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Cavity wall insulation and solid wall insulation

Cavity wall insulation and solid wall insulation

All you need to know about wall insulation. We explain the difference between solid and cavity wall insulation types, including how to tell which type of walls your home has.

What type of wall have I got?

The first question you should ask yourself, before thinking about wall insulation, is 'what type of wall have I got?'

  • Solid walls were built up until the 1930s in most parts of the UK. By 'solid', it means that there is no cavity inside them. A solid wall's brick pattern is recognisable by having lots of end bricks, which look like half-length bricks, in the middle of walls (i.e. not near ends of walls and not near windows and doors). These are in fact mainly cross bricks, which run from the front to the back of a wall. Therefore the depth of a wall - ignoring any plaster on the interior surface - is a brick's length, about nine niches.

  • Cavity walls consist of an inner leaf an outer leaf, and a gap in between. Ties, which depending on the era are made of metal or plastic, hold what is effectively two walls together. The brick pattern shows many more full length bricks. Obviously the depth of the wall is greater, as most cavities measure at least two inches.

There are some other types of construction, such as timber frame and no fines, but these are comparatively rare. 'No fines' homes were more often built by local authorities in the 1940s and 1950s for council housing, and can be treated as solid wall for insulation purposes, although (uninsulated) they perform a lot better than older solid wall properties.

Cavity walls lose less heat than solid walls, and are much easier to insulate.

Some parts of the country are fortunate in having relatively old homes with cavity walls e.g. North East England. Unfortunately for the East Midlands, solid wall construction continued into the 1950s.

Obviously the more external walls you have, the more heat loss you will experience. So a detached house loses more heat than a mid-terrace one, for instance. Also, some old homes with solid external walls may have extensions built with cavity walls.

Cavity walls and cavity wall insulation

It is not unusual for people to investigate getting cavity wall insulation when they already have it installed.

Homes built in the last couple of decades are likely to have had insulation put in the cavities when they were built. Otherwise, you may be able to tell if you've already got cavity wall insulation by looking through the papers handed over to you when you bought your home. Cavity wall insulation often has a 20-25 year guarantee. If you live in a low level block of flats, insulation installed by the owner of a flat above or below will also have insulated your property, as the cavity needs to fill from the bottom upwards.

Physically you can check whether you have cavity wall insulation by looking for drill holes, at about waist height, between bricks, at about one metre or one metre and a half intervals, all around the outside of your home. These will have been refilled with mortar (cement) but may show up as a slightly different colour. Another check is to go into your loft and see if you can take a look at the tops of the walls. It's not always possible but sometimes you can see them, and it is likely that some insulation will have spilled out of an open cavity.

Energy-saving measure Average insulation prices % saved on fuel bill Money saved per year Payback period (years)
Cavity wall insulation
(100% funded by a grant)
£0 19.3% £135 0.0
Cavity wall insulation
(partly funded by a grant)
£49 19.3% £135 0.36
Cavity wall insulation £350 19.3% £135 2.6

When you make enquiries about cavity wall insulation, you will be asked about the type and age of your home to assess its suitability.

It is also essential for companies to survey your home before suggesting going ahead with cavity wall insulation as your home may not be suitable or at risk of damp; this could be because of one or more of the following reasons:

  • A pre-exiting damp issue in the home
  • Your home is located in an area which experiences high wind-driven rain (often in the west of the UK, particulary Wales and the South-West)
  • Your home is in an unsheltered position
  • Solid wall insulation

    If you have solid external walls, you have two options for insulating them: 

    • Internal insulation, known as insulated dry lining, and external insulation (or insulating cladding). Insulated dry lining can be a do-it-yourself project but external insulation would not generally be a DIY option.

    • External insulation has the advantage of completely covering the facade of a home. Internal insulation is generally only installed within rooms. That leaves the opportunity for heat to seep out through other routes, e.g. via the voids between ceilings and floors, or in internal walls, and into the external walls, and then of course into the outside world. This escape of heat is known as the bridging effect.

    External solid wall insulation

    External wall insulation generally involves a wooden lattice being fixed to the exterior of the wall to hold some kind of solid insulation in place, and render or cladding over the top. There's a wide variety of finishes, including brick and stone 'slips', so there will be one to suit most tastes or circumstances.

    External wall insulation will add to the depth of your walls, so in some cases passageways and driveways will become narrower. Guttering and exterior drain pipes will probably need to be shifted outwards. There may also be the issues of the new exterior projecting over the public highway, or a home looking out of place in a terrace.

    External wall insulation will usually radically alter the appearance of your home.

    As a result you may need planning permission, so check with your local council. Windows will also change in appearance somewhat, as the insulation needs to wrap round into the window recess, to avoid the bridging effect.

    This kind of insulation is sometimes done when there is a need to repair an existing render. It can also help greatly with protecting walls, although problems with damp should be put right first (e.g. with damp-proofing). External wall insulation prices are usually in the several thousand pounds range, depending on your home. The savings each year on heating bills will be considerable but it could still take more than twenty years to recover the cost. Therefore, the work is often best done during major refurbishments or during major repairs to the walls of a home.

    Internal solid wall insulation

    Internal solid wall insulation, or insulated dry lining, can also be costly but there is the opportunity to 'do it yourself'. This brings down the 'payback period' by a great extent. If it's done professionally, there will be disruption - much more than with external insulation - and it will probably take well over a decade to pay back at current wall insulation prices.

    Insulated dry lining involves placing usually solid insulation against the inner side of a solid external wall, usually held in place with a wooden lattice. The lattice provides a mounting for plaster-boards, which go over the insulation.

    However just behind the plasterboard there will also be a vapour membrane, to prevent condensation (interstitial condensation) occurring inside the wall and insulation structure, as a result of warm moist air in the house entering into parts of the structure which will of course be cooler, the further it gets. Sometimes special plasterboards backed with a vapour membrane are used, but this involves sealing the joins between the boards with specialist vapour impermeable adhesive tape, to prevent the warm moist air getting through at the joins.

    Radiators, power and aerial sockets, shelves and other structures on the wall will need to be re-mounted on the new plasterboard.

    Because of the vapour membrane, which can be punctured, you can't put up shelves and picture hooks, etc. at a later date. Any work that might involve going through the membrane such as putting in new or extra power sockets, or mounting a new radiator, will also damage the membrane. Either the work should be avoided or the membrane re-sealed as part of the work. It is sensible to do the work as part of the fitting of the internal insulation.

    Internal solid wall insulation prices are around £5,500, but could cut your heating bill by almost 63.6% or up to £445 a year.

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