Monday, November 13, 2006

Interview: Ray "Mr U Value" Williams

On the last day of October, I went to the National Physical Laboratory to interview Ray Williams. He runs the department that measures the thermal performance of materials and structures and, as such, he is probably Britain’s leading expert on measuring U-values. Despite having such an important job, at the cutting edge of building science, he had never before been interviewed by a journalist. “It’s a sign of the times,” he joked. I think he was delighted that someone was at last showing an interest in the work he does.

I wanted to talk to him about the multifoil debate, in which he has been deeply involved. But I also wanted to see how they go about their testing work and in particular just what a guarded hot box actually looks like. It is the industry standard equipment, used for measuring U values, and it has played a crucial role in our understanding of how building materials hold onto heat.

The hot box (pictured here with Ray inside) is a much bigger piece of kit than I had imagined. It’s about 3m tall and it weights 3.5 tonnes: it’s also a very expensive. Ray reckoned that it would cost around £300,000 to replicate it. Because of this, there are now very few of them around. He only knows of two others in the UK, both at the BBA test site.

“Anybody can build a simple hot box,” Ray told me, “but the results they get from it will be subject to a huge margin of error, so much so that the results are worth very little. Here we have been refining and developing the apparatus (which complies with BS EN ISO 8990) and the techniques for measuring heat transmission over many years. This is probably amongst the most accurate guarded hot boxes in the world but, even so, our best measurement uncertainty is still only about ± 4.5%. That’s a large measurement uncertainty for scientific testing. It shows just how difficult measuring heat transmission through structures is.”

Thermal performance testing of building products was pioneered in the 40s and 50s when scientists began to systematically study the thermal resistance of all kinds of materials for the first time. However, the evaluation of the insulation properties of building materials and structures was given a boost by the energy crisis that developed in the 70s. The Thermophysical Properties Group at the NPL was established in the late 1970’s, brought about by the need to verify the claims of the manufacturers of low density aerated concrete that the thermal performance of these new materials were actually superior to traditional concrete blocks. A more recent surge in demand for these services came when the 2002 revision to Part L of the Building Regulations set a low U-value criteria for windows, making double-glazing incorporating low-e glass almost mandatory. It was important for manufacturers to have the U-value of their windows measured using the procedures specified in a then new hot box measurement standard known as BS EN ISO 12567-1 and many of them came to the NPL to have these measurements carried out. A hot box measurement at the NPL cost around £2,500 and takes about a week. The NPL hot box can measure element up to 2m wide by 2m high and is fully rotatable, so that it can work vertically, horizontally or at any chosen angle, which is important when the test element contains air cavities.

The question I really wanted an answer to was this: “Just how good are hot boxes at indicating what goes on on site?” Ray Williams wasn’t really in a position to answer this, as he doesn’t carry out site tests, but he did point out that there was really no reason why a structure should perform better on site than it would in a hot box. Which brought us on to the subject of multifoils, of which he has tested quite a few. What struck me was that he wasn’t at all dismissive of them, which I had half expected, but was really interested in the whole debate and was keen to devise a test that might show multifoils off in a better light, if at all possible.

“I have had a number of multifoil manufacturers here talking to me and while some are quite prepared to accept that hot box testing is the best way of quantifying the thermal performance of their products, others are not. Some really seem to believe that multifoils behave differently in dynamic temperature situations than when tested with a constant temperature difference across them (as in hot plate and hot box measurements) but they don’t have any data to support those views. I keep getting results that indicate the thermal resistance of an air cavity insulated with a multifoil mounted centrally, is the equivalent of between 50mm and 80mm of mineral wool, but no more.”

It is partly these continuing negative results which have finally persuaded Local Authority Building Control and the NHBC to issue guidance to their members to only use thermal performance values obtained from hot box measurements, at least until further notice. Even so, Ray is still open to carrying out further tests. “Some multifoil manufacturers want me to test dynamically. Thus far, every test I have done in the hot box has been steady state. That is to say that we test for the heat loss of a wall or roof with a steady 20°C temperature drop across it; designed to mimic the difference between internal and external conditions in winter. A dynamic test, in contrast, would look at total heat transmission through an insulated structure over an extended time period whilst the temperature difference is being varied, just as you might expect in the course of a day. The BRE have carried some in situ (hence dynamic) measurements of walls insulated with multifoil insulation and those results were broadly in agreement with hot box results. The DTI have now funded a small project for NPL to look at identifying the best way to evaluate these kinds of products. I have already had extensive contacts with many people involved in this debate and I must soon decide how best I can help to resolve these issues. I am of a mind to include carrying out some dynamic testing, either in a hot box or a hot plate apparatus to see if I can determine if different insulation systems can behave significantly differently in those circumstances. But I am at a loss to explain just how it could give a markedly different result. Dynamic testing takes longer and it’s far harder to interpret the results, but in principle I don’t mind having a go.”

