Multifoils and Private Building Control

It’s now nearly a year since the two most important building control bodies in the UK moved to stop the use of multifoils, at least as a substitute for conventional insulation materials. Under pressure from government, the LABC (Local Authority Building Control) and the NHBC (National Housebuilders Council) simultaneously stated that in future they would only accept insulation that met the standards laid out in BR443, which translates as using the U value figures derived from the guarded hot box test.

Multifoil insulation performs very poorly in guarded hot box testing. The manufacturers claim that it’s the guarded hot box test which is at fault. Their case rests on them performing well in comparison tests against conventional insulation, usually 200mm or 250mm of mineral wool. Such tests have been carried out several times by the multifoil manufacturers, to the satisfaction of some independent accreditation providers, but they have yet to win a European Technical Approval for their testing methods and they remain highly controversial.

I don’t want to dig too deeply down into the multifoil debate here — it has been covered several times on this blog already — but what is worth mentioning at this point in time is that there are still a number of private building control bodies out there who are more than happy to accept multifoil roof insulation. I was talking to a director of MLM last week and he remains an enthusiastic supporter of multifoils and is more than happy to sign off building works that use multifoil insulation. And I came away with the impression that MLM are far from unique in this respect, and that private building control saw this as a positive way of differentiating their services from the strictures of both local authority building control and the NHBC.

On Dynamic Insulation

Ever heard of dynamic insulation? It’s an idea that’s been knocking around for a long time, always in the category of academic curiosity, but now at long last someone is coming forward with a marketable product called Energyflo which, they hope, will bring the concept to the masses and, just possibly, sell in great quantities.

So what is it and how does it differ from ordinary insulation? In conventional building models, heat leaks out gradually through the fabric, be it a wall or a roof. Dynamic insulation seeks to capture that leaking heat and feed it back into the building. It does this by making the insulation layer air permeable (by punching loads of holes in it) and then de-pressurising the house so that air is drawn into the house, heating up as it passes through the walls or roof. In theory, if you get it right, you can recapture all the leaking heat and you could produce a wall with a U value of near zero, without having to use more than about 90mm of insulation.

To get it to work, you have to get a fan sucking like hell inside the building to pull the air inside. What happens to the air being sucked through the fan? Well, here it starts to unravel a little because it gets dumped outside. But in fairness, warm air is going to get dumped outside in any event because you need to have some form of ventilation built into the house and you may just be able to get a second bite at that dissipating heat if you plug in a heat recovery unit.

Last Wednesday (May 30), I sat in on a presentation given by Mohammed Imbabi and Andrew Peacock of Environmental Building Partnership, a spin out from Aberdeen University, which is planning to market Energyflo as the basis of a low energy building solution. They reckon that with no airflow at all, the U value of the 95mm expanded polystyrene panels would be around 0.35, but with the airflow working as planned, the U value falls to around zero: i.e. there will be no heat loss at all.

Of course, it’s early days for this product. It’s still undergoing tests, most notably at a CALA homes site in Edinburgh where it’s been installed in the roofspace. There are also plans for it to be used on a big apartment site in Dubai.

I don’t think the presentation met with quite the level of appreciation the backers were hoping for. Many of the questions expressed a surprising level of scepticism. To work as designed, the Energyflo cells have an air filter embedded within them: someone suggested that this would rapidly clog up in Dubai where there are frequent dust storms. And there appeared to be a finite life to the cells as well, which was determined by the site characteristics (i.e. how much pollution) and the thickness of the filter. But if you are building the insulation into the fabric of the structure, how are you meant to replace it?

Perhaps its churlish to be too critical. As a product, it’s only just making it’s first tentative steps away from the research labs and there is doubtless much to be learned en route. To establish a foothold in the insulation market, it will have to be monitored on a number of different buildings over a lengthy time period, something we in the UK are not good at doing. So I wish them well, but don’t expect to be seeing a whole mass of dynamically insulated buildings tomorrow or in fact anytime soon.

Multifoils: last throw of the dice

I went to London this morning to attend the first ever press conference of the Confederation of Multifoil Manufacturers (CMM), held at the Building Centre. The CMM consists of four members being Actis, YBS, Euroform and IPP who, combined, account for 90% of UK multifoil market.

