Friday, December 01, 2006

Role of internal foil layers in Multifoil

In summary the internal foil layers of a multifoil, defined as those between the inner and outer surfaces, are to a large extent redundant. This was demonstrated in an earlier experiment when 2 layers of foil either side of 12 layers of newspaper performed equally well in insulation terms as a typical multifoil. (see previous posting). Although it seems obvious to me why they have little of no effect, I attach a schematic diagram of what I think happens, without recourse to complex heat transfer calculations.

Aluminium foil is an excellent reflector of radiation with upwards of 90% efficiency in reflectivity often quoted. For this example I choose a conservative figure of 90% to illustrate my point, higher figures will make the argument even stronger.

So, assuming the foil reflects at least 90% of incident radiative energy then the remaining 10% of the energy will be absorbed by the 1st foil layer, 1% by the 2nd layer, 0.1% by the 3rd and so on(assuming all energy is transferred by radiation). With this argument alone is is clear that the second and subsequent layers have only a very small role to play. However, any radiation which is adsorbed by the first layer can be dissipated by any of the 3 heat transfer processes (convection, conduction and radiation). Now aluminium is an excellent conductor and will rapidly conduct the heat energy throughout the layer and most importantly to any cold bridge - eg a hole, a joint or any conductive bridge to the exterior. This will lead to even less of radiation energy penetrating the first insulating layer.

Nevertheless lets assume that some radiative energy is transmitted by the first foil layer into the interior of the multifoil, this will be reflected by the next layer with the same effeciency as the previous one (90%) and because aluminium is a good reflector this radiation will continue to be reflected back and forth with the same efficiency until it is eventually adsorbed and conducted away.

Therefore I conclude that radiative heat transfer though the interior of the multifoil cannot occur to any significant degree and conduction and convection must be the main heat transfer processes through the bulk of the material.

Note: The internal foil layers however do have a negative effect on the overall heat transfer process, because they exacerbate heat loss via conduction to the nearest cold bridge. This probably counterbalances their only known positive effect, namely that of reducing convective heat transfer between two insulating layers. However this could be achieved much more effectively, and no dount cheaper, by using a non-conducting film such as plastic.


Thursday, September 07, 2006

Charlie Duke's Data


The temperature Time curves for Charlie Dukes Hot Box experiments as supplied by Mark Brinkley are attached.

The early parts of the curves are very similar whilst later on in the experiments differences start to show themselves. (at least in the first 2 sets of experiments. The Curves do not follow the normal decay curve expected for an insulation experiment. There is a "kink" or change in the heat transfer process after 1 to 2 hours. This looks like 2 parallel processes operating with different time constants.

My view is that the early part of the curves are more or less identical to each other which suggests that this is the time taken for the whole system to reach steady state. Given the smallness of sample (350g) the relative low Delta T (<20c)>

Thursday, August 31, 2006

12 Layers of Newspaper

Attached are some additional results on the insulation properties of newspaper! This follows a suggestion by Tony in the green building forum.

12 layers of old newspaper were lightly crumpled and sealed together with tape and then used to encase a carton box. The overall thickness of the combined newspaper layers was between 10mm (base) and 20mm (side). This was similar to the multifoil dimensions. The carton box, and its contents were identical to that described in the previous posting. The only difference between this and previous experiments was a slightly higher external temperature of 23C. The experiment was run for approximately 6 hours.

In addition I also repeated the experiment with 2 layers of aluminium foil to provide a reflective barrier either side of the 12 layers of newspapers.

The results are summarised in the attached graphs. The insulation properties of 12 layers of paper are not surprisingly better than the carton box alone, but slightly worse than the multifoil. However, by placing one layer of aluminium foil on the inside and another on the outside of the newspaper, the insulation properties were identical to the multifoil material.

Figure 4a shows the temperature time data plotted on the same graph as in the previous posting. To facilitate comparison Fig 4b shows the same curves without the datapoints, but normalised to a common start temperature and start time. This provides a simple visual comparison of all the experiments and clearly illustrates how multifoil insulation is a very expensive alternative to newspaper and kitchen foil!

For those who are still not convinced the data is linearised in fig6a by plotting log(T-Te). The slope of the multifoil and the newspaper + foil are, within experimental error, identical.

Why should only 2 layers of foil have the equivalent effect of the 12 layers used in multifoil? The answer I believe is simple. The 10 inner layers of foil have no insulation properties, all the reflective effects are produced by the outside layers. There is absolutely no insulating benefit of having them there at all. On the contrary, the inner layers could well reduce the insulating properties of the foil. Since aluminium is such a good conductor the inner layers will conduct the heat away to the nearest cold junction (a compressed edge, a join or hole) rather than reflect it back 'through' the first layer.

Given the cost of waste newspaper (free to a good home) it would make a very interesting costbenefit analysis and no doubt an extremely good business case - also not a bad use of recycled paper.


Thursday, June 22, 2006

Comparison of Multifoil Insulation vs Celotex in a Model Experiment

Introduction

I am in the process of a having a barn converted and have been intrigued and also highly frustrated by the debate on the pros and cons of multifoil insulation material. The builder who is leading the work on my barn conversion suggested we consider using multifoil insulation about a year ago - it was very easy to apply, more expensive but much more efficient. It sounded very promising and ideal for a wooden barn conversion where there was little space and we wanted to expose as much of the timber work as possible. I became aware of the differences of opinion about this new material after doing a quick search on the web, but I put it down primarily to competitor infighting, not uncommon with a new technology.

