Low-e glass
The term ‘low-e glass’ is used to describe glass that has a coating added to one or more of its surfaces to reduce its emissivity.
Emissivity is an indicator of the amount of long-wave infra-red radiation which a surface (such as the façade of a building) will emit to its surroundings. According to Kirchoff's law the emissivity of a surface is equal to its radiant absorptivity at a given temperature and wavelength. It is expressed as a proportion of 1, so if a surface absorbs 40% of the long-wave infra-red radiation incident on its surface, it has an emissivity of 0.4.
All bodies which are hotter than 0°K emit thermal radiation and absorb thermal radiation. In general, the higher the temperature of a body, the lower the average wavelength of the radiation it emits. The range of terrestrial temperatures experienced within the built environment is comparatively ‘cold’ compared to the sun and so they are radiating at a much longer wavelength. This anomaly allows us to distinguish between short-wave solar radiation and long-wave infra-red radiation (terrestrial radiation).
Typically, glass is relatively transparent to short-wave solar radiation, but opaque to long-wave infra-red radiation. This is said to produce a ‘greenhouse effect’, where solar radiation enters a space, and heats it up, but the resulting long-wave infra-red radiation emitted by the hot internal surfaces is unable to escape.
However, glass has a relatively high emissivity. This means that although it does not transmit long-wave infra-red radiation incident on its surface, it does absorb it. This absorbed ‘heat’ is then re-radiated.
Low-e coatings can be used to reduce the effective emissivity of the surface of glass so that it reflects, rather than absorbs, a higher proportion of long-wave infra-red radiation.
In cooler climates this means that long-wave infra-red radiation that builds up inside a building is reflected by the glass back into the space, rather than being absorbed by the glass and then partially re-radiated to the outside. This reduces heat loss and so the need for artificial heating.
In hotter climates, a low-e coating means that long-wave infra-red radiation outside the building is reflected back out of the building, rather than being absorbed by the glass and then partially re-radiated to the inside. This reduces the heat build-up inside the building and so the need for cooling. In hotter climates, a low-e coating might be used in conjunction with solar-control glass to reduce the amount of short-wave solar radiation entering the building.
Low-e coatings can reduce the emissivity of glazing from more than 0.8 to 0.2 or even as low as 0.04. Coatings tend to be either hard ‘pyrolytic’ coatings which are applied to molten glass, or soft ‘sputtered’ coatings, which generally need to be protected within the sealed unit.
[edit] Related articles on Designing Buildings
- Angular selective shading systems.
- Brise soleil.
- Choosing the correct glazed facade heating system.
- Computational fluid dynamics (CFD).
- Curved glass.
- Double glazing.
- Double glazing v triple glazing.
- Emissivity.
- g-value.
- Glass manifestation.
- Solar heat gain coefficient.
- Thermal bridge.
- Thermal optical properties.
- Triple glazing.
- U value.
Featured articles and news
Cladding remediation programmes, transparency and target date.
National Audit Office issue report on cladding remediation.
HBPT and BEAMS Jubilees. Book review.
Does the first Labour budget deliver for the built environment?
What does the UK Budget mean for electrical contractors?
Mixed response as business pays, are there silver linings?
A brownfield housing boost for Liverpool
A 56 million investment from Homes England now approved.
Fostering a future-ready workforce through collaboration
Collaborative Futures: Competence, Capability and Capacity, published and available for download.
Considerate Constructors Scheme acquires Building A Safer Future
Acquisition defines a new era for safety in construction.
AT Awards evening 2024; the winners and finalists
Recognising professionals with outstanding achievements.
Reactions to the Autumn Budget announcement
And key elements of the quoted budget to rebuild Britain.
Chancellor of the Exchequer delivers Budget
Repairing, fixing, rebuilding, protecting and strengthening.
Expectation management in building design
Interest, management, occupant satisfaction and the performance gap.
Connecting conservation research and practice with IHBC
State of the art heritage research & practice and guidance.
Innovative Silica Safety Toolkit
Receives funding boost in memory of construction visionary.
Gentle density and the current context of planning changes
How should designers deliver it now as it appears in NPPF.
Sustainable Futures. Redefining Retrofit for Net Zero Living
More speakers confirmed for BSRIA Briefing 2024.
Making the most of urban land: Brownfield Passports
Policy paper in brief with industry responses welcomed.
The boundaries and networks of the Magonsæte.