They put on a 90-minute presentation consisting of four presenters and a short Q&A session in front of an audience of around 30, made up of people working in the multifoil industry and a number of trade journalists.

• Paul Newman, the secretary of the CMM, opened the proceedings and talked a little bit about the challenges facing them; in particular, the August 2006 release of a technical guidance notice from the LABC (Local Authority Building Control) advising against the use of multifoils as standalone products. Newman insisted that multifoils were being held back by old (redundant?) testing methods. He used a graph to show how multifoils had by then captured 6% of the total UK insulation market, up from just 1% in 2002, but this was now expected to fall; the graph suggested 5% in 2007.

• Paul Mitton, Technical Manager of Euroform, then spoke generally about the installation advantages of multifoils, highlighting speed of installation and minimal depth.

• Matthew King, Technical Manager Actis UK followed. He spoke at length about the failings of the guarded hot box (GHB) test used to evaluate conventional insulation and claimed that GHBs only tested for conduction. He claimed that 93% of heat transfer through building fabric was by radiation (a claim later challenged by Roger Bisby) and that somehow GHB testing was not measuring this. This is a controversial view and is disputed by GHB experts such as Ray Williams (not present) of National Physical Laboratories, who has told me that the GHB test doesn’t distinguish between conduction, radiation and convection. King also contested that the GHB measures a temperature difference in a steady state between 10°C and 20°C and that, in reality, temperature differences in the built environment are much larger than this. This was questioned by a lady from the AJ who thought that 20°C would cover all but the most extreme weather events. King further claimed that GHB lab tests do not take into account
- temperature variations which disturb the thermal system
- rain and humidity which influence thermal conductivity
- solar radiation
- wind strength and direction (convection)
- difference in pressure (interior, exterior)
- water and air tightness
- quality of the installation
- ageing of materials.

However, whilst emphasizing that site tests would be more realistic than lab tests, as has been show in the world of acoustics, he did not explain why site tests would bring about improvements over lab tests. In acoustics, elements on site invariably perform worse than they do in lab tests: here we are expected to believe that multifoils perform better on site than they do in GHB lab tests.

King then gave a lengthy exposition of how the in situ tests are carried out and how close attention is paid to ensuring that the controls are fair:
- two roof assemblies are constructed and tested without insulation to see if they are within ±3% of each other, as regards energy used to heat to a pre arranged internal temperature.
- then both assemblies are tested with 200/250mm mineral wool to see if they remain within the ±3% range.
- only then is multifoil placed in one of the roofs to carry out the comparison testing.

Results from the Actis tests have shown that whilst 200mm of mineral wool results in higher than predicted energy use, the Actis multifoil performs far better than the simple R-values derived from GHB testing would suggest. In fact the multifoil matches the results from the 200mm of mineral wool. This will come as no surprise to those who have followed this debate: it is the essential claim made by all multifoil manufacturers and the one disputed by virtually every other insulation manufacturer.

• The last speaker was Prof Nico Hendriks, professor of Building Materials at Eindhoven University of Technology and the Principal of the BDA Test Institution, a third part approval agency. Hendriks has been hired by the CMM to take on the independent testing of multifoils. Basically, he is charged with replicating the famous in-situ tests carried out by Actis and to use the (hopefully) positive results to persuade EOTA (European Organisation of Technical Approval) to grant multifoils an ETA (European Technical Approval), which would enable multifoils to be awarded an equivalent R value and thus to be used in all European territories.

He ran through the methodology that would be employed in carrying out the tests, but he was later questioned in depth on this by Roger Bisby of Professional Builder who noted that they were only planning to test against mineral wool and that in a configuration that would no longer pass UK building regs (basically wool stuffed between rafters). “Why not test against Pu foam instead,” asked Roger. Hendriks appeared to be unaware of the current requirements of Part L, saying that no one built like that in Holland anyway as they mostly used panelised roof sections. He also pointed out that there were both cost and time implications for further comparitive tests. “Why not three roofs, why just two?” went the questioning. “I am sure Kingspan would be willing to help with costs.” This observation met with a good deal of laughter from the audience, rather less from the panel.

