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Saturday, 17 December 2011

Pros and Cons of Sheep Wool Insulation

I really don’t envy the Babylonians. Apparently, about 6000 years ago they wore one of the earliest examples of cloth woven from sheep wool. Going by some of my sweaters it is no wonder they became the byword for everything nasty. All that itching and scratching must have driven them mad.

Even so, since this isn’t a blog about apparel, why should wool be a topic at all? The reason is we cannot talk about renewables and forget conserving energy. For someone with a heart for ecology both go hand in hand.

Air is Warmth

With the advent of synthetic fibres wool lost much of its importance and market share. I remember times when paying a shearer cost more than the fleece was worth. In certain regions that may still be the case but wool has found at least one new use. As a natural material it has gained in popularity because of its insulating properties and eco-friendly credentials. After all, sheep withstand the harshest conditions protected by it so it must be pretty effective.

The trick wool has up its sleeve is the waviness of its fibres. The crimp, as it is called, means they create a three-dimensional latticework that traps air in tiny pockets. Air, in turn, is a very good insulator, and that benefit can be carried over for walls, ceilings and roofs. From that point of view it hardly seems worth – or indeed appropriate – to question the qualities of the material. It is from a natural source, it grows back and is, therefore, truly renewable, not to mention recyclable. For environmentally friendly, low-impact buildings, such as passive or zero-carbon houses, these characteristics are perfect. Insulation works both ways. What keeps the cold out in winter, keeps the heat out in summer. In addition to that sheep wool can absorb moisture but doesn't become any less insulating, in contrast to artificial material such as glass wool. Some sources talk about a third of its own weight without compromising performance. A balanced climate is the result.

Sheep with wind turbines

Know Your R-Value From Your U-Value

So how good is sheep wool insulation really? Regulations regarding climatic performance of buildings vary the world over. Consequently, it makes more sense for me to give you a tool with the help of which you can evaluate whatever is available where you live. For that to work you should be familiar with the terms R-value and U-value. As we are becoming more and more environmentally conscious, you will encounter them on a regular basis.

R-value is the measure of thermal resistance. Therefore, it quantifies how much (or little) heat a material holds back. The unit generally used is m²*K/W (square-metres * Kelvin per Watt) or in America ft2*F*h/Btu (square-feet * Fahrenheit * hour per British thermal unit). Just take the variables of area (m2) and degrees Kelvin as the temperature difference between inside and outside. Then you have one Watt as the energy benchmark against which the others are measured. The higher the R-value is, the better.

U-value on the other hand is, according to John Brennan, Senior Lecturer in Environmental Design at the Edinburgh School of Architecture and Landscape Architecture, “…[the extent] of heat loss in a building element such as a wall, floor or roof. ... A low U-value usually indicates high levels of insulation. [It is] useful as … a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials.” What he means is, something like a wall consists of several components, for example, inner and outer insulation, bricks etc. With this measure at hand you don’t need to sit down and calculate the insulating properties of each part, instead you can see the performance of the entire wall. Mr. Brennan continues: “The U value is defined as being reciprocal of all the resistances of the materials found in the building element.” The unit is W/m2*K, and when you go back a bit you will see that it is the exact opposite of the R-value.

Of course, these formulas are all very well, but how are they applicable to sheep wool and its role as an insulation material? Unfortunately, that’s not simple to answer.  Not only does it depend on the thickness of the material but also the way it is processed and used. For example, British building regulations demand 270 mm insulation material between joists for insulating a loft. However, the timbers are usually no higher than 150 mm. If you put wool to the required thickness in between but then squash it so you can put boards on top to create a storage area you will have forfeited almost all the material’s insulating qualities by squeezing out the air, which we have seen already, is the most important factor. Only raising the height of the joists would help.

Those 270 mm of sheep wool would achieve a U-value of 0.16. As a comparison, a solid brick wall of about 220 mm weighs in at 2.0, double glazing at 1.6 - 2.5 and triple glazing at 1.0 - 2.0. Specially made windows for zero carbon houses can be brought down to 0.6.

