25 Mar 2020
The good news - Building Regulations
First of all, the good news: compared to the home you currently live in (assuming it was built before about 1990), the new home you’ll end up building – practically however you do it – will be less draughty and cheaper to run. The regulations controlling energy efficiency have tightened significantly over the past 20 years and knowledge of energy efficiency best practice has become more commonplace. The Building Regulations concerning energy efficiency are supposed to change again in 2021/2022, have already been consulted on and we hope that the government implement a significant step change in the required level of performance.
How far do you go?
Most self builders, however, want to do more than the bare minimum. After all, unlike developers, you will be the ones who benefit from lower bills in the long term. Indeed, some self builders go on to build homes to such high levels of efficiency that they cost well under £100 a year to heat. However far you want to go, whenever you’re thinking about reducing your running costs, it’s useful to think of the approach in terms of two sides of the coin – minimising the amount of energy your home actually needs, and then deciding how you generate that energy. Doing either in isolation is only likely to solve half of the problem. It’s pointless investing tens of thousands in a renewable heating system if it’s having to work three times as hard as it needs to. So, you need a holistic approach.
How much energy will you need?
By far the most important – as well as the easiest and cheapest – element of ensuring low future running costs is to concentrate on the amount of energy that your home requires in order to run. That’s a function of several different factors: its size, the number of occupants and so on, but the element you have most control over is your home’s ability to retain the heat energy created within it.
U-values and PSI-values explained
There are many things to consider here – not least the way the house is designed and built, and of critical importance is the amount of insulation in individual construction elements, ensuring that heat isn't lost through the roof, walls and floors. It is also important to manage so-called ‘thermal bridging’, which occurs at external wall junctions, and ensure that the amount of heat lost through these is minimised. The main indicators here are the U-value and the PSI-value. As you compare different build systems and suppliers, all will list the U-value of the walls, roof, and floors they sell. It’s a measure of the heat loss through the system or window, and the lower the number, the better. It is designed to help you compare the different choices. Although there isn’t a universal Building Regulations requirement for U-values, the notional target is 0.18 for walls. To offer some context on this, the requirement in 2006 was 0.3; many poorly insulated old homes will have U-values in excess of 0.2. From a thermal bridging perspective, PSI-values measure the heat lost along a metre of junction between two thermal elements, for example the junction between the windowsill and the wall. Because it’s relatively cheap and easy to install extra insulation at the construction phase, most experts agree it’s the first thing you should do if you really want to minimise your long-term bills – certainly much easier and more effective than fitting insulation after you’ve finished. Different types of insulation have different levels of performance, of course, but in general terms, the more the better. Modern forms of insulation such as those with phenolic cores perform far better for each inch of thickness than traditional forms such as expanded polystyrene or mineral wool.
Window: wall ratio
As modern homes tend to have a bigger window to wall ratio (as opposed to traditional cottage-style homes, which have more walls and smaller windows), it’s important to consider the U-values of the windows, too. It doesn't really make sense to have highly insulated walls only for the windows to leak significant amounts of heat. We are able to advise on the different levels of performance available, with double-glazed windows typically achieving a U-value of between 1.6-1.2 and triple-glazed windows in the region of 1.2-0.8.
Concentrate on airtightness
The other element of minimising the amount of heat loss from the building is to concentrate on airtightness- now a requirement under building regulations. The more airtight a house, the fewer draughts and gaps for warm air to depart and the cold to get in. As the amount of insulation incorporated within new properties increases, and the amount of thermal bridging reduces, improving airtightness becomes a significant way in which you can reduce the energy consumption of your new home. Generally speaking, leaky homes usually occur because of poor building system choice (i.e. the system could never be airtight) or poor site practice, such as a lack of attention to detail on the part of the builders. Factory-built panellised systems such as Kingspan TEK® and Kingspan ULTIMA clearly reduce the scope for things to go wrong in this area. In airtight dwellings, it is important to ensure that sufficient ventilation is provided, and this may mean that a mechanical ventilation system with heat recovery (MVHR) is required. In an airtight, energy-efficient dwelling, the inclusion of an MVHR system will not only ensure a continuous supply of fresh air, but will also save on long-term running costs.
How to generate energy for your new home
Having minimised your heat requirement by concentrating on the four core areas of energy efficiency – thermal envelope design, insulation, window performance and airtightness – you have mastered the ‘fabric first’ approach, and can then decide on how you generate your energy. Simply put, the conventional choices have limited upfront costs but an element of uncertainty about future running costs. The main options for renewable systems include (but are not limited to) biomass (wood pellets or logs); heat pumps (air or ground source); and solar (creating hot water or electricity as PV panels). They tend to have lower long-term running costs but a higher upfront investment, ranging from £5,000 to £15,000 in most cases, depending on the particular system and its size. There are some government incentives designed to mitigate the burden of these high-capital costs. Building your own home is all about making decisions. For most people who want to do the sensible things to invest in low running costs, the best approach is to get the basics right – the things you can’t change later. And that is the compelling case for a fabric-first approach.
A measure of the heat loss through a square metre of thermal element such as a wall. The U-value (the lower the number, the better) calculation accounts for the thermal resistance of each of the materials concerned, along with an assessment of the repeating thermal bridges that exist.
A measure of heat loss along a metre of junction between two thermal elements. PSI-values are concerned with non-repeating thermal bridges and are based on standard construction details. The default PSI-values presented within the approved documents that support Building Regulations can be enhanced by thermally modelling the junctions. Our systems all benefit from thermally modelled junctions that provide an improvement over the default figures.