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Establishing a precise window specification will ensure tenders for the fenestration package are compliant with Part L – but don't rely on the tables in the approved documents, warns Alex Smith. The moral is: if in doubt, consult your manufacturer

U-value variations
The U-value required for timber and PVCu windows (2.0 W/m²°C) in England and Wales is less than that required for metal-framed windows (2.2 W/m²°C). This difference takes into account the improvements in solar gain which smaller metal frames can provide by increasing daylight areas for a given hole in the wall.

Part L: handle with care
Don't rely solely on Tables A2 and A3 in the approved documents Part L1 and L2. These are indicative U-value tables and should only be used for guidance. They do not take into account the shape, size and type of windows, and can be misleading. Use these with caution in the early design stages, but do specific calculations for the project to establish the actual heat loss/U-value for the windows, before going out to tender. If in doubt, consult your window manufacturer for advice.

Weighted area average method
Standard U-values for doors, window and rooflights can be exceeded using the elemental method, provided that the average U-value of these elements does not exceed the standard U-values in table one, section one of Part L1 and L2. When using this weighted area average method, remember:

The size and shape of windows, along with the number and configuration of opening vents, will affect the U-value performance. Larger windows and fixed lights, followed by open-in windows, generally perform best.

There is scope for a trade-off between cost and efficiency of frames and glass specifications. To tailor the project to fit your budget, use a narrower module frame with high-specification glass or vice versa.

Establish a clear specification of the above points before going out to tender. This will ensure consistent and Part L-compliant bids for the fenestration package.

Whole building method
Consider using the whole building method to calculate heat loss and gain if all or part of individual fenestration design cannot produce a weighted area average below 2.2 W/m²°C and within budget. You may be able to maintain your chosen fenestration design if the fabric as a whole complies with Part L.

Installation details
Pay attention to the installation details, which are critical to thermal performance. The perimeter of windows includes all the interfaces and fixings at sills, into cladding and head details, which must be carefully considered to ensure there is no thermal bridging or air leakage.

Minding the gaps
Double-glazed windows, doors and rooflights with a 6 mm gap between panes generally won't satisfy the new regulation if the designer uses the elemental method of calculation. Window and door units with 12 mm and 16 mm gaps between panes will be the norm.

Low-E glazing
"Traditional" air-filled or argon-filled double-glazed windows will no longer satisfy the elemental U-value requirements of Part L of the Building Regulations unless they incorporate low-emissivity glass.