Cavity Wall Insulation

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Cavity wall insulation has been around since the 1970’s, and many of us have already had it installed now. If you have cavity walls, it is the single most effective energy saving measure you can have installed in your home.

It can save around 35% of the heat loss through your walls, and can save as much as 30% from your heating bills. It also saves around a tonne of carbon dioxide a year.

Homes and extensions built since around 1996 will usually have had cavity wall insulation built in as standard under building regulations but this is not always the case, on smaller housing developments in particular.

Cavity wall insulation should only be installed by registered installers, who will offer a 25-year insurance backed guarantee with the work.

The cavities should only be insulated if they are suitable, and the installers should check the following:

  • Condition of the pointing, bricks, render and guttering • Cavity width must be between 50–150mm
  • Safe access to all external walls (e.g. scaffolding over glass roofs)
  • Identification of any damp issues (not condensation)
  • Checking for any rubble in the cavities
  • Severe exposure to the elements (wind driven rain)

How do I know if my property has a cavity wall?

In most areas of the country properties were not built with cavity walls until around 1930, however in some areas of Dorset (and elsewhere) cavities were built as far back as around 1875, so it’s important to check your walls for the presence of a cavity.

The methods of determining whether you have a property with a cavity wall construction are as follows:

  • If walls have stretcher bond bricks (bricks laid long ways side by side) then it has probably got a cavity.
  • If it has an English or Flemish bond (stretcher then header, or row of stretchers interspersed with rows of header bricks) then it’s likely you have solid walls, although this is not always the case.
  • With English or Flemish bond construction, measure the width from the inside to the outside wall (called the reveal), if this is 10.5” or more it should then have a cavity and this could be insulated.

But cavity walls were built for a reason so why fill them?

Yes, cavity walls were built primarily to stop rain penetration getting onto the inside wall of the property and causing damp. The materials they use now, allow the air to circulate around the cavity and keep it dry, and do not bridge the cavity in any way. The other reason cavities were built was because of the lower build price of two separate walls compared to building one solid wall, a difference in cost of up to 30%.

The installers no longer use white foam (Urea Formaldehyde) to insulate cavities because if miss installed, could break down and disintegrate over time.

What materials do they use today?

Installers use two different products to insulate the cavities depending on which is most suitable or which the installer prefers. The first is a is a polystyrene bead product, which is blown into the cavity with an adhesive to bond it together. This material is preferred where there are external obstructions around the property, where the cavity is very wide and where the risk of water penetration is higher due to high exposure rates from the elements. It is also a more thermally efficient product. Because the beads are round, if any water gets into the cavity it will roll off the poly beads to the bottom of the cavity where it will dry out.

The second is a mineral fibre material, this is manufactured using sandstone, limestone and coke, which is melted down and spun into fibres. These fibres are ground up into loose material which is then blown into the cavities. The fibres are impregnated with an oil (usually sunflower oil), to discourage water penetration through the material. Because the fibre is loose it still allows a degree of air circulation which circulates and allows the cavity to remain dry. This material should not be installed in homes which are exposed to wind-driven rain, or are poorly maintained.

How long does it take to complete and what do they do?

The work on average takes around 2 – 6 hours depending on the size of the property and any obstructions (conservatories, car porches, garages). There is usually a team of two technicians who will drill a series of holes in the outside wall as set out in the manufacturers guidance for the product.

The drilling usually lasts between 30 minutes to an hour, and holes are drilled in between bricks or into render at approximately 1.4 metre intervals depending on the material being used.

All vents are maintained both before and after the installation.

The insulation material is then blown into the cavity until the area is filled. When the cavity is fully insulated, the holes are made good using mortar between bricks or an off-white caulk on render. These should match the existing mortar colour or exterior coating.

The installers should leave the work area clean and tidy and sweep up and hose down any dust as best as possible.

So what grants can I get toward the cost?

There is funding available, some people may be entitled to it free of charge, or with a small contribution, whilst others may not qualify for any assistance. Please enquire as the situation changes regularly.

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