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Passivhaus: Q & A

Put simply, what is a Passivhaus?

A Passivhaus is a building which is both energy efficient and comfortable.

How is this quantified?

  1. Heating energy to be <15kWh/m²yr. This figure doesn’t mean much without a bit of context: according to the Passivhaus Trust, “Passivhaus buildings achieve a 75% reduction in space heating requirements, compared to standard practice for UK new build.”
  2. Primary energy to be <120kWh/m²yr. This covers energy consumed for everything else (i.e. appliances, devices, hot water and lighting).
  3. No draughts: airtightness is to be less than 0.6 air changes per hour, measured at 50 pascals.
  4. No cold surfaces: internal surfaces of the building fabric, including window glazing, to be no less that 17°C on the coldest day of the year.
  5. Comfortable summer temperatures: overheating is controlled, permitting the internal temperature to exceed 25°C for only 10% of the summer months.
  6. Fresh indoor air quality: mechanical ventilation with heat recovery provides a fresh air supply of 30m³/person each hour.

How are these standards achieved?

  1. Shape: Design an efficient shape, keeping the amount of external surface (heat loss) area in comparison to useable floor area low.
  2. Glazing: The closer to south-facing the better; easy to shade against overheating in the summer, yet drinks in the solar gain in the winter.
  3. Thermal: Insulate, insulate and then insulate some more.
  4. Airtightness: Build quality is vastly improved in comparison to standard UK practice; no more leaky buildings.
  5. Ventilation: Fresh air is brought into the building and distributed in a controlled way with a Mechanical Ventilation Heat Recovery (MVHR) system.

Does the air get stale inside?

No… good air quality is one of the key characteristics of a Passivhaus. The MVHR draws in fresh air from outside and extracts the stale air from inside. The clever part is that the heat from the stale air is harvested and used to warm the incoming fresh air.

Are you allowed to open the windows?

Yes… if you want to open the windows nothing and nobody can stop you. It’s a myth that the windows in a Passivhaus have to stay shut, but people often find that they simply don’t need to open them.

Does it cost more than mainstream construction?

Yes and no… there are many factors at play here, including economies of scale and complexity of design, but essentially the initial outlay is likely to be in the region of 0-10% more than traditional build (source: Encraft). This is mainly down to the high standard of construction and quality of the components. However, a Passivhaus’ whole-life cost is demonstrably lower than a traditional build, due to the dramatic reduction in energy consumption.

Why follow Passivhaus principles?

  1. Simple principles: build robustly, wrap it up warm, use the sun for heating and maintain good air quality.
  2. Affordable solution: no requirements for expensive technologies to meet the performance criteria.
  3. Reduction in fuel poverty: minimal (sometimes zero) heating bills.
  4. Health: airtight construction with controlled ventilation keeps the risk of condensation and mould growth very low.
  5. Longevity: the high build quality ensures that the integrity of the structure will be long lasting.
  6. Because it works: the post-construction analysis of Passivhaus projects speak for themselves.

Why get Passivhaus certification?

It can be tempting to simply follow the principles and mantras of Passivhaus criteria but not bother with certification. However, with the knowledge that the targets don’t have to be met, designers and contractors are likely to cut corners, which may result in a building that misses the targets entirely. Large sums of money could be wasted on decent insulation if the details are not designed thermally bridge free and the construction is not of a high standard. Getting Passivhaus certification provides a quality control system, ensuring that the Passivhaus intentions are followed through from paper to practice.

Want to know more?

If you have any more questions about Passivhaus or are thinking of building one, just drop me an email at hello@theecoarchitect.com

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