Insulating Cavity Walls

Before you can insulate your walls you must find out what kind you are dealing with. As a rule, houses are built of brick and most recently built homes have cavity walls, which are made of two thin skins of brick with a gap between.

These walls can be recognised by their thickness, usually 270mm, made up of two 110mm brick skins with a 50mm cavity between them. If you measure your wall at a window or door opening, it should be about 280mm thick, allowing for 10mm of plaster, if it is a cavity wall. If the wall, with plaster, measures 230mm or 350mm then it is probably a solid wall. A cavity wall can also be recognised by the fact that on the outside there are only stretcher bricks whereas a solid wall will show stretchers and headers.

The legal aspects

No matter what sort of walls you are dealing with, you will need permission to insulate them. In the UK, filling a cavity wall officially contravenes the building regulations unless the firm that does it has a certificate allowing the appropriate regulation to be relaxed. The insulation of solid walls will require full permission according to the building regulations, and perhaps planning permission as well if it changes the appearance of the house.

The best course is to go and see the building inspector at your local council offices, and explain what you are planning to do. The inspector will give you some quite simple forms to fill in and, probably, some useful advice as well. We have found building inspectors without exception to be helpful and friendly, and it is obviously well worth being on good terms with your local inspector rather than treating him or her as a faceless bureaucrat.

If you are contemplating external insulation on a solid wall you will have to talk to someone at your local planning department as the work could change the external appearance of the building. Planning is a more tricky area to deal with as there are no rules — the building inspector must pass an application if it does not contravene the building regulations but a district council under the guidance of a planning officer can refuse anything quite arbitrarily. This is not to say that the council always will; if you ask the planning officer before you apply for permission you may get some idea of local attitudes. If all else fails you can always put the insulation on the inside.

Insulating cavity walls

If you have cavity walls there is nothing you can do to insulate them by yourself, unless you treat them as solid walls as described later. Your easiest solution is to turn to the Yellow pages and look under ‘Insulation Contractors’. You will find three common ways of insulating a cavity wall: ureaformaldehyde (or uf) foam, mineral fibre and polystyrene granules. All three methods start with holes drilled in the wall through which the insulation is put into the cavity. The holes made in the wall are mortared up once the job is done and the complete process takes about a day for an average house.

Uf foam is made by mixing two chemicals and blowing the mix into the cavity where it expands to fill the space and sets into a fairly rigid foam. The foam is cheap but not strong enough to be made into sheets like expanded polystyrene; when pumped into a cavity it does not matter that it has no strength as the brickwork will hold it up. Once in place, uf foam cannot be removed without taking down the wall whereas mineral fibre and polystyrene granules have the advantage that they can be removed if problems occur.

Unfortunately damp can be a problem with filled cavity walls. The purpose of the cavity is to stop rain from reaching the inside of the building. Usually the outer leaf of the wall becomes very wet when it rains; the water runs down the inside of this leaf and drains out at the bottom. If you fill the cavity with something you must be sure that whatever it is does not conduct the moisture across the wall to the inside. Because of the risk of damp some kinds of cavity fill are suitable only for sites that are sheltered from driving rain.

Cavity wall insulation is covered by the Agrement Board, a body which tests building materials and their fitness for use. Never go to a cavity insulation firm which cannot show you an Agrement certificate approving the firm and the process used. The certificate grants a waiver of the building regulations which forbid the bridging of the cavity, and it will also show the degree of exposure under which the material can be used. There are some cowboy firms operating in this field, but if you go to companies with an Agrement certificate you should be fairly safe. The best plan is to go to several firms and obtain a free quote from each before you make the final decision.

Cavity wall insulation has the disadvantage that the thickness of the insulation is governed by the thickness of the cavity, usually 50mm or 75mm. If you want a better U value than this amount of insulation can provide (0.4 to 0.5W/m2degC), you will have to apply extra insulation to the wall as if it were solid.

If your house has still to be built you have other options. The width of the cavity can be increased to 100mm, according to the building regulations (Regulations D15 and D16 Schedule 7 rule 12, in case you need to know), provided that you use ‘vertical twist type wall ties’, spaced 750mm apart horizontally and 450mm vertically. Insulation can be put into the cavity as the wall is built up, allowing slab materials to be used, such as expanded polystyrene r a resin-bonded glass fibre which has been treated with a water repellent to prevent the transmission of moisture across the cavity. The inner leaf of the wall can be built of insulating lightweight concrete blocks to further improve the insulation. If you want more than 100mm of insulation in a new wall the inner and outer leaves of the wall will have to be designed as separate free-standing walls, without conventional wall ties connecting them.

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