Part L1B of the Building Regulations – Extensions

Part L1B of the Building Regulations – Extensions

Under the new Part L1B 2021 for Extensions, Alterations & Conversions, there are some key changes you should really be made aware of.

Below is a summary table for the changes for new extensions for extensions with an initial notice lodged after 15th June 2022:

The general improvements are standard,. But the external wall stands out as a challenge because the planning drawings may be drawn with a 300mm cavity wall.

The original U value for Part L1A 2013 was 0.28 W/m2.K. Typically, a 100mm cavity within a cavity wall would achieve this.

Under the new Part L1B 2021 regulations, this will need a 35% improvement to achieve 0.18 W/m2.K.

Typically, you should allow a 150mm cavity to achieve this.

Some options are presented below with cavity width ranging from 100-150mm:

OPTION 1 – FULL FILL MINERAL WOOL WITH AIRCRETE BLOCKWORK – 150MM CAVITY

• 102.5mm brickwork
• 150mm cavity with;
• 150mm Knauf Dritter 32 insulation (0.032 W/mK)
• 100mm aircrete blockwork (0.15-0.18 W/mK);
• 12.5mm plasterboard on dabs

OPTION 2 – PARTIAL FILL PIR INSULATION WITH LIGHT MEDIUM DENSE BLOCKWORK – 150MM CAVITY

• 102.5mm brickwork
• 150mm cavity with;
• 100mm Celotex CW4000 insulation (0.022 W/mK)
• 100mm light medium dense blockwork (0.49 W/mK);
• 12.5mm plasterboard on dabs

OPTION 3 – PARTIAL FILL PIR INSULATION WITH AIRCRETE BLOCKWORK – 135MM CAVITY

• 102.5mm brickwork
• 135mm cavity with;
• 85mm Celotex CW4000 insulation (0.022 W/mK)
• 100mm aircrete blockwork (0.15-0.19 W/mK);
• 12.5mm plasterboard on dabs

OPTION 4 – PARTIAL FILL PIR INSULATION WITH BLOCKWORK – 125MM CAVITY

• 102.5mm brickwork
• 125mm cavity with;
• 75mm Kingspan Kooltherm K108 Cavity Board insulation (0.018 W/mK)
• 100mm aircrete blockwork (0.15-0.51 W/mK);
• 12.5mm plasterboard on dabs

OPTION 5 – FULL FILL PIR INSULATION WITH AIRCRETE BLOCKWORK – 100MM CAVITY

• 102.5mm brickwork
• 100mm cavity with;
• 100mm Xtartherm CavithTherm (0.021 W/mK) OR 90mm Kingspan Kooltherm K106 Cavity Board insulation (0.018 W/mK)
• 100mm aircrete blockwork (0.15 W/mK);
• 12.5mm plasterboard on dabs

For other elements, please see below for typical build up advised to achieve the target U values:

ROOF INSULATION – CEILING LEVEL = 0.15 W/m2.K

• 100mm Loft Roll 44 between ceiling joists and 200mm Loft Roll 44 over ceiling joists

ROOF INSULATION – SLOPING LEVEL = 0.15 W/m2.K

• Assumes Minimum 150mm deep rafters at 600mm centres (if 400-450mm centres, then increase insulation);
• Minimum 50mm ventilated air gap over insulation;
• 100mm Celotex GA4000 between rafters;
• Underline in 77.5mm Celotex PL4065 insualted plasterboard (65mm PIR; VCL; 12.5mm plasterboard)

ROOF INSULATION – FLAT ROOF LEVEL = 0.15 W/m2.K

• Roof finish
• 150mm Roof grade PIR insulation (such as Kingspan TR27) (0.024 W/mK);
• Vapour Control layer;
• WPB Plywood;
• Joists;
• Plasterboard

GROUND FLOOR = 0.18 W/m2.K

• Assumes P/A of 0.60 or below
• 50-75mm screed
• Vapour control layer;
• 100mm Celotex GA4000 PIR Insulation (0.022 W/mK). Ensure 25mm Celotex upstand at screed perimeter
• 100mm concrete slab
• DPM

There are other options to achieve the target U values but these build up are an initial guide to help you consider the construction.