Insulation

The Hot Cafetiere Test

There has been much debate over the use of multifoil insulation, much of it ill informed because information is scarce. If heat escapes by conduction then a material with low thermal conductivity, such as air, would seem to offer good insulation. But if heat leaves by infrared radiation then a reflective surface on an opaque material, such as aluminium foil, should do the job. It is suggested that several layers of foil separated by thin layers of non-conductive material form a worthwhile insulator in buildings. If this is correct, then multifoils offer a viable option, with the advantage that less space is required than the traditional foam and fibre materials.

Multifoils are being increasingly used in buildings, particularly in space-limited situations such as loft conversions. But some regard their use with suspicion, since, it is said, there is a lack of support in the form of test data provided by organizations that are independent from the manufacturers.

To begin to get some idea of the relative performance I conducted the experiment described below. The experiment is limited, very simple, using equipment that was available in my kitchen, and I make no claim for accuracy and reliability. Rather, it is hoped that the work encourages others to replicate and improve on the study, leading to the production of useful information.

The experiment compared the cooling rate of water in a container wrapped in multifoil and in polyethylene foam. A glass cylinder, from a coffee making cafetiere, was placed of a double layer of aluminium cooking foil on a cork tile on an oak board. A lid, not very tight fitting, was made of a piece of cork tile and a double layer of foil. A laboratory thermometer was pushed through a hole in the lid. 1.7 litres of water was boiled in a kettle and poured into the container. The temperature was recorded at one minute intervals for two hours.

In the first test run no other insulation was added.

In the second test run a jacket, about 10mm thick, comprising 11 layers of foil separated by ten layers of newspaper, was wrapped round the cylinder.

In the third test run sheets of fine-celled polythene foam packaging material forming a jacket about 50mm thick was used.

The experiment was conducted in a room with no strong draughts. Or the first two runs the room temperature was about 22°C but for the third run the room temperature had risen to 24°C. This ambient temperature difference would have resulted in a very slight slowing of the temperature fall in the third run. A further error may have been introduced if the lid had fitted more or less tightly in the different test runs, allowing varying amounts of evaporative cooling.

With no insulation the temperature dropped to 50°C after two hours. With the multifoil insulation the temperature dropped more slowly, reaching 59° in two hours. On the third run, with the foam jacket, the temperature dropped very slowly, only reaching 74°C after two hours.

It is evident that, while the 10mm eleven layer multifoil produces a significant insulating effect, it was not as insulative as the 50mm of polythene foam. The preliminary conclusion has to be to cast doubt on the efficacy of multifoils for insulating buildings. However, it could be that the choice of newspaper as the separating material between the foil layers was ill advised. If the newspaper allowed the transfer of heat by conduction from one foil to the next this would have adversely affected the multifoil’s performance. Perhaps a better separator would be a thin openly woven fabric or thin foam sheet such as used in commercially available products. Fewer foil layers may then be found sufficient.

There may be an optimum number of foil layers and interleaving foam sheets, which would make use of both the infrared opacity and reflectivity of foil and the low conductivity of foam, while making efficient use of space. Further studies should be encouraged in this area. The suppliers of one of the leading products, Tri-Iso Super 9, claim that laboratory hot-box tests do not adequately represent conditions, and hence product performance, in real buildings.

The Hot Cafetiere Test (Part 2)

In the light of the experiment described above, I decided to run a fourth test, using a combination of foam and foil layers. A thinner foam jacket, of about 25mm, with four interleaved layers of foil was constructed. The resulting cooling curve was very close to that of the foam only jacket - but this was with a jacket about half the thickness. I therefore conclude that alternating foil with thin layers of material of low thermal conductivity provides an overall insulation performance equivalent to a conventional insulator but occupying significantly less space. The claims of the makers of Tri-Iso Super 9 should be taken seriously, though with prices of over £10 per square metre such materials may only be worth while in situations where space is at a premium.

Let me know if you want the raw data on an Excel file.

The Hot Cafetiere Test (Part 3)

And now, thanks to the kind generosity of MJ of Daventry who sent me an off-cut of Tri-Iso Super 9, I have been able to run a fourth test with the real deal. A jacket of the TIS9 was wrapped round the cafetiere as before. The new cooling curve, the pale green one, goes between the home-made multifoil and the two jackets using foam. The insulation was probably a little more compressed, at 20mm, than ideally and the limitations of the 'hot box' described by Actis may apply equally to a 'hot cafetiere' test. The room temperature, at 19°C, was cooler than in the other tests. The first conclusion, however, is that TIS9 does not work as well as a thicker foam insulation but if space is seriously limited then this may be a useful material. Closer inspection of the cooling curves shows a curious anomaly. The TIS9 curve flattens out more quickly than the others, i.e. the rate at which the rate of cooling slows is quicker. If the test run had been continued and the trend persisted, the curves would have crossed each other. The behaviour of the TIS9 is showing a subtly different dynamic. So, as they say, more research is called for.