How do I take a green retrofit to passivhaus standard?

What elevates a green retrofit to passivhaus standard? And is it worth it? Tom Pakenham made the leap. What was the result? Net energy bills were just £189 in year one, the house is comfortable throughout and free of drafts, cold patches and condensation. In short, Tom now owns a Victorian terrace which, he says, is near perfect to live in…

From green retrofit to passivhaus

Tom Pakenham's London SuperHome - a green retrofit  elevated to passivhaus standard

Tom Pakenham’s passivhaus standard SuperHome

Turning an existing building – particularly one that is almost 150 years old – into a passivhaus is about the toughest conversion project out there. It requires an awful lot of planning, good design, flexibility, patience and money. It takes longer than a normal conversion and presents unforeseen challenges that test a project team’s ingenuity to the limit.

To achieve success, the build team needs to be properly trained and to have bought into the relatively extreme objectives of the project. And the client needs to recognise that things will go wrong, quite regularly, but that once you’ve started it’s almost impossible to go back.

Some people might therefore ask whether it is worth doing, to which my answer is a resounding “yes”. This is for the simple reason that a passivhaus makes for the perfect domestic environment.

The benefits are not merely in kilowatt hours and £££s saved, but perhaps more importantly, in comfort, air quality, sound levels and usability. Together these create a living environment that is balanced, calm, fresh and warm, an environment that is almost impossible to achieve through mechanical systems alone.

Planning for passivhaus standard

Super insulated roof and triple glazed roof windows - all needs to be planned ahead

Super insulated roof and triple glazed roof windows – all needs to be planned ahead

So, what does it take to retrofit to passivhaus standard?

Firstly, as much time and resource as the project can spare must be devoted to design and planning ahead of work commencing; even more than for a standard build.

All contributors to the project – architect, structural engineer, builder, energy engineer, interior designer (if there is one) etc – should be advised of the objective and consulted fully and collaboratively from the very beginning.

Drawings should be circulated to all parties, until finalised, and enough time should be left to complete the design before the build begins. This may sound burdensome and indeed, may add a small layer of cost to the project, but in the long-term will make it go significantly more smoothly and most likely cost less overall.

Targets for passivhaus standard

Triple glazed tilt replacement sash windows help deliver a passivhaus standard retrofit

Triple glazed tilt replacement sash windows help deliver airtightness

Passivhaus is actually a standard, rather than a technique, which means that you can build in any way you like as long as you meet the following performance targets:

  • The overall energy consumption for heating and cooling (including hot water) is at or less than 15 kilowatt hours per annum per square metre (15 kWh/m2/annum)
  • The overall source/primary energy consumption across all of the building’s energy requirements (i.e. including lighting, appliances etc) is at or less than 120 kilowatt hours per annum per square metre (120 kWh/m2/annum)
  • The building is airtight to less than 0.6 air changes per hour when pressurised @50 pascals. In other words, not more than 60% of the house’s air can leak out per hour when it is pressurised.
Solar PV helps achieve the passivhaus standard

Solar PV helps achieve the passivhaus standard

Achieving passivhaus standard

Although there are no prescriptive “rules” for achieving these targets, there are certain approaches that are typically observed:

  1. Super-insulation
  2. High levels of airtightness, with ventilation provided by a mechanical ventilation system with heat recovery (“MVHR”)
  3. Very high-performance windows
  4. Low or zero thermal bridging
  5. Low-energy lighting and energy efficient appliances
  6. Installation of renewable energy micro-generation systems

A reader versed in low energy building will observe that these are common to all energy efficient building projects. However, when building a passivhaus they will typically be pushed (close) to the limit, such that there is no longer a need for a wet heating system in the building. Instead, any heat required in addition to passive gains – such as sunshine, the fridge and body heat – can be put in via the ventilation system, usually via a small heat pump or electrical element heating the circulation air.

MVHR and airtightness

It is worth noting at this point that the airtightness and MVHR account for the freshness of the internal air, which is replaced in its entirety every 2 1/2 hours or so. Stale air is extracted from kitchens and bathrooms and new air is pumped into living areas, creating a perfect airflow that removes smells and moisture and provides constant fresh air. The incoming air passes through at least 1, sometimes 2, filters before entering the house. These are replaced every 3-4 months and are always impressively grubby with the level of particulates and other pollutants they accumulate; particularly in an urban environment.

No radiators but one comfy sitting room!

No radiators but one comfy sitting room!

Living in a Victorian passivhaus

So, what is it like to live in a passivhaus and how does it perform? There is a simple money answer to this – net energy bills in the first year were £189 all in. And this was not a cold house, rather a comfortable and fresh house in which every room could be happily and comfortably used at any time of the year.

There are no drafts, no cold patches, no windows with rivers of condensation. It’s true, the internal engineering meant that most of the original features have been lost, but apart from this it really is the perfect house to live in.

© Tom Pakenham Feb 2013.

Tom’s SuperHome won the Refurbishment Project Award at the 2013 Building Performance Awards run by the Chartered Institution of Building Services Engineers. Keep an eye out for future open days. Tom is also Chairman of greentomatoenergy. He has opted to donate his writer’s fee for this article to a school in Ghana through the JustGiving website.

Also See:
MVHR at free eco open house events in September

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