ECO: why it matters where you start from

Over the summer Ofgem issued a ECO2 Cavity Wall Checklist which sets out the new rules of evidence that must be observed when wanting to overwrite the default RdSAP U-value for a wall with a higher U-value. In the consultation document they justify the new evidence requirements on the grounds that

we are concerned that wall U-values for cavity wall insulation measures (CWI) are being overwritten to values that we consider to be unreasonably high for the premises in question, and as a result the calculated savings for a measure are artificially inflated. This could lead to fewer households benefitting under ECO.

So at the heart of this is the claim that higher starting U-values will result in more savings (which may well be unjustified). But, surely if we are assessing the impact of insulation the starting U-value doesn’t matter? Does it?

Strangely, that is the one piece of the explanation which is missing from the consultation and from the final checklist; because while it is true that a given thickness of a given insulation will always have the same thermal resistance, it is not true that the benefit of that insulation will be the same.

Let’s unpack that a bit.

At its simplest, the thermal performance of a wall with regard to heat transfer, can be expressed as a thermal resistance. If, for a moment we ignore thermal bridging, we can calculate the thermal resistance of a wall by adding up the resistances of the layers of which is it composed. A cavity wall made up of an outer leaf of brick and an inner leaf of 100 mm dense aggregate block work with 20 mm of plaster to the inside might have a total resistance of 0.617 m²K/W. A similar wall with an inner leaf of 100 mm aircrete blockwork might have a resistance of 1.090 m²K/W. The wall with the aircrete inner leaf has a higher resistance than the wall with the dense blockwork inner leaf.

That’s fine as far as it goes, but it doesn’t allow us to quantify the important aspect of performance, the rate of heat transfer. For that we need the U-value, which is the reciprocal of the thermal resistance and expresses the rate of heat transfer in watts per metre squared kelvin (W/m²K). As an equation:

$U = \frac{1}{R}$

So our two walls have U-values of 1.620 W/m²K and 0.918 W/m²K respectively.

What happens if we add insulation to each of those walls, say 120 mm of insulation with a thermal conductivity of 0.030 W/mK? We can evaluate that by going back to the thermal resistance in each case. The insulation has a resistance of 4.0 m²K/W, so the wall resistances are now 4.617 m²K/W and 5.090 m²K/W, giving U-values of 0.217 W/m²K and 0.196 W/m²K. But it’s not just the final number we’re interested in, it’s the change. The wall with the dense block inner leaf has gone from 1.620 to 0.217, a drop of 1.403, the other wall went from 0.918 to 0.196, a drop of 0.722. So, although the final U-value is lower in the aircrete wall, the reduction in U-value is much greater in the dense block wall.

You can see that visually on the graph in this post, where the same thickness of insulation added to a high U-value gives a bigger drop than the same amount added to a low U-value. (There is a more detailed discussion of U-values in my book How Buildings Work.)

As ECO is all about savings, a higher starting U-value is better. I took a standard house type I use in SAP training and experimented with the effect on emissions of using the two starting U-values I calculated earlier. The annual carbon dioxide savings are 1,080 kg/CO₂ for insulating the dense block wall and 669 kg/CO₂ for insulating the aircrete block wall. The insulation is the same, but the difference of 411 kg/CO₂ is entirely down to that higher starting U-value.

SAP 2016

The future trajectory of energy efficiency standards was uncertain even before the EU referendum: the abandonment of the expected Part L 2016 changes left an unfilled void. We have no idea what the policy landscape is going to look like, and whether regulation 25B (nearly zero energy buildings) is ever going to come into force. But in the middle of all the uncertainty SAP 2016 is still going to happen1.

SAP, the UK’s domestic energy assessment tool2, has been revised every three or four years since the first version back in 1993, so a revision is due. We are expecting the consultation draft sometime in July, and BRE already have a SAP 2016 page on their web site (there’s not much there at the minute, but we may as well book mark it now).

So what can we expect in SAP 2016? For an authoritative account we will have to wait for the consultation, but we can get some hints from a paper presented at the CIBSE Technical Symposium in April this year by Deborah Morgan and John Henderson. (The paper is also a good summary of the assumptions underlying SAP.)

