Thermal Bridging

What is a thermal bridge?

Why should my detailing improve with low energy design?

A major factor in the performance of the building fabric is not simply the amount of insulation you install, but how it interconnects with other components and the other insulated elements within the design. Building junctions, where building elements meet such as at corners or reveals are less well insulated than the main element.

It has been estimated that up to 30% of the heat loss in a well-insulated building is through these ‘Non Repeating Thermal Bridges’ at wall/floor junctions, corners, reveals, ceiling heads and sills etc. It is critical that when calculating the performance of low energy buildings that this heat loss is measured and minimized.

With Unilin Insulation’s Advanced Detailing and engineered jointing of boards the insulation continuity of your project is assured.

What is a Y-value?

How big a factor is it in my BER/SAP rating?

As with every element/component within the energy strategy of a building design, U-values, air tightness, boiler efficiency etc, the ‘Continuity’ of the insulation layer has a numerical value within DEAP/SAP and is called the ‘Y’ value.

From the changes to Part L regulations, the Y value must be actually calculated by the building designer rather than being assumed. The details used to calculate the Y value should be witnessed on site and included within the ‘As built’ BER/SAP.

If my junction detail is better than average where do I get the resultant PSI value ‘score’ from?

Guidance for the list of junctions to be accounted for in any design can be taken from appendix D in Part L.

For each of these junctions, appropriate detailing to ensure the ‘Continuity of insulation’ can be satisfied by following details printed in the publication of the Acceptable Construction Details published by the Department of Housing. The ACDs show each of the details with its components and gives a sequence on how the detail should be constructed, they also allow that detail to be signed off on site to the satisfaction of the energy assessor/designer.

Using these ACD details will allow the assessor to ascribe a PSI (?) value to each junction within the design; for example the wall/floor junction has a default value of 0.196 taken from Table D1 Column 3 in Part L . If no detailing is specified and witnessed on site the default values apply.

The designer/assessor should measure the length of each of the junctions in the designed building and multiply the length by the respective PSI value. The total of all heat losses through the junctions are then totaled and divided by the total heat loss area of the building to determine the Y-value for input into DEAP/SAP. It has been estimated that every 0.01 reduction in the Y-value will result in a 1% energy reduction in the design.

How can simply using Unilin Insulation’s products give me a thermal bridging advantage?

Unilin Insulation has taken the ACDs and replaced generic square edged insulation materials with high performance Unilin Insulation PIR that have engineered jointing and ancillary products such as pre-formed corner boards to improve the continuity of the insulation layer. We then recalculated the PSI values (Our team where the first to be qualified by the BRE on their Thermal Modelling Course.) to achieve performances that give significant improvements over the Acceptable scores from Appendix D.

Good U-values in these elements coupled with good detailing and decent air tightness ensures that, no matter where that energy comes from, conventional or low or zero carbon technologies, the heat loss from the building is minimized.

By improving Unilin Insulation’s jointing detailing and providing higher performance materials we simply improve the performance of the tried and tested ACDs.The industry has been using the Acceptable Construction Details for many years now. They have been developed by the Government to deliver the continuity asked for and clearly identify the build sequences that are required retaining traditional methods and skills – but with the inclusion of Xtratherm Partial or Full Fill the thermal performance of those details are greatly improved.

There are a lot of different specifications – can I get a score for each?

On this site, we have provided a range of PSI values depending on whether you are partially filling your cavity wall with Unilin Insulation Products or using our built in full fill solution CavityTherm. They have been used in wall specifications using dense, medium, or aerated block work. Some specific details for Northern Ireland building techniques are also given.

You can see the advantages that Unilin insulation’s products with engineered detailing actually provides.

Thermal bridging actually gets worse the better the U-values in the walls and floors – what do Unilin Insulation’s performances fall within?

The list of junctions that need to be accounted for in any design is given within Appendix D, Part L.

We have also given resultant PSI values for each junction that has been accredited within a spread of target wall U-values, from a current regulation target of 0.21W/m2K to a Passive level of 0.12W/m2K. Individual certificates illustrating the resultant PSI values can be downloaded on ACDs.