When is a PassivHaus not a Passive House?

There have been many attempts to design dwellings with a reduced (or zero) carbon footprint. Several example buildings have been constructed, and many are being used in real life. In the UK the designs have sometimes been referred to as ‘Passive House’ as the design and construction has been predicated on making them ‘energy neutral’, that is requiring minimal energy input and maybe incorporating energy generation such as wind or solar PV.

This minimal energy configuration is achieved through making good use of natural factors such as building orientation, solar gain for natural heating, high insulation to minimise losses and use of natural light to minimise energy demand and natural forces such as convection to drive the ventilation. Examples of these can be viewed at the BRE Innovation Park, there are tours several times per week, but at a price.

A PassivHaus, by contrast, has a more active approach to the design and construction of low energy homes. It does this by describing certain low energy targets that must be met for a building to reach the PassivHaus standard, and requiring the provision of frsh air with heat recovery through a very efficient warm air heat exchanger.

A key benefit of this approach is held to be that it allows the designer to use a greater freedom in the design area to provide PassivHaus buildings in all sorts of environments. Indeed, just recently the Green Store has received Passivhaus certification for a ‘conventional’ cavity wall built building in the UK as opposed to the timber framed buildings popular on the continent.

This approach is based on a key target, that is to get the specific space heating requirement in the building down to 0.15 kwh/m2 per year. This can only be achieved by using high levels of insulation and taking great care over airtightness so that practically all the air comes in through the Mechanical Ventilation and Heat Recovery (MVHR) system. In this way around 75 to 80% of the heat normally lost through draughts is recaptured and used to warm the incoming air.

When required on cold days the air is then warmed by a supplementary source. Because of the low energy demand, the air is never heated to over 50oC. This is to ensure that overheated air, which can smell of burning and release toxins from building materials, is never encountered with the required low levels of ventilation. Yet the building remains warm and comfortable.

Now we live in the real world and taking a 1930s bungalow with a 1960s extension accompanied by some 1980s refurbishment is not an obvious candidate for reaching this exacting standard. To do that would involve a “demolish and start again” approach, not a practical option within budget. However, there is no reason why many of the principles involved cannot be taken and adopted/ adapted to make the best of what is there.

The key components are…
1. Super insulation levels – U values for walls, floors and roofs to be 0.15 W/m2/oC
2. Low U values of 0.8 W/m2/oC for windows and doors.
3. Minimisation of thermal bridging
4. Airtightness of 0.6 h1 @ 50 Pa or better
5. MVHR system with 75% heat recovery

Quite some challenge!

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