How Much Do You Value U-values?

Published in the Winter 2013 issue of AT Magazine, for members of CIAT.

Let’s Start With A Quiz…

Published article about U-values in AT magazine.

You’re detailing a domestic timber frame building. The wall is a conventional design that has to meet a U-value of 0.22 W/m2K, so you check the literature of your preferred insulation manufacturer and copy the thicknesses specified.

The Building Control Officer, however, wants proof that the insulation specification will do what is claimed. You get a written U-value calculation from the manufacturer, which seems to do the trick because the BCO starts pestering you for water usage calculations instead.

Here, then, are your questions:

  • How much trust do you place in the calculation provided?
  • Will the various materials perform as claimed, and as expected?
  • How confident are you that the BCO could read the calculation?
  • And that they read it correctly?
  • How much do you trust the contractors on site?
  • Will they build the wall how you and the manufacturer expect?
  • How much confidence do you have in your responses to these questions?
  • More or less than you expected…?


Why I’m Asking

For two years, I’ve been a person who does those calculations. Every day I speak to architects, contractors, builder’s merchants, BCOs and the public about insulation. Some people have a good level of knowledge; some don’t but are happy to learn.

Some people simply aren’t interested and just want an answer. Then there are people – often better educated than you’d expect! – who think insulation is magic and the answer to all their bad detailing.

The ten years previous to that I was an Architectural Technologist, working on domestic projects and specifying insulation thicknesses based on manufacturer information and calculations. My interest, like the people I speak to now, was finding the thinnest insulation to obtain the best U-value in any given situation.

As long as the manufacturer said it met the U-value, I gave little thought to realities like what the contractor might buy instead, or how the product would be installed.

Competence Is A Preference

If you do think about those realities then you are my hero and can stop reading now. But our customers have requested 64% more calculations in 2013 than in 2012, so there’s still plenty of faith being put into what we send out. People rarely ask about a calculation – unless it doesn’t meet an expected target, of course.

Once a figure is put on paper and issued we essentially lose control of it, but it practically becomes gospel! In an effort to improve the quality of U-value calculations being sent out, the British Board of Agrement (BBA) set up a competency scheme. Membership is voluntary and subject to regular audit, and I believe it to be a good thing.

In fact, I’d like to see the scheme made compulsory and more rigorous. That way, calculations might eventually become a statement of what is likely to be built rather than a statement of what might be achieved if everything is perfect (which, of course, it never is).

That’s not to say calculations are a free-for-all, despite the competition that naturally exists between manufacturers. The scheme requires reference to various standards, including a document called BR 443 (2006). It’s free to download and worth a few minutes of anyone’s time to help understand the factors taken into account when a calculation is requested.

Prior to switching career, I understood little more than calculations being a series of layers. I’d never heard of BR 443, nor suspected there was a need for a competency scheme. Does it matter to you if a calculation is labelled with a scheme logo?

Model Behaviour

Example of a U-value calculation for a typical wall construction.

Occasionally a construction arises that is too complex to be properly modelled by our software. We might give approximate advice, but we also make clear that joints or fixings or certain materials can’t be fully accounted for. It’s unlikely such a calculation would be formally issued, but similar can happen even with simple constructions.

We are rarely provided with all the necessary information, though we try to obtain it. And when we clarify assumptions, it’s impossible to know whether, say, an architect or technologist feeds those assumptions back into their design. For example, if you ask what thickness cavity wall insulation is needed, do you know the wall ties that will be used? If not, will you make sure they match what’s been factored in? Or are you happy to accept an extra 10 or 20mm insulation to cover the worst case?

Another example: until I started this job, I never thought about the reflective nature of foil facings on insulation boards and their impact on U-values. How many people – architects, BCOs, customers – could recognise the inclusion of a low emissivity cavity in a calculation? How many would question why two calculations might use different values for similar cavities?

Mind The Gap

There’s one big reason I think the content of U-value calculations should be demystified, and that is the performance gap.

Buildings constructed on site don’t achieve the performance levels determined on paper. Whether it’s poor workmanship and site supervision, bad product substitutions, or low standards of knowledge and advice, our built environment isn’t meeting expected heat loss standards.

That means greater demand on heating systems and increased cost to the user. And the scale of the discrepancy is probably worse than you imagine – the startling truth is that average heat loss from new buildings is far in excess of even the requirements of Approved Document L 2006.

With a new Part L coming into force in 2014, and the Government sticking to an increasingly unlikely target of zero carbon housing in 2016, it makes you wonder whether residential development will ever live up to our expectations. Will ‘as-designed’ ever reflect what ‘as-built’ can actually achieve?

The Future

Performance gap issues won’t disappear overnight, and transition arrangements mean large housing plots will continue being built to old regulations. Issues like that can’t be solved by U-values alone, or whole-building assessments like SAP.

Wider ranging factors come into play, such as geography and climate. Exposure on one site can cause a building to perform differently to its neighbour half a mile down the road. Research is ongoing as to how these influences can be accurately accounted for at design stage – new correction factors for U-values; amendments to SAP to reflect site locations; lab testing of products supplemented by in-situ testing.

Any or all of these could happen. What we can be certain of is that getting to grips with as-built performance will take some time. A change of mindset is required, and that will also take time. For now, I believe we could all do each other a favour by being more realistic about our built environment.

Too many people do things as easily and cheaply as possible, only for the end user to pay more in increased running costs. Are those the values we really want to work to? If we’re serious about reducing the performance gap, maybe we could start by all being a little more competent when it comes to the U-value performance we seek at design stage.