New Build Cavity Construction

What is

New Build Cavity Construction

New build cavity construction has been widespread in the UK since the 1920s and has adapted constantly to changes in regulations and competition from other building methods.

The inherent strength and durability of masonry, coupled with high fabric energy efficiency, ensures that new build cavity construction is in line with the demands of the industry.

In addition, the availability of products, the ease to change design whilst on site, and the simplicity of adding to the building at a later date means that it is still an attractive method of building.

Cavity Construction Diagram

Why do we build with cavity walls?

Cavity wall construction was originally designed to prevent moisture ingress penetrating through the fabric of the building to the inside. The cavity would allow for moisture to run down the inner layer of the outer brickwork and prevent the crossing of the moisture to the inner leaf.

How do we build with cavity walls today?

The process of building the cavity wall basically remains the same; however, developments in products and insulation have taken place over the years.

The cavity is now used not only for protection against moisture ingress, but to facilitate the use of insulation for providing a warmer property.

What is a typical size of a newly built cavity wall?

Depending upon the insulation specified for the cavity, most schemes are now built using an overall wall thickness of 300-350mm which would include for a 100-150mm cavity.

What U-Value would a 300-350mm cavity wall achieve?

This is again dependent upon the actual design and the insulation specified, but generally no matter which combination is chosen, the U-Value should be around 0.20-0.18W/m²K.

Will cavity wall construction meet the new Future Homes Standard?

The beauty of new cavity wall construction is that if required, the increase in the side of the cavity to allow for more insulation is possible.

Also, the use of insulation materials with very low thermal conductivity, such as PIR, PUR or Phenolic foam makes this change easier.

Cavity wall construction is already based around a U-Value of 0.20-0.18W/m²K and the new Future Homes Standard will set wall U-Values at least at 0.18W/m²K.

What is the difference between partial fill and full fill cavities?

A partial fill cavity has to leave a minimum residual 50mm clear cavity between the insulation and the external leaf of masonry. Full fill insulation fits tightly between the internal and external masonry skins with no gaps.

When shouldn’t you use full fill cavity construction?

Full fill cavity construction is not recommended in areas of severe weather exposure, where random sandstone or other irregular walling is being used, or where recessed mortar joints are proposed.

What are wall ties and what different types are there?

Wall ties provide structural stability by allowing both leaves of masonry to work together to provide strength and resist loading. Wall ties are categorised into four basic groups: light duty, basic, general purpose and heavy duty.

Which insulation materials are used in new build cavity wall construction?

The main insulation products are: PIR, PUR, Phenolic, EPS, Mineral wool and glass wool.

The foam-based products such as PIR, PUR and Phenolic, generally have better thermal conductivity, therefore allowing for a reduced thickness, whilst glass and mineral wool insulation have better fire and breathability properties. EPS boards usually provide the most cost-effective solution.

What types of blocks are commonly used for the internal leaf?

The three most commonly used blocks are Aircrete (lightweight blocks which contains millions of air bubbles), Aggregate concrete (these are aggregate blocks available in three densities) and thin-joint systems.

What is a thin-joint system?

A thin-joint system uses a very thin bed of adhesive mortar to bond the blocks together. The advantage of this is that a standard mortar bed would be 10mm, and this accounts for approximately 7.5% of the wall area.

The standard mortar bed has greater thermal conductivity, therefore reducing the overall thermal efficiency of the wall.

Why would you use blown insulation into a new cavity wall?

Although blown bead and fibre is more commonly associated with existing cavity construction, the products can also be used in new build cavities.

Some of the benefits associated with blown insulation are: reduced waste and site storage, quick installation and a reduction in time spent using batt systems in addition to being able to fill intricate details.

Do you require movement joints with masonry?

Yes with masonry movement joints will be required. The NHBC Standards (Chapter 6.1 table 1) recommends the following:

Clay bricks – normal spacing should be 10 – 12 metres
Dense concrete bricks – normal spacing should be 7.5 – 9 metres
Lightweight concrete block or brick – normal spacing should be 6 metres
Dense concrete block – normal spacing should be 7.5 – 9 metres
Natural stone masonry – normal spacing should be 15-20 metres