PassivHaus standards are not always achievable for older homes, but the new EnerPHit certificate helps retrofitters bridge the gap.

Are you familiar with the term ‘retrofit’? I suspect most people in the building business (and that includes self builders) have an inkling of what it’s all about. Retrofit is a form of renovating, usually with a specific goal in mind: to greatly reduce energy consumption. Therefore it involves adding lots of insulation, improving glazing and – in all probability – upgrading the heating system. It’s not about stripping out the old kitchen and bathroom and simply replacing them with something a little more tasteful. Nor is it about extensions and alterations, although the typical retrofit usually involves far more than just energy-efficiency improvements.

Green Deal

So, immediately you can see that retrofitting is part of the green agenda and thus there is a political element to it all because the Government (or at least parts of the Government) are keen to promote green growth. To this end, they have championed the Green Deal which is part-loan, part-grant scheme, and encourages us to undertake all manner of energy-efficiency measures, although for private householders these principally concern external or internal wall insulation. Now, for those embarking on a retrofit, this might not be particularly useful because the Green Deal doesn’t really seem to go very far and it doesn’t have any specific targets in mind.

It’s all very well saving some energy, but the chances are that you won’t even notice half-hearted efforts and that, whilst your rooms might be a bit easier to heat, your fuel bills won’t be greatly reduced. This is partly because the typical older house never took to central heating that well in the first place and the majority of older homes were only ever partially heated.

Getting Serious

Really extreme retrofit is a little bit different. It involves stripping the existing structure down to the basic building blocks and rebuilding the fabric — the walls, floor and roof, not to mention doors and windows. It also involves attention to arcane factors like thermal bridging and airtightness, and requires attention to be paid to the ventilation as well. It’s about as far as you can go short of demolishing and rebuilding.

In fact, critics of extreme retrofit make this point. If you need to go to that much trouble to bring an existing structure up to current levels of energy-efficient construction, why not start again from scratch? The costs are not so very different and you are likely to get a better designed, more buildable outcome with a new build — not to mention a substantial VAT refund, not available on renovations or retrofits.

But there are, of course, other factors at play here. It’s not always possible to demolish an existing house because it may be listed or in a Conservation Area, or it may be part of a terrace or a semi whose neighbours wouldn’t take kindly to a gaping hole appearing where there was once a house. And the chances are that if the house is attached to another, the replacement would have to look very similar to the existing so there might not be much to be gained from demolishing the existing structure, only to replace it with something very similar.

Insulation Issues

Perhaps the biggest issue facing retrofitters is where to place the insulation that’s needed to bring the old house up towards current energy-efficiency levels. Wherever it is placed, it is likely to cause issues.

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. Now, if the existing wall has a tendency to hold moisture (many do, for instance if the brickwork is slightly porous), insulating the inside of the wall may upset the balance which had previously enabled moisture in the wall to evaporate. 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, but it is one that has been highlighted by the Society for the Protection of Ancient Buildings (SPAB). They have been carrying out research to ascertain when and where interstitial condensation may occur with a view to making some more certain recommendations. Generally, the advice now seems to be that if the wall is in a very exposed location (i.e. one that is likely to see a lot of driven rain) and the external surface is not inherently waterproof (such as stone work and some brickwork too) then internal wall insulation is best avoided. This goes for buildings of any era, not just the ‘ancient’ ones which SPAB seeks to preserve.

These issues have been addressed in some detail in a new book, Old House Eco Handbook by Marianne Suhr and Roger Hunt, written in association with SPAB, which looks at how you can go about insulating solid walled homes without causing them any damage.

The EnerPHit Standard

It’s all very well adding some insulation to the structure, but increasingly people are wanting to know how much and to what end. To answer these questions, you need a yardstick to measure against and compare performance with known benchmarks. The PassivHaus standard is perhaps the best known of the energy-efficiency fabric standards and yet it is felt that it’s just too exacting for an existing structure to meet. In response, the PassivHaus Institute has introduced a slightly less taxing standard, known as EnerPHit, specifically for retrofits.

The EnerPHit standard includes the following requirements: Annual specific space heating demand of 25kWh/m² (as compared to 15kWh/m² for full PassivHaus); airtightness ideally to PassivHaus levels (0.6ach) but will allow 1.0ach; windows need to be PassivHaus certified, with triple-glazed panes; and calculations to demonstrate moisture is adequately managed.

