Passivhaus also keeps you cool

Our 15^th Anniversary celebrations have us in a reflective mood. We thought that this would be a great opportunity to revisit some of our most popular posts to see if they were relevant today. With temperatures hotter than Ibiza and Majorca last week, and the mercury creeping up again ahead of this weekends Passivhaus Summer Open days and temperatures, where better to kick off then … “Passivhaus also keeps you cool.”

Published just last year (2022) ahead of the record breaking June heatwave, which saw temperatures in excess of 40 degrees for the first time in the UK. The article focused on the physics behind Passivhaus and that certified buildings are comfortable all year round. Physics doesn’t change too much, so we’ve left the article intact below. However, the article didn’t look at why UK homes can’t cope with heatwaves and what you can do to stay cool while you’re waiting to start your Passivhaus retrofit.

In the UK just 7% of homes were built in the last 10 years, over a third were built prior to 1945, with one in six actually built prior to 1900. The age of the housing stock is a significant contributor to the energy efficiency of homes in the UK (ONS: 2021). Though modern building regulations are doing little to improve this. In the time since most of our homes were built, the frequency, intensity and duration of heatwaves have increased. In the 60’s heatwaves were record temperatures in the 20’s, today they are in the 30’s. Back then they lasted for 5.3 days, compared to today’s average of 13.2 days (Met Office: 2018). Quite simply, most UK homes are ill-equipped to deal with todays heatwaves as they were never designed to do so.

As Passivhaus specialists, it’s not surprising to hear that we believe that this is the gold standard and that we recommend starting with a whole house plan which prevents locking out future improvements. But there are things you can do now to manage this summer, even if you’re not in a position to make changes to the structure of the building. Here are our top tips:

Night purging – once a traditional method of cooling buildings, now boasting multiple technical names. Night purging is simply opening the windows when the outside temperature is cooler than the inside temperature, usually early morning and overnight. Cooling is optimised by cross ventilation, opening windows or doors on different sides of the building. Having high and low openings can also help, utilising the stack effect, so opening windows upstairs and downstairs and leaving doors wedged open, to pull cool air in downstairs and expel it upstairs. Even an hour in the morning before the temperatures begin to rise too sharply can have a significant impact. Night purging can also have the added benefit of improving air quality ( Kilpeläinen:2022).

Shading – Keeping curtains and blinds closed during the day can help to reduce radiant heat, but external shading can be even more effective. Imagine bougainvillea on a Greek Island, when in full flow the plant spreads windows reducing the heat entering the home. It might not be possible to grow a bougainvillea of sufficient size this summer, though it might be possible to reorientate the garden parasol or invest in an awning.

Relocating Tech – We all have a lot of tech, beyond ensuring there are sufficient sockets we rarely consider it’s location, but it’s always there generating heat. With most of us relying on wifi, the location of some of this technology is now less critical. Relocating the tech that can’t be turned off to rooms that are not required could help with heat levels. Switching to more energy efficient models when upgrading will also reduce waste heat. Retiming can reduce your day time peaks – can you do your washing over night?

Our original post from 2022

Raised internal temperatures are not comfortable, plus they are associated with a rise in mortality rates and increased exacerbation of chronic diseases, especially amongst the frail and vulnerable. Quality of sleep can be impaired, and the sense of general wellbeing is decreased.

Due to climate change our summers are becoming warmer, but Passivhaus offers a solution. The standard sets maximum levels that help to maintain comfort from extreme temperatures in the summer, without this a building cannot be certified.

Buildings overheat because of uncontrolled solar heat gains through windows or glazing which rapidly warm a space to uncomfortable temperatures. It’s perceived to be more of an issue in well insulated and airtight buildings where the heat can get easily trapped into the house. We’ve all been in those horrid office blocks where the windows were sealed to stop winter drafts, but there hasn’t been a lot of thought given to summer cooling.

Passivhaus offers a solution to this problem by not only creating an energy efficient home that is warm in winter, but by ensuring the building is also cool in the summer. They offer a consistent, comfortable temperature year round, reducing the risk of overheating.

The standard defines overheating as when the percentage of hours per year that the interior ambient temperature exceed 25°C with a maximum limit of 10%. If the percentage is higher, it is not possible to achieve certification. However, it is generally considered that an upper limit of 3% of hours is more appropriate, particularly important when considering climate change with many places getting warmer.

There are several design measures and strategies that can be implemented to minimise this risk, and we will explore these with you. The options include:

Accurate design of the building aspect, shape and internal layout. This is one of the pillars of bioclimatic design, i.e. a design approach that takes account of the microclimate, geography, and topography of the site to ensure thermal comfort.
Solar radiation control through:
- The use of shading elements. These devices work by blocking solar radiation before they get into the house and can include shutters, brise-soleil or canopies etc.
- The use of vegetation. Deciduous plants can be a good solution as they guarantee shading in the summer, they then also let solar radiation and light in the winter. Also, Climbing plants on the walls can absorb the heat and prevent this from getting transferred inside.
Glazing with low g-values, i.e. the percentage of solar radiation that can enter into the glass can help control overheating. However, these will also reduce the useful solar gains into the house in winter.
Accurate design of the windows and their orientation. Orientation is really important in terms of heat gains and particular attention needs to be paid to the south and west elevations for our hemisphere. Here in the late afternoons in the summer, when the sun gets low and the temperature rises, it might be difficult to find adequate shading. If this is an advantage in the winter months, it might be the cause of overheating in the warmer ones.
Use of cladding materials with light colours. Darker colours attract heat, others have the capability to repel this, so a white render might be a better choice if the risk of overheating is high.
The use of “massive” construction or thermal mass in the building. Massive construction; including heavy masonry, concrete, earth blocks and cob, can have the potential to absorb the excess heat during the day and then release the stored heat slowly when temperatures cool.
Natural cross-ventilation strategies. Cross-ventilation is a very effective means to allow natural cooling in a dwelling. This can be achieved by ensuring the building is designed to have a double-aspect plan. If an increase in height can be achieved between the air in and air out, then the cross ventilation will be enhanced due to the stack effect. Openable roof lights at the top of stairs can be particularly effective.
Reduce the number of devices that might increase internal heating in the house.
Use of artificial cooling. Although we do not intend to encourage this type of cooling, if this is necessary, efficient systems such as air source heat pumps (ASHP) which may be providing the heating can be used in reverse to provide some cooling. If combined with Photo Voltaic panels, this will use excess generation in the summer months and could be a carbon-neutral solution.

As you can see, there is a great deal of potential solutions that can be implemented in existing and new homes to reduce the risk of overheating. The Passivhaus approach and calculation method helps identify and mitigate the risk of overheating. This is a very good tool that allows you to model different solutions and inform the design at the early stages of the project, maximising the performance of the house and avoiding choices that can have a detrimental impact on thermal comfort.

If you are considering a Passivhaus project, get in touch here so we can find out how we can help you with your plans.