U-value Calculations

Cross-Sectional Areas of Wall Ties for U-value Calculations

For the accurate calculation of U-values for walls it is important to use the correct information for stainless steel wall ties.

Using the actual cross-sectional area and the thermal conductivity value of a tie, rather than allowing a program to apply a default value, can make a considerable difference to the calculated U-value. It is widely accepted that default values for wall ties over-estimate the thermal bridging effect.

The required information for the main Ancon and Ancon Staifix wall ties are shown in the table below.
The Ancon Teplo range is manufactured from pultruded basalt fibres set in a resin matrix. These ties have a thermal conductivity of just 0.7W/mK.

Fixings with a lambda value of less than 1.0W/mK are excluded from u-value calculations to BS EN ISO 6946, so Teplo wall ties are always excluded irrespective of their cross-sectional area.

Chi values are available for the Ancon Teplo-L-Tie which has a basalt fibre body and a stainless steel upstand for surface-fixing.

Our stainless steel wall ties have a thermal conductivity of 17W/mK. They are value-engineered; all unnecessary material is removed to minimise the thermal bridging effect.

**Type 1 performance declared in M2 mortar only

Masonry-to-masonry wall ties are usually installed at 900mm horizontal centres and 450mm vertical centres; this is equivalent to 2.5 ties/m². Timber frame ties are installed at closer centres which is equivalent to a density of 4.4 ties/m².

Ancon 25/14 Brick Restraint System for Light Steel Framing Systems (SFS)

With the Ancon 25/14 system, only the channel fixing screw passes through the insulation to create a thermal bridge; the channel and wall tie are located outside the insulation. Ancon stainless steel fixings screws, referenced HTSS, have a cross sectional area of 15.2mm² and a thermal conductivity of 17W/mK. Ancon 25/14 channels are typically fixed at 600mm horizontal centres to suit steel layout and 337.5mm or 450mm vertical centres depending on insulation depth.

Separating (Party) Walls

Ties in separating walls should comply with Approved Document E.

U-value Calculations to BS EN ISO 6946

BS EN ISO 6946 permits the corrections due to wall ties, air gaps etc to be omitted if the corrections amount to less than 3% of the uncorrected U-value of the element.

U-value calculation programs either include a function to allow this 3% rule to be applied (e.g. BRE U-value calculator) or automatically apply it where necessary (e.g. BuildDesk U 3.4).

BuildDesk U 3.4 is available for download as a 30 day free trial by registering here. This program includes Ancon wall ties which can be selected from a drop down menu in preference to the default ties to reduce the U-value calculated. For more information on BuildDesk U visit www.builddesk.co.uk/software/builddesk-u/.

Video File

This BuildDesk U demonstration video entitled 'Changing Wall Ties' highlights how the selection of Ancon wall ties can improve the calculated U-value of a masonry wall.

Building Information Modelling

For BIM Objects of these products, please click here.