Energy saving at home

Energy saving at home

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From the very earliest days of CAT we have focused on building and renovating to high levels of insulation, so that we consume less energy. Reducing our energy demand makes it much easier to meet the remaining energy needs using renewable energy sources. Domestic energy saving measures are vital if we are to achieve the goal of a Zero Carbon Britain.

The energy we use in all our homes makes up about one quarter of the UK’s carbon emissions. Space heating accounts for about two-thirds of all domestic energy use. Of the remaining third, about half is for water heating and half for everything else – lighting, cooking and other appliances.

A few simple measures can help you to avoid many tonnes of carbon emissions while also reducing your bills. These actions will also make your home more comfortable and healthy by reducing draughts, damp and condensation. This page gives an introduction – see the links and questions below for more further information.

Installing insulation (demonstration)
Energy conservation course at CAT


Draughts occur around window & door frames, through letterboxes & cat flaps, down chimneys, at skirting boards, between floorboards, and where services enter. Even very small draughts can make a room uncomfortable during cold weather and have a big impact on heating bills.

Taking draught-proofing measures is often a simple low-cost DIY job that you can do whether you own or rent your home. Do bear in mind that you mustn’t block intentional vents – for example when the room has a fire or stove that draws in air, or trickle vents on windows.


In most cases, loft insulation can be a DIY job. About 270mm of standard insulation will meet current standards but where it’s easy, put in more – perhaps 350mm. You could recoup the cost of DIY loft insulation within a couple of years. If there’s already some insulation, then topping it up is still worthwhile as it may recoup costs within a decade or so.

You can insulate in the roof slope if you already have (or want) a room in the roof. There are a few ways of doing this, and you’ll need to get quotes to compare prices. For a flat roof, adding insulation on top is perhaps the easiest and cheapest way, but insulating underneath is possible.

Insulation board
Insulation board for energy conservation

If your home has cavity walls you can easily get insulation blown in, which should take less than a day and cause minimum disruption. It should cost you only a few hundred pounds, which you should then save on heating bills within 3 or 4 years.

It’s currently much more expensive and disruptive to insulate solid walls, but this can be worthwhile in the long term. With internal insulation you do lose a little room space and will have to move sockets, radiators and so on. External insulation is less disruptive, and it means you keep the ‘thermal mass’ of wall inside. These then act as heat storage – absorbing heat and slowly releasing it to help keep your home at a comfortable temperature.

For a suspended ground floor, aim to reach about 200mm of insulation. You can add insulation between the floor joists either through access to the underfloor space or by lifting the floorboards. For a solid floor you may need less insulation but it’s usually more work, because it involves lifting tiles and digging out a little, or raising the floor level.

Wrap all your hot water pipes in insulating foam sleeves and ensure a hot water cylinder has a cosy jacket.

Central heating

Good quality controls are vital for maximising the efficiency of your central heating. Having control gives you heat where and when you need it, and avoids over-heating. You’ll usually need a room thermostat, timer/programmer, boiler & cylinder thermostats and thermostatic radiator valves (TRVs).

If your boiler is very old, then upgrading to a modern, high-efficiency condensing boiler could cut a third off your fuel bill. This means much lower carbon emissions as well. For even more carbon savings, you could consider options such as a heat pump or modern biomass heating.


If you need to replace windows, the most efficient units use low emissivity (low-E) glass and are argon-filled. Low-E glass has a coating inside the gap that reflects heat back into your house. Using argon gas between the panes improves performance, as argon does not conduct heat as well as air.

If your existing windows are in good repair, they can be improved with secondary glazing. This can be anything from a clear plastic film to a second window fitted inside the original.


Related Questions

How could I insulate a pitched (sloping) roof?

A ‘cold loft’ roof, with the insulation at ceiling level (laid flat in the floor of the loft) is generally the most economical, and easy to install. However, if you want to insulate in the slope of a roof in order to make use of the space, then here are some tips.

The most economical way of achieving a good thickness of insulation in the roof slope is to have two layers of timber, one supporting the roof finish and another supporting the insulation and ceiling finish. To reduce cold-bridging, timber I-Beams can be used in new constructions. In an existing roof (with rafters supported on roof beams), the second layer of timbers (ceiling joists) can be hung off the rafters using hardboard, ply or timber ‘hangers’, or nailed crosswise to them, or they can span between the roof beams. This technique can also be used with flat roofs.