The paper describes the update cycle for SAP, and in doing so describes four parts of the methodology which were considered for revision. Those being:

• Default values for distribution losses in heat networks. The evaluation of community heating systems must account for distribution losses through the DLF (Distribution Loss Factor). Research on actual performance, and modelling of losses indicates that the current default DLF values are too low. Consequently, higher default values will be introduced, which will give more conservative results, and encourage assessors to use design values or metered data for the system.
• Secondary heating fraction for storage heaters. In dwellings heated by storage heaters a fixed percentage of the space heating requirement is deemed to be provided by another, secondary, heat source. The current value of 15% was thought to be too high, given that the design of storage heaters is based on 10% secondary heating. Re-analysis of an existing data suggested the current default sits within the range of actual use (0.11-0.16), and as such remains valid, with no need for change.
• Ventilation rates associated with chimneys. The current allowance for air change produced by chimneys is 40 m³/hr. Testing has indicated a value of 80 m³/hr is more appropriate.
• Assumptions on heating patterns. Every version of SAP has used the same assumptions for heating: the living area is heated to 21ºC while the rest of the dwelling is at 18ºC, on weekdays there are two heating periods, totally 9 hours, while Saturdays and Sundays have one daily heating period of 16 hours. Analysis of survey data showed that in practice most households either used two heating periods during the week and at weekends, or used one heating period during the week and at weekends. Incorporating those figures into the calculations made a difference of about 4% to the overall results. As the overall assumptions within SAP remain broadly appropriate it not proposed to change them.

So, of the four items considered, two are likely to result in changes in SAP 2016, while two will not. It will be interesting to see what other changes the draft proposal will bring, but overall, it looks like tweaks, rather than wholesale changes such as we saw with SAP 2009.

1. The fact that we can get one without the other is a result of the curious split in responsibilities between government departments. Although DCLG (the Department of Communities and Local Government) is in charge of the Building Regulations and the associated Approved Documents, SAP – the key measurement tool – is maintained and managed by DECC (the Department of Energy and Climate Change).
2. SAP is the Standard Assessment Procedure: not the most useful of names, as it doesn’t tell the uninitiated reader what is to be assessed.

R-values or U-values?

NBS has recently published an article by Anthony Lymath explaining the terminology around U-values: it is a useful summary and worth a read. (There are some points which I would frame differently, but I’ll address those at the end of this article). As I was going through the piece I found myself reflecting once again on the relationship between U-values and R-values and the respective benefits of each method of representing the thermal performance of a building element.

At first sight, the R-value, which is the total thermal resistance of an element, is straightforward: all you need to do to calculate an R-value is add up the resistance of the layers in the construction. That also makes it easy to establish how much insulation you might need to reach the performance level required by national or local building regulations or codes, particularly when insulation is marketed by its own R-value. If you need to hit an R-value of 3.5 m²K/W and you know that you currently have a resistance of 1, then insulation batts at R2.5 will do nicely.

The R-value is also straightforward mathematically: the conductivity and thickness give you the resistance of a layer, so if you double the thickness of those R2.5 insulation batts they go from R2.5 to R5.

But achieving that simplicity has costs. First, the use of basic R-values means we lose sight of thermal bridging. Those R2.5 insulation batts are likely to be fitted between between timber studs, resulting in a 20% overestimate of performance (based on the 15% bridging fraction required by BR 443 Conventions for U-value calculations).

Secondly, in terms of analysis, the R-value is a dead end. Because it is an expression of resistance to heat transfer, rather than a measure of heat transfer itself, we can’t go any further with it. In contrast, we can use the U-value of an element in the thermal analysis of a building: if we know the U-values and areas of the building elements, and the temperature difference between inside and outside we can make a reasonable estimate of the rate of heat transfer. That, combined with data on thermal mass, solar and internal gains, enables us to model energy demand (something I cover in more detail in my forthcoming book How Buildings Work). The R-value can’t do that.

So the U-value lets us carry out the more detailed analysis we need to design energy efficient buildings. But the downside with the U-value is that it is not as straightforward to understand as the R-value. Mathematically, the U-value is the reciprocal of the R-value, which means that increasing the amount of insulation in an element will not give a nice straight line reduction in the U-value.

If we plot the R-values and U-values of a construction with increasing amounts of thermal insulation we see the R-value increaes in a straight line with a constant gradient. However, plotting the U-values of the same situation gives a curve, with rapid reductions of the U-value to begin with (say, increasing 50 mm of insulation to 100 mm), but less change once U-values are lower (say, increasing 250 mm of insulation to 300 mm). The graph goes a long way to explaining why we will probably never want to get U-values much below 0.10 W/m²K.

Personally, I think the R-value’s time is up: the benefits of simplicity are outweighed by the inability to go any further in analysing the performance of a whole building.

