Wall Insulation
Uninsulated walls could be responsible for 35% of your home's total heat loss so insulating them should be a priority.
The major distinction is between solid walls and cavity walls. If your house was built before 1920 it's more likely to have solid brick walls. If so, these will usually be about 225mm thick and have a brick pattern with both long and short brick faces (the short ones are the ends of normal bricks turned sideways). If your wall is thicker (typically 270mm or more) and has a brick pattern with all long brick faces, then you probably have a cavity wall.
Cavity Wall Insulation
All houses (or extensions) built since 1995 should have insulated cavities because that's when it became a Building Regs requirement. That leaves a significant number of homes built between approximately 1920 and 1995 which may have uninsulated cavity walls. If you fall into this bracket, it is possible to retrofit cavity wall insulation. This usually involves a specialist contractor drilling a grid of small holes in the wall and blowing in glue-coated polystyrene pellets which fill the void.
This method is effective but it's reliant on the cavity being clear of obstructions. If the pellets can't get to certain areas then you could be left with cold spots. A thermal camera can be used to check for cold spots after the work. Cavity wall insulation is usually quite cost-effective at approximately £10 per square metre of wall.
Early forms of cavity wall insulation were prone to settling and allowing damp to across the cavity. These problems have largely been solved by modern practices where the insulation pellets are coated in glue. Be sure to appoint a specialist contractor who offers a guarantee through the Cavity Insulation Guarantee Agency, CIGA, or similar trade body.
Difference in brick pattern between solid wall (top) and cavity wall (bottom)
Solid Wall Insulation
Solid walls can be insulated either internally or externally, i.e. by placing new insulation either inside the rooms or on the outside of the house. The pros and cons of each method are compared in the table below. It sometimes makes sense to combine both approaches on different areas of your property - for example you might use internal insulation on the front elevation of the house to retain its appearance but use external insulation on the side and/or rear face where the appearance is less important.
External Wall Insulation
Insulating the outside of the walls is almost certainly the better technical solution. In contrast to internal wall insulation, no floor space is lost. The brick wall is kept inside the heated envelope of the house which usually ensures that it stays dry. The wall also contributes 'thermal mass' which helps to regulate the internal temperature. While the heating is turned on the bricks will absorb some of the heat. The external insulation stops the bricks from losing that heat to the outside, so when the heating turns off it can be re-emitted to the room to maintain a more consistent temperature.
Why then, doesn't everyone choose external insulation? There are a few drawbacks –
The insulation will completely cover the house. If your house has attractive brickwork or architectural features these will be lost.
If your house is terraced or semi-detached, external insulation will cause a step in the front face of the house where it meets the adjoining properties.
It's usually more expensive than internal insulation due to additional labour, scaffolding, and so on.
The increase in thickness of the walls could require roof eaves to be widened which will further increase the cost.
Gutters and other drainage features will need to be moved or replaced.
Windows may need to be moved to avoid them sitting too deep in their openings.
In summary, external wall insulation is often not suitable, especially on the front of a house, but there are still many houses where it’s the best solution. The judgment on what's attractive is subjective but we'd argue that much 20th century housing could actually be improved by the addition of external wall insulation and a fresh coat of render. Even where external insulation isn't suitable for the front face of a house, it may be possible to use it on the side and/or rear aspects where it's visually less intrusive.
Example of successful external wall insulation project on a mid-20th century property
Internal Wall Insulation
The main challenge with internal wall insulation is avoiding damp issues. After installation of internal wall insulation, anything outside the insulation layer – including your existing brick wall – is no longer heated. This can disrupt the seasonal equilibrium of wetting and drying of the brickwork, potentially allowing water ingress or condensation to form on the inside face of the wall. At best this can reduce the effectiveness of your new insulation, and at worst it can cause mould growth and rotting of timber floor beams.
There are a large number of possible insulation materials and installation techniques, but we would group them into two main approaches –
Typical arrangement for “vapour sealed” internal wall insulation (image credit - www.greenspec.co.uk)
Vapour-Sealed Internal Wall Insulation
This is the most common approach. Modern high-performance materials are used in a system that can be quick, easy and cost-effective to install. The thickness of the complete system can be less than 100mm, meaning you lose a bare minimum of floor area.
The philosophy here is to completely seal the surfaces to keep water and water vapour out of the insulation system. This is logical, but achieving a perfect seal can be difficult in practice and is wholly dependent on the skill and care of the installer. If there's a break in the airtightness layer, humid air can get behind the insulation and condense into liquid water where it meets the colder bricks. If this happens there's not usually an easy way for it to dry out so it can accumulate and cause damp issues. This is often hidden so it can be difficult to know there's a problem and over time defects such as rotting of timber joists can develop.