An air space of 50mm must be left between the insulation and the tiling felt, unless the felt is of a low-vapour resistance type. If using a breathable membrane, with insulation up against it, then above the membrane you would put counter battens (top to bottom) as well as the standard battens (side to side), for adequate ventilation beneath the tiles. Sometimes, a breathable membrane is used with only standard battens, with the membrane slightly draped between rafters to allow ventilation – in this case an air gap of about 25 mm would be needed between membrane and insulation.

Pitched roof insulation
Pitched roof insulation

Thin wood-fibre boards (22 or 35mm thick) can be used as an alternative to a membrane under tiles. When re-roofing, the fibre-board is laid over the rafters, and then counter battens (in line with rafters), and then standard battens to fix the tiles/slates to. Thicker wood fibre boards can also be used, to give more insulation and achieve a lower U-value (to minimise heat loss).

A ‘warm roof’ will have waterproof insulation on the outside of the structure (so the main timbers are on the warm side of the insulation). It’s a useful way of upgrading an existing roof when internal room height is at a premium. On a sloping roof, the tiling battens are supported by rigid insulation and fixed through to the rafters by special screwnail fixings. The insulation must be waterproof, such as cork, foamed glass or closed-cell plastic foam board – these will tend to be more expensive than the standard insulation materials for internal use.

There are many different types of insulation material to choose from. Please do contact us if you need to find suppliers of natural & renewable insulation.

You can also read more at the webpage of the Energy Saving Trust, on Greenspec’s pages about ventilated and unventilated roof insulation options, and in a guide from Historic England.

How could I insulate a suspended timber ground floor?

If you can access the floor from below via an unheated cellar or basement it will be easier, otherwise you’ll have to lift the floorboards (which requires care to avoid damaging them).

225mm of a renewable or mineral fibre type of insulation is a decent amount. Make sure you keep good ventilation to the underfloor space beneath the insulation – with vents at either side for air flow.

Renewable insulation will need to be protected by a breathable membrane to protect it – if the floorboards are not well sealed. See the website of the supplier of insulation you’re using (or call their advice line) for advice on the type of membrane that would be needed.

Loose fill insulation can be carried between the joists on a membrane or netting nailed to floor joists or on a low-vapour resistance board (for example softboard, a fibreboard bonded by heat rather than glues – this is good for Warmcel insulation made from recycled newspaper).

For other insulation materials (e.g. standard mineral fibre types) it will also be worth looking up the website of the manufacturer (e.g. Rockwool or Knauf) as they’ll usually have guidance sheets on how the material should be installed and what limitations there may be.

You can also see some information and examples on the Energy Saving Trust, Superhomes and Historic England websites.

How could I insulate a solid floor?

Traditionally, solid floors were laid directly onto soil. This relies on the ground underneath being kept dry, usually by it being higher than the ground outside the building, and by having adequate drainage.

The most common method now used is to have a thick concrete slab laid on a damp-proof course (e.g. a polythene membrane). A layer of polystyrene insulation is then finished with sand/cement screed and tiles or board.

For a low-impact alternative to the above you could look into using recycled aggregate in the concrete (rather then newly quarried material), and perhaps using stabilised earth as the screed. You could also consider using recycled polythene or bitumen for the damp-proof course.

A solid floor of stabilised earth or limecrete should have a solid insulation material below it, such as cork, perlite or foamed glass, with recycled polythene vapour check and damp-proof membrane (DPM) below this.

Try to achieve at least 150mm of insulation for a solid floor. Insulation should be placed around the edge of the floor, and the floor finish supported on some sort of rigid insulation. Possible materials include cork, perlite (volcanic glass), lightweight expanded clay aggregate (‘Leca’), foamed glass (slabs or granules), fibreboard, mineral wool boards, or plastic foam of some sort. A vapour check layer will normally be required to prevent condensation occurring within the insulation layer.

Another possibility is a hemp & lime (or ‘hempcrete’) floor. Lime has a much lower environmental impact than cement, so if you can use it place of cement in mortars or concrete you will be reducing the ’embodied energy’ of the floor and the carbon emissions from construction. The hemp provides the insulation. See for example the details of how we insulated the WISE building at CAT.

