How does double glazing work?

Nothing is more important when designing your home than the window & door selection. The 3 most important ‘S’ factors to consider are: Sustainability (Energy Efficiency), Style and Security. The whole character and thermal performance of your home will be decided by the choices you make at the design phase.

Beautiful windows are possibly the most complex and interesting elements in the fabric of our homes. They provide light and fresh air and offer views that connect the interior with the exterior. However, ordinary single glazed windows can also represent a major source of unwanted heat gain in summer and significant heat loss in winter.

Today, remarkable new framing and glazing materials have changed the energy performance of windows in a radical way.

Energy efficient windows will make your home more comfortable whilst dramatically reduce your energy costs and help create a brighter, cleaner, healthier environment.

Glass performance definitions

SHGC = Solar Heat gain co-efficient. SHGC measures how well a product blocks heat caused by sunlight. SHGC is expressed as a number between 0 and 1. The lower the window’s SHGC, the less solar heat it transmits.

U-Value = Thermal insulation properties expressed in watts x m2 per degree Celsius. U-Value measures how well a product prevents heat from escaping. It is the measure of rate of non solar heat loss or gain through a material or assembly. U-values generally fall between 2.0-10.0 W/m2 K. The lower the u-value, the greater a window’s resistance to heat flow and the better its insulating value. The U-value for a window takes into account the components that make up the window e.g the framing material. Timber and uPVC being an inert material will always have a lower u-value compared with aluminium. Heat transfer through a typical single glazed aluminium window = 100% compared with 54% for uPVC or timber framed double glazed window.

IGU = Insulated Glass Unit (double glaze unit)

Low-e glass = The ‘E” in low-e refers to emissivity. The emissivity is a measure of a materials ability to radiate energy. Glass with ‘low’ emissivity absorbs and radiates infrared energy poorly which is the key factor in reducing heat transfer. Where greater solar and thermal control is required, reflective and low-e glass can be used. A pyrolytic coating is applied during glass manufacture. When low-e glass is used, care must be taken when cleaning the glass. Under no circumstances can abrasive cleaners be used.

What is double glazing?

Traditional window glazing consisted of a single sheet of glass within your window sash. Double glazing refers to a sealed glass unit comprising 2 sheets of glass separated by a spacer bar around the outside of the glass. The spacer bar creates a nominated mm air gap between the glass panes. It is this still air gap that creates the insulation break between the inside and outside of the window.

Double glazing also controls noise while not impeding the view.

Which windows and door should be double glazed? The answer is all of them. Double glazing allows the low winter sun to stream through and warm your home and once the suns radiant heat has passed through the double glaze unit and is off the glass, the room retains the ambient heat, preventing heat loss back through the glass.

What is the optimum air gap between the 2 panes of glass? For maximum performance the space between the 2 panes should be about 9 mm however increasing the air gap to over 15mm will not provide any extra significant thermal benefits.

What about sun on double glazing?

Double glazing does not impede solar heat gain therefore it will still allow winter sun penetration. Unprotected double glazed windows will still require appropriate summer shading.

The best solution for managing solar heat gain is to actually shade your North and West facing windows in summer, but allow the sun to stream through in winter. Tinted glass will help reduce heat transfer in summer but equally blocks the valuable solar heat gain in winter.  Once the sun is off the glass the double glazing effectively insulates your home from heating up in summer and prevents heat loss in winter.

Are double glazed windows very expensive?

Double glazed is more expensive than single glazed windows. However, the initial cost should be offset against savings made by reduced energy required to heat and cool your home and the subsequent carbon emissions. Also, double glazing can negate the need for expensive heavy duty curtains. Double glazing will adding capital value to your home. It’s good all round – all round the home and all round the year!

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