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Construction details

An important aspect of building performance is the detailing of construction elements (foundations, walls, floors, roofs etc) and the junctions between them. Help is available for all aspects of detailing including  U-values for walls and floors, thermal bridging at junctions, fire compartmentation and  acoustic separation. In many cases, a single detail may be required to satisfy a combination of these needs.


U-values and thermal performance guidance for a range of different wall configurations, finishes and block types is available for concrete aggregate blocks through the U-value Calculator for Cavity Walls and for aircrete blocks guidance can be found from manufacturers websites or by contacting their technical teams. For a broader overview of wall U-values refer to Thermal Performance: Part L1A (2013) from The Concrete Centre and its Dynamic Thermal Properties Calculator, which can be used to determine indicative U-values of construction elements, along with other thermal performance information.

Thermal Bridging Construction Details

Construction details show the geometric layout and the expected heat loss through standard junctions, which is presented as a psi value.  An f value may also be presented, which relates to likelihood of internal condensation: a value above 0.75 is “safe” with regards mould growth.

The Masonry industry has come together to produce a free and easy to use resource housing a database known as Recognised Construction Details ™ (RCD). The independently assessed thermal junction details contained within the database have been certified for use in demonstrating compliance with Part L of the Building Regulations.

Recognised Construction Details is a project commissioned by members of MPA Masonry and delivered by the Building Alliance and architects Studio Partington.

To access this resource visit:

In addition to providing an assessment of Ψ values Recognised Construction Details also provides a guide to constructing homes that meet the requirements of Approved Document B1: Fire Safety.

Other key providers of masonry construction details are the Local Authority Building Control (LABC).

Construction details for aircrete blocks generally mirror those for aggregate concrete blocks and are based on standard cavity thicknesses. The LABC details are particularly aimed at the SME market and use products that are readily available from the builders merchants. The LABC psi values are generally better than those given by Constructive Details Ltd, which are more generic. It is recommended that designers and builders initially refer to the LABC website, but if unable to find details that meet the required cavity thickness or insulation type they should then refer to Constructive Details Ltd.

Fire Details

Masonry is non-combustible and as a result fire detailing of wall elements and junctions is far simpler than for other materials.

Acoustic Details

Building regulation Part E stipulate minimum acoustic performance for airborne and impact noise between adjacent dwellings.