0345 873 2828
0345 873 2828
Cavity Wall Insulation Guide
Insulating against heat loss through walls can save between 33 and 40% on domestic energy bills. Whether you need to insulate an existing structure or are constructing a new building, here’s our cavity wall insulation guide to help you work out your next steps.
UK Building Regulations state that all new traditionally built residential and commercial buildings must have cavity wall insulation installed. Evidence also has to be provided by builders and developers, under Part L of the UK Building Regs, to confirm the insulation is continuous.
If you are extending an existing property, the regulations state that the new construction must also have the correct levels of insulation to all areas to be compliant.
What Is a Cavity Wall?
If your home was built after 1920, it would probably have been built with a cavity between two walls, an inner structural leaf, and an outer rain screen. The cavity detail was developed to prevent moisture from transferring through the wall from the outside.
Brick-built British late Victorian and Edwardian terraced houses were among the first to adopt the design, and it became standard in the early 20th Century after the First World War. The cavity helped to prevent damp by introducing ventilation to the structure and helping to keep timber floor joists dry.
Ever since the 1980s UK Building Regulations have gradually refined, and often increased, cavity wall insulation requirements for all new building projects.
What is Cavity Wall Insulation?
Cavity wall insulation makes up an essential part of the thermal envelope of a building and should be continuous with the loft, or any underfloor insulation while allowing ventilation where required.
It is easiest to fit cavity wall insulation during the construction of a building. As the inner walls go up, the insulation is cut to fit and fixed to the inner leaf before the outer leaf is lifted. Retaining clips fitted to wall ties hold the insulation in position.
To ensure the continuity of the thermal envelope, cavity closers are installed around windows to prevent thermal bridging and maintain the integrity of the insulation performance.
How to Tell if My Cavity Wall is Insulated?
Have you got a cavity wall? That’s the first question. If your house dates to between 1910 and 1980 it is very likely that your house was built with a cavity, but without insulation. Houses built after 1980 could probably benefit from upgrading any existing insulation too.
If your house already has cavity wall insulation installed there are a couple of easy ways to tell.
An Energy Performance Certificate
If you have recently bought a property then you will have been issued with an EPC (Energy Performance Certificate), which will detail the energy performance of the building, including whether insulation is present in a cavity wall.
If you can’t find, or do not have access to, an EPC you can find yours on the UK Government register. Simply input your postcode, and if your property has an EPC the details will be available to view immediately.
A Visual Inspection
If your home has had cavity wall insulation retrofitted there will be tell-tales on the exterior walls of the house. Insulation that has been pumped into the cavity will have been installed via holes drilled at a height of about 1.2 m.
Brick Walls
In a brick wall, these holes are usually made in the mortar joints and then filled. The new mortar is usually a different colour from the existing sand and cement mix. If a wall has been completely repointed, look for irregularities in the brickwork mortar joints where the drill bit cut into the bricks.
Rendered Finishes
The installation evidence will be similar for rendered walls. Even if the external wall has been professionally redecorated there will be at least some evidence that the wall has been drilled. New material will always be different from old render in both mix and basic chemical make-up, so should be fairly easy to spot.
Eaves Loft Inspection
Expanding foam has been used to insulate cavity walls in the UK since the 1970s. One of the problems that this new technology brought with it was that it would sometimes block the roof ventilation at the eaves.
The insulation would have to be cut out, but it would remain visible at the top of the walls in the loft space. If your loft is easily accessible, this is also a reliable way to confirm visually, the extent of any cavity wall insulation.
What Is Used to Insulate a Cavity Wall?
There is a range of insulation products available, with varying properties, but each has similar energy-saving performance. The best insulation materials for new structures are mineral wool batts and foam board insulation.
Mineral Wool:
Spun from super-heated natural stone at temperatures in excess of 2000°C, Rockwool mineral wool is an excellent insulator that does not support mould growth, is rapid drying, and fireproof. Mineral wool batts have excellent structural stability, which means that they won’t slump, lose their shape, or shrink over time.
It is a sustainable choice that ticks all the Part L requirements and can be used in masonry or timber frame construction. Rockwool also has excellent noise mitigation and acoustic properties. As such it is perfect for use in party walls and internal partitions for privacy as well as comfort.
Foam Board Insulation:
PIR, or phenolic insulation boards, such as Celotex or Kingspan are excellent insulators that can double up as vapour and air barriers when the boards are properly joined together with proprietary sealant and tape materials.
Celotex foam board insulation is difficult to recycle, as such it is better for continuous insulation as part of the thermal envelope wrapped around a blockwork inner skin to reduce cutting, joints and waste.
What About Other Cavity Wall Insulation Materials?
There is a fair amount of choice when it comes to cavity wall insulation. Some materials are better than others in certain circumstances.
Fibreglass:
Fibreglass insulation is readily available, and inexpensive, but is better suited to loft space insulation where it is less likely to come into contact with moisture. Wet fibreglass quickly sheds any insulating performance, and it can promote mould growth too.
Not recommended for cavity walls, especially in maritime, or exposed regions.
Hemp & Wood Fibre insulation:
Hemp and wood fibre insulators tick all the right sustainability boxes and are excellent when used to insulate timber frame constructions. The materials are breathable, natural, and treated with flame retardant.
Hemp is not widely available and can be expensive to source, while wood fibre has excellent acoustic qualities when used for internal partition walls.
Polystyrene beads:
Polystyrene beads work well as an insulator but should be installed by a contractor in order to get the right mix of adhesive and beads. It can reach and fill all the hard to access cracks and imperfections within a wall.
Polystyrene is 98% air, which is why it is such a good insulator, however, exposure to wet conditions will call a halt to its insulating performance until it is able to dry out.
Spray foam insulation:
Open-Cell Spray Foam:
Open-cell spray foam insulation has been in the news recently, dividing opinion as to its safe use in retrofitting loft insulation. This is due to the foam covering the roof structure, making it impossible to check for issues with rot, leaks, etc.
Due to its ‘open’ nature, it’s not recommended for cavity walls.
Closed-Cell Spray Foam:
Closed-cell spray foam, however, is often recommended for use in timber frame buildings as it can double up as an air and vapour barrier when applied to a base material between the wall studs.
In new construction works, closed-cell spray foam can be applied extremely quickly and deliver outstanding insulation performance. It is, however, probably the most expensive option. Ideal for areas with limited space to accommodate insulation as it has the highest insulating performance of any material.
How do I Insulate an Existing Cavity Wall?
An entire retrofit industry has grown up over the last half-century that will install insulating material in the existing wall cavity of your home. From polystyrene beads to expanding foam, blown wood fibre to fibreglass, there is a dizzying array of insulants to choose from.
Foam insulation is good at finding and filling every nook and cranny within your cavity wall, while other, blown-in insulants can sometimes settle over time, leaving space for heat to escape.
It is possible to carry out retrofit works yourself, but it will often entail the sourcing of specialist equipment.
What is the Insulation Process for Existing Cavity Walls?
Be prepared for a number of holes to be drilled in the outer wall of your building for the retrofit installation of the insulation. There will be a fair amount of making good and perhaps even redecoration to be carried out after the work is complete.
Most companies will repoint, or fill holes with render, but it will be up to you to organise the redecoration. As already discussed, it is unlikely that the making good will ever be completely invisible!
Comments
Leave a comment