If you are redoing a floor, then you may have the chance to consider underfloor heating. Because it runs at a much lower temperature than standard radiators, wet underfloor heating is more efficient and provides a more comfortable type of heat. It’s particularly appropriate for use with heat pumps, as these need to supply low-temperature water to run efficiently.

You can also see some information and examples on the Energy Saving Trust, Superhomes and Historic England websites.

How could I insulate a flat roof?

The first two of the following four methods involve adding insulation to the outside of the roof, so will be suitable if there is little headroom underneath, or if access is difficult. The second two involve adding insulation underneath the roof. If insulating on top of a flat roof, make sure that it still drains well so that water does not pool on top.

Upside Down or Inverted Roof

An ‘upside down’ roof uses waterproof insulation on the outside of the building structure. The insulation is laid over the existing waterproof membrane and held down with something – which could be pebbles, turf (for a green roof), paving slabs, etc. Suitable insulation materials will tend to be a bit more expensive, and include cork, foamed glass and closed-cell plastic foam. You’ll need to check that the structure can bear the weight of the insulation and finish. This option keeps the existing membrane, but there is a risk that water will percolate through the insulation and so cool down the roof deck – causing condensation.

Warm Roof

A warm roof will have the insulation laid over a vapour control layer (itself over the roof deck), with a membrane laid over the insulation and suitable finish on top. You’ll need to check that the structure can bear the weight of the insulation and finish. If you are replacing the roof membrane anyway, then this will be a better solution than the ‘upside down’ roof (above), as it will keep water above the insulation and so keep the roof decking warm. You could still keep the existing membrane underneath the insulation if it would be difficult to remove.

Cold Roof

The insulation is put between the roof joists. A ventilated gap needs to be retained between the top of the insulation and the roof decking, to avoid condensation build-up. It can be difficult to get adequate ventilation, so this method is often not recommended.

Internal Insulation

A method similar to dry-lining of walls can be used, with a plasterboard/insulation board added to the underside of the internal roof, below the joists.

How do I choose between insulation materials?

There are basically three different types of insulation material:

  1. Organic – those derived from natural vegetation or similar renewable sources, which tend to require a low energy use in manufacture (a low ‘embodied energy’). Examples are sheep’s wool, cellulose, cork, wood fibre, and hemp.
  2. Inorganic – derived from naturally occurring minerals which are non-renewable but plentiful at source. Likely to have a higher embodied energy than organic materials. Examples are mineral/glass fibre, perlite and vermiculite (from volcanic rock) and rigid foamed glass.
  3. Fossil organic – derived by chemical processes from fossilised vegetation (oil) – a finite resource. Fossil organic insulation materials such as expanded polystyrene and polyisocyanurate or phenolic foam are highly processed, resulting in a high embodied energy.

Which is best?

If possible it is better to choose insulation materials that have not been heavily processed as this will reduce the carbon footprint and environmental impact of your home. But it is far better to install cheaper inorganic or fossil organic materials with the right physical properties and a low thermal conductivity than to install nothing at all.

In many cases, organic insulation material can be applied instead of inorganic or fossil organic, but there are exceptions. For example, there is not an organic insulation material suitable for cavity wall insulation.

Think also about the ease of installation. Loose fill insulation is quick to put in in lofts, but cannot be DIY installed in anything other than a flat place. Rigid boards and batts will come in certain sizes, but need to be cut to shape if you have some unusual spaces. Some materials can be cut with a knife, but a few will need a saw. Some mineral wool now comes in a thin foil or plastic wrap, to protect from the fibres. You should still wear a face mask when installing any type of installation, as small fibres of any kind are best not inhaled.

Is cavity wall insulation a good idea?

If you have cavity walls and they haven’t been insulated, up to a third of the heat produced in your home could be escaping. Insulation should reduce your heating costs and carbon emissions from your home significantly. The insulation itself needs to be suitable for the conditions inside a masonry cavity, and so choices are limited to three options: blown mineral wool, plastic beads or plastic foam.

