What are U-Values?
The U-Value rating or 'overall heat transfer coefficient' measures the total amount of heat that
transfers through a buildings fabric. Each element of a building has it's own U-Value rating whether
that be the brick/stone walls, roofing or building openings i.e. windows/doors.
Because the construction of any element can differ so much depending on the design and materials
used, the U-Values can vary too and as a result each element needs to be individually calculated. In
practise, ideally the manufacturers used will give guidance to what their product achieves when
coming to heat transfer.
The units used to express U-Values are watts per m2 Kelvin (W/m2K). To explain this, Bereco's
window U-Value is 1.37(W/m2K) which means for every degree of a temperature difference between
the air on the surface inside the window and air on the surface outside. 1.37 watts of heat would
pass through any m2.
The current and future Building Regulations are now striving to make new homes more energy
efficient and U-Values are a big part of this. The lower the U-Value, the slower heat will transfer
through an element and therefore the element will possess better insulation properties, keeping
house hold bills (heating costs) to a minimum.
Bereco Standard Ratings.
Both the glass and frame are taken into consideration when calculating the U-Value for any of the
Bereco's products. Bereco offer excellent U-Value rates on all our products (whole unit):
- Casement - 1.37W/m2K
- Sliding Sash - 1.37W/m2K
- Tilt & Turn - 1.37W/m2K
- Flush Entrance Doors - 1.57W/m2K
- French Doors - 1.57W/m2K
The above U-Values are achieved with standard soft coat low 'E' argon filled double glazed units,
giving the Bereco product a centre pane U-Value of 1.0W/m2K. The Low-E coating is microscopically
thin and has optically transparent layers of silver, sandwiched between layers of metal oxide coating.
This specific process is known as 'sputter coating' and is also referred to as soft coating.
The Low-E coating allows most natural light to enter freely but reflects a significant portion of shortwave
heat energy. In summer, long-wave heat energy radiating from objects is reflected back
outside, lowering cooling costs. In contrast, in winter, internal long-wave heat energy is reflected
back inside, thus lowering heating costs.
In relation to the Low-E coating the double glazed units are gas filled to improve the glazing
properties further. Filling the pane space with gas minimizes the convection current within the
space, as the gas achieves excellent insulation levels. As a result the glazing will achieve an overall
reduction of heat transfer between the internal and external areas of the property. As standard
argon gas is used in Bereco's double glazed units achieving the above U-Values, however depending
on your project you may need to achieve a lower U-Value and Bereco offer a range of solutions to
increase your fabrics insulation efficiency.
Warm Edge Spacers.
Thermal bridging is a major factor to losing heat through windows/doors and can be found inside
insulating glass units which incorporates traditional metallic spacers. Bereco offer a range of warm
edge spacers that have a thermal conductivity of 0.19W/m2K, which is 1,052 times less than the
thermal conductivity of aluminium.
This recent generation of insulating glass spacers, made of engineered plastics, thermally break the
insulating glass edge and create a 'warm edge', reducing condensation and keeping more heat inside
Heat transmission through the centre of an insulating glass unit happens in 3 ways:
- Radiation: this transport mechanism can be reduced with Low E-coatings to the glazing.
- Thermal Conductivity: this can be diminished by reducing the thermal conductivity pf the gas
fill, by the use of noble gases like argon or krypton of xenon.
- Convection: this reduced to a minimum by optimisation of the cavity.
Krypton Gas Filled Glazing Units with Warm Edge Spacers.
When purchasing your Bereco product you may want to consider Krypton gas filled double glazed
units. Krypton gas has many excellent characteristics, one been sound insulation. Because the gas is
dense and heavy the acoustic properties are much more effective than air or even argon filled
Another property that Krypton gas increases is thermal conductivity. Due to the dense and heavy
particles within the gas a further reduction of transferred heat is achieved in comparison to the
standard argon filled units. The Krypton gas performs more effectively in narrow cavities (approx 8-
10mm), just like in Bereco's Heritage Range. For this reason Krypton gas is often used in
conservation areas and listed buildings where the appearance of double glazing needs to be kept to
In utilising Krypton as the filling gas and using warm edge spacer technology for unit spacers, the UValue
can be reduced to 1.25W/m2K (centre pane U-Value of 1.1W/m2K of glass). Bereco realised the
advantages of this technology and have been offering this on all products for the past 4 years. Now
with the current and future Building Regulations becoming increasingly more devoted to energy
efficiency it is something that everyone should consider.