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Further guidance in the Building Research Establishment ’ s BR 443 [ 2 ] tells us that U-values for roof windows and rooflights are usually quoted for the in the vertical plane . This allows comparison of different products that could be used at different inclinations . However , for the purposes for calculating heat losses from buildings , U-values should relate to the plane of the component as installed in the building .
This can be done by calculating the U-value of rooflights and roof windows , allowing for the angle of the roof in respect of both surface resistances and gas space resistances . Alternatively , the following adjustments can be made to U-values assessed for the component in the vertical plane :
Inclination of roof U-value adjustment ( W / m 2 k )
Twin skin or double glazed
Triple skin or triple glazed
70 ° or more ( treated as vertical ) 0.0 0.0 < 70 ° and > 60 ° + 0.2 + 0.1 ≤ 60 ° and > 40 ° + 0.3 + 0.2 ≤ 40 ° and > 20 ° + 0.4 + 0.2 ≤ 20 ° ( treated as horizontal ) + 0.5 + 0.3
Figure 1 U-value adjustment table Conventions for U-value calculations ( 2006 Edition ) Brian Anderson BRE Scotland
WHAT IS THE BEST U-VALUE THAT YOU ARE LIKELY TO ACHIEVE ?
It is important to understand that there is much ambiguity surrounding rooflights , skylights and roof windows and the U-values quoted by various manufacturers . The much-coveted U-value figure has become a powerful sales tool for those claiming to be ‘ the best performing ’ or ‘ most thermally efficient ’ rooflight on the market and you will often see figures advertised that are misleading at best , at worst , completely untrue . Some companies might even try and confuse you by quoting the centre pane figure for the glass , rather than a whole frame U-value , which is the legal requirement . In any case , caution is advised when researching the various products available and you should always request evidence to support the thermal performance claims made .
U-value calculations for roof windows and rooflights must be undertaken by an approved UKAS accredited product certification agency , who will calculate accurate thermal performance figures based on the individual make up of components in the product ’ s construction .
Given that Stella use only the finest quality materials in our manufacturing process , from the highest specification glazing , to our high quality handmade hardwood liners , through to the best quality rubbers and seals , we are confident in claiming that our roof windows are among the best performing on the market , with our triple glazed , fixed product achieving a U-value of 1.1 W ( m 2 K ).
A HUGE CAVEAT FOR CONSERVATION PROPERTIES – EXEMPTIONS FOR HISTORIC AND TRADITIONAL DWELLINGS
While it is not always the case , it stands to reason that the majority of conservation rooflights are installed in traditional properties , barn conversions , and Listed buildings . If your project involves such a property , you may find that there are exemptions to these new regulations .
The Approved Document states that the energy efficiency of historic and traditional dwellings should be improved only if doing so will not cause long-term deterioration of the building ’ s fabric or fittings .
New extensions to historical and traditional dwellings should comply fully with the energy efficiency standards , unless there is a need to match the external appearance or character of the extension to that of the host building . In which case , the work should comply with standards in the approved document to the extent that it is reasonably practicable .
In determining whether full energy efficiency improvements should be made , the building control body should consider the advice of the local authority ’ s conservation officer .
Additional guidance is available in Historic England ’ s Energy Efficiency and Historic Buildings : Application of Part L of the Building Regulations to Historic and Traditionally Constructed Buildings . [ 3 ]
If you would like to know more about the rooflights , roof windows and thermal performance contact Stella Rooflight on 01794 745445 , email infor @ stellarooflight . co . uk or visit www . stellarooflight . co . uk
[ 3 ] https :// historicengland . org . uk / images-books / publications / energy-efficiency-historic-buildings-ptl / heag014-energy-efficiency-partll /
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