Frequently Asked Questions - Thermal Insulation

1 What effect does gas filling have on the heat efficiency?

The standard gas in insulating glass units is dry air. The space between the glass panes can be filled with other gases that have different properties that improve the thermal resistivity. Argon and krypton are the most likely alternatives. Krypton is beneficial if the space is limited. Typically argon reduces the U value by approximately 0.2 W/m2 K depending on the cavity size.

2 What is a WER?

A Window Energy Rating (WER) is measurement and category for the efficiency of a window system as a whole. It considers the frame, the centre pane U value, the solar transmission of the glass, the effect of the spacer around the edge of an insulating glass unit and the air permeability of the window. It can be used to demonstrate compliance with the building regulations for energy efficiency and the simple code allows consumers to know the relative efficiency between products.

3 Is triple glazing better?

Triple glazing can be used to create more opportunities for solar and thermal control but it is heavier, thicker and offers little in terms of acoustic performance. In most cases adequate performance can be achieved with double glazing.

4 What U value should I specify?

The building regulations require U values for the complete window and not the centre pane U value for the glass. Generally good timber or PVCu frames with a centre pane U value of 1.7 W/m2 K for the glass will suffice but you will need to liaise with the window manufacturer to be precise.

5 Where can I get more information on the regulations?

The Communities and Local Government website has a lot of freely available information, click here to view. Communities and Local Government was formerly known as The Office of the Deputy Prime Minister (ODPM)

6 What does energy balance mean?

Energy balance is the measurement of the efficiency of the window as a whole. The solar gain through the glass is measured as a positive contribution and the heat losses as a negative. The net result is the effective U value for the window. This is a far more realistic way of looking at the contribution of glass to a building and given the right choice of products the window can be energy neutral i.e. it lets as much heat in as it loses over the year for a given location.

7 How do I control condensation?

Condensation forms when moisture laden air meets a cold surface where the surface temperature is at or below the dew point. At the dew point moisture will condense from the air. Warm air will hold more moisture than cold air. Keeping the air temperature up changes the dew point and keeping the humidity down. Ventilation helps to keep the humidity down and fitting low e glass raises the internal glass temperature so condensation is less likely to occur. Low e glass also reduces the outer pane temperature which can lead to external condensation on the glass. External condensation is less of a problem for windows and can be seen as evidence that the glass is providing thermal benefit.

8 What is the difference between a soft coat and a hard coat?

Soft coats are applied off-line i.e. away from the float line where the glass is produced. Chemicals are deposited on the glass under controlled conditions and they are often vulnerable to damage and oxidisation once made. The glass is always used as part of an insulating glass unit. Hard coats are more durable and made in line with the float glass manufacturing process. The coatings are applied whilst the glass is still hot and bonds to the surface. Pilkington Activ™ and Pilkington K Glass™ are examples of hard coats. Pilkington Suncool™ and Pilkington Optitherm™S4 are soft coats.