Passivhaus Overheating: Design it out

Passivhaus overheating shouldn’t happen: it’s one of the criteria of the international Passivhaus standard. Even so, people sometimes ignore this requirement during the early stages of the design process.

It shouldn’t be ignored. Overheating is a key design issue and should be addressed from the beginning as an integral part of the Passivhaus design process.

But how?

This blog post is a simple guide to preventing Passivhaus overheating by design.

Overheating is a serious issue that has come to prominence in recent times. It is sometimes claimed to be an issue with well-insulated buildings such as those that meet the Passivhaus standard. It is true that some well-insulated buildings do overheat. But then so do some buildings that are not well insulated – such buildings are too cold in winter and too hot in summer.

Overheating can be uncomfortable and result in additional energy being used to air condition a building.

Overheating can also have much more serious consequences that impact on occupants’ health. It can even be fatal if it causes a heart attack, stroke, or sudden infant death.

The international Passivhaus Standard defines overheating as when indoor temperatures exceed 25°C. The Passivhaus Planning Package verification page reports a percentage of hours in a year that exceeds 25°C. For certification purposes, the limit is 10% of the annual hours. However, it is better to aim for below 5% of occupied hours and ideally 0% if possible to ensure the best summer comfort. This is particularly important with climate change in mind as many places will get warmer.

As with many things, prevention is the best cure.

Most importantly: Keep the sun out!

Get the building orientation right

It is well known that to optimise beneficial solar gain it is best to face a building towards the equator. Buildings in the northern hemisphere should face south, and buildings in the southern hemisphere should face north. For simplicity in the rest of this blog post, I will only refer to buildings facing towards the south.

Residential Passivhaus buildings benefit from this optimal orientation. However, it is still possible to meet the Passivhaus standard when this orientation isn’t possible. And for non-residential Passivhaus buildings, it can be even less important. In both cases, this is because the solar gain is only part of the Passivhaus heat balance, Passivhaus is not solely dependent on it.

When it comes to designing out overheating, facing south or close to it is best. In winter, the maximum beneficial solar gains are from the south. In summer, however, the greater risk of overheating is from the lower sun in the east and west. Midday sun coming from the south is very high and therefore presents less risk and is easier to shade against. Most buildings will have some inherent shading from the overhead sun provided by roof overhangs and window reveals. Buildings that face more west or east will have a greater risk of excess solar gain and present more challenges to shade adequately.

Don’t try to heat the building with the windows

Let’s be clear: Passivhaus is not ‘Passivhaus Solar Design’. Solar gain should not be maximised in a Passivhaus building, it should be optimised – that is, carefully balanced so that it is beneficial.

Despite what is sometimes thought, solar heat gain through a window is not literally ‘free’. A window costs more than the wall it is placed within. The UK’s Passivhaus Trust calculates the cost of solar heat gain for south facing windows at around 50p/kWh, based on the window lasting 20 years. In comparison, at the time of writing, the average price of gas in the UK is 4.18p/kWh and the average price of electricity is 13.86p/kWh. (Figures from here, accessed April 2016)

Windows have other functions besides letting in heat from the sun. Windows also provide daylight, ventilation, views, and make up an important part of the external appearance of a building.

Windows in a Passivhaus building should be designed with all these functions in mind, not with the sole intent of heating the building.

Stick to a sensible glazing ratio

Windows let heat in and let heat out. Windows are a weak point in the thermal envelope. Well-insulated walls and roofs, on the other hand, keep heat in and equally keep heat out.

The more windows in a Passivhaus building, the greater the heat loss and heat gain they will contribute. Increased heat loss from windows can be balanced by increasing the insulation performance or thickness in the walls, the floor and the roof. The consequential increased heat gain in summer can be managed by shading.

However, this is a slippery slope away from a cost-effective design. Increased glazing brings increased costs and requires additional shading which in turn increases costs even further.

The UK’s Passivhaus Trust recommends aiming for windows in a south-facing wall to be a maximum of 25% of the external wall area. This should be adequate for daylighting except in the case of very deep rooms. Windows in other walls should be considerably less than 25% of the relevant external wall area and designed around views, daylight, ventilation, and aesthetics, as per the previous point.

The UK’s Passivhaus Trust also offers as a rule of thumb that glazing (excluding frames) should be around 15 – 20% of the Treated Floor Area of a Passivhaus building, as a starting point.

Aim for rooflights to be 10% or less than the room floor area.

Shade the glass

If the windows are not adequately shaded by the roof overhang and the window reveal, external shading needs to be included in the design. This could be because the building has no roof overhang by design, or it could be due to the size of the glazing.

