ABOVE: This formerly tired 1960s Swansea self-build received a dramatic overhaul by Hyde + Hyde Architects, who added on a steel ‘sky frame’ which cantilevers out toward sea views. Huge sliding doors were added in the main living space, although with their slim profile, the doors created the challenging issue of dealing with the cold bridge between the glazing and the steel frame. The solution? A warm roof with insulation wrapped around the edge.
What is a Cold Bridge?
At its simplest, a cold bridge is a weak spot in the insulation surrounding a house. Cold bridges (also known as thermal bridges) occur whenever there is a break in the continuity, or a penetration of, the insulation. Examples of cold bridges include:
- Junctions between walls and floors, and walls and roofs
- Reveals around windows and doors
- Holes made by pipes and cables
- Studwork in timber frame walls (interrupting the insulation)
- Steel wall ties in masonry construction
Why Does it Matter?
It depends how seriously you take your energy conservation measures. Some people think that worrying about cold bridging is altogether too nerdy and that you are bound to have some cold bridging when you build and you should simply suffer the consequences. But others see cold bridging (along with airtightness) as a key feature in the battle to save energy and that without paying attention to cold bridges, much of the extra money spent on thick insulation is wasted.
Why is That?
It’s because insulation itself is subject to the law of diminishing returns. The first 50mm of insulation does most of the heat saving in a wall, roof or floor. The next 50mm saves much less, and by the time you get to 150mm or 200mm of insulation, adding another 50mm makes little difference. At this point, other factors like airtightness and cold bridging become far more significant and you have to address them if you want to make the house more energy efficient.
So, What Can Be Done About Cold Bridging?
The most effective way of countering cold bridges is to design them out. You can’t eliminate junctions and openings, but you can design them in such a way that the heat leakage at these points is minimised. Many of our standard building techniques have been trending towards this end. For instance, where we once screwed windows into solid returns, we now tend to slot them into insulated cavity closers.
But There Must Be More to it Than That?
Once you start looking in detail at cold bridges, and how you might eliminate or reduce them, you’ll find there are dozens of products and techniques that can be used. Take the humble cavity wall tie, used to bind the inner and outer leaves of a cavity wall together, essential for the stability of the structure. Typically, a wall tie is a stainless steel wire configured in an 8-shape. Metal like this conducts heat (think of pokers) and it will continue to conduct heat along its length whether your insulation is 50mm or 300mm thick. So you can see how a detail like a stainless steel wall tie becomes relatively more important as the insulation standards improve. Consequently, we are now seeing a range of non-steel wall ties designed to reduce the cold bridging effect.
What Do the Building Regulations Say About it?
Quite a lot, although it’s not always easy to understand just what they are on about. As a self-builder, you could (if you wished) remain blissfully unaware of any of this as it’s something that would normally be sorted out between your designer, your energy assessor and your building inspector.
But it’s worth understanding a little of the background here. The most recent version of the energy efficiency regulations (Part L 2010 in England & Wales) draws our attention to a series of supplementary documents, ‘Part L Accredited Details’, which suggest ways of designing junctions so as to minimise both air leakage and cold bridging. If you use these Accredited Details (ADs), your energy assessor works out their thermal bridge values (known as psi values) and enters them into the overall heat loss calculations, along with many other details like the insulation used and the heating system. However, if you don’t use the ADs, then you are penalised with a bad score, equivalent to adding 15% to your overall heat loss totals.
Energy assessor Adrian Fell, of Compass Energy Solutions, commented to me that it was now very difficult to meet the new energy standards if you didn’t use the Accredited Details. “I think you would probably have to fit renewable energy systems or build with much greater insulation widths if you ignore the ADs,” he said. “The whole gist of the new regulations is that they really want you to take cold bridging seriously.”
Three great ways to eliminate cold bridging in your self-build:
|Cavalock insulated cavity closers (0870 120 3003) improve thermal performance and eliminate draughts. Made from post-consumer recycled PVCu, they are compatible with all types of building methods.|
|To avoid cold bridges where the floor meets the base of the wall, Marmox Thermoblock (01634 862277) is designed to replace the bottom layer of blockwork. The lightweight blocks can support buildings of up to two storeys.|
|TeploTie is a range of wall ties made of a composite material based on basalt fibre. It’s 20 times less thermally conductive than stainless steel (magmatech.co.uk)|