Masonry Walls

MASONRY WALLS

Introduction
Cavity walls first became a popular method of construction in the 1920s, when it was used mainly to reduce risk of wind driven rain penetration. This early method of construction, which involved a cavity being created between brick & block masonry leafs tied together with metal ties, did not include thermal insulation. This is in major contrast to the highly efficient full and partial fill systems used in modern construction.

As Building Regulations in relation to the conservation of fuel & energy evolved, the space created in the construction of an external masonry cavity wall offered an ideal void in which to introduce insulation and provide increased levels of thermal performance. Insulated masonry cavity walls, still among the most prevalent construction types in the United Kingdom, are recognised as a highly cost effective solution throughout the building industry.

Superglass Insulation Products

Superwall Cavity Slab is a lightweight, semi-rigid slab of glass mineral wool with a water-repellent additive. It is sized to fit between wall ties at standard spacing.
Insta Blowing Cavity Wall Insulation is a loose glass mineral wool insulation material, which is injected into existing or newly built cavity walls.

Thermal Performance
The U-Value of a cavity wall is calculated using BS EN ISO 6946, which is better known as the ‘Combined Method’ of calculation. With the new Building Regulations emphasis on the energy efficiency of a dwelling as a whole, the insulation incorporated into a cavity wall has a major contribution to make towards the dwelling’s overall energy performance.

Thermal Bridging at Junctions and Openings & Accredited Details
As a result of the improving U-values for walls and the need for air tightness at junctions with other elements, the necessity to ensure that there are no cold bridges at junctions and openings has become ever more important. Now, under the name of Accredited Details, these issues are taken into account when calculating the energy performance of buildings (SAP 2005). Guidance is given in ‘Limiting Thermal Bridging and Air Leakage: Robust Construction Details for Dwellings and Similar Buildings. Guidance is given on website http://www.planningportal.gov.uk.

Wall Ties
The correct specification of wall ties is crucial as they tie the leaves of the wall together. Apart from structural considerations, the type of wall ties used is also important in two other key areas.

It is crucial that the ties do not facilitate or aid water ingress across the cavity wall as this will compromise the performance of the insulation.
The ties are considered when calculating the U-Value of the cavity wall with respect to the number of ties per square metre, the cross sectional area and the lambda value of the ties.

Where cavities are 100mm or less, stainless steel wire ties are recommended as they have negligible effect on the wall U-value.

Where the cavity is over 100mm wide, a more robust tie is required, which can have a significant effect on the wall U-value.

Guidance on the type of wall ties required for a cavity width can be found in BS 5628 –3:2001.

Wall Ties should meet the specifications as laid out in BS DD 140-2:1987 or BS EN 854-1:2003.

The following manufacturers are have BBA approved ties:
Ancon Building Products www.ancon.co.uk
Catnic Profiles www.catnic.com.

Rain Penetration
Prevention of wind driven rain penetration from the outer to inner leaf is of paramount importance when designing cavity walls. As well as consideration of the exposure zone the selection of appropriate materials and pointing methods for the outer leaf are crucial at the design stage of the cavity wall. The United Kingdom has four defined exposure zones as follows –
1 Sheltered 2 Moderate 3 Severe 4 Very severe
BRE Report BR 262 ‘Thermal Insulation: Avoiding Risks 2002’ provides a simplified method in the form of a map which identifies the zones

Cavity Trays
Cavity trays divert water away from the inner leaf of a cavity wall, and should be provided:

Where the cavity is bridged by lintels, structural beams, floor slabs, pipes and ducts
Where insulation is present but not installed throughout the full vertical height of the cavity.

Cavity trays should step down and slope across not less than 150mm within the cavity, toward the outer leaf and weep holes. Stop ends should be provided to the ends of all cavity trays. Weep holes should be formed through the outer leaf immediately above the cavity tray at not more than 1m intervals. There should be not less than two weep holes over each opening.

Guidance on use of cavity trays and DPC can be found in BS 5628 –3:2001

Condensation Risk
Condensation areas that require consideration include –
1 Internal condensation 2 Interstitial condensation 3 Thermal Bridge Condensation
Condensation occurring within a cavity wall is detrimental to the thermal performance of the structure and it is essential that steps are taken to ensure this does not happen. Unfaced mineral wool products do not resist the passage of water vapour allowing the water vapour to pass freely through the cavity slab. The Technical Support Team at Superglass can carry out condensation risk assessment calculations on your behalf if required.

Masonry Cavity Wall Systems
There are three methods of construction which include:

Built in full fill using third party approved cavity batts
Built in partial fill using third party approved cavity batts
Retrofit full fill where third party approved blowing wool is injected into new build or existing cavities.

A wide range of U-values can be achieved with performances tailored to specific building ‘Model Designs’ as defined in AD L1A.

Design Considerations click here