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how does a passivhaus handle the summer heat?

This is our second summer of experience from one of our passivhaus retrofits. The good news is that, with a little bit of occupant management, both summers have been a great success – the house has stayed cool and fresh, with minor instances of localised over-heating.

When we modelled the property in PHPP, the possibility of it overheating was highlighted by the software. The PHPP model of the building estimated that there will be overheating 2.5% of the time throughout the whole year, this is when the internal temperature is above 25°C, so we were interested to see how it would perform against the prediction.

Data from the MVHR control unit, which is located in the living room, the hottest room in the house show that the room has been over 25°C for 7.3% of the year so far, this shows higher overheating than PHPP suggested.

However, CIBSE defines over heating as 26°C and above in bedrooms and 28°C and above in living spaces, the criteria for an overheated building is when 1% or greater of annual occupied hours are at these temperatures. Due to enormous pile of data we would have to wade through to assess a whole year, we have looked at the data from May until the end of August 2012. It shows that the living room was 28°C and above for about 4 hours during this period, all of these hours being on the same day when the maximum external temperature was 30°C. This gives the percentage of overheating defined by CIBSE, to be 0.1% for the year so far. This is nearly 10 times lower than what would be classified as an overheated building.

Occupants report that whilst the house can get hot on very hot days – like any other house – it is quick to cool down and the temperature can be quickly and effectively managed by simply opening windows.

It is worth explaining and exploring some features of the property pertinent to its summer heating profile:

South facing – the house faces more or less due south. There is a lot of glazing on the south façade, 53.7% to be precise. As a result, a great deal of direct solar radiation enters through the front of the building, causing rooms on the south side of the building to be noticeably warmer than those on the rear / north side.

Thermal mass – due to the wall build-ups and structural (read “budgetary”) constraints, it was not possible to include any thermal mass in the rooms on the south side, at any level of the 4-storey building. The kitchen at the rear of the building has a concrete floor, which appears to contribute to keeping that area cool, but does not provide any thermal mass to the hotter, south-facing rooms. We explored more affordable / lightweight options, but thermal mass plasterboard and clayboards were not easily available when we built the property.

Underfloor heat exchange – there is a ground-to-air heat exchange under the floor in the cellar. It is there to pre-heat incoming air before it reaches the MVHR in the winter; and pre-cool it in the summer. The air is taken in from the north side of the property, so that in its self means the air will be cooler in the summer than if taken from the south.  Unfortunately, we do not have sophisticated enough monitoring in place to record its effectiveness from a cooling perspective. Similarly, because the windows are often open to provide passive ventilation (in addition to the MVHR), it is hard to know whether this element contributes anything meaningful to the internal temperature.

External shading – because the house is in a conservation area, it was not possible to add any external shading without planning permission. So before going down that route we decided to see how hot the house became and tackle this issue in future, if necessary. Whilst it’s true that external shading would increase comfort in the house, overheating is nothing like a big enough problem to warrant all the effort and cost involved in adding external shading. It’s always an option in future if the climate continues to get hotter…hmmm.

Passive ventilation – The main temperature control technique in the house is good old-fashioned passive ventilation; i.e. opening some windows. Because the house is tall and the ground floor generally stays so nice and cool in the summer (particularly with the north-facing kitchen door open), we generate an excellent stack effect bringing cool air up through the house and expelling hot air through the velux windows on the top floor. This generally means leaving one small bathroom window open (on tilt, so secure) most of the time when we’re in and one window at the top of the house open. If it’s very hot, we will open more windows to increase the flow of air. It is an exceptionally effective way of keeping the house cool and makes one wonder why anyone ever uses air-conditioning (in most scenarios).

For rapid cooling of particular areas that are “off the stack”, we also occasionally use cross-ventilation, but this is less desireable because of the presence of a reasonably busy road on one side of the house.

Some people ask why we don’t just turn the MVHR off in the summer seeing as we have windows open. The answer to this is twofold:

A)     We don’t open all the windows, particularly not at the front of the house which is on a relatively busy road. Therefore, some continued ventilation is required from the MVHR in these areas; and

B)      Unless it’s extremely hot, we tend to close the windows at night and it would make sense to keep turning the MVHR on and off; particularly if doing so risked forgetting to turn it on again at night!

All-in-all, whilst it is during the winter that the house really comes into its own, the good news is that it is a comfortable place to be even on those rare hot summer days.