Terminology

As I mentioned at the start, there are a few points which I would put differently:

1. Lambda value vs k-value. The article refers to a material’s thermal conductivity as its k-value. There was a time when that was the case, but now BR 443, calculation standards and Agrément certificates all refer to conductivity as lambda (λ). By sticking with lambda we also avoid the confusion with the kappa-value (κ-value), which is a measure of thermal mass.
2. Cold bridging vs thermal bridging. A repeating thermal bridge (to use the full term) occurs when a layer of one material is regularly interrupted by another material having a different thermal conductivity. In many cases the interrupting material has a higher conductivity, resulting in increased heat loss: hence the term cold bridge. However, there are cases where the interrupting material has a lower conductivity, resulting in reduced heat loss. The main occurrence is in stone walls, where the mortar will usually have a lower conductivity than the stone; the greater the proportion of mortar, the lower the U-value and the lower the rate of heat loss. Given that, thermal bridge is the preferred term.
3. Wall-ties as cold bridging. The effect of wall ties and other mechanical fasteners is not treated in the same way as thermal bridging. Instead we use separate correction factors which are added to the U-value at the end of the calculation process. (To be compeletely accurate: correction factors are ignored if they are less than 3% of the initial U-value.)

Catching up with Part L

For the last couple of months the Department for Communities and Local Government (DCLG) has been busy making changes to the Building Regulations in England. As well as changes to the guidance to Parts G and M, there have been revisions to the regulations themselves around Part L and Energy Performance Certificates which came into effect on 1 May 2016. But strangely, despite a statutory instrument running to five pages (SI 2016/285), the revisions will have very little practical effect. What is DCLG doing?

The short answer seems to be ‘tidying up’. The longer answer is: taking EPCs out of the Building Regulations and tightening up the definitions around energy performance. Let’s look at each of those in turn.

Energy Performance Certificates

The regulations governing the issue of EPCs for new buildings – dwellings and non-dwellings – was added to the Building Regulations 2000 in 2008, while EPCs for existing buildings were addressed in the Energy Performance of Buildings (Certificates and inspections)(England and Wales) Regulations 2007 (SI 2007/991). Even though both sets of regulations have subsequently been revised, that original split has remained until now.

In a pair of changes, on-construction Energy Performance Certificates have been moved from the Building Regulations to the Energy Performance of Buildings (England and Wales) Regulations 2012. The only remaining requirement regarding EPCs is in the list of requirements a local authority must check before issuing a completion certificate.

The detail of the changes is given in the following paragraphs in italics. Feel free to skip them if you don’t need the detail.

SI 2016/285 removes Regulation 29 – Energy Performance Certificates –from the Building Regulations and removes all internal references to it. The only requirement associated with EPCs is in Regulation 17(2A) – Completion Certificates. The provisions the Local Authority must check before issuing a completion certificate now include the issuing of an EPC. New definitions of EPC and energy assessor have been added, pointing to The Energy Performance of Buildings (England and Wales) Regulations 2012.

The requirements for on-construction EPCs have been inserted into the Energy Performance of Buildings (England and Wales) Regulations 2012 by SI 2016/284 (The Energy Performance of Buildings (England and Wales)(Amendment) Regulations 2016). The new Regulation 7A is a close cousin to the now-deleted Regulation 29 of the Building Regulations.

There is, of course, a little problem here: Wales. SI 2016/284 applies to England and Wales, but SI 2016/285, as it addresses Building Regulations, which have been devolved to Welsh Minsters, only applies to England (leaving aside those wretched excepted-energy buildings). Presumably the Welsh government will issue further regulations to remove EPCs from the Building Regulations in Wales.

UPDATE 2016 05 04: I understand from the Planning Department of the Welsh Government that similar revisions will be considered for Wales following the Welsh Assembly election on 5th May.

Requirements for energy performance

The second set of changes centre on the calculation and expression of energy performance. The changes begin with amending the definitions of asset rating and operational rating of a building, then modify regulations 25 to 27A, to clarify that the target carbon dioxide emission rate and the target fabric energy efficiency have to be calculated using the method approved in respect of regulation 24.

Again, feel free to skip the detail if you don’t need it.

An asset rating is no longer a numerical indicator, but an energy performance indicator, which is determined from the amount of energy estimated to meet the different needs associated with a standardised use of the building.

An operational rating is now an energy performance indicator determined from the amount of energy consumed during the occupation of a building over a period of time and the energy demand associated with a typical use of the building over that period.

Regulation 26 now reads (new text in bold): Where a building is erected, it shall not exceed the target CO2 emission rate for the building that has been approved pursuant to regulation 25 applying the methodology of calculation and expression of the energy performance of buildings approved pursuant to regulation 24.

Regulation 26A now reads (new text in bold): Where a dwelling is erected, it shall not exceed the target fabric energy efficiency rate for the dwelling which have been approved pursuant to regulation 25 that has been approved pursuant to regulation 25, applying the methodology of calculation and expression of the energy performance of buildings approved pursuant to regulation 24.

