AIRCRETE

4.3

Chapter 4. Design

Page 4 of 18

4.3.1

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4.3 Thermal
The requirements of Part L of the Building Regulations became more stringent from 1 April 2002 in order to reduce CO2 emissions from buildings. There is greater attention to air leakage and cold bridging with the introduction of Robust Details developed with industry.

Aircrete blocks continue to provide wall and floor constructions that meet and exceed the regulatory requirements.

Thermal conductivity is the inherent thermal insulation property of a material which is used to calculate the thermal transmittance of a construction element such as a wall.

4.3.1 Walls
4.3.1.1 Solid and cavity
Thermal transmittance (U-value) of any wall depends on the thickness and thermal properties of the various layers. The calculation takes account of: thermal conductivity of the materials, type of mortar joint, type of internal and external finish, type of additional insulation (if any) and the degree of exposure. The insulation provided by Aircrete leaves in external cavity walls can be augmented using non-structural insulation e.g. partial cavity fill.

As an alternative to cavity walling, the use of solid external walls of Aircrete can also be considered.

Thin-layer mortar joints reduce thermal transmittance compared with conventional mortar joints and larger face size Aircrete blocks reduce thermal transmittance and improve airtightness due to the reduced number of mortar joints.

Aircrete also has beneficial thermal inertia properties which, although not explicitly included in the calculations, aid the conservation of fuel and power performance of the building. Non-structural insulation materials do not provide these properties.

In order to develop methods to take account of thermal inertia within the assessment of thermal performance, the Aircrete producers have taken part in studies at the BRE. These are ongoing and are the subject of a Partners in Innovation application/project due for completion 2004.

4.3.1.1.1 Examples of U-values of walls

(1)	Cavity wall with facing brick outer leaf, 75mm blown insulation fully filled cavity, 100mm Aircrete (620kg/m density) inner leaf with any internal finish 0.35W/m K or better
(2)	Cavity wall with facing brick outer leaf, 29mm Celotex board (low E) partially filled cavity, 100mm Aircrete (620kg/m density) inner leaf with any internal finish 0.35W/m K or better
(3)	Cavity wall with facing brick outer leaf, clear cavity, 100mm Aircrete inner leaf with 50mm Thermaline Board Plus internally 0.35W/m K
(4)	External render, 300mm Solid 460kg/m density Aircrete block wall in thin-layer mortar with 9.5mm plasterboard internally 0.35W/m K.

4.3.1.2 Below dpc
As heat losses from the perimeter of ground floors are significant, solid Aircrete blockwork should be considered from top of strip foundation to underside of dpc when solid external walling is being used above dpc, to enhance the thermal insulation performance of the ground floor. When cavity walling is being used above dpc, the cavity should continue the equivalent of two brick courses below dpc. Aircrete blockwork below dpc can provide improved perimeter thermal insulation and the lightness of the blocks enables rapid construction in solid walling eliminating the need for concrete fill in the cavity below dpc.

This minimises the period before trenches can be backfilled. Where external rendering is applied directly to the block work above dpc, Aircrete used below dpc should not be rendered but should be left without an applied finish except just below finished ground and dpc level where a bituminous finish can be applied if required.

The use of blocks with tongue and grooved perpend joints speeds construction, saves mortar and enhances thermal insulation.

4.3.1.3 Basement walls
When refurbishing existing basements Aircrete walls should be considered for lining the external walls to reduce heat losses.

4.3.2 Floors
4.3.2.1 Beam and block
Beam and Aircrete block floors should be considered for providing thermal performance for both ground floors and first floors of dwellings. This will allow reduced added insulation compared with other types of ground floor construction.