The temperature is ‘just right’ at Woodlands

When it comes to indoor temperature, you’ll be warm and comfortable all year round at Woodlands. We think of our passive house in terms of the Goldilocks principle: it’s not too hot and it’s not too cold; it’s ‘just right’.

Checking the temperature all the time

We’ve installed high-accuracy data loggers to record the temperatures inside the house and outdoors recording them every 30 minutes. The graph below shows the indoor temperature (red) in Woodlands and the outdoor temperature (blue) from when we started recording on 4 May until the change from British Summer Time on 25 October 2020 when the data system started a new record.

We’ll look at this in more detail below, but the immediate thing to notice is that despite some pretty cold outdoor temperatures (the lowest was 3.75°C in May) and some pretty hot ones (the highest was 33.09°C in August) the indoor temperature has almost always been between 22 and 24°C dropping to 20-21°C during the night, with just a few exceptions which we discuss below. The other thing, not reflected in the temperature chart, is how the temperature is much the same throughout the house; it feels comfortable – ‘just right’ – everywhere just about all of the time.

Temperature record from the 4th May 2020

Experiences of people in Passivhaus buildings elsewhere

Other people living in passive houses confirm the comfort of living in a Passivhaus. For example, in this video, the owners of the Denby Dale Passivhaus describe their indoor temperature being 20-21°C all the time; and in this video, a Passivhaus owner describes the ‘gentle radiated heat from all surfaces’ and how on coming inside on a really cold winter’s day ‘a grin comes over [your] face’ because of this. We get similar grins for the same reason. And in this video, you can hear the experiences of people living in, or using, Passivhaus buildings that aren’t individual houses.

Looking at the temperatures in more detail

The next graph looks in more detail at what happened between 18 June and 18 August 2020 when we experienced some of the hottest days of the year and the outdoor temperature peaked at 31.57°C on 12 August. The indoor temperature dropped to its lowest at 19°C at 05.43 on 21 July (it was 9.58°C outdoors at that time) but generally it stayed about 20-24°C. It only briefly exceeded 25°C on six occasions during this two-month period. On three of these, we were away from the house and thus not able to open the windows until much later in the evening.

This highlights two important points: (i) even in very hot weather, it stays cool inside a certified passive house (the standard is that it should not exceed 25°C on more than 10% of occasions throughout the year – our design standard estimated 3%), and (ii) you do need to manage the indoor temperature in the warmer months.

On hot days such as these in previous homes, we would have had fans on all night in the bedroom and slept badly. In our passive house, we have not used any fans and have slept well even on the hottest of nights.

Temperature check for the 18th June 2020

Managing the indoor temperature

Managing the indoor temperature is simple – in the summer you use outdoor blinds or other shadings over south-facing windows to keep out the hottest rays of the sun, keep windows closed during the day, and open downstairs and upstairs windows in the evening to let out some heat (this is called Mediterranean purging).

The picture below shows the two blinds extended over the balconies which themselves provide shading for the two downstairs rooms. There are also blinds on the two roof windows (they are not closed in this picture). The effect of the balcony blinds can be seen clearly: the floors of the two upstairs rooms immediately behind the windows are in shadow. The blinds contribute substantially to keeping the indoor temperature down when the sun is shining very brightly in mid-summer.

How warm is it in the colder months?

It’s been colder recently. The graph below shows the indoor (red) and outdoor (blue) temperatures between 25 October and 29 November.

Despite the outdoor temperature dropping to zero on 4 November and the outdoor temperature being more autumnal throughout, you can see that the indoor temperature has stayed remarkably constant. And our heating has only come on in recent days (you can see that it got a bit colder indoors in the last week of November because of a period of sun-free days) and, after an initial day, it is only on for an hour or so, if at all, each day.

Even in the coldest months, we still only use summer-weight dressing gowns and we use the same lightweight Chinese silk duvet all-year round as well.

It is slightly cooler indoors in the winter but a lightweight jumper is all that’s needed to feel comfortable in all parts of the house, including being up close to the windows even on the coldest of days. The thick jumpers that we used indoors in previous houses are now only required when leaving the house.

We’ll write a further blog when we’ve collected temperature data throughout the autumn and winter.

The house keeps the heat ‘just right’

There’s another thing: there’s no need to close internal doors to ‘keep the heat in’ because the temperature is remarkably even throughout the house.

This evenness of gentle warmth everywhere is because of five things:

the high level of insulation enveloping the house (including under the floor) reduces heat loss
the mechanical ventilation heat recovery system extracts 90% of the heat from the air going out of the house and warms the constant flow of fresh air coming in
the high degree of airtightness not only stops heat loss through the building fabric but also prevents draughts
the inner pane of the triple glazing is at a similar temperature to the inside of the room so there are no convection currents causing cold air to fall to the floor in front of the windows (which is also a cause of draughts)
on the rare occasions, the underfloor heating comes on it‘s at a much lower temperature than conventional central heating radiators and so gently warms the house without creating a ‘layering’ of heat that’s cooler at floor level and warmer at ceiling level

Is there a problem living in a Passivhaus?

Maybe just one. It’s very difficult to tell what the temperature is outside and thus what you need to wear without checking an external thermometer or sticking your nose outdoors. You feel so warm and comfortable indoors that it’s difficult to believe that it’s very cold or very hot outside because it’s ‘just right’ indoors all the time. Goldilocks would be very happy.