There are similar changes throughout Regulation 27.

Why these changes, why now?

The EPC changes tidy up an unnecessary division that should not have occurred in the first place. The immediate practical effect will be limited. However, by shifting EPCs out of the Building Regulations it also shifts them away from DCLG’s remit and firmly into that of the Department of Energy and Climate Change (DECC). It’s hardly a turf war, but it may make their lives a little easier.

The changes to the expression of energy performance will also have little practical effect for most people. I can only presume there may have been compliance problems somewhere which DCLG thought merited the modifications.

The only consequence of the changes that concerns me, is that the auditing of calculations of building energy performance is now formally separated from the Building Regulations. The link was never strong, following the introduction of EPCs in 2008, but it has been severed.

But having worked through the changes in some detail I am left wondering why they made these particular changes now.

ebook edition of Guide to the Building Regulations

One of the first comments I received after the first edition of Guide to the Building Regulations was published back in 2010 was that it would be very useful to have the book available in an electronic format, so it could be used on site. Back then, an ebook edition didn’t seem feasible: the book has so many large tables and illustrations that putting it out on something like a kindle or nook was impractical.

But the technology advances and RIBA Publications have issued a number of their books as ebooks, in PDF format using Adobe Digital Editions. One of the first books available in the new format is the third edition of Guide to the Building Regulations, making it easy to take the book on site.

Also, at the time of writing, there is a 20% discount on the ebook edition.

Change is the only constant

2015 was a good year for Building Regulations changes, with England and Scotland seeing significant changes (although things were quiet in Northern Ireland and Wales).

There was also a huge jolt of uncertainty introduced in the Chancellor’s budget statement, with the abandonment of the tightening of carbon dioxide emissions standards scheduled for 2016.

I wrote two articles for the RIBA Journal, the first summarising the changes to the Building Regulations in England, while the second looked forward to see what might be coming, now the 2016 changes have been abandoned.

The 2015 changes to the the Building Regulations in England

What next for building regs?

Goodbye to 2016

That’s not a goodbye to the year 2016, but goodbye to 2016 as the target year for zero carbon homes. The government’s newly published Productivity Plan makes that very clear:

The government will … repeat its successful target from the previous Parliament to reduce net regulation on housebuilders. The government does not intend to proceed with the zero carbon Allowable Solutions carbon offsetting scheme, or the proposed 2016 increase in on-site energy efficiency standards, but will keep energy efficiency standards under review, recognising that existing measures to increase energy efficiency of new buildings should be allowed time to become established

So that’s it. Code for Sustainable Homes: gone. Zero carbon homes: gone. Part L 2016: gone.

No doubt over the next few weeks there will be plenty of discussion about this, but for now we can just stare slack-jawed at the announcement for a few moments.

When an edge is not an edge?

The latest edition of Approved Document M is now available on-line, prior to coming into force at the start of October 2015. I will review the content in a little while, but for now, I just want to observe one of DCLG’s Humpty Dumpty moments.

In Lewis Carroll’s Through the Looking Glass Alice meets Humpty Dumpty; this is part of their exchange:

I don’t know what you mean by “glory,”‘ Alice said.

Humpty Dumpty smiled contemptuously. Of course you don’t — till I tell you. I meant “there’s a nice knock-down argument for you!”‘

`But “glory” doesn’t mean “a nice knock-down argument,”‘ Alice objected.

‘When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’

Which is exactly the approach DCLG have taken:

Leading edge (of door): The surface of a door which leads into, (or faces), the room or space into which the door is being opened – sometimes referred to as ‘the pull side’.

What could be more fun than deciding to define an edge as a face or surface? It’s not as if they have saved themselves any space: leading face would take up no more letters.

So the answer to the question in the title of this post is: when it is a face. Hardly fall-off-chair funny, but perhaps worth a wry smile.

Happy birthday Part K

It was only after I finished delivering a CPD seminar on changes to the Building Regulations for RIBA Northern Region yesterday that I realised the new Approved Document K is a year old. That set me wondering: how has the new AD K doing? Is it everything we hoped for, or is it still awkward in places?

Looking at the macro level, the revision of AD K, coupled with the euthanasia of AD N and the excisions to AD M, was meant to remove clashes between different requirements and bring together much of the guidance into one place. How did that go?

Parts of it have done very well: there really was no justification for having AD N and AD M both giving guidance on manifestation of glazing, and no sensible reason why there should be guidance on the layout and configuration of stairs in two different documents. Bringing all of those together in one place has been a Good Thing. But – always a but – the longer I look at it the more annoying it is to see that requirements for stairs and ramps on access routes as still there in AD M, even though those requirements are, save one point, identical to those given in AD K. With a bolder approach DCLG could have got all the guidance on stairs into one place.

