U Values, Part L (Building Regulations) and RdSAP







Article submitted to RICS Building Control Journal – January 2009

With an ever greater requirement to improve the thermal performance of domestic dwellings as a direct result of the European Directive on the Energy Performance of Buildings in 2002, Building Regulations were extended in 2006. The Conservation of Fuel and Power Part L1 (dwellings) was split into Part L 1A for new builds and Part L 1B for existing buildings. Until this change the thatching industry remained pretty much outside the scope of Building Regulations except for new builds and obligations under Part B - Fire Safety (The Dorset Model page 50, item 10.9). When Part L was being upgraded the Thatching Industry not consulted. There are a further 3 revisions of building regulations due prior to 2016, I hope that the thatching industry will be consulted this time for the sake of Britain's thatching heritage.

Thatch: There are two different thatching materials used in three different thatching methods. Namely:- Combed Wheat straw, Long Straw (threshed wheat straw) and Water Reed. There are major regional differences in roof pitch, roof shape and depth of thatch. Roof pitches vary from 45 - 60 degrees, the roof shape (eaves to ridge) changes from straight and angular in the eastern and midland counties to rounded in the westcountry and the depth of thatch goes from 300mm to over 2 metres thick. Often the traditional thatched cottage will be a one and a half story structure with sloping ceilings - now classified as "a room in the roof".

These houses were constructed from local and available materials, they were not built to be energy efficient, however thatch is a natural product and it provides very good insulation, and the thatched cottage is warm in winter and cool in summer.

The process of re thatching is one of general maintenance and will happen many times within the life of a thatched roof. It involves removal of some or all of the old coat of thatch because it has worn down to the fixings. (Amount of removal depends on local accepted practices and the condition of the timber roof frame). The laying of a new coat of thatch increases the depth of thatch by between 300mm to 480mm and therefore automatically improves the thermal performance of the roof. The total depth of thatch from the outside surface right through to the rafters makes up the thermal element of the roof, and not just the new coat on top. With such a diverse selection of criteria spread across the 60,000 or so thatched properties in the country common ground has to be found to assess the energy efficiency of thatch.

U Values: The "U" Value is a measure of how much heat will pass through a square metre of a structure when the air temperatures on either side differ by one degree. U values are expressed in units of Watts per square metre per degree of temperature. The lower the U value means a lower transfer of heat through a thermal element.

Part L 1: Requires that a pitched roof thermal element should have a Target Level of 0.2w/m2k, where an existing roof exceeds the Threshold Level of 0.35w/m2k, the requirement is to upgrade the roof to a maximum of 0.25w/m2k.

Part L 1A: New builds and new extensions. Using Elmhurst software a 450mm layer of thatch, with main construction and timber battens and a 15mm asbestos cement fire board produces a U value of 0.15w/m2k. (The fire board is usually Calcium silicate but this is not on the program).

Part L 1B: Existing buildings. The CIBSE Guide advises that the thermal conductivity of water reed is 0.09w/m2k and wheat straw is 0.07w/m2k. To achieve a U value of 0.2w/m2k for a roof made of Wheat Straw it will require a depth of 350mm and 450mm for Water Reed.

When to apply for a Building Notice for a re-thatch: The process of rethatching will improve the thermal efficiency of the roof and in many cases the existing thickness of the thatch prior to rethatching will exceed a U value of 0.2w/m2k.

If less than 25% of the roof area is to be rethatched and the existing U value is less than 0.35w/m2k (deeper than 350mm for wheat straw and 450mm for water reed) and a like for like thickness of thatch is being replaced, then there should be no need to consult Building Control.

If more than 25% of the roof area is to be rethatched and the existing U value is more than 0.35w/m2k, after having subtracted the U value of any existing loft insulation, then both the Conservation Officer and Building Control should be consulted. The U value of the roof will have to be upgraded to 0.25w/m2k.

Risks of improving a U Value: It is possible to reduce the overall U value by either increasing the depth of thatch or topping up loft insulation, BUT both of these carry risk.

By increasing the depth of thatch additional weight will be added to the roof frame, both the traditional roof shape and local thatching specifications could be lost and chimneys may have to be raised. These would all require Listed Building consent and changes would be detrimental to the local and historical character of the building.

By increasing the depth of loft insulation there is an increased risk of damage to original ceilings, a reduction in the breathability of both ceilings and thatch (where insulation is between rafters,) and an increase in the risk of interstitial condensation, again detrimental to the character of the building.

Exemptions - Listed and historic buildings: There are exemptions for historic and listed buildings where work carried out in order to meet Part L1B requirements would be detrimental to the appearance, character or breathability of the building. Character includes both internal and external features and the building does not necessarily have to be listed. The Conservation Officer can over ride Building Control to protect Listed and Historic Buildings.
There will be some buildings where it will not be possible to meet the requirements in any way without loss of historical character.

Energy Performance Certificates, RdSAP and Thatch: The 9.82 RdSAP update that was released in September 2008 included a new methodology for Thatched roofs. Where irrespective of the thatching materials used and the thickness of existing thatch a U value of 0.35w/m2k is applied to all existing roofs where there is an horizontal ceiling and a U value of 0.25w/m2k is applied to all existing roofs with a sloping ceiling (room in the roof). There are recommendations against the suitability of solar panels in thatched roofs and against both the installation of loft insulation or the topping up of existing loft insulation.

This is compiled from a talk given by Charles Chalcraft at the 2008 Annual Conference of The National Society of Master Thatchers and some additional material.


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