I think it might be time for a bit of explanation of why we want to wrap a perfectly good bungalow in straw. Because - like so many buildings - it isn't actually perfectly good. It is structurally sound and mostly weather proof, both of which are very good things, but it is also cold, very poorly insulated, would require huge amounts of energy to heat, and is massively prone to condensation-related damp problems. By wrapping it and extending it with strawbales (along with associated works to insulate floor and roof, and careful attention to details of design and construction) we aim to produce a home that is super-insulated, so requiring very little energy to maintain comfort levels, and remaining pleasantly cool in summer. When necessary in the coldest parts of winter top-up heat would come from a highly efficient and beautiful masonry stove in a central position (more on these at some point in the future) from where heat can easily circulate throughout the home.
But why straw and not some more common kind of insulation material or construction technique? I've covered this is general terms before, so this time here are some numbers to back it up. The key figures here relate to the insulating capabilities of materials and to their embodied energy. Embodied energy represents the amount of energy used to make a product, including extraction of raw materials, processing, transport, installation etc. There are different means of calculating this ranging from "cradle to gate" - preferred by some manufactures at it gives the lowest figure as calculations stop at the point the product leaves the factory, to "cradle to grave" preferred by most environmentalists as it provides the most accurate estimate of the energy consumed in a material's entire lifespan. Generally, the higher the embodied energy of a product the higher the associated carbon emission from it's production and use, though this could potentially vary depending on the processes and source of energy used. Even when renewable energy is used it is still important to move towards low energy use throughout production though, in order to ensure sustainability of energy supply.
The table below shows a comparison of the embodied energy of strawbales and other more common insulation materials, along with their thermal insulation capabilities (thermal conductivity = how good a material is at conducting heat, U-Value = heat loss through a material over a square metre and at a certain thickness; both are calculated with a difference in heat of one degree Kelvin from one side of the material to the other, and in both cases, the lower the number the better the insulation)
Although strawbales conduct slightly more heat than the other materials if used at equal thickness, the incredibly high relative embodied energy values of the other materials render them undesirable, especially if used in thickness comparable to a strawbale. I believe this table also shows the woeful inadequacy of relying on cavity insulation alone. The thickness of brick wall and cavity used above is based on the construction of the bungalow we plan to wrap, and the strawbale thickness is based on standard size of small bales in the UK.
For our bungalow, we will probably have to fill the cavity before wrapping to be sure no heat is lost through circulation of cold air through the cavity. This combined with the strawbale wrap should result in a very low U-value, and a lovely warm home. To insulate the cavity we will probably use blown glass-fibre manufactured from left-over loft insulation blanket, which is itself made from 100% glass (Instafibre yellow wool - although not clear on their website the manufacturers assure me this is how it is made). I can't yet find any info on embodied energy of this, but the thermal conductivity is 0.39 W/mK, similar to the mineral wool in the table above.
But why straw and not some more common kind of insulation material or construction technique? I've covered this is general terms before, so this time here are some numbers to back it up. The key figures here relate to the insulating capabilities of materials and to their embodied energy. Embodied energy represents the amount of energy used to make a product, including extraction of raw materials, processing, transport, installation etc. There are different means of calculating this ranging from "cradle to gate" - preferred by some manufactures at it gives the lowest figure as calculations stop at the point the product leaves the factory, to "cradle to grave" preferred by most environmentalists as it provides the most accurate estimate of the energy consumed in a material's entire lifespan. Generally, the higher the embodied energy of a product the higher the associated carbon emission from it's production and use, though this could potentially vary depending on the processes and source of energy used. Even when renewable energy is used it is still important to move towards low energy use throughout production though, in order to ensure sustainability of energy supply.
The table below shows a comparison of the embodied energy of strawbales and other more common insulation materials, along with their thermal insulation capabilities (thermal conductivity = how good a material is at conducting heat, U-Value = heat loss through a material over a square metre and at a certain thickness; both are calculated with a difference in heat of one degree Kelvin from one side of the material to the other, and in both cases, the lower the number the better the insulation)
| Material | Embodied energy in MJ/kg (megajoules per kilogram) | Thermal Conductivity W/mK (watts per metre Kelvin) | U-Value W/m2K (heat loss in Watts per square metre of material) | Wall construction used for U-value calcuation |
| Strawbale | 0.24 | 0.06 | 0.123 | strawbale wall 475mm thick, plus lime render and clay plaster finish |
| Mineral wool | 16.6 | 0.04 | 0.57 | Brick built cavity wall, (2 brick walls 102mm thick plus plaster, filled cavity 50mm thick) |
| Expanded polystyrene board | 109.2 | 0.04 | 0.57 | " |
| Polyurethane foam board | 101.5 | 0.028 | 0.44 | " |
| Brick (average UK brick) | 3 | 0.84 | ------- | ------- |
Although strawbales conduct slightly more heat than the other materials if used at equal thickness, the incredibly high relative embodied energy values of the other materials render them undesirable, especially if used in thickness comparable to a strawbale. I believe this table also shows the woeful inadequacy of relying on cavity insulation alone. The thickness of brick wall and cavity used above is based on the construction of the bungalow we plan to wrap, and the strawbale thickness is based on standard size of small bales in the UK.