I asked what the timescale was for their application to EOTA for an ETA. EOTA is like an umbrella organisation for national bodies, such as our own BBA, which have a remit to test innovative building products. It moves slowly and is still considering whether to even look at this testing programme, let alone pronounce on it. One of the drawbacks of in situ testing is that it takes three months to carry out a winter test and three months to do a summer one as well, and the CMM have already missed this winter as a test time, so the very earliest they could have results before EOTA would be mid-2008. EOTA has to reach a consensus on the matter so it is far from clear what the outcome will be, assuming that Henrdriks’ BDA test outcome is positive for multifoils.

Interestingly, there is a second independent testing station being used by the CMM for their in situ tests to place before EOTA. This was mentioned in passing by Prof Hendriks, without reference to who this might be. Later I discovered that this other test station was none other than our own BM Trada, who of course were the original independent verifiers of the Actis in situ tests back in 1997. Why Hendriks was flown in from Holland when they could have called on BM Trada from up the road remains a mystery. Indeed BM Trada didn’t appear to be present at the event at all. Maybe they are seen as being too close to Actis for comfort.

In the question and answer session that followed, the lady from the Architects Journal asked what the middle layers of radiant foil actually did. Paul Mitton gave a reply that was both convoluted and unintelligible. I report that I am none the wiser!

Another questioner asked about Web Dynamics, a multifoil manufacturer who are conspicuously not members of the CMM, having made a decision that multifoils were best used as a supplementary insulation layer. Web Dynamics have achieved BBA third party approval (a UK-only version of ETA) after testing their multifoil, Thinsulex, in conjunction with conventional insulation materials in the BBA’s guarded hotbox. The CMM speakers were dismissive of the Web Dynamics approach and claimed that their multifoil was an inferior product that couldn’t match the performance of the ones produced by CMM members. Mitton claimed that: “Their wadding is black and as you know black material absorbs heat, not what you want in thermal insulation.”

The last question was about the BRE tests that took place at a site near Aberdeen. These were essentially in situ tests of multifoils against conventional insulation and the results appear to confirm the GHB lab tests – i.e. multifoils do not match the claimed for 200mm of mineral wool insulation. Mathew King’s view was that the test was not as rigorous as the ones that they had carried out, nor as the ones that they want BM Trada and BDA in Eindhoven to undertake.

Over lunch I was able to question Andrew Whittle from IPP and Claire who had flown over from Actis in France. I asked Claire if it was true that Lafarge were in the process of buying Actis: this was news to her, so it appears to be just gossip. I also asked her how Actis sold throughout Europe. She said it was a difficult time for them, as authorities seemed to be closing ranks against multifoils for similar reasons as we are seeing in the UK. Interestingly, they have never been able to sell into Germany (why does that not surprise me?) because the Germans have always insisted on hard R-value figures, which means positive GHB test results. But most other countries have been receptive to multifoils.

In summary, the multifoil industry feels very aggrieved that it has been unable to establish that its products do actually work as they claim. They say that their UK market has been badly affected by the LABC advice to stop using multifoils on their own, but that many metropolitan local authorities are still happy to accept multifoils, especially in loft conversions where it is often virtually impossible to build in the correct depth of insulation using conventional materials. They seem to be pinning their hopes on a) independent testing verifying their own test results and b) consequent EOTA approval, which would give multifoils access to all European markets via an equivalent R value method which even the Germans would have to accept.

My own feeling is that they are being extremely optimistic about their chances. What I didn’t hear today was any convincing explanation as to why multifoils perform so poorly in the guarded hotbox test and yet appear to equal large depths of mineral wool in comparison tests. As happens so often with the multifoil debate, I came away with more questions than answers. It’s all very well carrying out in situ tests, but it’s not altogether clear what exactly is being measured. It may be that 200mm of mineral wool as an insulator is very much worse than the lab tests indicate, in which case the multifoil comparison tests are simply illuminating this point. And why always test against mineral wool? It is rarely used in roof applications these days because it’s hard to meet the requirements of Part L without an absurdly deep rafter. Why not test against more realistic options for today’s housebuilders? And why can’t they come up with a scientifically plausible explanation of the workings of multifoils? Whilst I don’t doubt the sincerity of the multifoil manufacturers, their arguments are still some way short of compelling and on the basis of what they showed us today I really think they have their work cut out if they are to win the approval of EOTA, certainly first time around. It is far more likely that EOTA will ask the same question as Roger Bisby. “Why just test against a roof stuffed with mineral wool? If you want our approval for this unorthodox test procedure, let’s see more tests against a wide range of insulation products.” It could be 2010 before EOTA reaches a conclusion by which time the multifoil industry may well be history.

