Eco renovation & Passivhaus
Most homeowners are keen to achieve lower running costs and reduce carbon emissions – but how far should you go? Nigel Griffiths looks at the merits of this eco standard for renovators
Nigel Griffiths explains the routes to reducing carbon emissions and running costs when updating your home, and how to be realistic ahead of starting a retrofit scheme
Whether you’re building a new home from scratch or taking on a major refurb, one of your key motivations will probably be to minimise the amount of money you need to pay out on energy bills. One of the leading standards for sustainable homes is Passivhaus; a German model that’s designed to encourage better energy efficiency, specifically in new build houses. It works really well for this purpose, adopting a fabric first approach to design and construction that creates a highly insulated and airtight structural shell, reducing the need for bolt-on technology.
Trouble is, it’s pretty much impossible to get existing houses to perform to the same standards as the best new buildings. This is because so many aspects of the property are already fixed, such as the form, orientation, window openings and foundation design. This leads to issues such as thermal bridging (where heat can transfer out of the living space by passing across building components). So is Passivhaus a realistic target for homeowners looking to renovate an existing dwelling in a sustainable way?
Passivhaus vs Enerphit
It is possible to use Passivhaus to evaluate retrofits of existing buildings, but it’s not straightforward. So in 2010, the Passivhaus Institut introduced Enerphit to provide a slightly more realistic set of goals for ultra-efficient renovation projects. Nevertheless, the cost and difficulty of achieving this standard is not insignificant.
In the UK, the Passivhaus Trust (a subsidiary of AECB; the Association for Environment Conscious Building) provides guidance and lobbies for changes to the Passive House Planning Package (PHPP) to reflect our climate. Very few Enerphit schemes have been completed on these shores to date; currently, just 10 projects are listed on the Passivhaus Trust website.
There are three potential routes to attaining the Enerphit standard. The first is to take a performance criteria approach based on energy usage, which is fundamentally similar to Passivhaus certification with new builds – albeit with some slightly different criteria. You can also adopt the building component method, which is more akin to the Building Regulations in terms of how it sets out performance requirements. The third route, stepwise retrofit, is a staged approach that allows you to meet the standard over the course of several years. Here’s how they work in practice:
1 Performance criteria approach
This method focuses on the end result of your eco project, which must be designed to reduce the amount of energy (expressed in kilowatt-hours) needed for space heating, ventilation and power. It also sets a maximum threshold for air tightness (expressed as the number of air changes per hour at a pressure of 50 Pascals).
This approach represents the crux of the new build Passivhaus standard, but the rules are slightly relaxed under the Enerphit scheme, as this table shows:
It’s worth bearing in mind that new houses constructed to simply pass the current UK Building Regulations will have a space heating demand of around 60kwh/m2 per annum. At 25kwh/m2pa the Enerphit standard is highly ambitious, even if it doesn’t quite reach Passivhaus.
For comparison, an older dwelling that hasn’t been refurbished in recent memory could easily have a space heating demand of more like 200kwh/m2 per annum. So fundamentally, the Enerphit standard is seeking to reduce this key metric by around 85%.
Note that the values given in this table apply to the UK only, which is classed as having a cool temperature climate. Different limiting factors apply to different climate zones across the world. An interesting point to consider is that there’s a wide climate variation across Britain, for instance between Scotland and the south coast of England. In theory, therefore, Enerphit should be easier to achieve in warmer parts of the country.
2 Building component method
Unlike Passivhaus, it is possible to meet the Enerphit eco renovation standard based on the technical performance of the components. This involves evaluating all the building fabric elements and the ventilation system against a set of individual criteria, according to their U-values. The U-value is a measure of how much heat can escape through a structural element, expressed as watts per m2 Kelvin, where lower values indicate better performance.
For the UK’S cool climate, the limiting thresholds are as follows. They are shown here compared to the maximum tolerances allowed in the Part L1B of the Building Regs:
One positive of this approach is that the Passivhaus Institut recognises that achieving very low U-values with internal insulation can be problematic due to the nature of the existing building fabric and the existence of thermal bridges. Hence the U-value thresholds for walls, roofs and floor treated internally are actually higher than those required under the UK regs. In some cases, the building component method may not result in a space heating demand below 25kwh/m2pa (the Enerphit standard set out in route one). Nevertheless, overall it’s more stringent than the current UK energy performance regs.
3 Stepwise retrofit
In 2016, the Passivhaus Institut introduced a procedure for staged approaches. This allows eco renovations to be carried out in several consecutive steps, possibly over a number of years, with pre-certification occurring before you actually start any work. A comprehensive Enerphit Retrofit Plan must be created that shows how the building will meet the criteria and over what timescales. You can submit this to a Passivhaus certifier to check; giving you reassurance that, if you follow the process as described, your project will meet the standard. The method recognises Enerphit is a very deep renovation, and that cost can be prohibitive for homeowners if everything is undertaken in one go.
