Saturday 25 June 2016

What is a Passivhaus? Architect Mark Stephens explains.

If you've been paying attention to TV programs like Grand Designs you'll have noticed a growing interest in building something called a Passivhaus.  You may also have been wondering exactly what a Passivhaus is.  

This guest post by Mark Stephens RIBA MRIAI, Registered and Chartered UK and Ireland Architect and Certified European Passivhaus Designer, explains how a Passivhaus is designed and constructed and gives a preview of a one of his projects in the West of Ireland. Mark works across Ireland and collaborates with UK architects on residential designs in London and the Home Counties.

We are hoping to collaborate on a project in the very near future.

What is Passivhaus (or Passive House if you prefer the English spelling)?

is a rigorous, voluntary, design and construction standard for energy efficiency in a building. It results in ultra-low energy buildings that require little energy for space heating or cooling and thereby reducing its ecological footprint. 

For a building to be considered a Certified Passivhaus, it must meet the following criteria:

1. The Space Heating Energy Demand is not to exceed 15 kWh per square meter of net living space (treated floor area) per year or 10 W per square meter peak demand. In climates where active cooling is needed, the Space Cooling Energy Demand requirement roughly matches the heat demand requirements above, with a slight additional allowance for de-humidification.

2. The Primary Energy Demand, the total energy to be used for all domestic applications (heating, hot water and domestic electricity) must not exceed 120 kWh per square meter of treated floor area per year.

3. In terms of Airtightness, a maximum of 0.6 air changes per hour at 50 Pascals pressure, as verified with an onsite pressure test (in both pressurized and depressurized states).

4. Thermal comfort must be met for all living areas during winter as well as in summer, with not more than 10 % of the hours in a given year over 25 °C. 

Passivhaus buildings are planned, optimised and verified with the Passive House Planning Package (PHPP).

All of the above criteria are achieved through intelligent design and implementation of the 5 Passive House principles: thermal bridge free design, superior windows, ventilation with heat recovery, quality insulation and airtight construction.


What does this mean in layman's terms?

The Passivhaus method of design and construction is focussed on three key areas:

1. Very high levels of insulation.

A typical u-value for the fabric of the building (walls, roofs, floor etc) would be around 0.14 W/(m2K), and frequently better. This is an insulation level double that required by UK Building Regulations

2. Very high levels of air tightness.

The level of airtightness required is very vigorous. The example in this guide:

compares UK compliant construction and Passivhaus construction. The maximum hole allowed for air leakage in a Passivhaus would be the size of a 5p piece for every 5 square metres of building envelope. The UK Building Regulations compliant house would allow a hole of a 20p piece for every square metre of building envelope.

But surely you'd suffocate if the building was airtight? Which is why we have...

3. Mechanical Ventilation and Heat Recovery (MHVR)

Forget the idea that you get clean ventilation by opening a window. The benefits of Mechanical Ventilation are immense:

• Better ventilation
• Heat recovery and fewer cold draughts (with airtightness)
• Better air quality (through controlled C02 concentration)
• Zero mould growth due to poor ventilation and condensation issues.

Opening a window is fine in the summer but you still need ventilation in winter.  The heat recovery system in MHVR recovers heat from the wet, moist rooms such as bathrooms and recirculates the heat into cooler rooms such as bedrooms.

These are the key elements that need to be considered in the construction but other characteristics including triple glazing and shading also need to be considered in order to achieve a comfortable living environment. All of these aspects are carefully calculated at every stage of the design and construction using the Passive House Planning software (PHPP).

Why have a Passive House?
1. Savings

It's a common fallacy that a Passive House requires no heating or heat supply.  However,  the heating costs are incredibly low, typically around £150 a year.  

2. Environment
By reducing the carbon footprint of your house you are doing your bit to limit climate change.

3. Comfort
A Passive House is incredibly comfortable to live in. They are designed to be at 20 degrees throughout the entire year, 24 hours a day, winter or summer.

The airtightness means there are no nasty draughts and the MHVR means you are breathing beautifully clean air.

Case Study
We have several Passive Houses in various stages of the design process.  The following describes one of these projects in the West of Ireland.  The build should start later this year. Below is a link to a Youtube movie showing a flyover of the proposed house (created using Vectorworks Architect and Landmark): 

The house is designed to fit into the landscape with a Sedum roof.  The Passivhaus aspects  are to maximise the living spaces to the south without excessive glazing to ensure it is not uncomfortably hot in the summer.

As well as using the PHPP software we also use the Energos commands in Vectorworks to test the design (see screen grab below).

The next stage in the process is to prepare construction drawings for the house,   including the specification of all the building elements (to meet the u-values described above) and to specifiy the MHVR units to ensure the house is correctly ventilated.

This project is to design and construct a new house to the Passivhaus standard, but it is also possible to renovate an existing house to a similar standard called Enerphit.  This is a slightly lower standard but still accredited by the Passive House Institute.

The roof is to be constructed in a 'Warm-roof' method.  This involves high levels of insulation on top of the rafters as well as between them.  It will then be finished in a Sedum matting in order to blend the house into the landscape.  

But what type of garden should have a house like this have? Linsey Evans will answer this question in a guest post on my blog.

For more information on our work, please visit:

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