U-value Calculations

For the accurate calculation of U-values for walls it is important to use the correct information for the 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 actual areas for the main Ancon and Staifix wall ties are shown in the tables below.

The Ancon TeploTie range is manufactured from pultruded basalt fibres set into an epoxy resin. These ties have a thermal conductivity of 0.7W/mk and are the most thermally-efficient wall ties on the market.

Ancon and Staifix 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.

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 are installed at closer centres which is equivalent to a density of 4.4 ties/m².

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 http://www.builddesk.co.uk/our+software/builddesk+u.

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