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.

Multifoils banned

Two weeks ago the government moved to prohibit the use of multifoil insulation in roofs. They wrote to all the local authority building control departments and to bodies like the NHBC, concerned with the policing of our building standards, and told them that they could no longer accept the use of Actis Tris Iso Super 10 and similar products as an adequate method of insulating roofs. In future all insulation must meet the standards laid out in BR 443, requiring hot box tests to be carried out. Multifoils do not perform well in hot box testing and instead have relied on comparison testing, where the material is used head-to-head with other insulators in similar roofs and measurements are taken of energy usage.

Mutilfoils have had a very good run in the UK. Most other countries – at least the ones that take these things seriously — have given them short shrift. But Actis gained a toehold in the UK by gaining third party approval from BM Trada, who witnessed the comparison roof tests and verified that the performance of their multifoil was as good as 200mm of mineral wool. This test, carried out nine years ago, has been the subject of much heated debate ever since: there have been many sceptics who suggested that the mineral wool was pulled well and truly over the eyes of the witnesses. Yet, on this basis, Actis and others have sold hundreds of acres of their multifoil products. Builders like them because they are no more than 25mm thick and are very quick and easy to fix and are especially good in situations like loft conversions where headroom considerations make other thicker insulation problematic. And indeed, I have spoken with several customers who have been delighted with the thermal performance of their multifoiled roofs. But this doesn’t prove anything, as it is notoriously difficult to measure the thermal performance of just a part of a structure.

Aware of their precarious position, the multifoil suppliers formed themselves into a Confederation of Multifoil Manufacturers with a brief to persuade the powers-that-be that multifoils were as good as they claimed and not some elaborate con. They did very well. They were given an extension till Jan 1st 2007 to prove their case. But sometime in September, the position of the legislators hardened and this extension has been removed.

Who'd be a pressure tester?

I spent yesterday morning with Keith Bartlett who runs a business called Air Leakage Testing based in Saffron Walden in Essex. Keith had a pressure test booked on a renovated house in Greenwich, SE London, and I was getting to ride shot gun with him, picking his brains as we travelled down the M11 in his van with all the kit in the back.

Keith’s background is running a building business erecting steel structures as industrial units. A requirement for air pressure testing for buildings larger than 1000m2 in floor area came into effect in 2002 and Keith took a view that this was the start of a trend and decided, with two partners, to start this new venture to get in on the beginning of a new business opportunity. Together, they have invested something like £150,000, not to mention thousands of hours labour, to get things up and running and they fully expected to be rushed off their feet by now, since the requirement for air pressure testing was extended to new homes in April this year, under the changes to Part L of the building regs.

But it hasn’t worked out quite that way. At least, not yet. Building inspectors have yet to get to grips with the changes in Part L and there is still only a tiny trickle of work coming their way, despite their being less than a dozen firms offering a similar service. In theory, when the new Part L bites, air pressure tests should be carried out on every dwelling type used in a development. That’s a difficult figure to put an exact number to but it must be of the order of 10 to 20 thousand a year throughout the country, not to mention a significant increase in commercial work as well as here the size limit for testing has been reduced from 1000m2 to 500m2. If Keith’s business is typical, it appears that the actual number of air pressure tests being carried out is less than 10% of this figure.

What appears to be happening is that as many as 80% of qualifying commercial buildings are passed by building control without a pressure test. It seems building control are happy to accept “robust details” as an alternative method of compliance, despite there being officially no allowance for this in Part L. This may also prove to be the case with domestic work as well, although the reasons here for slow take-up of pressure testing are to do with the delayed adoption of the 2006 Part L regulations by local authorities.

