Use Less Energy

The concept of insulating our dwellings against the cold is hardly a new one – the nomads of the Central Asian steppes have been wrapping their yurts in fleeces and felt for millennia – but unfortunately the era of cheap North Sea oil has meant that here in the UK we are still lagging behind our European neighbours in all things thermal (pun intended).

Thermal insulation is important in order to achieve thermal comfort and energy efficiency. Insulation reduces both unwanted heat loss on cold days/nights, and heat gain on hot days and nights, and thus decreases the requirements of heating and cooling. In the UK of course, the former is most important.

The thermal resistance of a material is referred to as its r-value. It is expressed as the thickness of the material divided by its thermal conductivity, or k-value. The u-value of a wall equates to the rate of transfer of heat through all the elements that make up the wall, and is expressed in Watts per metres squared Kelvin, or W/m²K.

In essence, insulation is all about pockets of air, which has a very low thermal conductivity (k-value), separated by material with a high resistivity (r-value). Theoretically a vacuum would be the best insulator, but since this is not practical the next best thing is to have lots of fluffy stuff – air pockets separated by thin strands of material with high r-value.

Oil-based insulation materials basically contain lots of little air bubbles separated by plastic and have good k-values, meaning a relatively thin board (50mm) will have a good r-value and enable good u-values for a reasonably thin wall make-up. There are more modern ‘space-age’ materials which have the best k-values, such as Aerogels, but they are prohibitively expensive to use in any thickness and so do not necessarily give good u-values overall. Oil-based materials aren’t necessarily best though – see Materials.

How much insulation your house should have is really a moot point – it is unlikely that you’ll be able to insulate enough for zero heat loss, due to practicalities and cost, and so the general rule of thumb is to try and have as much as you can. Building regulations now require good insulation levels for new buildings, however going beyond this is recommended if possible.

See Materials for the pros and cons of all insulation materials.


If you are planning to replace more than half of the internal plaster or external render of a wall, or if you are dry lining a wall, then you have to insulate to Building Regulations standards whether you had intended to or not.

The main condition to meet is the thermal performance of the insulated wall - if you live in England or Wales then it must have a u-value of no more than 0.30 W/m2K - as a rough guide you will need around 60mm to 120mm of insulation to achieve this, depending on the insulation material and your existing wall make-up.

Your installer should ensure that the insulation is up to standard and will arrange approval from the local Building Control Office for you, but check with them. If you are doing the work yourself, then contact Building Control to make sure you’re adhering to the standards.

Cavity wall insulation

If your house was built after 1920, then there is likely to be a cavity between the outer and inner brick or blockwork. Typically, house builders since the mid 90s have filled this cavity with insulation during construction, but up until then, and in many cases after, the cavity was probably left empty.

If the cavity in your wall is at least 50mm wide (typically it will be between 75 and 100mm), and it hasn’t been done already, you may like to consider having it filled with insulation – ask the previous occupants or your local authority’s Building Control department as they may know if the cavity has been filled already. Alternatively ask a local installer for a boroscope inspection, whereby they will drill a small hole in your wall from the outside and insert a camera for a peep.


Cavity wall insulation is installed by first drilling small holes at 1m intervals into your external walls, blowing the insulation into the cavity using specialist equipment, and then cementing the holes up – this is not something that you can do yourself!

It is however a very simple process, should only take a couple of hours, and is neither intrusive nor disruptive. Apart from having an empty cavity in the first place, the only other pre-requisite is that the installer must be able to reach all external walls and fill every part of the wall – the only way of knowing if your whole cavity has been filled is to take a picture of your heated house with a thermal imaging camera on a cold day/night.

There are other considerations though: your cavity is currently acting as a ventilation gap, allowing both your internal and external walls to ‘breathe’ into it and thus avoid damp issues - filling it may result in an increased dampness inside your house. For this reason it is important to use a ‘breathable’ insulation material. If you already have damp patches on the inside of your external walls then you should not insulate them until the root of the problem has been identified – see Damp Problems. If practically and financially possible, always consider using external insulation instead.

For more information contact or visit the National Insulation Association and always use a registered installer who uses a product certified by the British Board of Agrément (BBA).

For alternative cavity wall insulation materials, see here.

To find out about possible funding for cavity wall insulation, see here.

Roof Insulation

Everyone knows heat rises - in an uninsulated, house up to 25% of your heating energy is going straight through your roof. Since insulating your roof is amongst the simplest and most cost-effective energy efficiency measures you can take, there really is no excuse for not having it done, or doing it yourself!

Ideally you should have 270mm of insulation in your loft – if your loft is already insulated, check that you've got enough. If everyone in the UK had 270mm loft insulation, we would save nearly £500 million and 2.7 million tonnes of carbon dioxide every year between us, the equivalent of taking 100,000 cars off the road (Energy Saving Trust).

If your loft has easy access and is well ventilated, then it should be easy to insulate yourself – simply roll insulation blanket or batts between the joists, right into the eaves, and make sure there are no gaps, and then roll more blanket or batts perpendicularly across the joists to the required overall depth. Do not squash or crush the insulation as this will reduce its thermal properties.

To find out about funding for loft insulation, click here.

Alternatively, should you wish to use your loft for storage, insulate between the joists as above, and then either lay further joists across, insulate between and then board out, or you can lay structural/rigid insulation boards directly onto the existing joists and then board out.

In both cases remember to insulate the loft hatch as well, and fit draught excluders!

This creates what is known as a ‘cold roof’ – this is fine, but you must ensure that the loft space is adequately ventilated so as to avoid any damp problems that may eventually rot roof timbers. You must also ensure that any water tanks and pipework in the loft are well-insulated; otherwise they may freeze in the winter and burst – you won’t know til they thaw!

