Passivhaus? Or Michaelhaus?

Passivhaus 

The modern ‘Passivhaus‘ philosophy of building design involves building houses with exceptional levels of insulation – so that very little heating or cooling is required. Click here for videos explaining the concept

Making houses airtight is essential to achieving this low heating requirement. If the air in my house (volume 400 cubic metres) were exchanged once each hour with outside air then the heat leak would be equivalent to 148 watts for each degree of temperature difference between the inside and outside of the house. This would amount to more than half the current heat loss and make insulating the walls almost pointless.

So to achieve Passivhaus certification a low level of air leakage is required: the number of Air Changes per Hour (ACH) must be less than 0.6 ACH when the external pressure is changed by 50 pascal. The Passivhaus Institute have an excellent guide on all aspects of achieving airtightness in practice (link).

But with this low background ventilation, the general day-to-day activities of a family would likely lead to the build up of unpleasant odours or excess moisture.

So the air flow through the house is then engineered to achieve a specified number of air changes per hour (ACH) through  mechanical ventilators that capture the heat from air leaving the building and use it to heat air coming into the building. This use of  Heat Recovery Ventilation leads to fresh air without the noticeable draughts or heat loss.

Michaelhaus

Achieving the Passivhaus standard for newly built houses is not easy, but it is readily achievable and there are now many exemplars of good practice in the UK.

But achieving that standard in my house would require extensive retrofit work, lifting floorboards and sealing hundreds of tiny leaks. So what should I do?

I don’t know what to do! So I am adopting a “measurement first” approach.

1. As I have outlined previously, I am monitoring the energy use so after the external wall insulation has been applied next month, I should be able to assess how significant the heat loss associated with air leakage is over the winter.
2. And more recently I have been estimating the number of air changes per hour (ACH) in the house in normal use.

The second of these measurements – estimating the number of air changes per hour – is normally extremely difficult to do. But I have been using a carbon dioxide meter and a simple spreadsheet model to give me some insight into the number of air changes per hour – without having to figure out where the air leaks are.

Carbon dioxide meter

I have been using two CO2 meters routinely around the house.

• The first meter also reads PM10 and PM2.5 particulate levels. But it does not log results to a datafile.
  • Link to TEMTOP product on Amazon
• The second meter only reads CO2 concentration, but logs the data to a microSD card.
  • Link to TFA Dostmann product on Amazon

Each meter cost around £140, which is quite a lot for a niche device. But since it might guide me to save hundreds or thousands of pounds I think it is worthwhile.

Calibrating the two CO2 meters used in this study by exposing them to outside air. Both meters have a specified uncertainty of ±50 ppm but they agree with each other and with the expected outdoor CO2 level (~400 ppm) more closely than this (407 ppm and 399 ppm).

To estimate the number of ACH one needs to appreciate that there are two common domestic sources of CO2.

• Human respiration: people produce the equivalent of around 20 litres of pure CO2 each hour – more if they undertake vigorous exercise.
• Cooking on gas: a gas burner produces hundreds or thousands of litres of CO2 per hour.

So if there were no air changes, the concentration of CO2 would build up indefinitely. From knowledge of:

• the volume of the room or house under consideration,
• the number of people present and the amount of cooking.
• a measurement of CO2 concentration

It is possible to estimate the number of air changes per hour.

I have been studying all these variables, and I will write more as I get more data, but I was intrigued by two early results.

Result#1

The figure below shows the CO2 concentration in the middle room of the house measured over several days using the data-logging CO2 meter.

This room is a ‘hallway’ room and its two doors are open all day, so I think there is a fair degree of air mixing with the entire ground floor.

The data is plotted versus the time of day to emphasise daily similarities.

Click for larger version

I have annotated the graph above in the figure below:

Click for larger version

There are several key features:

• The first is that the lowest level of CO2 concentration observed is around 400 parts per million (ppm) – which is the approximate concentration in external air. This probably corresponds to a time in which both front and back doors were open.
• The second is that overnight, the concentration falls to a steady level of between 400 and 500 ppm. The rate of fall corresponds to between 0.5 and 1 ACH.
• The third is the rapid rise in concentration and high levels of CO2 (up to 1500 ppm) associated with cooking with gas.
• The fourth is that excluding the cooking ‘events’, the typical CO2 concentration typically lies in the range 400 to 600 ppm. With typically 3 or 4 adults in the house, this is consistent with between 3 and 4 ACH. During this time the weather was warm and doors were often left open and so this plausibly explains why the air change rate might be higher during the day than the night.

Result#2

The figure below shows the TEMTOP CO2 meter after reading in my bedroom (volume 51 cubic metres) with the door and windows closed on two consecutive nights.

It can be seen that the CO2 concentration has risen steadily and then stabilised at around 1900 ppm. With two people sleeping this corresponds to an air change rate of around 0.5 ACH.

What next for Michelhaus?

The data indicate that:

• For our bedroom, probably more airflow would be beneficial.
• For the bulk of the house, more airflow might be required in winter when doors and windows will likely remain closed.

So it seems that some degree of mechanical ventilation with heat recovery will likely be required. I will study the matter further over the winter.

What is empowering about the CO2 monitoring technique is that I now have a simple tool that allows me to estimate – rather than merely guess – the number of air changes per hour.

Tags: Passivhaus