Several techniques can control solar heat gain through roof glazing, the most common being solar control glass. This guide explains how solar control glass is designed to reduce solar energy, improve internal comfort and manage natural light in roof glazing systems.
Solar Control Glass Guide
In addition to admitting light, our roof glazing systems also allow the natural heat from the sun to enter a building.
During the winter this can be considered a benefit – offsetting heating costs by providing ‘free’ heat on sunny days during the winter season.
During the summer months, however, unless some form of solar control is considered, this heat from the sun could be regarded as a disadvantage and lead to overheating issues.
Several solar control glass types offer different levels of light and heat transmission. This is achieved by using body tinted glass or special reflective coatings. These options support energy-efficient glazing design, especially where levels of light transmission must be carefully managed.
Basic Principles
Glass resists solar radiation through reflection, transmission and absorption. These are defined by the following parameters:
Reflectance:
The proportion of solar radiation at near normal incidence which is reflected by the glass back into the atmosphere.
Absorptance:
The proportion of solar radiation at near normal incidence which is absorbed by the glass.
Direct Transmittance:
The proportion of solar radiation at near normal incidence which is transmitted directly through the glass.
Total Transmittance:
The fraction of solar radiation at near normal incidence that is transferred through the glazing by all means. It is composed of the direct transmittance, also known as the short wave component, and the part of the absorptance dissipated inwards by long wave radiation and convection, known as the long wave component. The proportions of the absorbed energy which are dissipated either inside or outside depend on the glazing configuration and the external exposure conditions. All solar radiant heat properties are angle dependent.
Shading Coefficient:
The solar radiant heat admission properties of glasses can be compared by their shading coefficients. The shading coefficient is derived by comparing the properties of any glass with a clear float glass having a total solar heat transmittance of 0.87 (such a glass would be between 3 and 4mm). It comprises a short wavelength and long wavelength shading coefficient.
Solar Control For Glass
Solar control can be achieved by the use of:
- Body tinted glasses with increased absorption.
- Reflective coated glasses with increased reflection.
- Combinations of body tinted and reflective coatings in a single glass
- Special high performance insulating glass units.
Body Tinted Glass
These types of glass are usually tinted grey, green, bronze or blue throughout their thickness. Their solar control properties and colour vary with thickness whilst their reflectance are slightly less than clear float. When used in double glazed units they are best positioned as the outer pane as the heat due to the absorbed radiation is more easily dissipated to the outside of the building.
Coated Glass
Solar Control can be increased by the use of coated glass which:
- Reduce solar heat gains with a full range of high, medium and low performance options offer a choice of high to low light transmittances.
- Provide varying degrees of reflectance including low reflectance.
- Are available in a wide range of colours and appearances to meet aesthetic design requirements.
- Are available as toughened or laminated options for safety and security.
- Offer a comprehensive range of solar control performance options. The numerous coating compositions available provide a wide range of performances, which is further increased by their combination with body tinted glasses. Thus, glass with a particular performance may be selected for specific applications. These solar control coatings are widely used in modern glass production and help optimise performance in both commercial and heritage glazing projects.
Design
Glazing design must balance several often-conflicting requirements, which cannot be considered in isolation. However, when considering the use of solar control glass, it is necessary to consider the location and orientation of the glazing within the building envelope.
Temperate Glass
Glass performance in temperate climates has to balance the need to provide solar control and reduce summertime overheating against the need to provide high levels of natural illumination and the benefits of passive solar heating. To allow for passive solar design, the performance range for a double glazed unit configuration could be:
- Total transmission 20% to 70%
- Light transmission 35% to 90%
- U value 1.0 to 1.2 W/m² K
These performance parameters for glass need relating to the specific application, since there is no one ideal glazing solution for all applications. However, as a general principle, high thermal insulation with solar control is a usual requirement for temperate climates such as the UK.
Solar Gains
Solar radiation increases indoor air temperature, so designers must ensure it does not cause discomfort to building occupiers by providing appropriate heating, cooling and ventilation schemes to provide comfortable conditions. To this end, the designer will have to undertake calculations to assess the effect of various glazing options on the solar heat gains that his ventilation or air conditioning equipment need to cope with, and to choose the best solution for the specific project. Solar gain is not the only source of heat which contributes to the ‘total heat’ within a building. Other sources include lighting, occupants themselves, electrical equipment etc. The choice of glass type directly affects the solar heat gain coefficient (SHGC) and how much solar energy enters the space
Solar gains into a building can be determined from a knowledge of the following:
- The position of the sun in relation to each elevation of the building. Levels of solar radiation are dependent upon whether or not the sun is relatively high in the sky and the orientation relative to the glazing.
- The intensity of the solar radiation incident upon the faces of the building.
- The surface areas exposed to the sun. A large glazed area will allow more solar gains to enter a building than smaller areas of glazing.
- The date and time of day. This is related to the relative movements of the sun and earth. Shading effects. Presence of blinds, overhangs, nearby buildings etc, may prevent solar radiation entering a building.
- Type of glass. Different glasses will transmit, reflect and absorb different proportions of the sun’s energy.
- Structure of the building. A building constructed of heavyweight materials will heat up and cool down more slowly than one made with lightweight materials.
Direct Radiation and Comfort
Whilst air conditioning can provide comfortable conditions for the building and occupants as a whole, the effect of solar radiation falling directly on people situated close to the glazing needs to be treated separately. An occupant receiving direct solar radiation can feel uncomfortably hot even when room temperatures are being maintained at a comfortable level by means of air conditioning or mechanical ventilation. As a general guide, highly reflective glasses with relatively low direct solar transmittances will be most effective at combating the localised overheating of occupants situated under the glass. Selecting a solar control glass with lower direct solar transmittance can reduce localised overheating while still allowing natural daylighting.
