Three experts on the best ways to retrofit housing

But, with potential fuel shortages in much of Europe this winter and soaring energy prices, are insulation and heat pumps the answers to our problems? Three experts share their perspectives

In the UK and Europe, we have a system that uses energy performance certificates (EPC). Where dwellings are concerned, it’s a very simplified, standardised approach that estimates performance in all dwellings, including older buildings. But in the UK it often makes recommendations that over-estimate the savings that will be made. When the EPC rating recommends certain measures, there could be unforeseen consequences because those measures are not suitable for the building itself or in combination with each other. The best way is to take a “whole-building approach”.

One online tool that is useful is the retrofit guidance wheel, which tells you what other measures you need to consider when planning a single improvement. Buildings can only take so much before there are unintended consequences – with insulation you need to think about whether the material is compatible with the building. In traditional buildings these materials need to be vapour permeable and with government-funded retrofit schemes, such as the recent Eco3, materials must have guarantees.

Traditionally, one of the biggest problems we have with buildings in the UK is moisture, so maintenance is key. As a rule of thumb, moisture is the cause of about 80% of problems in our buildings. But now we also have to think about solar shading as they do in southern Europe.

Retrofits should also be dealing with the consequences of climate change, such as overheating. This could include shutters on windows, which can help retain heat during winter but can deflect solar gain during summer. We need people who are better informed to advise the general public on this.

Then there’s the building use – is it the right building for that function? I’m thinking of barn conversions that were never designed to be weathertight. Consider the ways rooms are used – ones that receive lots of direct solar radiation stay warm more easily so are good living spaces.  

Only once you’ve done all those things should you start thinking about wall insulation. For my PhD I looked at retrofitting historic timber frame buildings, which are notoriously leaky, so addressing the airtightness is a major concern. You don’t want to make them completely airtight so that moisture can’t be removed but you need to improve it. Doing so makes a major difference to the amount of heat needed, whereas the amount of improvement you can make by insulating the walls is quite small.  

In solid masonry you can make quite large improvements, but it has to be done well using vapour permeable materials. There are also issues about window details, eaves details and even the space junction between boards. With vapour impermeable materials, if moisture gets in, it’s trapped in the existing building fabric. There are recent cases where mould growth has been a real problem, such as in Preston.  

That was primarily put down to poor workmanship and detailing but perhaps if the materials hadn’t been vapour impermeable, there would not have been as many problems. One reason for poor workmanship is that the whole industry has been driven by stop-start incentives from the government. It is very difficult to build up a well-trained workforce under these conditions.

When you’re looking at the thickness of insulation, you want to create a minimum internal surface temperature of 13^oC, ideally 17^oC, to prevent condensation. When dealing with solid walls, external wall insulation (EWI) makes more sense but doing one home in isolation can look odd. The government should be looking at encouraging insulating them in blocks or groups, so it would be more cost effective and there would be a consistency of appearance, especially in conservation areas.

Houses in different regions, of different ages and in different conditions need a variety of interventions. Is the house terraced, detached, or semi-detached? Does it have double-glazing? Does it have a cold or warm roof? What is the ventilation strategy? Then you can look at the energy it uses: an average house uses 150kwh/m² a year and a good retrofit might be able to get it down to 50kwh/m², but that takes money and effort.

Would it be realistic to get in a heat pump? If you were just to replace a boiler with an air-source heat pump without any other measures, your fuel bills would probably go up because the energy cost of running it is high if you’re not generating any power. Our clients want to embrace energy efficiency, but retrofitting is not straightforward.

Don’t miss a single Modus article – sign up for the newsletter.