Thus far the main take up in the domestic sector has been from architects and interested clients who are testing the water and trying to get to grips with the concept air pressure testing. Keith told me: “Architects are in fact often their own worst enemies because the buildings they design are over-complex and full of junctions, just the type of structures that perform badly in an air pressure test.”

The actual test normally takes a couple of hours but there is usually a costly transport element to be taken into account because testers are thin on the ground and one test frequently takes up a full day. So the cost is typically around £300 plus transport for a single house, though it can be much less if there are multiple houses ready to test on the same site. “Most builders feel that they have carried out a reasonable job and are deeply suspicious of pressure testing. If the readings suggest that the house is leaky, they start questioning the accuracy of the equipment. So then we do a smoke test and this shows precisely where the leaks are. Then they believe.”

I was hoping to witness and photograph the test in Greenwich. But the traffic around the Blackwall Tunnel was gridlocked and after sitting in the van for two and a half hours without even reaching our destination, Keith aborted the mission and rescheduled for another day. With more than a touch of irony, I reflected on how the intention of an air pressure test is to save energy consumption, but London’s chaotic road system had ended up with us wasting rather a lot of fuel, achieving precisely nothing. That’s the politics of energy for you.

Homemade Actis Test Yields Good Results

Are reflective insulation foils all they are cracked up to be? Or are they are con trick?

It’s amazing that we still don’t have a definitive answer to this question, despite them having been around for 20 years or more. Most countries don’t accept the claims made by their manufacturers and refuse to let them be used as thermal insulation. But the UK has taken a more relaxed approach and has said that if the foils can be shown to a reputable independent body that they are as good as the traditional forms of insulation, then they will be acceptable for use on building sites across the land.

The problem for these foils is that when they are subjected to the standard method for testing insulation, known as the guarded hot box test, they don’t perform particularly well. They are not useless, it’s just that they can get to an equivalent to about 80mm of mineral wool and no more (doubling the thickness of these reflective foils has only a marginal effect on their efficacy) and that’s not enough to meet the current U-value standards. But in field trials, the foils can be shown to perform as well as 200mm of mineral wool or 120mm of polyiso board and, if these field trails are to be believed, then the foils are good enough to use as an alternative to mineral wool or the polyiso boards made by the likes of Kingspan and Celotex.

Now, the credibility of the foils in the UK has, to date, hinged on one field trial conducted by the main manufacturer, the French company Actis, and witnessed by Geoff Pitts of TRADA. Actis built two identical roof spaces, insulated one with 200mm of mineral wool, the other with their Tri-Iso Super 9 foil and measured how much energy was used to keep each roof at a constant temperature during a couple of winter months in the French Pyrenees. TRADA, who are an accredited third-party certifier of building products, were satisfied that the Actis product was as good as 200mm of mineral wool and, on this basis, Actis has been able to sell acres of Tri-Iso Super 9 to be laid inside the UK’s roofs. It’s particularly popular for roofs where its simple and quick to fit and it gets over the problem of there not being enough rafter depth to accommodate the traditional insulation boards for the foil-based product is only 20mm thick.

Manufacturers of traditional insulation have seen red and have attempted to rubbish the claims of Actis but to date, as far as I know, they have only ever repeated the guarded hot box test. No one has published the results of another comparison field test.

So it was with great interest that I received a letter last week from Charlie Duke, a house designer who lives in Devon, who was also troubled by the reflective foil debate and wanted to see for himself whether it really worked. He has run a series of tests over the winter measuring how long water will stay warm in a variety of insulated shells. The tests were carried out in his garden shed using very basic materials but shouldn’t be dismissed because of this. He wrote:

The temperature of 350ml of water inside two empty cat food tins was measured in half hour intervals. Each tin was enclosed with insulation with a claimed thermal resistance of 5 (equivalent to a U value of 0.2). As the reflective foil requires a 25mm air gap on the heated side, a 100x100x130mm box of 2mm MDF was made with fins to maintain the air gap. To make things as equal as possible another box (without fins) was set inside the 120mm of polyiso insulation. All joints were taped to prevent air loss. The water filled tins were placed on a layer of 70mm polyurethane foam.

