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What is EnerPHit?

With Paul almost at the end of his own EnerPHit project, we decided to take a look at the process in more detail.

Excerpts of this blog have been taken from a piece Paul wrote for Homebuilding & Renovating magazine.

EnerPHit allows homeowners the ability to update the thermal performance of their existing home.

Many people love their existing home for a number of reasons – the location and the neighbours, for instance – but are often faced with a dilemma when the house fails to meet certain requirements.

This is when you might begin to think about taking on a renovation, remodel and/or extension project. While this type of project can deliver more space, what about comfort and thermal performance? Step in retrofit.

What is Retrofit?

Retrofit is a form of renovation, typically undertaken to reduce energy consumption. As such, retrofit involves a significant improvement in the thermal performance and comfort of your home.

It’s about improving the building fabric rather than simply the introduction of renewables.

However, it’s difficult to know how far to go and predict how much comfort and performance your retrofit will deliver. To this end, you need something to measure against and compare performance with known benchmarks.

The Passivhaus standard is perhaps the best known of the energy efficiency fabric standards for new builds, and the good news is that this standard has been adapted for existing homes. 

What is EnerPHit?

Unlike a new home built to Passivhaus standard, when considering a retrofit, many of the elements like geometry and structural approach are already there.

You may also have thermal bridges (or cold bridges: a path for heat to escape through gaps in insulation) that are difficult to completely eliminate.

The EnerPHit standard recognises this and sets the required performance at a lower level than Passivhaus to accommodate working with existing buildings.

To achieve EnerPHit you must:
  • Have a space heating and cooling demand of 25kWh/m2/year (compared to the Passivhaus standard of 15kWh/m2/year).
  • Instead of an airtightness performance of 0.6 air changes per hour you need to achieve 1.0 (the Building Regs for new homes require between 5 and 15 according to the Chartered Institution of Building Services Engineers).

This might use more energy to achieve the same comfort levels as the Passivhaus standard, but it’s a huge improvement in most existing homes and even new builds.

EnerPHit elements of Paul’s retrofit

The design process

EnerPHit offers a benchmark for renovators to work to. Like with a Passivhaus, the Passivhaus Planning Package (PHPP) design tool is used when designing an EnerPHit scheme. This will deliver more efficient buildings at an early design stage, considering orientation and geometry.

The PHPP allows us to make informed decisions about where to spend money. It also helps to understand what cost and energy implications there are in various retrofit measures and the alternatives to consider.

Retrofitting is not easy, however, and EnerPHit is a tough standard to achieve. The thermal and airtightness strategies are likely to be more complex and more difficult on site than with a new build.

You may have to balance the pros and cons of internal or external insulation, and potential moisture issues that come from changing the building fabric. More so than ever, it demands a skilled and informed design team.

That said, perhaps the ultimate benefit of aiming for the EnerPHit standard and for full certification is the rigour and quality assurance it demands. Following the criteria ensures that the works are completed in the way they are designed; that the airtightness performance is achieved and that there are no mistakes along the way.

What’s involved in an EnerPHit?

When carrying out an EnerPHit, there will be a list of measures required in order to meet the standard. These will involve:

  • High levels of insulation — either internal or external, although internal needs more care in terms of moisture risk.
  • High-performance triple-glazed windows and external doors.
  • Careful consideration of window installation.
  • An airtightness reading of 1.0.
  • A mechanical ventilation with heat recovery (MVHR) system.
As the standard you’re working to is a recognised benchmark, the process is exactly the same as working towards Passivhaus certification. To meet these requirements, the project must be designed using the PHPP and certified by an accredited Passivhaus certifier.

Although the products you’re including within the house don’t need to be Passivhaus certified, it does help, especially with MVHR.

The EnerPHit energy balance

How much does it cost?

The cost of an EnerPHit retrofit will vary as the more complex an existing house is the more complex the retrofit will be. It is wise to budget around £800-£1,200/m2 for deep retrofit/EnerPHit. Don’t forget the VAT as you’re dealing with an existing building.

For typical component or system costs, you can expect to pay around £10,000 for an MVHR unit. Windows and doors will cost approx £400-£600/m2.

The big costs are in labour — insulation installation and airtightness measures, which are time-consuming and needs to be done with care.

Insulation and retrofit

Perhaps the biggest issue facing retrofitters is where to place the insulation that’s needed to improve the energy-efficiency levels.

One of the biggest worries about internal wall insulation concerns the possibility of condensation and mould growth. Placing insulation inside an existing wall is bound to make that wall colder.

If the existing wall has a tendency to hold moisture (for instance, if the brickwork is slightly porous), insulating the inside of the wall might upset the balance.

As a result, the insulation now acts as a moisture barrier and water gets trapped within the new wall assembly.

Just how big a threat this is can be hard to fathom. It’s one that has been highlighted by the Society for the Protection of Ancient Buildings (SPAB). They have been researching where and when interstitial condensation may occur with a view to making some more certain recommendations.

Avoid internal wall insulation if:
  • It’s very exposed, for example if it’s likely to see a lot of driven rain
  • The external surface is not inherently waterproof (such as stonework and some brickwork)
  • This goes for buildings of any era, not just the ‘ancient’ ones which SPAB seeks to preserve

When to carry out an EnerPHit

Paul’s house – before and after

An EnerPHit retrofit makes the most sense when you’re already considering renovating or remodelling your house.

The economic argument for improving the performance of the roof when it needs replacing, or installing triple glazing rather than double-glazed windows when they need changing, is a much easier one to justify and often the numbers stack up.

The key thing is to have a whole house plan from the beginning so that each measure works together in the long-term, giving you the end result of a high-performing home.

If you’re considering an extension, this may not be the time for you to undertake a retrofit. But it would be frustrating to find that your extension has created a barrier to a more in-depth retrofit in a few years’ time.

If you’re already considering renovation or repair works to your home, now could be the time for an EnerPHit retrofit.

Find out more about EnerPHit here: passivhaus.org.uk

If you’re interested in creating your own EnerPHit home, get in touch.