Whatever your feelings about multifoils — if you have any feelings at all that is — I find it reassuring to know that the government’s chief thermal performance tester is open-minded about trying alternative testing regimes. Because if, in the future, we want to address issues such as solar gain, quality of on-site workmanship, building anomalies and dynamic thermal performance, in a truly rigorous way, the guarded hot box apparatus is unlikely to be able to provide all the answers. But in the absence of any widely accepted and validated alternatives, it remains the best testing method for comparing products that we have for now. Whilst it is easy to see why a product might not perform as well on as it does in a lab test, it remains a puzzle as to how exactly it could be persuaded to improve.

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10 Comments:

Blogger Frank Brown said...

From what I know of the hot box, I am very disturbed by the lack of control of the airflow from the hot side to the cold side. Micro turblences can drive hot air through a breathable membrane, such as glass fibre but not the foils. In real life loft insulation in older housescan have a gale blowing across the top of the insulation. The ability to stop the movement of the air at the insulation/air boundary is importent, as its the air which tends to carry most of the heat away. The ability to rotate the hot box is a sham ( unless the insulation is for an acrobatic airplane). There should be 3 classes of insulation, zero air movement ( under concrete floors), moderate air movement( modern lofts with breather membranes), high air movement (old fashioned lofts with wind speeds up to 50MPH). The testing regime should then try to simulate, one of the 3 conditions. The material should be presented in a wooden frame work made say of Nmm deep 25mm thick wood with an opening of 1m^2, so it sits in a recess in the testing machine. This is so the insulation manufacturer can take responsibility for the fixing system. The bottom surface will rest on a square of aluminium to support the insulation in the machine. The surround to the frame will be polystyrene to a depth of N+2 mm and 60mm wide. The ali base will have 4 surface mounted heating elemment, mounted at .5M centres with thermistor temperature control elements mounted 100m from its associated element. The servo electronics will keep the temperature of each element to better then +_.05 deg from ambient + 10 deg C, referenced to a thermistor sited above each heater 1mm from the top side of the insulation. Now the clever bit. To run the test for under concrete a 3mm Ali plate is put over the frame, stops air but not heat. A piece of breather felt is put over the frame, and a gentle air movement is generated(airspeed=.1M/S). For the old fashione loft, no cover and a gale is initiated( airspeed= 30M/S). If infiltration air is expectd, suitable a suitable number of small holes could be drilled in the heater plate. I see no reason why a piece of kit such as this should cost more then £5000, and have results every 2 Hrs. Every insulation manufacturer could then have one so they can do instant testing of their products (and their competitors?). It will completly de-mystify the testing of insulation materials.

10:14 pm  
Anonymous Nick Grant said...

As a multifoil sceptic I ave always assumed that the reason the thin multifoil proved as good as 200mm fibreglass was due to wind whistling through the latter.

On this basis it could be argued that clingfilm has the same insulation as say 150mm of fibreglass, however this doesn't show up in hot box tests since hot boxes don't reflect real world dynamic conditions...!

On a windy mountain top a cagoule might keep you warmer than 3 thick woolly jumpers but it's obviously nothing to do with U values.

Climbers and intelligent builders know that you need the woolly jumper (insulation) and the cagoule (eg breather membrane).

Interestingly Celotex who debunked multifoil by commissioning the testing of Tris Iso Super 9 at the NPL are now selling their own multifoil insulation.

There has been a lot of discussion of all this on the AECB forum at www.aecb.net

12:09 pm  
Blogger Ray said...

I have just read some of the comments on the article and they do make sense. Of course wind taking away warm air is going to reduce the insulation properties of a highly breathable membrane.

To take the Kagool on a windy mountain further imagine it was a black Kagool on a sunny windy day compared with a white wooly jumper, then remove the wind and the sun. The relative performance of the two would vary dramatically but their intrinsic properties would remain the same.

So how should this be catered for in a standard test which does not include all the variants of a site installation?

We should recognise that heat is lost in three different ways:-

Conduction, radiation and convection.

An insulation material will be tackling these three different methods to varying degrees and in different ways the test methodology will have a large influence over the comparative results.

My proposal is to separate these three properties and test for them individually.

This way we will be able to see the conductive resistance, the reflective resistance in particular (convection resistance is more to do with the design of the structure and how the insulation is installed rather than a property of the insulaton material per se) so I cannot see how this can be measured in a hot box test, you would need to have a mock up of the a structure and have the material installed in accordance with manufacturer's instructions to see the convection heat loss results.

We should then be in a position to design buildings with good resistance to all three types of heat loss and stop trying to compare a reflective membrane with an equivalent thickness of a bulk insulator. This comparison is just not valid.

The result would be that building regs would require performance criteria on each form of heat transfer as individual requirements and appropriate and specific test methods could then be developed to measure each one separately and accurately.

I have written a guide to reflective insulations which can be seen on www.importsuppliesdirect.com if anyone is interested.

Ray Foulkes

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