How green should your renovation be?

As is our Christmas tradition, the central heating boiler broke down within spitting distance of the joyous day.

Usually such failures can be remedied swiftly with an engineer’s visit. But this time was different. A leak to the flue gases vent had resulted in moisture dripping onto the temperature gauge and other internal parts, rusting them in the process.

The result was seven unheated days in the middle of a cold snap, which concentrated my mind wonderfully on the values of a warm, well-insulated house. And I wasn’t alone.

The government is currently consulting on its next upgrade to the energy-efficiency standards for new homes. It has two suggestions.

The first is to cut carbon emissions by 20 per cent by demanding very high insulation standards for walls, roofs and floors. The second is to cut emissions by 31 per cent by having slightly less rigorous insulation standards, but combining them with carbon-saving technology, such as photovoltaic panels or heat pumps. This is the preferred option, not least because the government’s Futures Homes Standard, announced last spring, includes the aim to remove all fossil fuels as an energy source for new homes by 2025.

The government’s Futures Homes Standard includes the aim to remove all fossil fuels as an energy source for new homes by 2025.

Ambitious as these measures are for new builds, their relevance for existing homes will largely be restricted to extensions and loft conversions. But existing houses are where most of us live, and, it’s estimated, are where we’re still likely to be living in 2050. That’s partly because only around three per cent are replaced every year, but also because of Britain’s extraordinary richness of well-loved domestic architecture. In survey after survey a ‘character’ property scores over new build, however energy efficient or sustainable.

Bringing these properties up to existing, let alone proposed, energy-efficiency standards is a major undertaking, demanding knowledge and skills which are currently in short supply. It’s clearly important in combating climate change, but it’s just as valuable in minimising, or even eliminating, fuel bills and making homes more comfortable and healthier.

Bringing properties up to existing, let alone proposed, energy-efficiency standards is a major undertaking,

So how do you best upgrade the period property you’ve fallen in love with or the current home you want to renovate?

The process is known as ‘retrofit’ or ‘deep retrofit’ (for the most complete version). If you’ve lived in your home for a while, chances are you’ve already covered the easiest options: cavity wall and loft insulation, ground-floor insulation – if you have a suspended timber floor – central heating, double glazing, simple draughtproofing and switching to low-energy LED lighting.

All those will have had a significant effect on fuel bills. But they won’t have eliminated draughts or cold spots, ensured even temperatures throughout the house or made the best possible use of your energy supply. So what should the next step be in the retrofit process?

The easiest answer is to look at ultra-low-energy new build. The gold standard for this is the German Passivhaus system. It’s a method of design and build that guarantees the energy performance of a building, unlike the building regulations’ rather vague SAP or EPC standards. Passivhaus is based on three inter-related principles: high levels of insulation, high levels of airtightness and carefully controlled ventilation.

These are relatively easy to achieve as a house is being built. Much less so with an existing house.

For example, the easiest and most effective way to add insulation is to apply it externally, enclosing the building in a kind of ‘tea cosy’ where any gaps are immediately visible. This includes highly insulated doors and triple-glazed windows. But if you have a period house, local planners are unlikely to approve such a dramatic change to a front elevation, limiting external insulation to the rear and less visible sides.

Insulation, however, can also be applied internally. This is usually cheaper, but it’s also more disruptive and reduces internal space.

Insulation goes hand in hand with airtightness to banish any hint of draught. It’s typically achieved with a membrane that needs to be as all-encompassing as the insulation. A sealed interior, however, requires a mechanical ventilation system to keep the internal atmosphere fresh.

A sealed interior requires a mechanical ventilation system to keep the internal atmosphere fresh.

Typically this consists of a central fan unit which draws warm, moist air from kitchens and bathrooms through a series of ducts. An integral heat exchanger can extract over 70 per cent of the exiting warmth and use it to pre-heat fresh air from outside, which is then distributed throughout the house via another series of ducts. Since the ducts are around 100mm in diameter, finding room for them can be a challenge. But the end result is a quiet, draught-free house where the air is always fresh, the temperature constant throughout and the annual fuel bill, whatever form of energy is used, is only a fraction of what it was previously.

If all this sounds excessively arduous, there is a retrofit version of Passivhaus known as EnerPHit which guarantees a similar, though less stringent, level of performance. As to cost, the most succinct indication I have heard comes from architect Robert Prewett of retrofit pioneers Prewett Bizley: “If you can afford a BMW on your front drive, you can afford to retrofit your house.”

Retrofit can be done in stages and much can be done yourself

And if that sounds beyond your current finances, retrofit can be done in stages and much can be done yourself – labour is usually the highest cost. What is essential, however, is the advice and skills of experienced retrofit professionals, to design, build or oversee your project.

And hopefully, given the emphasis on green issues in the recent election, it’s going to dawn on the building industry that this is where the great bulk of their work is going to come from for the next generation.

Janaury 2020