Thermal Values and U-Values for
our Concrete Blocks

Our wide range of block types and thermal conductivities give the builder multiple options for finding the most practical and economical solution for meeting target u-values. Furthermore, exceptionally low u-values can be achieved resulting in improved SAP ratings with the possibility of reducing the amount of additional and expensive energy saving measures.

The table below summarises the key technical properties of our block range including their thermal conductivity values.





3.6 & 7.3

3.6, 4.2 & 7.3

7.3 & 10.4

7.3 & 10.4

460 (±50)

530 (±50)

600 (±50)

730 (±50)

950 - 1050

1350 - 1550

1850 - 2100

1850 - 2100


















Download our U-Values brochure which contains 100's of examples of wall and floor constructions to meet specific u-value targets using our range of blocks



For Ultralite, Insulite and Dense Concrete blocks, use the Concrete Block Association (CBA) thermal bridging details, click here.


For Airtec blocks use the Aerated Products Association (APA) thermal bridging details. These modelled junctions are available by clicking here.


All aggregate and aerated block thermal bridging details are available as Construction Details by the NHBC, click here.

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  • Performance Requirements: Part L of the Building Regulations

    Approved Document L1A of the Building Regulations was updated in April 2014. There were several changes to the previous version including:

    • No change to limiting u-values for building elements
    • Introduction of the Fabric Energy Efficiency (FEE) ratings
    • New CO₂ target for the dwelling


    These measures are part of the ‘fabric first’ approach which basically attempts to get the building as thermally efficient as possible before adding on the renewables and other energy saving techniques. This is the most sensible approach and avoids poorly designed buildings passing SAP 2012 requirements through simply adding on solar PV etc.


    The two criteria which must now be met in SAP 2012 are:

    • DER < TER CO₂ emitted (kg/m² of floor area)
    • DFEE < TFEE Energy consumption (kWh/m² floor area)


    Our range of concrete blocks offer simple solutions to achieving target SAP ratings, minimising thermal bridging, maximising thermal mass, improving air tightness and significantly improving thermal efficiency of the construction throughout its lifetime.

  • Limiting U-Values


    Part L1A New Build

    Part L1B Existing Dwellings

    External Walls


    0.30 W/m²K

    0.25 W/m²K

    0.28 W/m²K

    0.22 W/m²K

    The table above shows the maximum permitted u-values for walls and floors in Part L1A (New build dwellings) and Part L1B (Existing dwellings) - 2013 Editions.


    In practice, lower u-values are aimed for to help the required % reduction in CO₂ from the SAP calculation be achieved easily. Currently for new build dwellings, walls tend to be 0.28 and below, floors 0.22 and below.


    View Our U Value Tables brochure to meet these targets using our range of concrete blocks.

  • Party Wall Bypass

    Party Wall Type

    U Value


    Cavity - Fully Filled with Effect Edge Sealing

    Cavity - Unfulled with Effective Edge Sealing

    Cavity - Unfilled, No Effective Edge Sealing

    0.00 W/m²K

    0.00 W/m²K

    0.20 W/m²K

    0.50 W/m²K

    A new concept introduced into Part L 2010 is the Party Wall bypass. The theory is that heat can be lost from the dwelling up through the cavity party wall and that therefore this heat loss must be accounted for in SAP.


    Robust Detail Party Walls as approved for use with Thomas Armstrong’s range of blocks have been updated to include fully-filled options and therefore allow a party wall u-value of 0.00 W/m2K to be applied.


    Furthermore, solid wall options (traditional blocks laid flat or solid Airtec blocks) allow a u-value of 0.00 W/m2K to be applied.

  • Air Permeability

    The limiting value for air permeability has been set at 10 m³/hr/m² tested at 50 pascals, and the requirements for on-site air permeability testing has been increased over previous regulations. For non-tested properties, the regulations say that an additional 2 m³/hr/m² must be added to the figure achieved for tested figure for a same house type. Therefore, in practice designers are aiming for a maximum of 8 m³/hr/m² to ensure compliance with Part L.


    Masonry block walls using any type of Thomas Armstrong Concrete Block have been shown to easily achieve these figures provided that good workmanship is employed. Figures of around 5 m³/hr/m² are commonplace and are desirable.


    A parge coat on the internal wall faces can contribute to exceptionally low air permeability figures of 2 or less which help to achieve Passivhaus and Code Level 4 requirements.


    Table: Air Permeability of Thomas Armstrong Concrete Blocks


    However, the designer has to be aware that air quality might suffer, and mechanical ventilation systems may have to be installed to avoid condensation and related health issues for the residents.

  • Linear Thermal Bridging

    Higher thermal performance and airtightness of the building fabric has increased the significance of heat loss at junctions and must be accounted for in SAP calculations.


    Simply put, the detail of each junction between each building element throughout the property (e.g. wall/window, floor/wall junctions) must be assessed individually to give a ψ “psi” value for that junction. The psi values are then added together to give an overall Linear Thermal Bridging figure for the property; its “y-value”. There are currently 3 options for determining the overall y-value of a building:


    Current Status:
    For aggregate and Airtec blocks, option 3 can be used. As part of the CBA and the APA we have a comprehensive range of modelled thermal bridging details resulting in significant improvements over the options 1 & 2 described below.


    All models are available as Construction Details from the NHBC here.

    OPTION 1

    Do nothing use a y-value of 0.15

    OPTION 2

    Use figures from table K1 in the guidance to SAP 2009

    OPTION 3

    Use specific modelled junctions

    Do nothing use a y-value of 0.15

    If the designer does not account for each thermal bridge, then a default y-value of 0.15 must be applied in SAP 2012 which will be significantly disadvantageous in achieving desired DER and % reduction in CO2 requirements.


    It is unlikely that SAP 2012 can be passed using the default y-value of 0.15 using anything other than expensive and unrealistic additional energy reduction technologies.


    In the absence of any approved government scheme, it is permitted to use the accredited values in table K1 found in the guidance for SAP 2012 for the purposes of SAP calculations.


    An overall y-value of around 0.08 would be expected.


    Once an Accredited Construction Detail (ACD) scheme is up and running, on- site inspections of the as-built vs design will need to be carried out by inspectors.

    ψ values for specific junctions using specific building materials can be calculated by trained, approved persons. These can lead to y-values of 0.04 or lower which can greatly reduce the need for Photovoltaic etc in order to achieve SAP target DER figures.


    No site inspection is required but a 25%, or 0.02 confidence factor (whichever is the greater) must be added to each ψ value.


    Nevertheless, using modelled junctions result in better values than any ACD scheme and can result in significant cost savings for the builder and is by far the best and most cost-effective option.