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U-Value....What is that worth?

U-Values can be worth a lot to you! If you have good one's.

In RdSAP (Reduced Data Standard Assessment Procedure) The software used to create an Energy Performance Certificate (EPC). U-Values are applied to the walls, roof, windows, floors and doors. If you house has proper insulation lets say in the loft space and the walls and you have double glazed windows and UPVC doors, you will have good U-Values.

If your property is Thermally fit, you will save money on your heating bills with either gas, renewable or electric heat.

So what exactly is a U-Value?

U-value, or thermal transmittance (reciprocal of R-value)

Thermal transmittance, also known as U-value, is the rate of transfer of heat through a structure (which can be a single material or a composite), divided by the difference in temperature across that structure. The units of measurement are W/m²K. The better-insulated a structure is, the lower the U-value will be. Workmanship and installation standards can strongly affect the thermal transmittance. If insulation is fitted poorly, with gaps and cold bridges, (an example of a cold bridge would be if they fit loft insulation between the joist and left the top of the joist exposed. This is why now they lay the insulation in between and over the top of the joists.) then the thermal transmittance can be considerably higher than desired. Thermal transmittance takes heat loss due to conduction, convection and radiation into account.

Although the main focus of environmental performance of buildings is now on carbon usage, there is still a need to consider thermal performance of the building fabric as a contributing factor. Thermal performance is measured in terms of heat loss, and is commonly expressed in the construction industry as a U-value or R-value. U-value calculations will invariably be required when establishing construction strategies. A number of the terms have subtly similar meanings, and conflicting interpretations can be found across the internet.

So U-Values are important, you want the lowest U-Value you can get. Get your cavity walls filled and your loft insulated! By having good double glazed windows, this allows for a lower U-Value and a higher G-Value.


The coefficient of the permeability of total solar radiation energy stated as %Composed of the direct transmission of energy and the secondary dispensation of heat of the glazed surface toward the interior (solar gain) or passive solar gain, which occurs on the basis of absorbed solar rays. ( When the sun shines in your window) The sun can heat a room only by glass and thus also contribute to heating without any additional expenses What is welcomed in winter can be unpleasant in summer, because gaining energy through solar radiation means heat The correct course is high thermal insulation, which means a low U value combined with a G value that is not too high The technological standard has become insulating glass with a plated layer Formed by at least two panes of glass, one of which is coated with an extremely thin layer of high-grade metal, which radically reduces thermal losses.

In the UK we want good high G-Values however in places that are hot for example Spain the would not want a high G-Value as they want to keep the heat out of their properties.

G-value (sometimes also called a Solar Factor or Total Solar Energy Transmittance) is the coefficient commonly used in Europe to measure the solar energy transmittance of windows. Despite having minor differences in modelling standards compared to the SHGC, the two values are effectively the same. A g-value of 1.0 represents full transmittance of all solar radiation while 0.0 represents a window with no solar energy transmittance. In practice though, most g-values will range between 0.2 and 0.7, with solar control glazing having a g-value of less than 0.5.