The positives and negatives of cavity wall insualtion
What is a cavity wall?
In the past, walls were simply built as one solid unit of two rows of bricks, but from the 1920's onwards and especially near coastal areas, cavity walls started to become the norm. It consists of two skins or leaves (walls) with a cavity in-between and held together with wall-ties. When water enters the first skin, it can't penetrate into the house, but collects at the bottom of the cavity and exits again at the ‘weeping holes'.
Problems with cavity wall insualtion
A cavity wall is not so effective in insulating a house and so this can be filled with various materials, including mineral wool and polystyrene granules. The problem with mineral wool is that water that penetrates the first skin gathers in the mineral wool and causes it to sink under its on weight and collect near the bottom. There it forms a dense mass that can temporarily gather water, which is further transferred to the second skin and ultimately into the house.
Polystyrene granules, or beads, are blown into the cavity and is said to be more superior to mineral wool, but it's more expensive. It is inherently waterproof, which suggests that no water get can through via this medium. It is claimed that water goes directly to the bottom past the individual beads, but the voids between the granules may hold water and this can eventually pass across the beads instead of straight down. Sometimes the beads don't bond as it's supposed to do and when an opening is made in the wall, the beads can pour out. In other cases some of the polystyrene gets blown out through airbricks.
There is also the option of foam injected into the cavity. This seems to work quite well, except that the expanding force of the foam might be strong enough to crack the walls. Since the 80's, through Building Regulations in the UK, new houses were to be built with cavity wall insulation and this foam is used and works well in keeping out rainwater. The Building Research Establishment found that single leaf brick walls always leak when receiving driving rain, which makes cavity walls essential with homes that receive a lot of wind-driven rain.
During construction, mortar gets dropped down the cavity and collects on the reinforcing, or wall-ties, and this may cause an obstruction to materials that are blown or poured down the cavity wall. When these insulating materials get wet and are in contact with the wall-ties, the wall ties corrode much faster. They're made from iron and steel and in dry conditions they can last for many years, but if there is too much moisture within a cavity wall, then these critically important parts of the wall can eventually disintegrate and affect the structural strength of the walls. To replace wall-ties is a very time-consuming and expensive process.
Another problem with insulation is that heat is kept on the room side of the wall, which leaves the outer wall cooler. When this happens, condensation may start to form on the outer wall when warmer moist air comes in contact with the outer leaf. The wall-ties attract the most condensation and when this moisture is trapped in the insulation, the corrosion is accelerated.
Probably the best way to fix this is to allow the cavity to dry out completely and then seal the wall on both sides, so that no water can enter from either side.
Before getting cavity wall insulation
The condition of the wall ties should also be inspected and assessed by a trained surveyor to determine suitability for insulation. This is important when applying for a grant to insulate you cavity wall. In the UK it is the British Board of Agrément (BBA) that deals with this issue. Homeowners over the age of 70 can have their cavity wall insulation done for free through a grant.
CWI installers should be registered with the Cavity Insulation Guarantee Agency (CIGA), which offers 25-year guarantees. When applying for a grant, it is best to do it in spring or summer when the waiting list for approval is the shortest. The installer usually also handle the documentation, the application for a grant and the claim on your behalf. The cheapest to install is mineral wool, but it may cause a problem with people allergic to this material and it is also impossible to completely remove out of a cavity wall.
Benefits of cavity wall insulation
Insulating your cavity wall is one of the most effective ways to save on your energy bill, with a saving of around 35 percent. You could save about £115 per year, while the cost of installing is around £500 for polystyrene insulation, with a possible grant of 40 percent to 100 percent, depending on different situations. To find out in the UK if you qualify for CWI you can phone the Energy Saving Trust advice centre at 0800512012.
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