Insulation

[ Go to other articles ]

The three important terms to get to grips with are k-value, R-value and u-Value. Together these define the degree to which types of materials (such as bricks) and whole buildings lose or retain heat. They are all mathematical principles, but the general idea is as follows:

k: The lower the better. This is the conductivity of a material and shows how easily heat can pass through it. k is a number expressed as W/mK, or the amount of energy that can travel through a material that is 1m thick and at a standard temperature. Materials with high k values will be worse insulators than those with low k values. Concrete is not a good insulator at all and has a k value of around 1W/mK; wood is much better and has a k value of around 0.2W/mK. k values are good to compare materials generally, in that concrete is a worse insulator than wood, but it does not take into account the actual thickness of these materials in real life. For instance, it would not tell you whether a 2.5m thick concrete wall would provide better insulation than a 0.4m thick wooden wall.

R: The higher the better. This is a measure of thermal resistance, or how little heat can pass through a material of an actual measured thickness. This gives a good idea of the insulating value of the actual materials being used in construction, such as concrete blocks or wooden beams. The R value for standard concrete blocks is around 0.1m²K/W and for a 250mm thick beam around 1.25m²K/W: a thick wooden beam insulates better than a thin concrete block.

u: The lower the better. This is the most important of the terms and measures the thermal conductivity of the final combination of materials used in construction. u is basically the upside down of R for all of the materials in, for example, a wall: bricks, cavity insulation, blocks and plaster. To get the u value of the whole wall, the R values of the individual components would be added together and divided into 1. So, for a wall comprising only of a double thickness of concrete blocks, the R value would be 0.2m²K/W and the u value would be 1/0.2 = 5W/m²K.

DIY things to do

Installing more insulation in you home is one of the best ways to cut energy bills, reduce your carbon footprint and become a bit of an Eco-Hero. Almost every bit of a house can be insulated, with many different materials on offer to do it. Some are unsustainably produced, but even the worst are better than nothing: the emissions they save outweigh the damage done through producing them.

The first steps for the DIYer should be the simple things: for houses with older windows and doors, put in draft excluder. This only cost a few pounds, but if done well can easily save £10 to £15 per year. Seal the gaps between floorboards and under skirting boards. This can be done with paper or sealant, and if you sand your floorboards it is worth saving the dust to mix with PVA and use that. This does make a noticeable difference, but is not as warming as carpets. Another good move is to put tin foil or reflector board behind radiators on external walls.

Make sure your hot water tank is properly lagged. A new jacket will cost around £15 and will make a big difference. Lagging hot water pipes can also be important if they run through cold areas of the house. Ditto cold pipes through warm areas of the house.

Once the quick, easy and cheap options have been exhausted, it is time to get down to some serious insulating. The first stop is to make sure that your loft is well lagged. This can take a bit of effort if it is full of junk, but is well worth it. If you take all the stuff you haven’t seen for five years to a car boot sale, it should just about pay for the insulation materials. Or try offering it for sale in our Market Place. There are some good eco-friendly options in the shop, but anything is better than nothing. Generally speaking, it is a good idea to have 27cm of insulation in the loft, which will cost about £200 if you do not have any to start with and give a saving of about £60 a year. Insulating your floors can also be a good idea and will cost about the same as the loft, but you may have to lift your flooring first to put in the insulation. Plank floors are probably the easiest, but make sure that any air bricks are left free as wooden floors rot without some ventilation.

Wall insulation

After the loft come the walls. For most of us with cavity walls, this is a fairly cheap and easy process of filling the cavity with wet foam, dry mineral wool shreds or expanded polystyrene (EPS) beads. For all of these materials, holes are drilled into the external wall to gain access to the cavity. The foam is then pumped or the mineral wool/ EPS beads blown into the cavity. This fills the cavity completely and provides good insulation. Costs vary due to labour, but the Energy Saving Trust quoted cost is £500 per house, whereas I have seen a quote of £195 for terraced houses.

For those of you unlucky enough to have solid walls, you have the choice of internal and external cladding. External is more expensive but probably better, as it allows the walls to act as a thermal store. This ‘storage heater’ effect keeps the house more comfortable in hot and cold weather.

For internal insulation you could either cover the outer walls with insulating board or render them with insulating slap. A waterproof membrane is usually needed between either of these options and the wall, but lime-based renders such as hempcrete are permeable, so you should not need the waterproof membrane. Alternatively, you can buy thick, foamy wallpaper to put up as a simple DIY remedy, but this is the cheap and cheerless option. High quality thermal board for internal insulation will cost around £40 per square metre, but may need additional damp-proofing between it and the wall. You will also need to re-plaster or paper over the top.

External insulation may require planning consent and is less of a DIY option. The two common types are insulating render or insulation board with a weatherproof covering. The latter usually requires batons or netting over the insulation to hold everything together, then render or weatherboard for the cosmetic finish. Costs vary massively between method and contractor, but you should be thinking around £60 per square metre. More information can be found at the Energy Saving Trust website under the Home Improvements tab.

If you are building a new house or extension using standard cavity wall construction, there are lots of standard rigid and semi-rigid wall insulating systems available. The usual idea is to line the internal wall of the cavity to keep the rooms warm, but leave a gap between the insulation and the external wall to allow airflow. The rigid wall insulation tends to be made from petrochemicals (crude oil) and costs around £10 per square metre. It is a very good idea to install this when building a traditional cavity wall, as it will give excellent insulation whilst retaining the ventilation and low moisture penetration of the cavity.

More modern construction techniques have the insulation built-in from the beginning, so I will leave that discussion to the Building Systems section.