Cavity wall insulation should cost only a few hundred pounds and recoup this installation cost within 3 or 4 years. With gas-fired heating, you’ll reduce annual carbon emissions by almost half a tonne per year. This makes it several times better than installing a solar PV roof in terms of both carbon savings per pound spent and payback time. Both costs and savings will be higher for a detached house.

The installation process must include an assessment to ensure that the construction is of a suitable type. Installers should work through an accredited scheme and guarantee the installation for 25 years. It may take about 3 hours to inject the insulating material into the cavity. For installers, see the Cavity Insulation Guarantee Agency (CIGA), National Insulation Association, or British Board of Agrément.

Some homes may be classified as ‘hard to fill’ cavities – perhaps too narrow or uneven to fill easily. Insulation may then be more expensive to install (perhaps 2 to 3 times as much), or it may not be feasible – external or internal insulation may then be better.

A Which? Magazine survey in 2011 found that some installers were not undertaking adequate assessments. According to industry guidelines, they should inspect all external walls thoroughly to check for cracks/defects, check all internal walls to check for any existing damp, and do a cavity check (with a drill hole) on all walls. A proper survey like this is likely to take an hour. Do ask a few companies around to give quotes.

Cavity wall insulation should not cause problems of dampness, but a proper assessment of the property is needed to ensure it would be suitable. If your home is towards the west coast of the UK, more prone to wind-driven rain, then it might be unsuitable for cavity insulation if it’s in a very exposed, unsheltered position and there are cracks in the external wall. For those few homes in this position, measures could taken to prevent damp risks – for example by putting extra protection in the form of boards or tiles on the exposed walls. There will be a cost to this of course, and it should be compared with quotes for options such as internal or external wall insulation.

BRE (Building Research Establishment) research in the 1990s showed that cavity wall insulation when assessed & installed properly does not lead to an increased risk of damp. The study found that the structural condition of the walls was the most important factor in damp problems – for example, badly filled mortar joints or ‘dirty cavities’ (where, during construction, mortar has dropped down inside the cavity – i.e. if too much is used). Over time, this can cause problems with damp-proofing.

What sort of glazing is best?

Double or triple?

When building from new to a very high standard (e.g. passivhaus), triple glazing will generally be appropriate, but for renovation projects money is better first spent on ensuring good insulation all round before going for triple instead of double glazing. The extra weight and thickness of having a third pane may be an issue in some renovation projects.

Glazing panes with low-E (“low emissivity”) coating and argon gas fill are now common in order to meet current building regulations. Low-E coating is a very thin layer of metal on the outer surface of the inner pane, reducing heat transfer across the gap. Argon gas in the gap between the panes improves the performance of the window, as argon does not conduct heat as well as air. Krypton gas is better but more costly – it might be used if a narrow gap is needed for a particular design.

The gap between the two panes of glass should usually be about 20mm – a smaller gap gives a slightly poorer performance, but a bigger gap won’t make much difference (although will give better sound insulation). Windows with lots of glazing bars (e.g. Georgian) are poorer: the multiple bars conduct heat out. U-values measure thermal performance, and a lower U-value means less heat loss.

Specification U-value
Single glazing 5.5
Standard double glazing 2.7
Double glazing, low-E coating 1.8
Double glazing, low-E coating
and argon-filled
1.6 to
Triple glazing, multiple low-E
coating and argon-filled
less than

Wood, metal or PVC Frames?

CAT’s eco-building experts recommend good quality timber frames, as these require little energy to manufacture, can last for over 50 years, and can then be renovated rather than replaced. In contrast, uPVC window frames require more energy to manufacture and lead to higher pollution problems at manufacture and disposal. Aluminium manufacture involves a lot of energy use and pollution, and some frame coatings make recycling difficult – not ideal for such a high-value material. If you have to use aluminium, look for anodized frames.

Wood frames have had a poor image in the past, as UK-made softwood windows were often of low quality. However, we’ve learned from the Scandinavians and it’s possible to get UK-made windows with high standards of airtightness, built from homegrown, durable, untreated wood. FSC-certified temperate (not tropical) hardwood or durable softwood would be first choice. A durable temperate hardwood (e.g. oak, sweet chestnut or larch) won’t need treating and should last 30 years. The preservatives used to protect softwood will cause some pollution when it eventually needs to be disposed of. If you choose a less durable wood, look for one pre-treated with a low-impact natural, renewable treatment. Any sustainable timber is preferable to uPVC, but avoid painting wood as this increases the environmental impact.

