A-Rated Energy Efficiency


Since the inception of float glass from Pilkington in 1959, glass technology has professed in leaps and bounds. Most glass now-a-days is energy rated, and the 2 most commonly used are Low Emissivity (Low-E) & Solar Reflective (SHGC).

Important ratings to check when buying windows & doors are the U-value, the R-value, the G-value and the SHGC.

The U-Valve

A U-Value is the measurement of the heat loss or gain (non solar) through a material or an assembly. With glass or sealed glazed units, the aim is to achieve the lowest U-Value as possible. The lower the U-Value, the less heat will escape from your windows or conservatory, when all the doors and windows are closed. U-value ratings generally fall between 0.4-10.0 W/m2K for most British products.

Technically, U-Values are calculated as the amount of heat lost through one square meter of glass for every degree difference in temperature either side of the glass through the whole assembly, which includes the effect of the frame, glass, seals and any spacers.

In descending order, these are typical U-values for glass:

To keep your heating bills low, you will need to invest in the lowest U-Value glass. There will be a point at which the investment reaches the point of ever diminishing returns, this will depend on the total volume of glass, how much you pay for your energy, as well as how much time you plan to spend in your conservatory or orangery in the winter months. One of our team will be able to help you with these calculations.

The U-value for a window takes account of the various U-values for the components making up the window, so you may see these in technical literature; UW is the value for whole window and because of its importance is usually abbreviated to U, UC is the value at the centre of glass, UF is the value for the frame.

U-Value, R-Value & G-Value

R-values and U-values are essentially two sides of the same coin.

R-value is usually cited when discussing things such as wall and ceiling insulation values. The term does not translate well to windows and other fenestration products. That industry prefers U-values.

The two are actually inversely related. The higher the R-value, the better insulated are the walls and ceilings. The lower the U-value, the better job a window does in keeping out the heat and cold.

G-value (or Solar Factor) measures the percentage of heat that passes through the glass. The lower the solar factor or G-value, the higher the solar protection and therefore the higher the performance of the solar control glass.

Solar Heat Gain Co-efficiency (SHGC)

SHGC measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation (Infra Red Radiation) admitted through a window, both directly transmitted, and absorbed and subsequently released inward.

SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits.

In very cold climates, windows with very low U-values minimise discomfort. If summer discomfort is expected very low SHGC values should be looked at, but bear in mind that lower SHGCs lead to increased comfort in the summer at the expense of less winter solar warmth.

What Now?

Talking through your project will cost you nothing, so to discuss your requirements, please call 0800 88 247 88