Windows

Replacing your windows is expensive and has quite a long pay-back time, although the really cheap and nasty stuff will probably pay for itself in 5 years or so. Double-glazing halves heat loss through the windows, from about 15% to 7% of total heat loss for a partially insulated house, and it also reduces noise penetration and condensation. The optimum gap between the sheets of glass is 20mm: more is a waste of space, less is inefficient.

A single glazed window will have a k-value of around 0.2W/mK and double glazing around 0.1W/mK, so you can see how poor this is compared to the insulation material compared below.

Heat loss can be reduced by almost half again if ‘low-E’ glass is used for the inner sheet of glass in the double glazed unit. This is where normal glass is coated with a microscopically thin layer of reflective material, which results in the heat from inside being reflected back whilst heat from outside can still pass through. Filling the gap with argon or a vacuum can reduce heat loss further, but be wary of these as they usually leak and revert back to normal double-glazing quite quickly.

Windows should be made from wood rather than plastic (uPVC), as the wood will last a lot longer if maintained and takes a lot less to manufacture. Almost all windows and doors will be made with FSC approved timber, but it is always worth checking. Try to avoid anything from rainforests, even if it claims to be sustainably logged; any logging in rainforests upsets the delicate ecosystem and leads to significant loss of habitat

Secondary glazing is a cheaper and easier option, and involves putting additional sheets of glass in front of your existing windows. These can be fixed, hinged or sliding units that vary from distinctly unpleasant to extremely posh. DIY fitting is often easy and the decent stuff starts at less than £100 per window. Try Selectaglaze or BB Group for DIY options.

Insulation Products Compared

The most common insulating material is mineral wool, which is often used to insulate lofts and floors. Mineral wool is made from either glass, rock or slag (smelting waste) that is spun into an unpleasant candy-floss type material. It is horribly irritating to skin, eyes and lungs, but on the up-side it is not sticky and does not rot your teeth. It takes a fair bit of energy to make, but the industry is beginning to use more recycled materials and boost their energy efficiency. It comes in rolls and in the loft you should lay it 270mm deep to achieve a really good level of insulation. Mineral wool is likely to cost between £5 and £15 per square metre to fully insulate a loft (270mm thickness or equivalent).

Rigid foam boards are excellent insulators, especially the foil-backed varieties, and are made from petrochemicals (crude oil). These are good for putting between rafters to insulate loft conversions, for internal cladding of solid walls and the thin varieties are good beneath floor slabs. They have the best u-values going, but lose slightly to the more eco-friendly varieties due to condensation: you have to allow some room for ventilation or put in a membrane with these. They can be cut to shape with a knife, but, being rigid, some care is needed not to leave gaps at the edges. They cost between £5 and £15 per square metre.

Warmcell is made from recycled paper and comes as loose wadding. It is mixed with natural salts to be fire resistant, mould resistant and vermin repellent, but will rot if it gets wet. It is good for lofts and for filling gaps as it can be poured. To get a good level of insulation it will cost around £6 per square metre.

Hemp, flax and wool insulation all have fairly similar characteristics; Isonat, Isovlas and Thermafleece are the most common makes. Carbon neutral and natural apart from the bindings, they have fairly good u-values but benefit from controlling condensation by being hydroscopic. This means that ventilation gaps are not necessary and they give a lot more warmth than their u-values suggest. They all have the vermin/mould/fire/nesting fairy resistance of Warmcell and they all help with soundproofing, although the flax rates most highly for this, followed by the wool. Wool has the best vapour properties, still works well when wet, absorbs pollutants like formaldehyde and is very resilient. The hemp is the most rigid, so is best for wedging between joists and noggins (totally fatuous use of the word ‘noggin’ there, I’ll admit, but I rarely get the opportunity so thanks for your indulgence). Hemp and flax will cost around £5 to £10 per square metre, wool about £7 to £14.

Fibreboard is made from wood fibres and is rigid, but has fairly similar properties to the other natural insulators. It costs around £12 per square metre. 

Some average thermal properties for various insulators are given below. The PU, XPS, PIR and PF are often in the form of rigid boards. Their k-values are improved (i.e. reduced) by about 0.005 W/mK when foil-backed.

Notes:

*Eco-rating is based on the BRE Green Guide to Specification, where impacts associated with extraction, manufacture, transport and disposal have been assessed.

1) A is best, E worst and ? is yet to be assessed. Some of the ratings above are no longer listed on the BRE website, so may be updated in the near future.
2) Vermiculite is a type of clay mineral that needs to be excavated and dried.
3) Cork is ecologically beneficial as cork-tree plantations are not intensive and harbour good biodiversity. It requires very little processing and is mainly worked as a cottage industry.
4) The higher rating for recycled mineral wool rather than straw bales may result from the use of recycled materials, longevity and k-value of the glass wool. However, glass wool does take a lot of energy to make and straw bales are probably better in the right applications. The less eco-friendly types of rock wool are rated 'C'.
5) Sheep’s wool is more or less a by-product of sheep farming, as the amount farmers get paid for a fleece is pitiful. The normal environmental strain caused by animal husbandry is therefore not entirely applicable for wool insulation and it probably deserves an A rating.
6) Straw bales do not appear to be very good insulators, but the k-rating does not take into account the thicknesses used. Bales are much wider than the other options, so will in fact provide excellent insulation.

What to do

Insulate everything, even the cat, and always insulate thoroughly. It is cost effective to insulate, with payback times of a few to several years. I like the idea of wool, but different applications will benefit from different products. I have foil-backed PIR behind foil-backed plasterboard on the roof of my kitchen, as there is no ceiling on which to lay floppy stuff like Warmcell.

Use hempcrete on floors rather than concrete, as it is warm and gives a cosy finish for your feet!

[ Back to top ]