Floor level glazing needs particular attention as the lower portion doesn’t contribute any useful daylight but will contribute solar heat gain. The lower portion of the glass is unlikely to be shaded by a roof overhang or the window reveal but will need to be shaded.

External shading can be brise soliel, canopies, balconies (free-standing is best to avoid structural thermal bridging or expensive details), shutters or blinds.

The cultural conditions of the shading design need careful consideration. Some people are perfectly familiar with closing external shutters and operating external blinds, for other people it is an entirely alien concept. Shutters or blinds will also restrict daylight and views. For external shading to help prevent overheating, it needs to be something that will work and will be used by the occupants.

Internal blinds and shutters will provide some shading, but should not be considered as the main source of shading. Once the heat from the sun has already passed through the window it is in the building and internal shading won’t reduce it very much.

But also: Control Internal Heat Gains

Account for realistic occupancy

The Passivhaus Planning Package uses a set level of occupancy to calculate the internal heat gains from the people. This is for the purpose of calculating the specific heat demand. Clearly, it is important not to design a building that is dependent on having a lot of people in it to stay warm. Also, occupancy might change over time so the PHPP takes a robust approach.

When it comes to summer overheating risk, though, a more accurate picture of the likely occupancy should be taken into account if it varies from the expected norm. An accurate number of people and how often they are in the building can be entered. If there are particular activities that give off a lot of heat these can also be accounted for. For example, frequent catering for large groups such as extended families. This can be more important with small residential buildings and non-residential buildings where the ratio of people to floor area can be higher than typical. Obviously having more people in the building contributes more heat and this needs to be known to check the overheating risk.

Use only energy efficient appliances and fittings

It goes without saying, that when designing an energy efficient building, low-energy appliances and fittings should be selected.

This is also important for internal heat gains, which in turn could contribute to an overheating risk.

Inefficient appliances and fittings tend to give off more heat as well as consuming more energy. And, if the appliances and fittings are large, or there are more than an average number of them, they can contribute to increasing internal heat gains.

Inefficient lighting, such as Halogen fittings, should be avoided for the same reasons.

Minimise building services heat loss: it’s also internal heat gain

Hot water systems often use more energy in a Passivhaus building than the heating system. Additionally, the hot water system runs all year round and can, therefore, contribute internal heat gains in summer. For these reasons, the design of the hot water system is important and should be started early as part of the integrated Passivhaus design process. This includes entering the hot water system design into the PHPP so it is part of the iterative design process. It is important that the internal heat gains are accounted for at the time when windows are being sized and any overheating risk is checked.

Obviously, hot water distribution pipes need to be insulated so the water is still hot when it arrives at the other end. This includes junctions and valves etc along the pipe, not just the straight runs that are easy to insulate. In larger systems, for example, communal heating in an apartment block, this is even more important.

If the design includes a hot water cylinder it should be located centrally to all the hot water outlets. This will allow for short pipe runs from the cylinder to the taps and potentially smaller pipe diameters also. It can also help eliminate the need for a pump to distribute the hot water. Similarly, the boiler or heating source needs to be close to the cylinder.

The pipes between the cylinder and the taps will be full of hot water when the taps are turned off. The hot water in the pipes will cool down (regardless of insulation) until a tap is used again. The shorter the pipe and the smaller the diameter of the pipe, the smaller the volume of water it contains. Therefore, the system will be more efficient as there will be less heat loss from the pipes. And, equally importantly, this means less heat is given off from the pipes into the building which contributes to internal heat gains.

Similarly, if the heating is provided via the ventilation, the ducts need to be insulated fully so that the heat is delivered to the intended room. If it’s not, it is heating other parts of the building and the occupants might turn the heating up in an attempt to get enough heat where they actually want it. This will not only use more energy but will increase the internal heat gains.

And finally: Remove excess heat

Mechanical Ventilation needs a summer bypass

In most cases, mechanical ventilation with heat recovery (MVHR) will be needed in a Passivhaus building for comfort and for energy efficiency.

Most MVHR units come with some form of summer bypass, either manual or automatic. The bypass allows the air flows to pass through the system without exchanging heat. When outdoor air temperatures are suitable – above 16°C and below 25°C – then fresh air from the outside can help keep it cool indoors. Realistically, in most cases around 20% heat exchange still takes place, though. It’s not a complete bypass as the air is still going through the unit which itself will have gained some of the heat from the outgoing air.

If the air is hotter outside than inside, the heat exchanger should be used, though, and not bypassed. In this case, the heat exchange will transfer heat from the incoming air to the outgoing air, cooling the air as it enters the building.

