One of the primary functions of external walls is to provide an efficient weathershield. They prevent the penetration of moisture, which can have a detrimental effect on the internal structure. The moisture-resistant properties of Thermalite® blocks make them ideal for the inner and outer leaves of external cavity walls, as well as solid external walls.
The combination of moisture-resistance, strength and thermal insulation makes a wide range of external wall solutions possible.
For two-storey dwellings, Thermalite® Turbo and Shield are more than adequate to satisfy the structural requirements of the Building Regulations. The introduction of Thermalite® Hi-Strength 7 and Hi-Strength 10, however, extends the range of Thermalite® applications to include 3 and 4-storey residential buildings, as well as commercial and industrial buildings.
The thermal insulation characteristics of Thermalite® blocks also enables external wall constructions to meet the requirements of the Building Regulations. Possible Thermalite® solutions include cavity walls incorporating insulation material that partially or fully fills the cavity or, alternatively, cavity walls with insulating internal lining systems.
External solid walls, which are growing in popularity, are another method of meeting the requirements of the building regulations. The external wall may be of solid Thermalite® alone or a combination of Thermalite® and internal or external insulation systems.
The construction of Thermalite® blocks should be carried out in accordance with BS 5628: Parts 1 and 3 and BS 5250, as well as the information contained in the British Board of Agrément certificate 00/3720. Lintels
The external wall should be set out to ensure that the ends of lintels bear onto full blocks, not onto cut pieces of block, and the length of the bearing should be at least 150mm.
Situations will occur where the stress beneath the lintel bearing is likely to exceed the design stress. In such cases either the lintel bearing area should be increased or concrete padstones or spreader beams should be positioned beneath the ends of the lintel.
The use of lintels can introduce thermal bridging. It is therefore recommended that the lintel either incorporates insulation material or be of a type that allows Thermalite® blocks to be included as part of the lintel. Wall ties
It is important that the correct type of cavity wall tie is selected. The wall ties most suitable for use with Thermalite® are those that will transmit axial load between the leaves of the cavity wall. At the same time they need to be sufficiently flexible to allow differential movement to take place between the two leaves.
Wall tiles should either comply with DD140 or BS EN 845-1 and should be made of material as references 1 or 3 in BS EN 845 Table A1, austenitic stainless steel. To satisfy the requirements of Approved Document A, ‘Structure’, wall tiles should be selected in accordance with Table 5 of that document.
In external cavity walls the wall ties should be bedded into each leaf to a minimum depth of 50mm.Elimination of thermal bridging
The requirements of the Building Regulations place particular importance on the elimination of thermal bridging, between external walls and floors, and at eaves level. The BRE report ‘Thermal insulation: avoiding risks’, discourages the use of clay bricks over other dense materials. The report does, however, actively encourage the use of low density blocks such as Thermalite® as a satisfactory means of reducing thermal bridges.
The continuous use of Thermalite® blocks, together with Thermalite® Coursing Bricks, across junctions with floors, will reduce thermal bridging.
See also BRE IP 17/01 ‘Assessing the effects of thermal bridging at junctions and around openings’ (no longer current, but citied in Building Regulations). Installation
Thermalite® blocks are suitable for the construction of external cavity and solid walls, when installed in accordance with the relevant Detail Sheet of Agrément Certificate 00/3720. The walls should be designed and constructed in accordance with the recommendations of BS 5628: Part 1, BS 5628: Part 3 and BS 5250.
In addition, the table below gives minimum thicknesses of blocks in solid rendered walls related to exposure conditions as defined in BS 5628: Part 3 and BS EN 1996-1-1 (Eurocode 6).Thickness of rendered Thermalite® solid walls for various exposure conditions
||Minimum block thickness (mm) |
|Moderate / severe
|Sheltered / moderate
Thermalite® is an ideal material for the construction of extensions to existing properties, with a range of block sizes and strengths that offer choice of structural performance, external appearance and internal finish.
Solid wall construction is also viable for the design of complete houses and other buildings, fully in compliance with Building Regulations and Standards, especially when associated with Thin Layer Mortar. High levels of insulation can be achieved and advantage can be taken of the thermal mass to assist with heat retention in winter and cooling in summer.
Render is a traditional external finish in many parts of the UK, and is easily applied to Thermalite® solid walls to give excellent resistance to water penetration by wind driven rain.BS 5628-3 describes exposure categories for wind driven rain
||Wall construction |
|3 - Severe
||215mm Thermalite blocks renderedto BS EN 13914-1 |
|4 - Very severe
||Solid wall with impervious cladding |
In addition to traditional and technical renders, Thermalite® will readily accept a range of impervious claddings fixed with either mechanical fasteners or adhesives such as:
- timber weatherboard
- tile hanging
- manufactured board cladding
- insulated render systems
- ceramic tiles
- brick slip systems