Should you wish to use your loft as a living space, then you need to create a ‘warm roof’. Do this by insulating under the roof between the rafters using boards, ideally leaving ventilation space between the boards and the underside of the roof surface, and then insulate with further boards across the rafters on the inside. Some insulation boards can be plastered, or you can use insulated plasterboard.

If however your loft is difficult to access or has irregular joists, you can use a loose-fill material, simply poured (or, if very difficult to access, blown) over the joists to the required depth. See Materials - Insulation for more details.

Flat roofs require a very different approach, and one best done by a professional. Ideally the insulation should be applied between the rafters and also from above, either above or below the waterproof layer.

Solid Wall Insulation

Solid walls typically allow twice as much heat to escape as uninsulated cavity walls - but they can also be insulated, either internally or externally. Very thick solid walls will have a better u-value than a typical brick wall, however they act as a massive heat sink (thermal mass - great if it’s hot). For a wall to have the same u-value as a typical 240mm brick wall with 100mm insulation, it would have to be 2 metres thick!

It is the received wisdom that internal wall insulation is cheaper to install than external, although in practice once everything has been taken into account the costs are broadly similar.

The budget costs that we use in our calculator (£100/m2) include for all material costs and labour of installing the insulation and completely finishing the wall.

Generally speaking, external insulation is the better option, as it can cover the whole wall and keeps the thermal mass inside the house, but there are pros and cons of both.

In many cases a mixture of the two will be the best solution - lots of older houses have an attractive brick frontage which wouldn't be suitable for external insulation, however the rear is generally less impressive (and less likely to be protected by conservation) and external insulation could be just the thing. The other walls can then be insulated internally, one room at a time if need be.

The alternative to solid wall insulation is to fix battens or framework to the wall, and then fill or cover with insulation; however there are no obvious benefits to the added complexity, apart from perhaps in listed buildings where the original wall surface must be protected.

External Wall Insulation

Rigid insulation boards are fixed directly to the outside of your walls using pins, and then rendered and if required painted, although pigments can be introduced to the render.

[Interesting fact – the colours of many rendered houses historically are down not to choice, but to the render additive available at that particular time of the season. Pig’s blood, rape, mustard, and cornflower were all used to improve adhesion and elasticity.]

There are many very good reasons for choosing to have your walls externally insulated. It can be applied with only minimal disruption, does not reduce your floor area, and keeps the thermal mass inside, thus ensuring a more stable internal temperature and avoiding over-heating in the summer. Also, because it covers all of your walls, external insulation eliminates cold bridging (points which penetrate the insulation) and areas inside where condensation is likely to form.

If you need to carry out external refurbishment works, it makes sense to install the insulation at the same time, particularly if you have rendered walls with damaged render or brick walls that need re-pointing – insulating at the same time may not cost you much more than the repairs. The new external rendered layer will renew the appearance of your walls and protect your brickwork, and improve weatherproofing, soundproofing, and draught-proofing.

Of course external insulation comes with its own considerations. It is likely that you will require planning permission, since it can alter the appearance of your house drastically (unless of course it’s already rendered), you will need enough space alongside the wall for access during installation, ideally 600mm, or 900mm if it is a passageway, and your walls will need to be in good structural condition.

All rainwater goods will need to be moved, and the detailing to roof eaves, window ledges, and door and window frames will all need to be thought out – however none of this is insurmountable!

Internal Wall Insulation

Rigid insulation boards are fixed directly to the inside of the external walls, and then plastered or papered and painted. This of course is very disruptive, although can be done room by room.

Radiators, pipework, skirting boards, door frames, and other fittings will all need moving, window sills will probably need extending, and the walls will then need redecorating, incurring further costs.

It is important to return insulation into windows and along internal walls, as well as insulating the walls between floors and ceilings so as to avoid thermal bridging and potential damp problems.

The room area will be reduced by the thickness of insulation and finish on each insulated wall, and it is also not ideal to then hang heavy items from the wall.

Unlike external, installing internal insulation does give the option of splitting the costs and doing your house room by room as required – you can also insulate a wall whenever you’re doing something else to it anyway, such as fitting a new kitchen or bathroom or just generally re-decorating, cutting costs and disruption.

Under-floor Insulation

If you have an uninsulated solid ground floor, up to 10% of your heating energy is being drawn into the ground. This is less of an issue if you have suspended floors, although draughts from below can prove just as energy sapping.

If you do have a solid floor you have 3 basic options – either to lift up the existing floor, dig down 100-150mm, lay a screed, insert rigid insulation, and re-lay the floor; or insulate on top of the existing floor and create a new floor (thus raising the floor by 50-100mm); or simply to fit thick carpets with an underlay.

Newer homes may have a ground floor made of solid concrete. This can be insulated if it needs to be replaced, or can have rigid insulation laid on top.

Damp problems can arise with all of these options, and it is one of the few situations when an oil-based material is the best alternative.

Suspended timber floors can be insulated more simply either from underneath or by lifting the floorboards and laying mineral wool insulation supported by netting or boards between the joists.

You don't need to insulate the floors of upstairs rooms in your house if they're above heated spaces, but you should think about insulating any floors that are above unheated spaces such as garages.

Hot water cylinder & Pipe-work Insulation

Modern hot water cylinders tend to be factory fitted with solid foam insulation or an insulated jacket. However if yours isn’t, or it still gets hot to the touch, you could start saving now by fitting a tank jacket or extra insulation. It’s easy to fit yourself, the materials for the whole lot will only cost you around £25, and you’ll save £60 a year.

While you’re at it, why not insulate or lag all hot-water pipe-work that you can access – in the winter this means that your heating will have to work a tiny bit harder (although your pipes are less likely to freeze), but in the summer you’ll save at least £10.00.

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