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This post was written by Paul in 2011, for more up to date information on Passivhaus click here: https://hemarchitects.co.uk/2018/02/passivhaus-retrofit-presentation/

To read about our latest Passivhaus projects, visit our Projects. To find out more about Sheffield’s first Passivhaus housing development, click here.

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The Passivhaus standard is growing in popularity in the UK; there are still only a few certified Passivhaus buildings in the UK.

Having researched the topic, I am convinced that it’s the way forward (at least in its main principals) for good sustainable construction.  The BRE set out the main Passivhaus requirements in their Passivhaus Primer document that can be found in the downloads area of their website as follows:

To achieve the Passivhaus standard the energy required for space heating must not exceed 15 kWh/m2/yr – this is the overarching requirement (In addition to this the total primary energy demand must not exceed 120 kWh/m2).

The figure of 15kWh/m2/yr has been arrived at from calculating the maximum amount of heat that can be delivered using the fresh supply air at the minimum required ventilation rate.

It is limited to 15kWh/m2/yr for two reasons:

1: The temperature of the fresh incoming air cannot exceed 50°C. If the fresh air is delivered at a higher temperature than this problems can occur with the indoor air quality (e.g. burning smells caused from the air scalding dust within the ductwork).

2: A comfortable indoor temperature (20°C) needs to be achieved in areas with low ventilation rates. This means that only a certain amount of heat can be supplied without exceeding the 50°C temperature limit.

Without achieving the PassivHaus space heating requirement of 15 kWh/m2/yr or less, criteria 1 & 2 will not be achieved and thus it is not recommended to try and heat a dwelling using the fresh incoming air.

Achieving a space heating requirement of 15 kWh/m2/yr or less means that the following headline features are specified:

•  Super-insulation: opaque U-values must be less than 0.15 W/m2 K

•  U-values for windows and doors need to be 0.8 W/m2 K or less (for both the frame and glazing). This requires the window frame to incorporate insulation and triple glazing

•  Thermal bridging needs to be minimised, and ideally eliminated

•  Airtightness: 1m3/hr/m2  @ 50 Pa or less

•  Whole house mechanical ventilation with heat recovery (75% efficient or better, with a low specific fan power).

This primer is very useful (and short) read for anyone interested in sustainable construction.

You still need to consider orientation, day lighting, passive solar gains, internal gains and overheating etc, but fundamentally, this means that the main definition of Passivhaus is the space heating requirements in terms of a fixed, absolute and measurable energy consumption level.  There are no ways to cheat the system; adding renewables doesn’t make it pass; increasing the u-value on the roof doesn’t mean you can have lower performance windows etc.

This aligns very much with my approach to sustainable design. I believe that we need to ensure that the building envelope is performing at its optimum before we start introducing other “green” technologies that can often hide the fact that the building isn’t designed well or performing as it might.  Once we have this right then the introduction of renewables can only make things better, and they’ll have so much less energy to produce.  As BRE state in the Primer PassivHaus is a great starting point to achieving a zero carbon building.

This ties in well with my post on Hector’s House last week.  My issue with that building was that the envelope wasn’t optimised prior to the introduction of a renewable heating source, but the presence of a “green” fuel hid the fact that it was using more energy to heat than it needed.