6 key things to consider when choosing insulation materials
Posted by Chris Newman on 13 January 2014 at 9:09 am
1. Insulation myths busted
It's never easy knowing which insulation materials to choose, especially with a lot of the guff you see on many TV programmes. In this blog we'll give you some hints and tips to guide you through the quagmire.
Starting with a few truths to keep in mind:
1) There are no magic solutions. If something promises to be a magic solution then it definitely isn't.
2) Most materials have pros and cons. You'll need to understand these to make the correct choice as they are specific to your situation.
3) No matter how good your product, all the good work can be undone with poor installation - and it's even more difficult to identify and rectify problems when they are hidden behind plasterboard or under floorboards.
So what are the differences between materials?
2. Insulation and water vapour
Hygroscopic and hydrophobic are technical words that describe how water vapour behaves when it condenses on the insulation material. Water will condense into droplets on a hydrophobic material. On a hygroscopic material it will be absorbed into the material with the potential to be released when ambient humidity levels drop.
Hygroscopic materials tend to be natural, eg woodfibreboard or sheepswool - and so in extreme moisture circumstances they can get damaged from prolonged presence. Hydrophobic materials, such as phenolic foam or mineral wool, also might be damaged by wet conditions and their ability to retain heat reduces when wet.
3. Can vapour pass through the insulation
Something like foil backed foam board, or EPS is vapour impermeable. Wool or fibreboards are vapour diffuse. These terms simply refer to whether or not a material allows vapour to readily pass through it. The common phrase 'breathable' is a bit misleading as it suggests a desirable quality which is not always accurate.
You don't tend to get hygroscopic vapour impermeable materials.
Putting both of these together, you either want a hygroscopic vapour diffuse product or a hydrophobic vapour impermeable product. A vapour diffuse hydrophobic material is a no no. If you go down this route you will need a dedicated vapour barrier for example, polythene sheet to prevent the passage of moisture from warm to cold surfaces, but this is prone to damage over the years from screws and structural movement.
4. Embodied energy
When it comes to insulation materials, we believe that embodied energy (ie the energy used to manufacture and transport it) is a bit of a red herring. In virtually all circumstances the amount of energy saved by insulating a home dwarfs the amount of energy used to make those materials no matter what the insulant. In actual fact many 'sustainable' materials can have higher levels of embodied energy than materials such as phenolic or polyisocyanurate - although these are still dwarfed by their savings.
5. Thermal conductivity
You want your insulation material to have the lowest thermal conductivity as possible. The better the thermal conductivity, the shallower depth of material required to achieve the required thermal performance. Thermal conductivity is measured in W/m2K, and is also known as the k value (also k, λ, or κ) and will be shown even on a packet of insulation sitting on the shelf at the builders merchant.
There are a few outliers but most materials range in value from ~0.019 for phenolic or PIR, to around 0.039 for mineral and sheeps wool, woodfibreboards and recycled fibres such as cellulose. Polystyrene is somewhere in between. The main outlier is something called Aerogel which has a low value around 0.014. However, this performance comes at a high price. This makes it suitable for areas where space is a problem, thermal bridging areas such as window reveals or for people without major budget constraints.
6. Getting the right insulation for your building
The right product is not just dictated by your budget and your material preferences but also by what your building needs. There are no hard and fast rules but if your building location or its construction materials make it high risk for moisture damage from outside and inside then you may wish to consider hygroscopic vapour diffuse materials.
Indicators that your building might be at risk include:
being in an area of high amounts of driving rain
walls that face south west
the bricks or blockwork are made of porous materials such as sandstone
the mortar is lime mortar or the point is damaged
the house is a heritage timber frame property
For example if you are in a brick house in London in good condition then PIR will probably be fine, but if you are in a 300 year old sandstone house on the south west coast then a woodfibreboard solution may be more appropriate. We definitely recommend staying away from vapour impermeable solutions for cob houses. As always, speak to an expert before making a final decision.
I hope this is has been useful and I can assure you that if you make the right decisions your warm and cosy house will be worth your efforts.
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About the author: Chris Newman leads on development and delivery of the Parity Projects Home Energy Masterplan.
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