Considering Overheating
If not well considered, the design of your home could lead to overheating at times. The key to avoiding these uncomfortable periods is through good design and making considered choices for not only the glazing but all of the products used in the construction of your home.
Introduction
Glazing is the only building material to pass heat from the sun directly into the building, and naturally it seems the key to controlling the overheating within our homes is through glass specification. Absolutely glass selection can have an impact on solar heat gain in your home, but overheating is a complex issue that is best tackled during the design phase, when all of the conditions and elements can be considered collectively.
Building Design
The key to avoiding overheating in your home is through good design, working with nature, understanding your site conditions, and making considered choices for not only the glazing, but all of the products used in the construction of your home.
Building Design Strategies to Minimise Overheating
It is suggested that a well designed building will,
- be airtight,
- have adequate insulation,
- be well ventilated,
- allow for opening windows and operating blinds, and
- the ability to be heated and cooled.
Other important elements to consider during the design phase of the building are:
- orientation and layout,
- how you will use the spaces,
- shading, either natural or as a part of the building, to the north and west facing windows, and
- size, placement, and configuration of windows and doors.
Windows
Windows and doors have an important function in the design of homes in Aotearoa New Zealand, many of which are set out in the Building Code. They provide access, ventilation (G4), natural light (G7), and appropriate levels of insulation (H1).
Optimising Window and Door Design for Comfort, Ventilation, and Thermal Performance
In addition to satisfying the regulatory requirements, windows and doors must of course also satisfy the needs of the occupier in terms of size, placement, configuration, functionality, and thermal comfort:
- placing windows to allow cross ventilation,
- including sufficient opening sashes/panels to allow for natural ventilation,
- sliding doors offer more controlled ventilation than hinged or bifolding doors,
- for upper levels consider modifying sill heights so that restrictor stays are not required, allowing greater ventilation,
- high level opening windows can offer secure ventilation,
- carefully consider the size of windows on each elevation,
- north facing windows allow heat from the sun in winter, but will require shading from the summer sun,
- east facing windows allow morning sun and some heat,
- south facing windows will provide consistent light and limited heat,
- west facing windows will heat a space in the afternoon and should consider size and shading where possible to minimise the heat gain.
Glass Selection
In addition to good building design and good window design the glass used in your windows must also be carefully considered to reduce the risk of overheating.
Choosing the Right Glass for Thermal Efficiency and Comfort
In terms of thermal performance, glass design considers three separate elements
- U-value, the measure of how much heat is transferred through the glass, basically its insulation value. The lower the U-value the better the insulation.
- Double glazing made using two panes of clear glass will have a U-value around U2.8, whilst a unit using a pane of Low E glass can achieve U1.1.
- The Association has an IGU calculator to help understand what the U-value of a particular combination will be
- The term “g-value” refers to solar gain transmitted through the glass and is expressed as a value between 0 and 1. The lower the g-value the better the resistance to heat gain.
- Double glazing made using two panes of clear glass will have a g-value around G0.75, whilst a unit using a higher performance glass can achieve much lower g-values
- VLT, or visible light transmittance, refers to the amount of light that passes through the glass. Normally nominated as a percentage where 0% means no light passes through the glass, and 100% where all light passes.
- Double glazing made using two panes of clear glass will have a VLT around 80%, whereas a unit using a pane of grey tinted glass might be between 40-55%, but the range is much broader for different types of tinted glass.
- Common Low E double glazing might be around 75% but is variable across a wide range of products available.
Glazing Specification Considerations
These three elements can make glazing difficult to specify and it is recommended that you discuss your building design with your glass supplier.
- The three elements must not be specified separately as the specified combination may not be achievable. It is recommended that one element be selected as the foundation of your glass design, which will inform the achievable performance of the other elements. U-value is the common starting point.
- Specifying a low g-value glass to reduce the risk of overheating can be a double edged sword, as the glass will also block heat during the winter months, potentially increasing the need to heat the building to achieve desired comfort levels.
- Some consider that specifying different glass performance for each elevation, and in some cases each window, is a method of optimising heat gain. The reality of this decision would be a negative impact on the not only the cost of your windows and doors, but also the aesthetics of them. Different glasses, including different Low E glasses, appear differently. The reality is that a buildings thermal envelope must be considered as an entity to understand how the building will perform. High performing areas will always be compromised by poorer performing elements as the envelope works as a whole. It is recommended that a glazing make up is designed to suit all aspects of the building’s orientation, layout, and location, and visual consistency considered.