You need openable windows in a Passivhaus

Yes, it is true. Passivhaus buildings should have openable windows in all habitable rooms. And you can open them, despite some misconceptions to the contrary.

MVHR is mainly for background ventilation and moves air very slowly and quietly. For this reason, MVHR can only remove a limited amount of heat. More air can be moved, and therefore more heat, by running the MVHR on boost. However, the boost setting isn’t intended for long periods as it uses more energy and the fans will be giving off some heat also.

In contrast, opening windows and allowing for wind-driven cross-ventilation can rapidly remove a lot of warm indoor air and replace it with cooler outside air. And it doesn’t consume any energy to do so. In summer, this is sometimes suitable for the daytime, but more often it is suitable in the evening, overnight or in the early morning.

Indoor air can be very warm from a whole day of the building being in use, combined with solar heat gain. Even if the building isn’t hotter than 25°C it might be relatively unpleasant. As the evening starts to cool, opening windows can rapidly purge out the warm indoor air and cool down the internal surfaces.

If security, air, and noise pollution aren’t issues, windows can be left partially open overnight to cool the building ready for a fresh start in the morning. This can be particularly useful in non-residential Passivhaus buildings like a school.

Some Thermal Mass is helpful

Thermal mass is not as relevant in Passivhaus buildings as sometimes thought. This is because the airtight thermal envelope provides significant thermal inertia that ensures indoor temperatures remain very stable.

However, thermal mass does have a useful role to play. Thermal mass does very little, in most climates, to reduce the specific heating demand. What it does to, is help reduce the daily temperature swing in a Passivhaus building in summer.

The amount of thermal mass needed isn’t significant, though, and can easily be provided in what is typically considered a lightweight building. For example, increased density wall linings and/or an exposed concrete floor slab can provide enough beneficial thermal mass in a timber frame Passivhaus building.

Incorporating useful thermal mass can help reduce the peak summer temperature but it might not reduce the total number of hours above 25°C. And for it to be effective, there needs to be a way for the heat to be released from the mass overnight. This could be through night ventilation, for example. If the heat isn’t released, the next day the thermal mass will absorb more heat and start acting as a heater, emitting or radiating heat, and contributing to the overheating risk.

In a hot climate active cooling might be needed

Active cooling just doesn’t seem very ‘Passive’, does it?

However, much as the international Passivhaus Standard is not a zero-heating standard, it is also not a zero-cooling standard. Passivhaus reduces the heating demand to a practical and economic minimum, whilst preserving the highest of comfort standards for people to enjoy and be healthy. Cooling is approached the same.

It is unlikely that active cooling would be needed for a Passivhaus building in many of the climates around the world. In very warm climates such as parts of Australia, or Indonesia, though, active cooling could be needed. In this case, the limit is the same as for heating demand: 15kWh/m2.a.

Passivhaus Overheating: eliminate it by integrated design.

Eliminating or minimising overheating risk is a vital aspect of designing any building, Passivhaus or not.

Excessive solar heat gain is the main cause of overheating risk in a Passivhaus building. This is easily managed in the design process. The risk of overheating can be eliminated by design. Solar heat gains should only make up approximately a third of the annual heat balance – typically close to, or less than, 15kWh/m2.a for a residential Passivhaus building.

Once solar heat gain is under control, internal heat gains should be carefully addressed and then the means of removing excess heat from the building.

The UK-based Passivhaus consultants and building certifiers, Warm, offer some advice on how to stress-test a design against overheating risk. They suggest testing the following changes in the PHPP and seeing what effect they have on the reported frequency of overheating:

  • Minimum user operated summer shading
  • MVHR operating in summer at its background rate (with summer bypass on)
  • No natural ventilation during the day
  • Only 0.1ach night ventilation

If these changes cause the frequency of overheating to increase more than a few percent then the design probably isn’t robust enough. It is likely to be too reliant on occupant behaviour. This leaves the design vulnerable to overheating if the occupants don’t behave as expected.

Finally, it should be noted that the PHPP assesses the frequency of overheating for the entire space within the building envelope. This is normally suitable for a single residential building. For buildings with multiple residential units, or non-residential buildings, this might not be granular enough.

For example, in a Passivhaus apartment block, units facing south-west or south-east will have much higher risk of overheating than apartments facing mostly south or north. In cases like this, the overheating risk needs to be carefully and accurately modelled in suitable software other than the PHPP.

Good robust Passivhaus design should eliminate the risk of overheating, as should all good design, Passivhaus or not.

Elrond Burrell

Passivhaus Overheating: Design it out [Online] Available at: < http://elrondburrell.com/blog/passivhaus-overheating-design/ > [Accessed 20 December 2016]