For our bungalow, we will probably have to fill the cavity before wrapping to be sure no heat is lost through circulation of cold air through the cavity. This combined with the strawbale wrap should result in a very low U-value, and a lovely warm home. To insulate the cavity we will probably use blown glass-fibre manufactured from left-over loft insulation blanket, which is itself made from 100% glass (Instafibre yellow wool - although not clear on their website the manufacturers assure me this is how it is made). I can't yet find any info on embodied energy of this, but the thermal conductivity is 0.39 W/mK, similar to the mineral wool in the table above.
(Sources for figures in the table above: http://www.homegrownhome.co.uk/pdfs/Lecture%20notes%20V1%2031%20March%202011.pdf; http://www.greenspec.co.uk/insulation-mineral.php; http://www.greenspec.co.uk/insulation-oil-derived.php; http://www.cba-blocks.org.uk/tech/tech_uvalue.html; http://people.bath.ac.uk/cj219/)
Whole house U-value calculations are affected not just by insulation but by airtightness - if there are lots of holes then hot air will escape and cold air will get it. These can be avoided with careful design and construction. The problem then is that moisture vapour (from breathing, or evaporation of any liquids in the house, and baths, showers, cooking etc) can't get out. Another wonderful thing about strawbale walls, plastered with clay and rendered with lime render, is that they are moisture vapour permeable - they won't let the rain in but they will let the damp out, so avoiding condensation and associated mould. Heat Recovery ventilation can aid this, but that's one for another blog.
While researching the statistics above I read one of the most ridiculous sentences I've come across in the area of environmental impact assements for materials: "Embodied energy becomes important only when high levels of operational energy efficiency have been achieved... In these instances embodied energy can increase to around 10% of overall energy expenditure" (http://www.greenspec.co.uk/insulation-introduction.php).
Sorry, what? Presumably embodied energy of construction materials "isn't important" in other cases because it is so dwarfed by the amount of energy consumed in poorly insulated heating buildings. Okay, possibly there is a fair point there that the most pressing case in those instances is to increase insulation and reduce energy use of the building. But, essentially it's also saying that if a building is going to be poorly insulated and consumes masses of energy and as a result causes the release of lots of CO2, it might as well consume loads of energy in its construction too. This does not seem to me to be a sensible approach. Surely any reduction in energy use and concurrent CO2 emission at any stage is to be encouraged and welcomed? Rather than "don't bother, it's a rubbish house anyway". I get extremely frustrated by this kind of woolly logic.
Whole house U-value calculations are affected not just by insulation but by airtightness - if there are lots of holes then hot air will escape and cold air will get it. These can be avoided with careful design and construction. The problem then is that moisture vapour (from breathing, or evaporation of any liquids in the house, and baths, showers, cooking etc) can't get out. Another wonderful thing about strawbale walls, plastered with clay and rendered with lime render, is that they are moisture vapour permeable - they won't let the rain in but they will let the damp out, so avoiding condensation and associated mould. Heat Recovery ventilation can aid this, but that's one for another blog.
While researching the statistics above I read one of the most ridiculous sentences I've come across in the area of environmental impact assements for materials: "Embodied energy becomes important only when high levels of operational energy efficiency have been achieved... In these instances embodied energy can increase to around 10% of overall energy expenditure" (http://www.greenspec.co.uk/insulation-introduction.php).
Sorry, what? Presumably embodied energy of construction materials "isn't important" in other cases because it is so dwarfed by the amount of energy consumed in poorly insulated heating buildings. Okay, possibly there is a fair point there that the most pressing case in those instances is to increase insulation and reduce energy use of the building. But, essentially it's also saying that if a building is going to be poorly insulated and consumes masses of energy and as a result causes the release of lots of CO2, it might as well consume loads of energy in its construction too. This does not seem to me to be a sensible approach. Surely any reduction in energy use and concurrent CO2 emission at any stage is to be encouraged and welcomed? Rather than "don't bother, it's a rubbish house anyway". I get extremely frustrated by this kind of woolly logic.
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