The results were remarkable. The heat loss performance from the two tins showed that whilst 120mm of polyiso board consistently beat one 20mm layer of Actis Tri-Iso Super 9 foil, it was never by more than short head. And that the effect of using a double layer of Actis actually gave a marginally better performance than the 120mm of polyiso. Charlie Duke also tested two layers of Actis, with 25mm air gap between them: in theory this should be better than two layers touching each other but it didn’t show any appreciable difference.

Now just how much credence should be paid to Charlie Duke’s tests I am not sure. He makes no claims to being a hot-shot physicist and the nature of his tests, using Whiskas cat food tins for his water container, is probably a bit too Wallace & Gromit for some. But full marks to him for carrying out these tests and contributing to the debate. I spoke to him on the phone on Friday and was convinced that this is a genuine attempt to understand the complex science of heat loss and to make a reasoned comparison of two different systems: i.e. he is not being set-up by Actis. I hope that others are tempted to follow in Charlie Duke’s footsteps and try similar experiments themselves. Indeed, I am very tempted to try and replicate it here in Weston Colville but I suspect it may have to wait for another winter.

Bill Dunster's walls

Bill Dunster is, for want of a better word, an ecotect. That is an architect with a passion for green design. He sprang to fame with a project known as BedZed, a development of live-work units built on a disused sewage farm in Beddington, South London. It’s had acres of publicity and is rightly held up as an exemplar of how multiple housing units should be built in the future. But BedZed has now been finished for three years and Dunster has found it a hard act to follow. Developers and social housing landlords have not been falling over each other to repeat “the experiment” and he has been frustrated by planners who haven’t been prepared to loosen the planning corset just because a scheme is green.

But at last, Dunster has another scheme to showcase his talent. This time it’s a four-storey block of key worker flats on the St Matthews council estate in Lambeth, South London. It’s not quite as big or prestigious as BedZed but in some ways it’s more technically advanced. Building carried a feature on it this week, which caught my eye: in particular, the wall detailing. The outer walls are no less than 550mm wide, compared with just under 300mm in conventional housing. They are made up of 150mm blockwork inside 300mm of expanded polystyrene insulation filling the cavity, behind a 100mm brick skin, which is what them outside world sees. The plus point is of course that, with this much insulation, you have got a tremendously low U value – reckoned to be just 0.1W/K/m2. But against this, on a 60m2 apartment, you are loosing up to 25% of the footprint to walling.

Yikes. That’s a hell of a lot, especially considering we are being encouraged to build smaller and smaller units. Dunster is a big fan of heavy mass construction, which means little or no timber frame and little or no off-site pre-fabrication. He believes in the importance of thermal mass (concrete in other words) in regulating the temperature characteristics of a home and in reducing the effects of summer over-heating. But is thermal mass really such a wonderful concept that you have to loose 20% or more of your floor area just to accommodate it? If there wasn’t quite so much south-facing glazing — another of his betes verts — then perhaps the designs wouldn’t require quite so much thermal mass and the walls wouldn't have to be quite so thick.

Another problem comes with the adoption of a 300mm cavity, shown here in diagramatic form, (ref Building magazine). How do you manage the openings? In particular, how do you ensure that the water penetrating the outer brickwork is directed back out of the external wall rather than dripping down into the joinery? You’ve left the world of conventional construction far behind here: there are no off-the-peg wall ties this long and there are no pre-formed cavity trays this wide. Dunster’s solution has been to design a wrap-around cavity tray around each window. To me, it sounds just like the sort of detail which is likely to fall victim to sloppiness on site. Done perfectly, there should never be any problem but construction isn’t a perfect world.

One other feature of this article stands out. The costings. £1600/m2. This isn’t, in fact, far out of line with many other innovative social housing projects being built around the country at the moment, but it’s about twice the rate that selfbuilders hope to complete their projects for. Why the huge discrepancy? A good question, which will have to wait for another blog.

Temparature Rises in Insulation Debate

Testing the effectiveness of insulation is notoriously difficult. Mark Brinkley reports on the merits of laboratory vs field testing.

The world of thermal insulation is well used to professional rivalries and knocking copy, but a current spat between rival manufacturers threatens not merely competing products but the whole system of third party accreditation which underpins construction innovation in Britain.

