and other factors. Where your project is subject to a long air-conditioning season, it is most important to reduce solar gain and therefore reduce air-conditioning loads. Try not to confuse SHGC and SC numbers as many designers do, and some HVAC software uses one or the other.
SC (Shading Coefficient) - The Shading Coefficient (SC) is the ratio between the solar heat gain (SHGC) for a particular type of solar control glass and that of 3mm clear float which has a SC of 1.00. As a guide the SC = SHGC/0.86. A very high performance glass can have a SC as low as 0.20.
Tips; The SC is a key design number but it can vary a little depending on the calculation method with the more common NFRC method (SHGC/0.86) used in USA and Asia regions, compared to the EN method (g/0.87).
U Value (Insulation) - This is a measure of heat transfer through the glass due to thermal conductance and the differential between interior and exterior environment and temperatures. U value is expressed as Watts per m2 per degree Kelvin (W/m2K) or W/m2C.
Tips: Single clear glass has a U value of about 6, an IGU about 3, a low e IGU about 2 and a very high performance IGU about 1. The lower the U value the better but this can create some external condensation problems in cold climates. The U value is measured and denoted at the centre-of-glass (cog). Please try not to use R values for glass or confuse with R values for windows (Rwindow) as they are not the same.
LSG Ratio – The light to solar gain ratio (LSG) compares the VLT and SHGC of a glass.
LSG = VLT/SHGC. It is also known as the coolness factor (CF) or Selectivity (VLT/g)
Tips; LSG around 1.7 - 2 is very high performance, and some at 2.2 are cutting edge products. These glasses are often referred to as high selective or “high VLT – Low SC” glasses.
Acoustics – Sound control (acoustic) data is calculated in a range of indices with the most common being;
- STL – Sound Transmission Loss
- STC – Sound Transmission Class
- Rw – Weighted sound reduction index (with sub index Rtr + Ctr)
Tips; It’s very important to understand the noise source, pressure and frequency as different glasses behave differently across the full spectrum of 100 – 4000 Hertz. Some glasses like laminates are very good for speech privacy but not so good for road noise. In addition every ones hearing is different and small changes 1-2 dB cannot be detected. As a guide the ear will pick up 3dB difference or more, a change of 7 dB is always detected and 10dB change is half as loud or twice as loud.
The combinations of glass to meet a specific project requirement are significant. In order to meet the specific needs we have generated the attached for to assist in our proposing the right glass solution. Please click here view the PDF that can be completed and returned to enquiry.akl@woodsglass.co.nz. The sort of information we generally seek is;
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VLT % |
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VLRe % |
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VLRi % |
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Tuv % |
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SHGC |
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SC |
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U value |
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Acoustic |
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Colour |
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GANZ – The Glass Association of New Zealand is an industry body formed by glass and related supply companies and focus on wider industry issues such as standards and code compliance.
GANZ also provide educational material which is available on the web site www.ganz.co.nz and provides a GANZ Accredited Standards Advisor programme, where members sit a comprehensive 100 question exam of NZS 4223 Part 3 and require a 100% pass mark for certification.
IGUMA – The Insulating Glass Unit Manufacturers Association is an industry body formed to set the testing and quality standards for the manufacture of IGUs in the New Zealand market. IGUMA have introduced IGU testing to BS EN 1279 using BRANZ as the provider and have introduced an advanced member QA manual based on BS EN 1279 Part 6.
IGUMA provides educational material and this can be accessed via the GANZ or WANZ website
WANZ – The Window Association of New Zealand is the umbrella group for GANZ and IGUMA and provide industry guidance for the entire window and doors industry.
Refer www.wanz.co.nz