Responding to the draft TGD L(2010)


A draft version of the new TGD L(2010) is currently on Public Consultation, which is aimed to achieve 60% reduction on energy consumption from 2005 standards.


It has good points but many weak areas:

·         In general this document does not represent a good transition point on the way between TGD L(2005) & TGD L(2013), given that the latter will be 70% more onerous than the last Regs under which this Industry built many dwellings.

·         There is still far too little guidance on retrofit. This is still predominantly a new-build guidance document, despite the fact that the retrofit market is dominating domestic construction at the moment.

·         Compliance is measured against a ‘Reference Dwelling’ being linked to geometry and perimeter/area ratio. This makes it harder to comply for dwellings with more compact envelopes which is inappropriate (seerelated post).

·         There is no minimum airtightness standard for new extensions.

·         For internally insulated walls, there is no understanding of the fact that low (i.e. ‘good’) U-values are linked to higher risk of unacceptable levels of inter-stitial condensation (see related post).

·         Also, the Glaser method is given as the normal method of assessing inter-stitial condensation, with hygrothermal simulation listed as an option. In fact, hygrothermal simulation is the ONLY assessment method suitable for heavyweight structures (e.g. drylined hollow block).

·         It is increasingly clear that U-values for walls should be differentiated into those suitable for internal insulation, cavity wall insulation and external wall insulation. We believe we’ve already exceeded the limits of sensible values for internal wall insulation, but there is further room for better cavity and external U-values. It’s crazy that only one value is given.

·         We suggest that because U-values lower (i.e. better) than 0.50 W/m²K
are increasingly risky for internal insulation (see related post) it no longer be an acceptable choice for new build dwellings in TGD L (2010), which currently requires performances of 0.21 W/m²K or better for new-build walls.  Obviously as with other ways of building considered to be outside the usual, dependable norm the specifier could continue to use that system IF they were prepared to prove how it makes sense in their exceptional circumstance.  We suggest internal insulation would remain in use for retrofit work with the lower limit of 0.50 W/m²K  suggested.

·         Designers no longer need, nor should be allowed, to use 0.08 W/m²K as a default value for a Y-factor when using ACDs, given the creation of Tables D1-D5. In doing this we would be following what has just happened in the UK. It will now be easy to calculate a much more accurate Y-factor for dwellings that comply with ACDs using these tables. ACDs should also be expanded to allow for a Y-factor of 0.04 W/m²K to help compliance in new build (see related post).

·         The default thermal bridging Y-factor of 0.15 W/m²K is too generous for retrofit, values can be many times worse. We suggest 0.25 W/m²K is a more realistic default value for retrofit. A default Y-factor of 0.15 W/m²K does appear to be reasonable for new dwellings that don’t comply with ACDs and uninsulated existing dwellings (see related post).

·         Given the jump in regulations we are seeing, dwellings which exceed the requirements for compliance by, let’s say more than one energy rating, should have a lower, or no, renewables obligation. They’re spending the money in the right place: on the fabric. Equally, the current situation where even more renewables can be used to ensure compliance of a building with large perimeter or non-optimised fabric, is like a ‘back door’ approach that is neither sustainable nor economic long-term.

Please feel free to look at the various posts on this subject, and if you wish copy and paste or adapt items in your own submission to the Department of Environment. It’s important that as many people in the Construction Industry as possible make clear detailed submissions. We need better Regs.


Send your comments to (closing date is 29th October 2010).


Friday, October 15, 2010