u-value

The energy efficiency of a double glazed unit can be compared with other windows using something called the U Value.

This is a calculation into how much heat is retained into any given installation and helps homeowners choose the most efficient panes. When calculating heat loss, the lower the U Value the better the efficiency.

What’s clear is that double glazing has a better U Value than single paned units, so double glazing your conservatory will be a worthwhile project.

By using the U Value, you can individually measure each window in your home and work out where heat is being lost.

And by adopting changes, you can reduce your carbon emissions by 50% and cut your home’s energy bill.

While double glazing has the added benefits of improved security and noise pollution reduction, the U Value only takes heat loss into its calculations.

In Britain the U Value takes into account the whole window frame, including the glass, frame and spacers.

But a U Value is available that only measures the performance of the glass. This is known as the centre-of-glass U Value.

Double glazing companies have been known to only measure the pane’s heat reduction, so ensure you get a calculation for the whole fixture.

These are the general U Values for specific glazing units:

• Single glazing U value = 5.6 w/m2K (Watts / square metre Kelvin)
• Double glazing (with air cavity) U value = 2.8 w/m2K
• Double glazing (with argon gas cavity) U value = 2.6 w/m2K
• Double glazing with low emissivity glass (with air cavity) U value = 1.8 w/m2K
• Double glazing with low emissivity glass (with argon cavity) U value = 1.5 w/m2K.

Then to measure the heat loss, simply calculate the following:
Glass width x Glass height x U Value x Temperature difference from inside to outside = Heat loss per hour (Watts).

While the U Value deals with window installations, the R Value covers other energy efficiency measures, including insulation in walls floors and roofs. The R Value requires a different calculation and only covers properly installed insulation.