Meeting Part L 2010

If you’ve been thinking about designing your property with an eye on minimizing its environmental impact, you may well be aware Part L of the building regulations (which deals with thermal efficiency) has this year been updated. In fact the revisions come into force as of October 1st 2010, which creates some fresh challenges and opportunities for our private clients and residential developers.

This is an area we take very seriously at Granit and we encourage all of our clients to meet and where possible go beyond the minimum requirements.

With the update to Part L just around the corner, our environmental consultant Richard Bendy wrote the following letter which gives a clear overview of the changes and what this means for Granit’s clients. Richard runs consultancy Natural Action.

We thought this was too good to keep to ourselves and you can find the entire letter below. If you would like to know more about how to ensure your home is thermally efficient and therefore keeping your carbon footprint to a minimum, we of course are happy to help.



Dear All

I’m sure you’re all aware that the new Part L comes in on October 1st 2010 so I thought I’d write and give you a very brief overview of the changes from Part L 2006. In addition, with three weeks to go, what the changeover rules are as you may have some clients who wish to take advantage of the older Part L.

Dwellings can be built to Part L 2006 if:

1: Work has already started on site before October 2010

2: Work done under a competent persons scheme (new boiler/windows/electrics etc) had a contract entered into before 1st October 2010 and work commences before 6th April 2011

3: A Building notice, Full plans Application or initial notice is submitted before 1st October 2010 and work on site starts before 1st October 2011

The aim of Part L1 2010 is to give 25% improvement in carbon emissions over Part L 1 2006. This gives a 40% improvement over Part L 2002. It uses the outputs of SAP 2009 instead of SAP2005 to show compliance.

I’m happy to give you more detailed advice for individual projects but in general the main changes from Part L1a 2006, which you need to bear in mind right from the beginning of the design stage, are:

1: The Design Stage SAP calculations which were optional under Part L 2006 are now a legal requirement and they have to be submitted to a Building Control Officer at least the day before work starts on site. To repeat, this is a legal requirement!

2: As insulation standards improve thermal bridges become a much greater part of overall heat-loss, so now instead of being able to put in a default of 0.08 for thermal bridging if using Accredited Construction Details, all non repeating thermal bridges (wall corners, roof wall junction, floor wall junction, party walls, lintels etc) have to be individually calculated as part of the SAP calculations.

With thermal bridges making significant difference to carbon emissions in a Part L 2010 dwelling, care needs to be taken in the design to reduce use of elements such as solid-steel lintels (insulated ones are much better), flat roofs with parapets, gable ends with roof insulation at ceiling level (at rafter level is much, much better) and use the best details possible for floor wall junctions as these are the worst thermal bridge offenders.

If ACD or Enhanced Construction Details are not used, there are substantial carbon penalties added to the calculation and likewise if quality control in their use and construction cannot be proved there are also substantial carbon penalties. This is therefore an area to take quite seriously.

3: Party Walls are now taken into account as it’s been discovered that they have heat loss as well. To get a party wall to a zero U value, as was just assumed previously, is quite difficult and needs to be designed in right from the start. There is a lack of ACD/ECD party wall details at the moment but these should be available very soon.

4: Thermal mass is being taken into account for the first time as well. This is a little understood phenomenon however, in brief, in a poorly insulated dwelling you don’t want much thermal mass but in a highly insulated one you do because used intelligently it can give significant benefits in heating and cooling. With a dwelling just passing Part L 2010 however it has an effect but it is not that great.

5: The minimum air tightness requirement of 10m3/hm2 requirement remains the same unless building a development of more than two houses, however reducing air permeability is a very good way of reducing carbon emissions (see conclusion below).

6: Whereas under Part L 2006 secondary heating was assumed to be electric (which added considerably to the carbon emissions) under Part L 2010, only secondary heating actually specified will be used in the calculations; therefore anything burning wood in a controlled manner will help but gas points and gas fires will make the DER worse.

7: All low energy lighting whether in dedicated or non dedicated fittings will now be taken into account; there is a plethora of excellent low energy lighting now available, including LEDs which actually give a warm light so this is something worth talking to your clients about at an early stage.

8: Heat loss from swimming pools must now be limited with minimum U value of their walls being 0.25

9: The worst possible area-weighted averages of all elements of that type have been reduced to:

Roof -0.2 (was 0.25)
Wall – 0.3 (was 0.35)
Floor – 0.25 (stays same)
Party Wall – 0.2 (wasn’t in previous Part L)
Windows & doors of all types – 2.0 (was 2.2)

In conclusion having done a few examples with the new software, all these changes come together to make it quite difficult to get a dwelling designed using the standard rule of thumb U values above and built using current building methods to pass on its own. To do this usually requires some form of renewable energy.

Now may be a good time therefore, instead of designing to just pass the Part L regulations by tacking on some solar panels, to instead take your clients in the direction of a building fabric designed to a much higher spec right from the start. This will save them money in the long run.

Likewise should you have any clients who are wanting to go well beyond compliance to Passivhaus standard or similar I am now qualified to carry out the necessary calculations using PHPP (Passivhaus Planning Package) and to give the necessary design advice in this area. This applies to both new build and refurbishment.

There has not been much change to Part L1b (extension/refurbishment) except a general tightening of required elemental standards, a small change in rules for conservatories and the addition of swimming pools to the regulations.

Roof -0.18 pitched, 0.16 flat (was 0.20 pitched, 0.16 flat )
Wall – 0.28 (was 0.30)
Floor – 0.22 (stays same)
Party Wall – 0.2 (wasn’t in previous Part L)
Windows 1.6 (1.8)
Doors – 2.0 (was 2.2)<

In practice for Granit extension designs, which tend to have large areas of glazing, this means that the 2x SAP calculation method is even more likely to be the best method of ensuring compliance with Part L1b.

I think that’s about it, I hope it helps demystify the whole thing a bit and do let me know if you’ve got any questions.

Best wishes


Richard Bendy
Natural Action