Getting ventilation right
Achieving high levels of air tightness is all very well when it comes to saving energy on space heating, but we all need a supply of fresh air. What’s more, in this scenario the ventilation method can become a significant source of unwanted heat loss. So Enerphit quite rightly requires the installation of a mechanical ventilation and heat recovery (MVHR) system. This is why the efficiency of the ventilation setup is specified in the elemental approach (route two).
As Enerphit combines MVHR with minimal fabric heat losses, it delivers a superbly comfortable internal climate, with constant living temperatures and plenty of fresh air. MVHR does add to the cost of a renovation project; but it can be the best solution in the right circumstances. It’s also essential that these systems are well-maintained and function correctly at all times. If they fail for any reason, then indoor air quality may be compromised. Enerphit includes rigorous testing and commissioning criteria for MVHR units to ensure the risk of such issues is very low.
Challenges for meeting Enerphit
Achieving this standard involves making huge changes to a building, as most existing houses weren’t designed to attain the required levels of insulation and air tightness.
Air leakage paths are surprisingly complex, and it’s often necessary to take a building apart in order to insert the requisite membranes or seal up the junctions between structural elements. If you’re getting to this stage, you may find you’re reducing the property to its shell to such an extent that it’s actually more effective (whether in terms of cost or energy) to demolish and rebuild.
It’s worth bearing in mind that sealants don’t last forever, so a building that achieves the required air tightness at the time of testing may not necessarily perform as well in 10 or 20 years’ time. That’s a particular concern where dissimilar materials are being joined, as these have differing rates of thermal expansion and contraction, accelerating the breakdown of seals.
New Enerphit-ready systems are being developed for projects where it’s possible to insulate externally. One example is Energiesprong, a wholehouse refurb approach that’s currently focused only on social housing projects but could be expanded into other sectors. Essentially, the building gets wrapped in a prefabricated, highly-insulated and airtight facade (giving an attractive makeover), alongside other upgrades. Some versions can even supply sufficient ventilation from the outside.
This approach means the building doesn’t need to be stripped internally, so the owners can remain in their home during the work. External wall insulation (EWI) is often inappropriate if a property’s appearance is important, however; such as with listed buildings or traditional brick and stone terraces. Narrow pavements, the location of key services and other issues can also prevent the installation of EWI.
Meanwhile, internal wall insulation (IWI) reduces the amount of useful floor area available inside a home – and British houses tend to be smaller than the equivalents in Germany, where the standard originated. But the biggest challenge with IWI is thermal bridging, where heat finds a path around the insulation. Sometimes these simply can’t be addressed without rebuilding, so Enerphit accepts some thermal bridges can be tolerated in a retrofit.
Embodied energy & costs
In 2010, the Grand Designs TV show featured one of the earliest Passivhauses in the UK – and the episode was aired again recently. Kevin Mccloud looked at the sheer quantity of insulation going into the building, before hitting the nail on the head when he said there surely comes a point when the impact of the additional material outweighs the savings in operational energy. It’s just common sense, and it reveals the limits of using the narrow metric of energy and emissions from buildings in use (and comfort).
We now know that the embodied energy that goes into creating and fitting out a building in the first place is highly significant in its life-cycled performance. Research from 2013 (by Crawford & Stephan) even shows that a new property designed on passive principles can use more energy over 80 years than a standard house of the same size, structure, finishes and number of occupants – and that this is mainly due to the additional materials required.
The point applies equally to Enerphit: a little insulation goes a long way, but when you get to a certain level it takes a lot more insulation to achieve little more saving.
Retrofit costs
Taking a building apart to carry out a full-scale refurb is very expensive, and the financial investment may not be repaid over the occupants’ lifetime. To my mind, the cost of achieving Enerphit is likely to be prohibitive for many renovators’ budgets. Given economies of scale and the simpler structural form of apartment blocks compared to detached buildings, however, larger public-sector projects might see a favourable return on investment.
Should you aim for Enerphit?
Going back a few years, I might have said there are some useful aspects to the Enerphit standard, as there was no alternative. Things have moved on in the UK, however. A new standard for retrofit (PAS2035) is due to be released later in 2018, based on a thorough process that includes consideration of heritage, moisture risk, ventilation and health. If it’s implemented, it will help ensure every building becomes as good as it can reasonably be in terms of energy use – ie without trying to meet arbitrary targets.
If we can achieve good energy savings with a more holistic approach (with lower embodied energy and less cost) this would seem sensible. Even if you can reach Enerphit on your scheme, it should perhaps be combined with other methodologies such as Building Biology or the WELL standard. These both encourage the use of natural materials, while considerations such as lighting design are also factored in to create a truly healthy building.
It’s often said that best is the enemy of good – and this holds true for retrofit projects. There’s only so much money available to refurbish our building stock, and each property is different. In the words of the government’s Clean Growth Strategy, we need to do what is practical and affordable.