Turning to the Approved Document itself, one of the big changes was the move to ‘plain english’, and a one column layout better suited to reading on screen. That gets a thumbs up. Much as I might like the passive voice – mainly I think for its (?erroneous) sense of neutrality – it can become an obstacle to understanding. Personally, I find it difficult to judge if the new-style AD K is easier to understand, as I have spent so long reading and working with the old-style approved documents. I think, for guidance, it does work better, certainly where there are definite rules. A ‘do this’ is better to work with than an ‘ensure that’. Reading through, there are points when I catch what seems to be an unreformed sentence (the maximum dimensions for … are shown in Diagram 5.2), but on the whole the new style of language works.

One part of the revision which does seem problematic is the division of guidance by building type. For example, when looking at handrails, the guidance addresses:

• All buildings
• Buildings other than dwellings and common access areas in buildings that contain flats and do not have passenger lifts
• Buildings other than dwellings
• Dwellings

That is quite a set of distinctions to work round. It also, on occasion, leaves some gaps. What happens in common access areas in buildings that contain flats and do have a passenger lift? Should they follow all the guidance for buildings other than dwellings, or do we just go with the all buildings guidance?

There are five categories overall, not each of which is mentioned in each set of divisions, which leaves me wondering what building type a particular piece of guidance applies to, or how it applies.   If I wanted to give DCLG anything as a corporate Christmas present it would be Occam’s Razorentia non sunt multiplicanda praeter necessitatem (entities must not be multiplied beyond necessity). The Department simply loves making lots of different classifications of building types.

Finally, there is the guidance itself. The promise was ‘There are no new technical requirements’. That is true, but there have been changes within the requirements. A minor change has been the reduction in soffit height which requires guarding from 2.1 m in AD M, to 2.0 m in AD K 2013. That isn’t a substantial change. However, as someone pointed out at the CPD seminar, the widths of stairs have changed. AD K 1998 did not specify a minimum width for stairs, it merely require stairs wider than 1800 mm to be divided with handrails: the limits on widths of stairs were in AD B; 1000 mm for institutional buildings, 1100 mm in an assembly building and 800 mm in other non-dwellings. AD K 2013 requires a minimum width of 1200 mm between walls, strings or up stands and 1000 mm between handrails. That is an extra 200 mm in many cases, which, to my mind constitutes a significant change.

So how is Approved Document K 2013 doing? Overall, it gets a pass, and in places reaches merit standard, but still falls some way short of a distinction.

Wales – a sneaky side-step

I’ve written before on the separation of building regulations in Wales from those in England following the devolution of powers to Welsh Ministers at the end of 2011. At that time I was expecting the Building Regulations in Wales to be unchanged from the 2010 England and Wales regulations until the Welsh Part L (Conservation of Fuel and Power) was revised later this year. However, it turns out I’ve been blindsided by a small but unexpected side-step last summer.

The unexpected move was a response to the European Union’s Construction Products Regulation, which introduced compulsory CE-marking for relevant construction products from 1st July 2013 (relevant in this case is a short-hand for products for which there is a harmonised European Standard or European Technical Assessment). In England, the necessary changes to guidance were proposed in the 2012 consultation on Building Regulations, together with a draft Approved Document to Regulation 7 which would implement the changes. Following the consultation the new Approved Document came into effect on 1st July 2013.

And what about Wales? Not a peep. Until 29 April, when a consultation on changes to the Approved Document supporting Regulation 7 opened. The consultation also addressed changes to remove the Warranty Link Rule, which requires Approved Inspectors to check a new dwelling is registered with a warranty provider before carrying out any building control function, and to establish a central register for registration and insurance details for Approved Inspectors. The consultation closed on 21 June 2013. The new Approved Document in support of Regulation 7 came into force on 1 September.

What’s changed? Well, the main change is that the Approved Document reflects the implementation of the Construction Products Regulation, with the Ways of establishing the suitability of materials  section updated. There are a couple of other tweaks, but that’s the bulk of it. The Approved Document also has a new format: one column, sans serif type face, headings on a grey background, but mainly very blue – with the surprising exception of defined terms which are bold and green (they really would have been better in blue, but for some reason the notes to the document have been set in blue: hmm).

Some might say a coming-into-force date of 1 September is a bit late: two months late. It’s not great – Northern Ireland managed the change by 1 July 2013, but it still took until 1 October 2013 for Scotland to recognise the Construction Products Regulations in its Building Standards.