Other examples are

  • Bricks                  =         0.56 – 0.77
  • Concrete             =         1.13 – 1.93
  • Plasterboard        =         0.25
  • Mineral wool        =         0.042
  • Polyurethane      =          0.025
  • Cellulose             =         0.12 – 0.35
  • Hemp + lime        =         0.1

Please note that these are guides only. As mentioned already, thermal conductivity is very much dependent on thickness and density of the material. The higher the density the easier energy can travel and, consequently, the poorer the insulating effect. The same raw material achieves discrete U-values in individual products.

Environmental Credentials

All is well then. Sheep wool is natural, renewable and great for retaining heat in winter and keeping it out in summer. It is also inherently flame retardant and self-extinguishing. These benefits are not the only side to it, though. While wool itself is without doubt an environmentally beneficial material, looking at the bigger picture reveals some troubling aspects.

Some sources talk of resin with which the fibres might be bound so they can be made into building products. However, talking to a representative of the Centre for Alternative Technology I was assured that such a binder is not normally used, at least here in Europe. What has been put under the spotlight is Borax, a naturally occurring substance that is used to treat the wool for prevention of attack by insects, especially moths. It is also antifungal. There is no evidence for any acute risks to humans, even small amounts ingested cause effects no more severe than skin rash, nausea or diarrhoea. High doses have triggered developmental and reproductive damage in laboratory animals but going by current knowledge the risks from treated sheep wool seem extremely small.

It depends on how far you want to go down the manufacturing path backwards to check the environmental credentials of sheep wool insulation but there are other factors that might influence you decision. The raw material has to be cleaned and prepared. That process in itself uses chemicals but there are also residues from substances on the wool. I am thinking of organophosphates from sheep dips. They are necessary to protect the animals against parasites such as scab mites. The active ingredient is a contentious issue, as it has been linked to severe health problems in farmers. ‘Why not stop using it then?’, you might ask. Believe me, if you ever see a sheep with scab and the suffering the disease causes it will make you think again.

That leads us right down to the basic issue of livestock farming and impact. Do we damage our environment by encouraging farmers to keep more sheep because they can sell the wool at a good price to companies that make insulation products?

Devil or Angel?

As usual, there are pros and cons, and you will have to decide for yourself whether the positive qualities are enough to outweigh the negative aspects. In my opinion you can forget about the question of farming and having an indirect influence on it as a customer through buying insulation. Apart from Australia, where sheep farming is really geared towards wool production, in most parts of the world it is just a by-product. Therefore, you’d be purchasing something that is there anyway. No one would keep more sheep as a result of this new opportunity, because the wool alone could not offset the costs and risks involved. It only means a useless material can be made into something of value.

What you might take into account before you make a choice is the question how far the raw materials and end-products have travelled. After shearing wool might be sent to another country for preparing and manufacturing before being carted back to where it started from for sale in the shops. Only, all these concerns apply in one way or another to every insulation product. Nevertheless, if all that makes sheep wool insulation a far too scary proposition, consider cellulose or hemp.

I will keep an eye on the question of Borax and will update you on developments as they become available.


See you next week.


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I have used Sheep Wool Insulation in my previous house. It had both the pros and cons. It would vary person to person what exactly they want when it comes to total insulations.

It is very crucial to look at all the heating and cooling aspects of an insulation type as the house inhabitants would be experiencing the outcomes for many years to come. This post is really informative. Thumbs up.

Thanks for your kind comments and interest. It would be great to have you back in the future. As I always say, please don't hesitate to suggest topics. I am always open for suggestions.

The beauty about insulation is that many technologies are still in the early stages, whereas saving energy by covering walls or roofs, for example, can be done NOW.

I totally go along you! I have often felt that way but nobody really would like to take it as seriously as we do apparently. Excellent blog anyway, I am going to have to stop by more frequently.

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