In April 2014 new changes in Part L came into effect. This means that instead of using Accredited Construction Detail letters of confirmation for new build SAP compliance, a set of Y-value calculations or Thermal Bridging calculations will be required for Part L Building Regulations.
What is a thermal bridge?
Thermal bridging can occur in three different ways:
An example would be the effect of rafters penetrating the insulation layer on a sloped roof every 600mm. Another example would be the break made by timber framing when the insulation is placed between the studs. This type of bridge is predictable and is accounted for when U-values are calculated to EN6946. Your insulation supply will calculate the build-up whilst proportioning those particular thermal bridges and take them into account. A set of conventions are available within the BRE publication BR443 ‘Conventions for U-value calculations’. It sets the rules for completing U-values competently and provides guidance for real scenarios, for example on how wall ties should be accounted for and the default add-on penalty if square edged insulation boards are used instead of engineered jointed or overlapped.
Non-repeating thermal bridging typically occurs at the junctions between plain building elements, e.g. at wall/roof and wall/ﬂoor junctions, and around openings, e.g. at window jambs, where the continuity of the insulation is interrupted or compromised because of the junctions detail such as at corners of the building. This thermal bridging increases the heat loss and also the risk of condensation due to the lower localised internal surface temperatures. It has been estimated that in a well insulated house around 30% of the heat loss can occur due to non-repeating thermal bridges.
A more difficult thermal bridge to plan for and detail, but when they occur they can lead to not only an increase in heat loss at that particular area, but the increased possibility of condensation forming on the cold surface and resultant mould growth. These random thermal bridges, like for example meter boxes, should be insulated effectively and accounted for in the overall heat loss calculation.
PSI Values explained…
Accredited Details (ACDs) England, Scotland & N. Ireland
Like all other inputs into a building energy calculation, the way that insulation is installed to avoid thermal bridging has a numerical input into the software – which is called a Y-value. A set of ‘good practice’ details have been available in the form of ‘Accredited Details’ ACDs published by the DCLG in England. These ACDs are a set of design drawings for the junctions listed in Table K1 of SAP which are most prone to heat loss. They detail how the insulation should be installed at these junctions in order to improve not only the heat loss but also airtightness results.
The Scottish Government has produced its own set of Accredited Construction Details (Scotland) 20103. Differences in traditional build methods have led to slightly different details being used, not only in Scotland, but also in Northern Ireland. Typically the traditional window detail placed the framing further into the construction, whereby a different PSI value will be achieved. For this reason the Y-Calculator program shows
a speciﬁc Northern Ireland choice when choosing the wall block type.
The Y-value is the term used to describe the sum of all (or to be more correct those identiﬁed in Table K1 of SAP) the non-repeating thermal bridges divided by the Total Heat Loss Area of the building, and is expressed as W/m2K. Its relevance or impact can be described most clearly if you view the Y-value as a U-value ‘penalty’ that is added onto the average U-value of your design to account for the thermal bridges. So let’s take an example of a well insulated property where the designed U-values for wall, ﬂoor, roof and openings average 0.20W/m2K. The Y-value is added to this average, so if your detailing has not been particularly good, a Y-value of 0.15 takes your average back to 0.35W/m2K which is the equivalent of taking away approximately half of the insulation that was put into the building! So reducing the Y-value down to around 0.05 or better (the target set to achieve Full FEES standards for 2016) has real advantages.
The Y-value is determined by quantifying this extra heat loss at junctions through thermal bridging by way of its linear thermal transmittance or Psi (Ψ) value in units of (W/mK). The PSI value target is set within Table K1 of SAP, for instance the target PSI value for a window reveal is set at 0.05, but this can be improved to around 0.02 using proprietary closers or wider traditional reveal insulation. When all the junctions are quantiﬁed (in lineal metres) they are multiplied by their individual PSI Values. The sum of all the L x PSI are then divided by the Total Heatloss Area for the building (i.e. the area of walls, ﬂoor roofs and openings) and this results in your Y-value.