Planning and building control

Replacing either glass units (e.g. broken windows or faulty double-glazed units) or some rotten wood in the frame or sash will not need Building Control approval. When replacing an entire window frame and the opening parts of a window, or a door that is more than 50% glazed, you will need approval. However, by employing a FENSA registered (Fenestration Self Assessment Scheme) joiner or installer you don’t need to get the work checked. Instead, the installer issues a certificate to verify compliance with Part L of the Building Regulations.

For properties such as listed buildings the requirements can be strict, but high specification windows or secondary glazing are available. “Conservation grade” replacement units (e.g for sash windows) replicate the appearance of old windows but have much greater levels of insulation & draughtproofing.

The BFRC energy label for windows gives an overall assessment of performance, so can be a help when choosing windows. There’s also useful further reading on the Greenspec website.

How could I improve windows with secondary glazing?

You can improve the performance of old windows without going to the expense of double-glazing. A simple, cheap alternative is to fit secondary glazing, which can be an additional window fitted on the inside of the existing frame, or a magnetic or adhesive pane to fit to the frame, or even just clear plastic film stuck to the interior frame.

Clear plastic film (polythene) should be available from DIY stores. Attach it round the edge with double-sided sticky tape and then heat it to make it first stretch and then contract back as it cools – to clear the wrinkles. It won’t last a huge length of time – probably one winter – but is very cheap.

Rigid (or slightly flexible) plastic sheets are also usually available from DIY outlets. These can be stuck on, or held magnetically (to enable easier cleaning). More advanced options could be a single pane in an aluminium frame attached on hinges or sliding runners, or perhaps an old window cleaned up and fitted inside the frame.

There are also specialist window films that improve the insulating properties of glass by letting through certain wavelengths of light in only one direction.

The most advanced type of secondary glazing is to fit a new second window inside the existing one. This will often be better and cheaper than replacing windows altogether. You could hire a local joiner to make them, or proprietary systems are available – for example from DIY stores.

Can a conservatory make my home more energy efficient?

A conservatory can be a great way to use solar power. And as well as saving energy, it will provide a pleasant extra room. The big thing to remember is that a conservatory should never be heated – or all the benefits will be lost!

A conservatory is a ‘buffer’ against the outside weather – the temperature will stay a few degrees warmer than it is outside. For much of the year a conservatory is a very nice place to be. However, you’ll need blinds or shutters to prevent overheating in high summer. Growing seasonal vines or creepers across the roof is also a great way to get summer shading. A south-east facing conservatory is best, as it gains from the morning sun but will be slightly shaded from the westerly sun at the warmest time of day.

A conservatory can also act as a lobby for coming and going from the house, so reducing draughts and heat loss. Fresh air coming in to the house via the conservatory will be warmed on its way through. This is good, because lots of the heat loss from a well-insulated house is through ventilation.

A danger with conservatories is that they come to be relied upon as an extra living space in the home. As the coldest months come around, a few degrees above outdoor temperatures is still quite chilly. This leads to the temptation of putting a heater or radiator in, a move that would make your home an energy guzzler! It is impossible to insulate such a highly glazed room sufficiently,so if you want to maintain energy and monetary savings, you’ll need to resist temptation and keep it unheated.

Shutting the conservatory off from the main part of the house with solid doors, or glass doors with thick curtains, will stop heat escaping at the coldest times of the year. Plenty of ‘thermal mass’ within the conservatory will store the heat gained for longer. So if it is being added to an existing brick wall, much of this could be kept for this purpose. Alternatively, a solid stone or brick floor will soak up and then slowly release heat as the evening cools down.

For a sunny but heated space in your home, consider instead a sun-room. This would have double glazing throughout with an insulated solid roof and well-insulated curtains or blinds. An insulated, heated sunroom would need to meet Building Regulations, so the balance of windows and insulated roof or walls would need to be designed carefully.

A conservatory may seem cheaper to build than a proper extension, but this is not always the case – they can be more costly per square metre than the rest of the house. And if it ends up as a heated room then the less obvious heating costs will not be at all cheap!

How can I reduce condensation in my house?

