Part L changes ramp up the need for ‘fabric first’

With the revisions to the Part L regulations now in force and a list of 15 urgent performance gap reducing actions recently identified by the Zero Carbon Hub, the emphasis on fabric first is being stepped up. Thomas Wiedmer, architect and technical manager at Actis, the first company to dual test its insulation products to address both thermal bridging and the performance gap, explains why compliance needn’t prove onerous to today’s builder.

Changes to Part L are aimed at ensuring an overall 6% carbon reduction across the build mix compared with current regulations. The introduction of the Target Fabric Energy Efficiency (TFEE) rate, alongside the existing CO2 emission rate, strengthens U-value requirements for domestic new builds. Compliance will therefore be predominantly down to the nature of the insulation used in the construction.

The revisions state: “Insulation should be reasonably continuous over the whole building envelope. The building fabric should be constructed so that there are no reasonably avoidable thermal bridges in the insulation layers caused by gaps within the various elements, at the joints between elements such as those around the window and door openings. Reduction in thermal performance can occur where the air barrier and the insulation layer are not contiguous and the cavity between them is subject to air movement.

Among the 15 areas for urgent action highlighted in The Zero Carbon Hub’s report, Closing the Gap between Design and As-Built Performance, those which refer specifically to the thermal efficiency of a building are:
* addressing unrealistic calculation assumptions for both fabric heat loss (U-values) and thermal bridging (Psi-values) .
* clarifying modelling conventions and calculation of U-values, thermal mass, and thermal bridging
* improving construction of the building fabric, typically cavity wall insulation and eaves to wall junction insulation, which when incorrectly installed, reduce the actual performance of the thermal envelope.
In the summer the Hub will produce a further report on how to reduce the gap and move towards its aim of ensuring that by 2020 90% of new homes perform in real life as calculations predict.

The Actis Hybrid range, specifically designed to perform in real life as it does in the lab in order to address the performance gap, includes a new insulation material, Hybris, a vapour control layer with built-in thermal performance, HControl Hybrid, and an insulating breather membrane, Boost‘R Hybrid.

All three products combine both insulation and air tightness properties, with HControl Hybrid and Boost‘R Hybrid in particular addressing the thermal bridging and air tightness issues.

Using just 45mm of Hybris along with HControl Hybrid gives a U value of 0.22. Increasing the Hybris thickness to 90mm reduces the U value to 0.17 in roofs and 0.18 in walls. Throw Boost R Hybrid into the mix and the U values can reduce further. Even just using Boost‘R Hybrid and HControl Hybrid without the ‘official’ insulation give a U-value of 0.20 in walls – better than required under current building regulations.

Not only do the materials address the performance gap and thermal bridging concerns, but their thinness and lightness makes them easy to transport in a van and manhandle up and down loft ladders. All are also quick, clean and easy to install, with no need for respiratory masks as the material contains no irritants and does not produce dust.

Because they are compressible and flexible, if installers cut the pieces slightly too large they can squeeze the material in between the rafters. There are no gaps as there could be if cutting a solid piece of insulation to fit an awkward shape – and no odd remnants which can’t be used for anything else. You can use the whole roll which means wastage is minimal.

A free online simulator at enables specifiers to play around with different combination to estimate the likely U-value.

Try the simulator here!

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