As a physical chemist I could see the theoretical benefits of using reflective materials to reduce heat loss and was very tempted to go ahead, despite there be no conclusive evidence of it’s effectiveness. The new insulation material clearly has a potential advantage for barn conversions and I was eager to get on with it - that was until I read the discussion on Multifoil insulation on the green building forum http://www.greenbuildingforum.co.uk/. The posting by "Biff" on his kitchen experiment to measure the insulation properties of foil vs bubble wrap and a few other materials, left me with some doubts as to the insulation benefits of the multifoil. This together with some of the arguments by the different suppliers led me to carry out my own experiments which I hoped would give me the reassurance that the foil insulation was the way to go. I therefore set up an experimental model to simulate heat loss from a sealed container in which only the insulation material was varied.

The suppliers of the particular multifoil material claimed that it was equivalent to 100ml of celotex. Despite this claim, I chose to compare the foil with a layer of 50mm celotex, expecting to achieve a superior result.

Experimental Approach

Details of the various setups are shown in figure 1.

A 5 litre plastic container filled with hot water was centrally located inside a similar shaped 10 litre plastic container and insulated with either celotex or multifoil. The smaller container was isolated from the larger container by wooden spacers, creating an air gap of c. 20mm between the two containers. After the hot water was placed in the inner container, a 50mm thick tightly fitting lid was put in place and the temperature recorded with a digital thermometer with the sensing tip located c. 20mm below the surface of the water. The temperature of the hot water was monitored over a period of 5 or 6 hours and in 2 experiments overnight.

The lid and base support for the experiments were always the same and made from Celotex. This was purely for convenience since there was a risk that heat loss round the thermometer area could have been different for the different materials. Keeping the lid constant avoided this complication. The principle variable was the insulation material around the sides and bottom of the 10 litre plastic container.

After a number of feasibility experiments 4 controlled experiments were carried out.

1. No insulation material - The 10 litre container was simply encased in a cardboard box (the same box as used to support the multifoil in a later experiment). The top and base was Celotex as in all subsequent experiments, but there was no insulation material around the sides.

2. Multifoil insulation – A layer of a propriety multifoil insulation material (20mm thick, 14 layers of foil) was wrapped around the carton box as used above. The edges of the foil were overlapped by about 5cm. and sealed with "duck tape" (Figure 2)

3. Celotex insulation - A configuration in which the 10litre plastic container was encased by a Celotex box fabricated from 50mm Celotex board material. All joints were sealed with duck tape(Figure 3) In this configuration, no carton box was used.

4. Multifoil Insulation repeat - A repeat of exp. 2 with multifoil but to avoid possible effects of crimping of the foil by the tape, the insulation material was sealed without any compression by simply “butt jointing edges using using silicone sealant. As far as possible the insulation material retained a constant thickness of c. 20mm.

For each of the experiments water at approximately 80C was poured into the container, the lid and thermometer inserted and the temperature monitored over a period of 4 to 5 hours. In both experiments 3 and 4 the temperature was also measured the next day (about 18 hours after set up).

In addition to measuring the temperature of the water, the external temperature was also periodically checked. The ambient temperature varied from 11C to 18C depending on the time of day. By using a high initial water temperature, the effect of external temperature could be minimized. Furthermore In experiments 3 and 4 the ambient temperature variation was almost identical.

Results

A summary or the experiment results is given in the graphs (figures 4 and 5). The starting temperatures varied slightly and therefore the data is adjusted along the time axis so that all graphs intersect at a common temperature for ease of visual comparison. Figure 4 showing the detailed temperature change in the first 4 to 5 hours and figure 5 showing that these differences are maintained over an 18 hour period for experiments 3 and 4.

All curves show a logarithmic decline in temperature as one would expect for a heat loss experiment. However, there is a slight deviation for logarithmic behavior in the first 15 to 20 minutes of setting up the experiment. This is attributed to the time it takes for the heat flux to equilibrate. The temperature of all materials needs to come to equilibrium and this leads to a slightly higher apparent heat loss in the first few minutes. After everything has settled down then the curve is logarithmic. For the technically minded this is demonstrated in figure 6, in which the logarithm of T-To ( ambient temp) vs time is clearly linear.

The data is quite conclusive - the multifoil insulation is not as good at retaining heat as the 50mm celotex although clearly it is better than no insulation at all. The repeat experiment for the multifoil insulation with a slightly different sealing approach gives almost exactly the same results. The foil behaves almost 50% worse than the celotex despite claims to the contrary. The time for the system to drop by a fixed temperature is nearly twice as long for celotex vs multifoil foil.

Conclusions

This set of experiments was designed to compare the two insulation materials at heat retention under as near identical conditions as possible. My objective was to provide data which I could use to make a simple decision on which material to use for insulation of my barn. Despite repeat experiments under near ideal conditions the Multifoil insulation was clearly inferior to the celotex. Although I was disappointed with the outcome, because of the practical consequences, the decision on which sort of insulation material to use was obvious.

In hindsight I am not surprised by the result, since if the foils claimed benefit is due to it’s reflective properties, then logically only the first layer will have any real effect. Any subsequent layer reflects heat back into the material itself – this heat is trapped and the foil heats up. Heat loss would then be by conduction along the various sheets of aluminium foil.

The controversy of multifoil as an insulation material will no doubt continue in the building trade. However, as a simple consumer and end user of the building it has to protect, I am sorry to say that multifoil insulation would not be my first choice. Only if space was limited and the structural configuration complex, would I consider multifoil as an alternative, but then as a last resort. It is just not cost effective compared to other materials.


Fig 1a - Experimental Set up.

Fig 1b Plastic Containers used to provide the heat source and the air gap



Fig 1c Plastic Containers Assembled with lid and Digital thermometer


Fig 2 - Multifoil Insulation Set Up


Fig 3 Celotex Insulation Set Up


Fig 4 - Graph of Temperature vs Time 6 hours duration


Fig 5 - As for fig. 4 including 18hour data for 2 experiments


Fig. 6 Logarithmic graph for T-To vs Time