Condensation is due to excessive moisture, cold conditions, cold surfaces or inadequate ventilation. It can cause mould, heat loss and building damage. To address these issues, the room should be properly insulated and adequately heated (to keep the surfaces warm).

So do take all feasible insulation and draught-proofing options, and look into improving single glazing with either replacement glazing or with secondary glazing (a cheaper option).

Condensation may still occur on replacement windows, as they’ll still be the coolest surface. New windows will be more airtight than old ones, so warm moist air will be no longer be escaping through cracks in the frame and around the seals. This means that existing damp issues may become more pronounced. Many windows will include trickle vents in the frame, to allow a small amount of ventilation, but to keep the house warm and dry you may need to take a few other measures to avoid producing lots of moist air.

Drying clothes indoors can easily cause problems of damp and condensation, leading perhaps to mould, etc. So if you need to dry indoors, it should be in a room that can be shut off and ventilated (perhaps with heat recovery, as mentioned below).

The bathroom and kitchen in particular should be able to be ventilated in a controllable way, to stop moist air circulating into the rest of the house. For example, after a shower or bath, leave the bathroom door shut and the window open for a while until moist air & condensation on the window/mirror has cleared. Do the same when cooking if you can; if your home has an open plan layout at least stop the moist air circulation where possible (e.g. to the upstairs rooms).

To avoid heat loss from a room like a kitchen or bathroom, where lots of moist air is regularly produced, you could consider a heat recovery extractor fan. It may be worth getting a slightly bigger heat-recovery fan unit than you need, as they can be a bit noisy if they are operating on full power. This may be fine if are just switching it on for a short while to clear the bathroom, but it could be obtrusive in the kitchen. If a fan unit is not supplied with more solid covers over the plastic slatting, it could be worth fitting something if you live in a house that is a bit exposed to the wind (as they could let in draughts when not in use). An openable wooden casing could be fitted quite easily.

If drying clothes indoors is not an issue, and you’re already careful about venting away moisture from bathroom & kitchen, then excessive condensation may be from some other cause, such as a water leak somewhere (e.g. from a pipe under the floor or in the loft), or water penetrating the structure from outside (such as rainwater coming in cracks in masonry, or if gutters are broken). If problems persist, it would be worth investigating these issues, as over time they’ll cause damage to the building.

Is underfloor heating good, and how is it installed?

Wet underfloor heating is definitely something to consider if you are replacing your heating system, undertaking renovation work, or building from scratch. It gives a very even temperature over the floor area, and works well with renewable energy sources and well-insulated buildings. Underfloor heating is slower to respond to changes or to heat up from cold, so is best suited to well-insulated buildings and works well if you have good levels of thermal mass.

The system should be sized to run with a flow temperature of 35°C, compared to radiators which may run at 60°C or more. To allow for a low flow temperature, the pipes should be spaced at 100mm or less. It’s therefore a good match for a ground or air source heat pump as these are more efficient when supplying low flow temperatures. Underfloor heating also works well with a modern condensing boiler, because this will then run in condensing more more often.

The radiant heat given off by the floor results in high comfort levels. In our experience this means that, in practice, you can run it at lower temperatures and save more energy. It should be possible to have the house at a temperature 2 or 3 degrees lower than with conventional radiator use. It’s great for rooms with high ceilings because the heat goes up the centre of the room. You’ll also have more free wall space without radiators.

There are several ways of installing underfloor heating – see below for examples based on what we’ve done at CAT. It’s possible to do it yourself but you’ll need good building and plumbing knowledge & skills. A professional installer can do a pressure test to check for any leaks and ensure it is stable before covering it over. 15mm bore pipe is better than 10mm, as it’s easier to then pump the water around. A manifold connects the pipes to the heating system and heating controls just as in a normal heating system.

Solid Floor

Start with a limecrete or concrete subfloor above a damp-proof membrane. Above this, add a layer of solid insulation (could be cork), which should also run up the edge of the floor to stop heat escaping into the walls. Then lay the underfloor heating pipes (cross linked polyethylene or barrier pipe), running back and forth along the length of the floor. Various fixing systems are available to hold pipes in place. Over this lay a screed of limecrete or concrete, covering the pipes by at least 50mm. Finally, add a floor finish such as tiles, slates or stone.