'Dot and dab' installation for foil-faced PIR insulation boards (image credit: Celotex)
The most common form of insulation for this approach is rigid foil-faced foam boards (e.g. PIR) of 60-80mm thickness (the thickness needs to be calculated to achieve a target U-value as defined in Building Regulations). These are impermeable to water and the foil acts as a vapour barrier. The joints between the panels and around any openings should be sealed with aluminium tape or similar to complete the airtight seal.
The foam panels are often mounted on top of or between timber battens, with plasterboard fixed on top, and finished with a thin "skim" coat of plaster before painting or wallpapering. A popular alternative is direct fixing where the insulation is essentially glued to the wall with adhesive. This method is commonly known as "dot and dab".
Insulation boards are also available with plasterboard pre-fixed to the internal face saving the need for a second step fixing separate plasterboard. If your existing wall is dry and even, direct gluing of insulated plasterboard is a very quick and easy way to add insulation. If the wall isn’t flat then it’s advisable to mount the boards onto a frame of timber battens which can smooth out any irregularities.
Insulated plasterboard is produced by market leaders Kingspan and Celotex and can be purchased for as little as £25 per square metre.
Breathable Internal Wall Insulation
This approach is gaining in popularity, especially for older houses where dampness is more of a risk. In contrast to the previous approach, this accepts that water may find its way into the insulation and uses breathable materials to allow it to evaporate again. Some insulation materials are even hygroscopic meaning that they will absorb water from the air during high humidity and release it again when the humidity drops. This effect can actually improve the air quality in the room. The drawbacks are that the insulation materials are usually less efficient, meaning the overall thickness is greater, and more expensive. Specialist finishing materials such as lime plaster and breathable paints are also recommended, limiting choice.
N.B. Although this approach is more tolerant to some moisture, it's still not appropriate for a situation with active water penetration through the external wall may occur so any defects in the wall should be rectified beforehand.
Step-by-step installation instructions -
“Friction fit” of semi-rigid wood fibre installation between timber battens
The most common materials for this approach are mineral wool or wood fibre, usually in semi-rigid “batt” form. This is installed between timber battens, and covered with plasterboard and ideally a breathable lime plaster finish.
Some wood fibre boards are designed to be attached to the wall and plastered directly, removing the need for the timber battens and plasterboard. You will need to anchor the boards with non-conducting wall ties drilled into the brick wall. At least 140mm thickness of wood fibre board is required to achieve a U-value of 0.3. The market-leading products in this category are Steico or Pavatex which will cost approximately £45 per square metre.
Installation detail for rigid wood fibre insulation blocks (image credit - www.greenspec.co.uk)
Internal Wall Insulation Detailing
Whichever approach you use you will need to remove anything attached to the walls – electrical sockets, light switches, curtain rails, radiators, pipes, skirtings, covings, kitchen cabinets, fitted wardrobes, etc. These will need re-fixing to the finished insulated wall. This could be a good opportunity to move any radiators away from their traditional position under windows onto internal walls. Not only will this reduce the amount of heat the radiator loses through the external wall but it will also allow you to hang full-length insulating curtains over the windows, potentially saving the cost of a window upgrade.
Electrical cables and any pipes will need to be accommodated somewhere in the new wall. If you aren’t moving the electrical fixings then you may be able to leave the cables and pipes embedded where they are and simply drill small holes in the insulation for them to poke through (don't forget to seal the holes afterwards to maintain airtightness). If you need to move the services, you may need to carve out new channels in the existing plaster wall before fixing the insulation. An alternative is to use timber battens that are slightly deeper than the insulation system, leaving a small 'service void' between the insulation and the plasterboard. Notches can be cut in the battens to allow the services to cross over.
Window reveal detail with thin layer of Aerogel insulation to prevent thermal bridges (image credit – Proctor Group)
Floor void insulation – worth considering to reduce thermal bridging which could reduce the effectiveness of the insulation and lead to condensation
(image credit - www.greenspec.co.uk)
Window openings must also be insulated to avoid thermal bridges causing condensation. The thickness of most insulation systems makes them unsuitable for window reveals (they would be deeper than the window frame) so special materials may be required in this area. High-tech Aerogel insulation (marketed as SpaceTherm) is available pre-bonded to thin sheets of magnesium oxide board which can be plastered directly. The thinnest combined system is only 13mm thick and sells for approximately £100 per square metre. This wouldn't achieve the required U-value for an entire wall but would be perfect around a window frame which might otherwise be impossible to insulate without replacing the window.
Finally, an area which is often neglected during insulation projects is the floorspace. If you don't insulate the wall between the ceiling and the floor this will become a cold bridge, potentially causing condensation and rotting of the timber floor joists in the long-term. The only way to insulate this area is to lift or cut the floorboards along the wall and insert insulation in the gap. The easiest way to do this is to stuff some flexible mineral wool insulation into the gap against the wall. A more thorough method would be to plaster the wall for airtightness and insert sheets of calcium silicate board insulation (e.g. Calistherm) which has excellent moisture-control and anti-mould properties.