How Important is Thermal Bridging?
In our latest video, Jason Martin, our Specification and Product Development Manager, looks at the importance of thermal bridging in the Irish construction industry, and how effectively dealing with thermal bridges can help easily achieve nZEB and passive house standards.
With greater emphasis on lower energy building design, we are hearing more and more about thermal bridging. Unfortunately, in many sections of our industry it is not fully understood. Firstly, we will look at what is thermal bridging? To explain this, we need to cover U-values.
A U-value measures heat lost per square metre out through all of our building elements. That is, out through our walls, down through our floors, up through our roof and out through our windows and doors. But unfortunately, what U-values don’t cover is the heat loss at the junction between these elements.
For example, at floor-to-wall junctions, wall-to-roof junctions, corner junctions, and around windows and doors. The heat loss through these junctions is what is known as thermal bridging.
Figure 1 shows an example of one such junction, the wall to floor junction. Here we have floor insulation, and wall insulation, and whilst our U-values measure the heat loss per square metre down through the floor and out through the wall, it doesn’t measure the heat loss at the junction between those two elements. So, because the insulation in the two elements don’t meet, we have significant heat loss out through that junction. Again, that is what is known as thermal bridging.
Thermal bridging in BER calculations
Thermal bridging is measured in terms of Psi values (Ψ), which measure heat loss per linear metre of each junction, and are calculated using thermal modelling software. So how do we deal with this in BER calculations? In BER calculations, we have to apply a thermal bridging factor, which is known as the Y-factor. To calculate the Y-factor, we take the Psi value of each junction and multiply by the length of each junction. The combination of these for every junction in your building, divided by the envelope area (which is the area of your floors, walls and roof), gives the Y-factor.
Thermal bridging and Irish Building Regulations: Using default Y-values
So how do we deal with thermal bridging within Part L? Basically, we have three options. The first two options involve using default values:
- The first option is to use a default Y-factor of 0.15. We would use this Y factor if we didn’t follow any particular set of construction details. But in almost all cases 0.15 is a gross over estimation of the heat loss through thermal bridging and it should be avoided.
- The second option is to use a Y-factor of 0.08. We can use this Y-factor if we use the DOE acceptable construction details and whilst 0.08 is a lot better than 0.15 it can still be significantly improved upon.
Looking at an example of how these Y-factors affect the heat loss within the DEAP software, we have taken a house with an average U-value of 0.16. Essentially, what the DEAP software is doing is adding our default Y-factor to our average U-value:
|Default Y-value||Average U-value||Heat loss||% Heat loss|
When we apply a default Y-factor of 0.15, we get an overall heat loss of 0.31W/m2K. That’s a massive 48% heat loss due to thermal bridging, and that is in no way accurate, therefore this default value should be avoided.
Looking at the example of the default Y-factor of 0.08, we get an overall heat loss of 0.24W/m2K. That equates to 33% of heat loss through thermal bridging. Whilst this is a lot better than 48%, it is still not accurate.
The third option for dealing with thermal bridging within Part L is to calculate the Y-factor based on the method we have shown, and this is the option which we encourage BER Assessors to use.
To calculate the Y-factor using this method we need the Psi value for each junction in the building. Thermally modelling each junction can be costly and time consuming, so in Quinn Lite we have done that for you. We have taken the DOE acceptable construction details and, where possible, introduced Quinn Lite blocks into those details to improve the performance of the junction, as shown in the example in Figure 2.
So, if we look again at the wall to floor junction we looked at earlier, we have introduced two Quinn Lite blocks at the key location to provide continuity of insulation across the junction. By introducing those two Quinn Lite blocks, we have improved the thermal performance at the junction by x10.
All of our junction details have been developed and approved by an NSAI Accredited Thermal Modeller.Quinn Lite Accredited Construction Details
So, looking again at the example of a house with an average U-value of 0.16, using our Quinn Lite construction details, we can calculate Y-factors as low as 0.015 for a typical semi-detached house. So, when we add this to the average U-value of 0.16, we get a heat loss of 0.175W/m2K.
|Default Y-value||Average U-value||Heat loss||% Heat loss|
Comparing this to our default values, the percentage heat loss using our Quinn Lite construction details is 8.5%, which is a significant improvement on the two default values. This is the method which we should be using.
As well as reducing heat loss, which results in long term savings in energy bills, improving your thermal bridges also reduces your construction costs, and because of higher surface temperatures at junctions, we also eliminate the risk of mould growth and improve the comfort factor of dwellings.
Compliance considerations for lightweight blocks in Ireland
So how does Quinn Lite thermal block compare to the other alternatives on the market? The main alternatives being Foamglas and lightweight aggregate blocks (or medium density blocks)
If we look firstly in terms of Technical Guidance Document Part A structure: Part A states that blocks used in Ireland must have a minimum compressive strength of 7.5N/mm2. If we look at Appendix D of Part L where the enhanced acceptable construction detail has been used, the thermal conductivity of that lightweight block must be 0.20W/mK or better.
The maximum compressive strength of a Foamglas block is 3N/mm2 so it doesn’t meet the strength requirements of Part A. The thermal conductivity of a Foamglas block is better than 0.20W/m2K, so it meets the requirements of Part L Appendix D.
Moving on to lightweight aggregate blocks: 7.5N/mm2 is achievable, so it meets the strength requirements of Part A. However, the thermal conductivity of a lightweight aggregate block is around 0.33W/m2K, so it doesn’t meet the requirements of Part L Appendix D for the enhanced detail.
Looking at Quinn Lite, 7.5N/mm2 is achievable with the B7 block, and the thermal conductivity of our B7 block is 0.19W/m2K. So, based on the strength requirements of Part A, and the thermal conductivity requirements of Part L Appendix D, Quinn Lite blocks is the only product of the three that ticks both boxes.