Green External Wall Insulation ?
Written by admin on April 20, 2009 – 3:12 pm
NBT – Natural Building Technologies – EWI
This system is based on Wood Fibre insulation and hence contributes to reducing the CO2 compared to materials such as expanded polystyrene (EPS) that has embodied energy. According to Bath University’s Inventory of Carbon & Energy (ICE) high impact polystyrene has embodied carbon of 2.8kg CO2 /kg which is petroleum based. Whilst NBT’s Diffutherm is made from waste wood and therefore carbon negative – it locks up “1.2 tonnes of CO2 for every tonne of boards produced”.
This sound like a good direction to go, providing it doesn’t cost me an arm and a leg.
Have you considered using a hemp & lime mixture for the external rendering? 50mm thickness of hemp & lime render applied to a 9in solid brick wall could halve its u-value, insulation board can also be applied between the solid wall and external render to further reduce u-values. Details can be found at this website
http://www.lime-green.co.uk/images/ProductMedia/Hemp_and_Lime_Brochure.pdf
Thanks for pointing me to this very interesting material, and I will investigate further. The brochure doesn’t give a u-value for a cavity wall filled with rockwool, so will need to get some calculations. In particular I’m interested in rainscreen cladding and would like to know whether this material can be used with this approach. Ultimately, it will depend on how much it costs – if its double the price of say, Celotex, then I have to decide what other energy/CO2 saving option drops by the wayside and whether holistically, that is the right decision.
50mm of hemp lime reduces the insulation value to 1.05 according to the website Tanith quoted. 50mm of Rockwool reduces it to 0.52, half again, and 50mm of Celotex reduces it to 0.4. Hemp line is not a very good insulant and it is very expensive, more so than Celotex which is more expensive than Rockwool.
“So you pays yer money and yer makes yer choice.”
If we don’t use oil to manufacture insulation it will be burnt in car engines. So, surely it is better to use it for insulation and sequester it for the life of the building, which could be hundreds of years.
It would be very interesting to see a comparison of thermal efficiency on materials with a high embodied versus those with lower ones. Phenolic foam (eg Kingspan) usually has superior thermal performance to PIR’s such as Celotex, and glass fibre usually has superior perfomance to rock fibres. All of these materials would have relatively high embodied energy, especially rock and glass fibre, but if their thermal efficiency is superior then would the life time CO2 reduction of the product perhaps work out as more environmentally friendly than a less efficient green product? I haven’t seen enough information either way to be certain, but looking quickly at the u values Diffutherm quote, they would appear to be inferior to boards such as Celotex and Kingspan.
This is a very interesting comment with lots of potential for an application or documentation to give accurate advice. It seems to revolve around the following factors :-
Embedded Energy
Thermal Efficiency – savings on CO2 / Kwh
Purchase Cost as related to U-value
Lifetime
The CO2 factor will vary depending on the baseline. For example, if the current heating system is oil based, then the CO2 saving will be more than gas. Using kw hrs has less variation – you know how much you were using before and can monitor what you are spending afterward the change.
The end goal for such an application would be to give a “price / performance” which will vary depending on the initial baseline (as mentioned above) and chosen factors.
If kwh is the criteria (and CO2 will vary depending on the base line), then you will get a kwh saving which then has to relate to the kwh of the embedded energy of the material used over the given lifetime.
CO2 and energy cost savings will vary according to the base line which then need to be compared to the purchase cost.
Hence for a given lifetime :-
energy saved > embedded energy
Costs saved > energy cost over given lifetime
There is yet another factor which is the amount of CO2 used to manufacture the insulation which could be used to compare with the (variable) CO2 saved over a given lifetime.
Who ever said this was easy ?
Embodied energy , longevity and u values are all important, but there are other pollutants involved in the manufacturing. Not so much with regards to hemp.