Suspended Floor

Insulate between (and perhaps also below) the joists to prevent heat loss, and then lay a subfloor over the joists. The underfloor heating pipes lie on top of this, between battens. To ensure that the heat is evenly distributed, you can either fill the gaps with a weak sand/lime mix, or fit aluminium plates to the pipes to dissipate the heat evenly through the floor. The floorboards then sit on top of this, held by the battens.

The timber must be really dry – if it isn’t, it may shrink and crack with the heat. The moisture content of timber for a wooden floor with underfloor heating should be about 8% for retrofitting and 10% for a new build. Alternatively, lay it loose for the first year, so that adjustments can be made for any movement. The surface temperature shouldn’t go above about 30° or the timber may distort, and you should leave a gap around the edge of the timber floor to allow for expansion.

Should I sign up for a green tariff?

Signing up for a ‘green tariff’ from a company focused only on renewable energy is a great way to support the renewable energy industry. Changing your supplier is now very easy, and in most cases won’t make any difference to your supply.

One issue is that the small companies that specialise in renewable energy may not be part of the ‘Warm Home Discount’ scheme (although if they get enough customers they will be brought into it). This scheme gives a rebate to people at risk of fuel poverty, such as those receiving Pension Credit Guarantee Credit and some others. If this applies to you then you’ll need to stay with a larger provider to get this rebate.

Which green tariff?

We recommend choosing a company that only supports renewable energy. This means your money will not indirectly go to operate or build fossil-fuel power stations.

All electricity providers are required by the government to include some electricity from renewable sources. If they just offer a green tariff as one of a range of tariffs, then they may be simply charging a premium for electricity they’re legally required to produce! This is why we recommend companies that invest your electricity bill payments only in more renewable electricity.

If enough people sign up for renewable energy tariffs with these suppliers, then demand for renewable electricity will rise above the minimum government requirement. Therefore, as well as signing up yourself, encourage others to do the same.

The Ethical Consumer website gives a ranking based on the ethical and environmental record of electricity & gas suppliers. You have to be an Ethical Consumer subscriber to see the whole report, which gives more details.

I’m on a green tariff – so can I use as much electricity as I like?

It’s important to bear in mind that signing up to such a tariff does not mean you can leave all your lights on because it’s all zero carbon!  If you use more electricity through your green tariff it means that less renewable electricity is left for those that are not on green tariffs. This means that more fossil fuel will be burned to meet their share of energy use.

Also, every means of generating electricity has some environmental impact, including the energy and materials that go into manufacture and installation. Energy saving measures are vital, because it’s them much easier to meet our electricity needs with energy sources such as wind farms, and wave & tidal power. Our Zero Carbon Britain project has a lot more details about how we can meet all our energy needs using only renewable energy.

What is a U-value, and what is thermal conductivity?

The U-value is a measure of how many watts (representing the rate of flow of energy) pass through one square metre (m²) of a construction detail (such as a wall) for every degree difference in temperature between the inside and the outside. Temperature is measured in kelvin, and 1K = 1°C (degree centigrade).

As an example, a U-value of 6.0W/m²K for a single glazed window means that six watts will be escaping through each square metre of glass when the temperature difference is one degree. If it is 20°C in the house and 0°C outside, then the heat loss is 20 x 6 = 120 watts per square metre.

U-values are generally used to describe the thermal performance (heat loss) for a section of construction that involves several materials – such as a wall made up of timber, insulation, board & render.

Thermal conductivity

For individual materials, such as a type of insulation, you’ll come across the term ‘thermal conductivity’, also known as a k-value or λ-value (lambda). This is the rate at which heat flows through a particular material, and good insulation will have a low thermal conductivity. It is measured in watts (heat flow) per metre (depth of material) per degree difference (inside to outside), so the unit is W/mK.

Most natural insulation materials (hemp, wool, recycled paper or textile) will have a thermal conductivity of about 0.035 to 0.040 W/mK, which is similar to the performance of conventional mineral wool insulation. Foil-backed plastic foam insulation boards are slightly better, with thermal conductivity about 0.023 W/mK. So about 100mm of the plastic foam board will give equivalent insulation value to about 150mm of the various woolly types.

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