New Build Extensions

Newly built houses must meet the energy targets in the Code for Sustainable Homes. This uses a SAP process to calculate the carbon emissions of the whole house.

Air permeability and thermal bridging

From 2010, private sector houses must meet Code level 3 and public sector houses Code level 4. In 2013 this changes, and private sector houses must meet Code level 4 and public sector houses Code level 6. From 2016 all new houses must reach Code level 6. The U-values of each element of the building are flexible, provided the whole house meets the standard, but the U-values need to comply will be typically:

Code level   Pitched roof   Air permeability   Wall   Thermal 
3   0.18   0.24   Reduced   Reduced
4   0.16   0.20   Reduced   Reduced
6   0.16   0.16   Maximum possible reduction   Maximum possible reduction

TLX Silver helps to meet the required U-values when used in conjunction with other insulation such as mineral fibre or rigid board. TLX Silver is particularly effective at reducing air permeability by sealing across timber studs and rafters and by wrapping around junctions.

Pitched roofs

A breathable underlay is fitted above rafters. PIR or mineral fibre between the rafters can go almost up to the top of the rafter space. TLX Silver is fitted below rafters, battened and plasterboard added.

Pitched roof solutions

An alternative method for insulating a new pitched roof is to use a layer of PIR beneath rafters, PIR between rafters and TLX Gold insulating breather membrane above rafters.


Timber frame walls are ideal for insulation with TLX Silver. Rigid or fibrous insulation is fitted between the studs. TLX Silver across the front of the studs provides thermal insulation and a very effective air and water vapour barrier. A batten forms a service cavity before plasterboard is fitted.

Wall solutions


Extensions are treated in a similar way.

A pitched roof must be constructed with an elemental U-value of 0.28 Wm².K, a flat roof and a pitched roof with a sloping ceiling must have U = 0.18 Wm².K.

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