Part L “Conservation of Fuel and Power”

The recently published revision to the Approved Document changes the approach to providing energy efficient buildings. The required overall performance is now the product of a calculation method developed by the BRE which considers all the various elements that contribute including up stream energy source, heating and ventilation efficiencies, solar gain, and insulation of elements amongst others. In addition there are minimum structural element U values that cannot be reduced. Given this scenario the REQUIRED U value for an external wall, for example, cannot be determined until the calculations using all the contributing elements are complete. Experience to date suggests that external wall values tend to be between 0.25 and 0.3 W/sqmK.

Prestoplan recommends a 140 mm stud solution for external walls. This provides a number of options depending on the exact U value required as the examples below illustrate.

Notes

1. Wall thickness assumes a masonry clad wall with 50mm cavity but excludes plasterboard.

2. Glidevale TF200 Thermo breather membrane is a reflective membrane with the required vapour permeability

3. Glidevale Protect VC foil is a strong impermeable refective foil membrane which requires the internal cavity provided by the batten to contribute to the thermal characteristics of the wall. This replaces the usual polythene vapour check layer

It should be noted that Prestoplan buildings incorporating PresWeb joist floors have a lower timber fraction within the external walls due to the omission of solid timber lintols directly above openings. This allows the use of insulation to be extended into areas previously occupied by structural members.

Air Tightness

Part L now includes specific recommendations in connection with air leakage (ie the uncontrolled and undesigned movement of air through the building's fabric). There are two options available which mimic the approach adopted by Part E i.e. testing or adopt "Accredited Details". The UKTFA has assembled a significant body of test evidence of buildings built to normal standards without any specific air leakage prevention methods, e.g. seals around panels. This evidence demonstrates that timber framed construction regularly achieves between 6 and 8 air changes at 50 pascals which is significantly better that the minimum performance of 10 air changes required by the AD. Your attention is drawn to the introductory paragraph in Approved Document F which states that ventilation designed in accordance with this AD is only valid for buildings with air leakage coefficients of 3 or more.

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