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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;

 B1 – Structure    Wind loads, snow and live loads for glass design
 B2 – Durability  Glass and glazing system and material life
 D1 – Access Route       Balustrade and handrails design
 E2 – External Moisture    Window, façade and balustrade fixings
 F2 – Hazardous Building Materials   Human impact safety and safety glazing
 F4 – Safety from Falling    Glazing safeguarding  a fall and barrier design
 G7 – Natural Light   Transparency and light transmission of glazing
 H1 – Energy Efficiency    Insulation of glazing housing and buildings

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.

 NZS 4223     Glazing in Buildings 
   Part 1   Glass selection and glazing 
   Part 2   Insulating glass units 
   Part 3   Human impact safety requirements
   Part 4   Wind, dead, snow and live actions 
 NZS 1170  Part 0-5        Structural design actions 
 NZS 4211     Specification for performance of windows 
 NZS 4232  Part 2   Fire resisting glazing systems 
 NZS 4218     Thermal Insulation – Housing and small buildings 
 NZS 8500     Safety barriers and fences around swimming pools, spas and hot tubs
 AS 1288     Glass in Buildings – Selection and installation 
 AS/NZS 2208       Safety glazing materials in buildings 
 AS/NZS 4666     Insulating glass units 
 AS/NZS 4667     Quality requirements for cut to size and processed glass 
 AS/NZS 4668     Glossary of terms used in the glass and glazing industry
 BS EN 1279  Part 1-6   Glass in buildings – Insulating glass units 

There is a wide range of international standards that are often used in special applications


Glass has been used for centuries and it basic composition of clear glass has not changed significantly. The basic composition involves soda, lime and silica and glass is often called “soda-lime-silica” glass.  Silicon dioxide (silica 69 to 74%) is the key component then sodium oxide (soda 12-16%) then calcium oxide (lime 5-12%) with some magnesium (0-16%) and aluminium oxide (0-5%) in the mix to help with melting and clarity. Other oxides are added to make tinted glass and special glasses.

  • Strength – Glass in its pure form is a “very strong, perfectly elastic, non –crystalline brittle solid” The actual strength is governed by surface flaws, cracks and defects, which are inherent in the process. Glass cannot be permanently deformed due to its pure elasticity and brittleness like metals and plastics so it fails without warning.
  • Chemical Resistance - Glass will resist most acids except hydrofluoric and high temperature phosphoric. Alkalis can attack the glass surface, so it crucial not to get run off from concrete and the like as they will etch the glass surface. The same can happen with some weathering steels and some sealants release chemicals that can stain glass. Hard water with high calcium can be a problem as can geothermal areas for some ceramic glasses.
  • Impact – Ordinary annealed glass will not resists high stresses from impact objects and it can shatter in large sharp dangerous pieces. It can be made stronger by heat treating into heat strengthened or toughened (tempered glass) and made stronger by laminating with special interlayers. In special security application glass can be laminated with polycarbonate to increase impact and penetration resistance.
  • Safety – Glass can be processed into Toughened Safety Glass (TSG) or Laminated Safety Glass (LSG).
  • Wind Resistance – Different glass type have different strength under uniform loading such as wind and this is dealt with design charts in NZS 4223 Part 4. The strength of the glass does not however effect the deflection but thickness does.
  • Thermal Strength – ordinary annealed glass can thermally break if the temperature difference in a pane is significant, but heat treating can overcome this. Care is required if using annealed float and laminated glass, especially in IGUs as the temperature increases within the unit. Thermal Safety checks are often done to determine the risk of fracture.
  • Density - Glass is heavy with a density of 2500 kg/m2. A simple way to determine weight of panes is 2.5x thickness (mm) x area (m2). For 1 m2 of 6mm glass -= 2.5 x 6x 1 = 15kg
  • Solar Radiation – The properties of glass vary with thickness, type and coatings, and the typical glass performance data is listed in the following section.
  • Sound Control – The acoustic performance of glass increases with thickness (mass) and when using laminated glass and IGUs. Refer performance data section.
  • Fire Resistance – Special glasses are required to withstand fire tests, and these vary from simple wired glass to highly tempered to intumescent laminates and IGUs


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. 

Glass Selector

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 The sort of information we generally seek is;

  VLT %    
  VLRe %    
  VLRi %    
  Tuv %    
  U value    

 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 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.



Woods Glass source a wide range of hardware and fittings from local and overseas suppliers for the following applications. We also design and make be-spoke fitting for projects.

  • Pivot Doors
  • Sliding doors
  • Folding stacking doors
  • Auto doors
  • Entry systems
  • Showers
  • Balustrades
  • Pools fences & gates
  • Canopies
  • Display cases and doors
  • Furniture and fixings
  • Spider fin wall
  • Tension truss wall

Selecting the correct hardware is only part of the puzzle and the design must be appropriate and durable, and often design producer statements are required for the complete glass and hardware systems, including the fixings.

Woods Glass specialise in finding the right hardware and glass solutions to solve the designers puzzle while ensuring it complies with the New Zealand Building Code. 

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