Within buildings the heat loss through junctions is known as ‘thermal bridging’. In a poorly insulated building, heat loss through thermal bridging is not significant. In highly insulated walls – with low ‘U-values’ – the heat loss through junctions, where several components are assembled together, can be very significant if they are not well designed.
For the first time in 2010, Building Regulations also began to measure heat loss through junctions, and measured this through Y-values – also known as ‘psi’ values. The 2010 Building Regulations ensure compliance with SAP 2009, meaning that building design must not only consider these but also meet TER (Target Energy Ratings) and TFEEs (Target Fabric Energy Efficiency).
To do this, a SAP assessment will assign credits through a calculation relating to well-designed and insulated junctions. This approach works on existing properties and also helps to tackle common issues like surface condensation.
Importance of planning
A ski jacket provides the ideal analogy for the contribution of Y-values to heat savings. The wearer may be as warm in a thick jacket with the zip open, as they would in a thin jacket with the zip closed. When Y-values aren’t maximised, thermal bridging allows heat to escape through the path of least resistance. Not planning for Y-values can be costly. The challenge now being faced is that many builders, architects, and SAP assessors aren’t familiar with Y-values, let alone being able to apply a solution to minimise them.
If this knowledge gap isn’t filled, 33% of heat loss through junctions will continue to occur on typical dwellings built to 2006 standards. Worryingly, around 60% to 70% of new properties are still built to this standard and so a large proportion of homes are still being built with this inherent level of inefficiency. Being able to improve Y-values is therefore vital to reduce reliance on other energy saving measures in the build whilst still being able to pass SAP and vastly reduce cost by using less insulation.
If the architect or builder doesn’t consider them, the SAP assessor will use ‘default’ Y-values which assume the junctions will lose as much heat as having a 3m2 hole in the wall. Having this hole, which is the size of a pair of French doors, will of course mean that much thicker insulation is required to meet required regulations.
A number of ‘approved’ details are on the market. Whilst these have slightly better Y-values they still create a relatively large hole which is around the size of a standard front door.
Independently modelled drawings, which I will detail further in this article, provide the most effective savings due to their unbiased recommendations across an extensive range of products that will formulate the best solution.
The SBS Standard Construction Drawings (SCDs) have been designed for new-build and retrofit with the objective of being an information resource and advice service to customers with the ultimate aim of maximising the fabric’s contribution to SAP.
Using these drawings not only reduces the need to use default SAP figures but will make some very simple, but significant, changes to standard build methods which mean the fabric can be used to its maximum capability. The drawings have also been independently tested and verified by BRE, LABC and the HBF. As SBS has access to the full range of products available from numerous brands across the Travis Perkins Group, the best nominated product is put forward with 3D thermal modelling. Not only do the drawings provide a lower Y-value and therefore cost savings, but remove the need for the builder or designer to model their own junctions, which can be incredibly expensive.
There are two notable examples of how the SBS team has worked with builders and architects to deliver cost savings through improved Y-values.
A leading UK national house builder was looking to comply with 2013 Building Regulations and SAP 2012 through an update to its standard housing design. In addition to minimising build costs, a building with improved performance would offer added value to the future purchaser.
SBS was tasked with reducing the cavity thickness from 125 to 100mm. By successfully implementing a design with Y-values that would decrease the thickness by 25mm, the build costs were reduced whilst the volume of the living space was improved. This simple change also negated the need for renewable technologies meaning that the originally planned solar thermal systems were no longer needed. In total, build costs were reduced by £3500 per property which is equivalent to £4.11 per square foot.
For an independent architect working on three terraced houses, ease and efficiency of the build was paramount. Each junction followed the SBS Standard Construction Drawings and allowed a gas boiler to be combined with solar PV. The lower Y-values allowed cavities to be reduced by 50mm with the number of PV panels halved in number to provide cost savings of around £1300 per plot.
Significantly for the architect, it also meant that time was saved on the design with SBS providing a full specification directly to the builder, in addition to specification support for the solar PVs and detailed estimates.
My view is that if a house is built well and with the right products it will last for hundreds of years. Bolting technology onto an unprepared shell won’t make much of a difference. It is a positive step that a ‘fabric first’ approach is now recognised in industry as the ideal.
The real challenge that I see going forwards is in bridging the knowledge gap around the importance of Y-values and the actual size of the hole that is created when the default values are replied upon.
It is the responsibility of the merchant to support this requirement for education and to provide a suitable solution whilst encouraging a closer working relationship with the architect or building designer.