The most important of these standards is the first — the overall space heating demand. To work this out, the design for the project is put through the PassivHaus planning package (PHPP) which produces an end result expressed in kWh/m², which is a measurement of the typical amount of energy needed to provide space heating over the course of a year. The EnerPHit standard of 25kWh/m² per year is much lower than a new Building Regulations house (around 60kWh/m²), and far better than a typical untreated older house might hope to achieve (maybe 120-200 kWh/m²).

For some designs, it’s virtually impossible to get down to such a low figure and in these instances the EnerPHit standard permits you to use an elemental approach. So, for instance, if your external walls are insulated with a U value lower than 0.15 and your roof or top floor ceilings have a U value lower than 0.12, then you will still be deemed to meet the standard. The standard is explained in great detail in a 16-page document available for free download at

The advantage of working to a standard is that you get a quality audited design which takes account of all the technical details such as cold bridging and airtightness, and you should therefore produce a building with not only low, but predictable energy usage. Without a standard such as EnerPHit, retrofit measures tend to be hit or miss and the outcome is more than likely to be unpredictable.

The problem with exacting standards is that they are often technically very challenging and consequently very expensive. To get a wall U value down to as low as 0.15, you might need to add something like a 200mm thickness of insulation — more or less doubling the width of the original wall. The typical British house is smaller than the German equivalent (where the EnerPHit standard hails from) which can make the addition of large thicknesses of insulation problematic. And, as Suhr and Hunt point out in their book, big changes to the fabric of older homes are made at a cost to the architectural value. In consequence, EnerPHit is probably best suited for dealing with tired 20th century housing, rather than anything earlier although – as the case studies on this page show – it doesn’t stop people trying and, of course, there’s nothing wrong with aiming for high standards when it comes to minimising heat demand.

Enerphit Case Studies

The thermographic image (above) makes it clear which of the houses on this street has been renovated to the very best in PassivHaus principles. Architect/builders Simmonds Mills, with energy consultant Peter Warm, managed the eco retrofit of this 1869 townhouse which aims to reduce its CO? emissions for heat and power by up to 85%. The house has been externally insulated with nearly a foot of expanded polystyrene insulation with render on top, over a foot of insulation between new rafters on top of the original roof, new triple-glazed windows and a ventilation system with heat recovery. As a result, the heating demand has been reduced to a level below a tenth of the average UK house.


Jane Caccavale and her husband Luigi are in the middle of carrying out an EnerPHit renovation of a London townhouse, using predictive modelling to work out the key areas that need addressing when it comes to insulation and airtightness. You can check up on their progress here.

Where to Insulate?

  • Roof: Ceiling insulation is easy to install but can lead to problems as the loft space above becomes cold, so roof ventilation needs to be addressed. Insulation in a sloping roof space is difficult because it is to some extent limited by the depth of the rafters, which is unlikely to be enough to accommodate the depth of insulation required for an extreme retrofit. Often the best technical solution is to take the roof cover off, place insulation above the rafters and fix the new roof higher than the old one.
  • Floor: Solid floors often have to be dug up because there is rarely enough depth for insulation to be laid on top, without causing all manner of problems around door openings. Partition (internal) walls also present problems because they become cold bridges (where heat leaks down into the ground, because they are not insulated too).
  • External walls: Technically, insulating the outside of a house is often preferable, but it’s not straightforward. Even if it’s aesthetically acceptable, there may not be space to incorporate insulation (if the house is next to a pavement, for instance) and there are frequent complications around existing openings. Downpipes and soil stacks often have to be moved, which requires adjustments to the drains, and boiler flues and window openings have to be worked around carefully. Perhaps the biggest issue is coping with an inadequate roof overhang. The newly thickened walls often protrude beyond the existing eaves. Thus it often makes sense to replace the roof at the same time.
  • Internal walls: This avoids many of the problems of an external solution, but runs into a host of other ones. Passageways and stairwells are possibly not wide enough; radiators, sockets and switches have to be moved; tiles and shelving may have to be replaced or removed. There are also lots of cold bridges — every time an internal wall meets the external wall, not to mention any intermediate floor/ceilings.

Find out more on EnerPHit:

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