At the heart of the dispute is the claim by the French firm Actis that it's top selling product, Tri Iso Super 9, a reflective multifoil only 25mm thick, is equivalent to 200mm of mineral wool. This claim, substantiated by BM Trada and thereby widely accepted by building control and the NHBC, is hotly disputed by manufacturers of competing products and now one of them, Celotex, has commissioned independent tests which it hopes will disprove it.

Behind the dispute lies a battle for the future of roof insulation. As the thermal regulations become ever tighter, designers and builders have been struggling to accommodate the necessary insulation within the roof fabric. Mineral wool is now rarely used because it's simply not efficient enough to give a sufficient U value within the rafter void. Foam board manufacturers, principally Kingspan and Celotex, have been benefiting hugely from the legislative changes but have also found that they have been loosing market share to reflective multifoil insulation.

Richard Crisp, marketing director for Celotex UK, said: "We became acutely aware that sales of Actis and other multifoils were growing. If it works as advertised, then we would like to be making it or, at least, selling it. So we set out to see if it was as good as it claimed to be." Celotex paid for samples of the Actis to be independently tested by the National Physical Laboratory in Teddington. There, it was subjected to the standard test for thermal insulation, known as the guarded hot box test. To be equivalent to 200mm of mineral wool, the R value measurement would have to have been around 5.0: in fact, Actis scored just 1.7. Used in a roof application, this would give a U value of between 0.5 and 0.6, whereas the latest Part L requirement is much lower, at just 0.2.

The NPL labs results were in line with test data from Germany, Canada and the USA for reflective multifoils. So how has Actis managed to achieve acceptance in the UK market and why have BM Trada seen fit to give it their stamp of approval? The answer lies in what is being tested and the relative importance of conduction, radiation and convection.

Actis claims that the guarded hot box test looks principally at rates of thermal conduction whereas their product works by stopping other channels of heat loss, principally radiation, convection and unplanned ventilation. They prove their product by building comparison roofs and measuring the heat loss from each. If a roof insulated with Tri Iso Super 9 can be shown to perform to the same standard as one built with 200mm of mineral wool, then the thermal characteristics must be equivalent. Actis last undertook such test in 1997. It was verified by BM Trada who checked the design, construction and the test data and subsequently gave it their stamp of approval.

On the back off this one test, multifoils have been given the green light to sell into the UK market as being equivalent to 200mm of mineral wool. Other multifoil manufacturers have only to prove themselves to be as good as Tri Iso Super 9 for them to be accepted. One such, YBS Super Quilt, has even used the National Physical Laboratory to show "a 13.7% better performance than its nearest competitor" without publishing what the actual test results are.

During the next 12 months, Part L is due to be tightened again. The multifoils are faced with another hurdle, for it will no longer be enough to be equivalent to 200mm of mineral wool. Rather, it is believed that the new standard will be around 250mm of mineral wool. Actis already have a new product in production, known as MultiPro TS 250, and testing is underway to show its performance characteristics. It's essentially the same product but with eight reflective layers instead of six. Industry watchers are awaiting the test results with interest, but few are anticipating anything different to the previous tests.

www.celotex.co.uk
www.insulation-actis.com (site in French)
www.ybsinsulation.com


Housebuilder's Update background:

Insulation: the murky science

Ever since thermal conductivity rates were first measured and compared, there has been debate about just how relevant they are. It is accepted that heat moves through the building fabric via four different methods, conduction, convection, radiation and ventilation, and also that the more complex the structure, the more difficult it is to model what is happening within it. There is also likely to be a huge discrepancy between laboratory results and as-built performance, similar to what has been found with sound insulation designs. The regulatory authorities accept laboratory measurements as gospel: there is remarkably little post-completion testing carried out, if only because a whole building is such a complex system that it's impossible to isolate the effect of one particular component.

The field is therefore open to anyone who can show that their product performs as well as, or better than, a competitor.

Reflective multifoils are being sold as the answer to every roof builder's problem: how to build in really good insulation levels without having to deepen the roof profile. They hardly rank as an innovation, having been around for decades, but until recently they were always seen as being too expensive compared to the wools and foams which builders more normally work with. But with Part L demanding ever increasing depths of insulation, multifoils have come into their own and are they are now taking an increasing market share.