Masonry cavity walls are the most commonly used form of wall construction in the UK. Originally designed to resist the risk of rain penetration, the cavity between the external and internal wall leaves was not insulated and therefore delivered minimal thermal performance. Today, however, the installation of Superwall cavity products and Insta blowing cavity insulation in the cavity wall provides a cost-effective means of delivering excellent thermal performance. Indeed, while Building Regulations now focus on the efficiency of a building as a whole, the insulation used in a cavity wall has a major bearing on the overall energy efficiency of a building.
There are three different methods used for insulating masnory cavity walls with Superglass products:
Full Fill: Built In
Using either Superwall cavity slabs or Superwall Roll delivers the best u-value to wall width ratio and the lowest cost option.
Full Fill: Injected
Insta blowing wool insulation is suitable for new-builds and existing cavity walls.
Superwall cavity slabs in conjuction with a minimum 25mm clear cavity is low cost option but has a reduced thermal performance.
Superglass Insulation Products
Superwall 36 is a BBA approved dual purpose cavity slab treated with water repellent additives. Superwall 36 is intended for new masnory walls in either full fill or partial fill applications. With a thermal conductivity of 0.036W/mK.
Superwall 32 is a BBA approved dual purpose cavity slab treated with water repellent additives. Superwall 32 is intended for new masnory walls in either full fill or partial fill applications. With a thermal conductivity of 0.032W/mK.
Insta blowing wool cavity insulation is a loose glass mineral wool insulation material, which is injected into existing or newly built cavity walls.
Superwall Roll 36 is a BBA approved cavity wall roll treated with water repellent additives. Superwall Roll 36 is suitable for use in full fill applications and provides a thermally efficient, cost effective means of achieving current Building Regulations.
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 2009). Guidance is given in BRE Appendix K.
Guidance is given on BRE website.
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.
Wall Ties should meet the specifications as laid out in BS DD 140-2:1987 or BS EN 854-1:2003.
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 –
- Very severe
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 EN 1996 –1:2005.
Condensation areas that require consideration include –
- Internal condensation
- Interstitial condensation
- 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.