Whenever we are working on a project which is struggling to meet SAP targets, we rarely go to renewable technologies, or even look at insulation levels (that comes later).
The first thing we typically look at is Thermal Bridging and more specifically the use of a scheme like Accredited Construction Details (ACDs).
All too often when we mention thermal bridging to a client, the reply is “what’s that” or “never heard of them”. So a brief post on the subject may be in order.
Thermal Bridging and ACDs
Thermal bridging is a key contributor to heat loss within a building – for a good basic understanding head over to our page on Thermal Bridging.
Bridging occurs when two exposed thermal elements meet; this allows heat to transfer through two highly conductive materials to the outside. Junctions like those shown in the picture highlight the problem of ‘cold bridging’.
Junctions exist in a variety of places, including ‘linear’ junctions such as stud work or timber framing, but in this context we are talking about non repeating junctions affecting external walls. For example these may be where:
- A ground floor meets an exposed wall
- An external wall meets a balcony
- Lintels and sills
- Window or door jambs
There are many possible junctions – the number and type will depend on the design of the dwelling. Within the SAP calculation there are 23 junctions available to select, but rarely are more than 10 used in any one project at the same time.
ACDs – The Slightly Technical Bit
Each junction has a psi value associated with it (heat loss value expressed in W/mK) which in combination with it’s total length creates the total heat loss across that junction (heat loss co-fficient).
As a rule, the more complex a building’s geometry, the more thermal bridges will apply. If a property does not use a standardised set of construction details or the thermal bridges have not been independently modelled, then a standard, default heat loss (y-value) of 0.15 is applied to the entire dwelling. This in effect applies a ‘penalty’ to the SAP performance and will have a significant effect on the emissions target, and more recently, on the fabric efficiency (TFEE) target.
To minimise the heat loss within junctions, the thermal bridges must be broken up so that the two highly conductive materials no longer meet. To achieve this strips of insulation are used, creating a thermal break.
Improved junctions such as these have been designed and accredited under various schemes, the most established being the tried and tested Accredited Construction Details scheme (ACDs). These enable SAP assessors to use improved values for those junctions, and therefore gain a much improved SAP performance….ta da!
So what are ACDs and how do you use them?
Accredited Construction Details
Accredited Construction Details are a set of standardised junctions which architects and builders can follow to ‘design out’ thermal bridges. The full set is available to download online at the government’s Planning Portal.
If the specification checklist is followed, the site manager then signs the sheet, and on completion the SAP assessor may use a better performing psi value for that junction. This process can then apply to every other junction in the dwelling, replacing the default psi values with the ACD version.
The ACD sheets have a description of how to construct each detail, with differing construction methods depending on the build type, i.e masonry or timber.
Enhanced Construction Details
The Energy Saving Trust has also produced a separate, improved set of junctions, called Enhanced Construction Details (ECDs). Although there are far fewer details in this scheme, they are more specific to those particular junctions which do not achieve such large gains under ACD’s, i.e lintels and gable details.
You can use ECDs in conjunction with ACDs, and the process works in exactly the same way. The details can be found on the Energy Saving Trust website.
There are now several schemes offering a set of junctions to follow, for example from insulation manufacturers such as Celotex, Knauf and Xtratherm. These generally offer improved levels of performance over ACD’s but do rely on a specific set of proprietary products being used.
So why is thermal bridging so important? Well ACDs and ECDs can enable an otherwise failing building to pass. All this with no alteration to insulation levels or deployment of possibly expensive renewable technologies.
Due to the recent Part L changes, thermal bridging is now even more crucial – most standard builds will simply not pass current building regulations without some consideration of thermal bridging and this is partly due to the new TFEE requirements.
We have lots of other advice around the latest regs on our Blog so why not head over to:
- Part L 2013 – What Will It Take To Pass?
- How to pass the fabric target in SAP
- What Are SAP Calculations?
And of course, if you need further help with any of these topics feel free to give us a call, or drop us a line. See you next time!