One of my friends remarked today that their colleague was asking about that “German eco home, the one where you can’t open the windows…”
The idea that you can’t open the windows in a Passive House comes from a misconception regarding the clever way that Passive Houses regulate the heating and cooling of a home. Passive Houses are built “airtight”, which means they have no uncontrolled air draughts in the windows, through the walls or under the floorboards. This is crucial in retaining heat in the winter months and keeping the Passive House cool in the summer.
Passive Houses are airtight buildings with lower than 0.6 air changes per hour (ACH) at a simulated pressure of 50 Pascals, which means that every two hours the entire volume of air will change just once in a strong breeze. UK homes built to current building regulations are at 10 ACH per hour at 50 Pascals, which means every six minutes a strong breeze will push all of the warm air completely out of the home.
The airtightness of your home is important because every time you heat the air in your home via your radiators, underfloor heating or fireplace you risk losing it within six minutes. So you have to heat it all over again six minutes later. This is why new-build homes that meet UK building regulations never feel truly warm for long and have expensive heating bills.
So aren’t Passive Houses stuffy?
If the home is almost completely airtight then surely Passive Houses are stuffy and smelly, right?
Well, no, Passive Houses use a ventilation system called a Mechanical Ventilation and Heat Recovery (MVHR) system to deliver fresh, filtered air from outside into each of the rooms in a controlled way.
How do MVHR systems in Passive Houses work?
- MVHR systems extract the warm, moist air created by family activities like cooking and cleaning from bathrooms and kitchens.
- They take this warm, moist air to the heat exchanger (in the loft, basement or utility room), where it passes over tiny pipes that allow the heat to transfer to fresh, filtered cold air brought in from outside. No air is mixed.
- The fresh, filtered air is now warmed, and the extract air leaves the home
- This fresh, warm, filtered air is supplied to the living areas like bedrooms and lounges
The MVHR system monitors for healthy temperature and humidity levels around the house to make sure that the rooms aren’t too humid or dry. The temperature is normally set at 20DegC in a Passive House, as it’s considered the optimal temperature for human comfort.
The best thing about an MVHR system is that because it is recovering heat that comes from cooking and washing it’s basically free to run. Passive Houses have heating bills that are ten per cent of traditional UK homes. The system, installed correctly, is whisper-quiet and super-cheap to run – the cost of two LED light bulbs in fan power.
One apartment that my consultancy converted to EnerPHit standards is expected to save £500 a year in heating bills for the next fifty years because of its MVHR system (it just won an energy efficiency award, too).
Can you open the windows in a PassivHaus?
Of course you can open the windows in a Passive House, but the question should be:
Why do we open our windows at all? For ventilation…
- If you live in London or another major city, or even if you live in the countryside and hate the smell of manure, you might keep the windows closed to keep out the smog or the stink.
- If you live on or near a busy road, or near a school or high street, you may not want to open your windows because it’s noisy.
- If you live in a crime-risk area you don’t want to leave your windows open if you’re not in the house.
- If you have hay fever or allergies you don’t want the window open in the summer months.
- In the winter months you don’t ever want to open your windows because it’s cold and all the cold air and rain would blow in.
- If you have children and you don’t want them to fall out of the window, you keep windows closed.
With a Passive House the MVHR system provides warm, clean and fresh air to your home all year round, so if you have any of these problems you don’t ever have to open your windows and you’ll still have fresh, filtered air. MVHR guarantees good quality ventilation with the windows closed.
In fact, Passive Houses have better air quality in places like London and New York because they have surgery-grade cleaning filters on their ventilation intake ducts, so the air inside is super-clean.
So why would you want to open your windows at home?
- To clean them
- To purge the air after you burn your food
- To purge the air after a party
- To let cigarette smoke out
- To make your escape during a fire (this is Building Regs for ALL homes, by the way)
- To sell beers from if you live in Notting Hill during Carnival
In a normal home you can open your windows and doors whenever you want, and in a Passive House you can open your windows and doors whenever you want, too! But purging the air in a Passive House can be done even more easily than by opening the windows – you just turn up the fan in the MVHR for 30 seconds and the air’s fresh again. That’s important if air quality, safety or time is an issue.
So PassivHaus gives you more choice in your life, and let’s you decide when you want to open your windows – if at all.
Please contact me if you have any other questions about Passivhaus, and I’ll be happy to help, or if you have a build project that you’d like MVHR or building airtightness guidance for.
Hey Patrick, I’m super interested in Passive House design. What brought me to your page was that having windows/doors open in the summer can be really nice for ‘inside/outside’ living during the summer – would there be a way to accommodate this? Possibly ‘thermally isolating’ one room that has large sliding doors or similar? Thanks in advance.
If I follow you correctly, you’re thinking of the impact that large sliding doors (nice to have in summer) would have on the winter heat losses (because large glazing area)?
So you’d want to further insulate the interior walls of the room with large sliding doors to stop this cold from getting to the rest of the room? That wouldn’t be necessary in a Passivhaus. Instead, the normal strategy is to use the heat load requirements for that specific room with the large sliding doors (which are available in the PHPP software) and then mitigate accordingly as necessary – normally with localised heating (eg, a wall radiator or underfloor heating if solar gains weren’t enough). If the sliding doors are triple-glazed then the internal surface temperature won’t drop below 13 DegC anyway, so there’s little risk of condensation or cold spots.
Did I answer your question correctly? Let me know if not – patrick [a] heatspaceandlight.com
Hello, I’m hoping to restore an old house to a PassiveHouse and I’m wondering if i have sliding doors open occasionally will it increase the cost of my heating bills.
If you heat your home but you leave the sliding doors open on a cold day then yes, the heat will escape and you’ll have to reheat the home again. It doesn’t matter if it’s a Passivhaus or a normal home. If you keep the doors closed on a winter’s day then you’ll keep the heat in the home for longer.
In summer you can keep the doors open for as long as you want, with no issues in a Passivhaus.
Renovating a home to become a Passivhaus is exceptionally difficult. The equivalent for retrofit scenarios is EnerPHit, which has slightly more manageable targets to reach in terms of energy efficiency, airtightness and heating system. Please contact me on Patrick[@]heatspaceandlight.com if you’d like to get your project started.
Hi, first of all great article! Really helped me gain more understanding of MVHR systems.
So, how does a PassivHaus work if its “seamlessly” integrated with a green house, living with nature sort of scenario..?
Glad the info has been of help. A green house, being glass, will heat up significantly in summer (as it is designed to do). In winter, given the heat performance of even the highest quality glazing, it will be cool or cold. Therefore it shouldn’t link to the home at all, as it will disrupt the comfort requirements of the home seasonally – as will orangeries and sun rooms. It should be a standalone place of nature in the garden, much like rather than an open fireplace in the home, a firepit is better in the garden.
Hope this helps, I’ve written about low energy building design for passive cooling in summer here: How to design and build a new home in 2021 which stays cool in summer
Ah that makes sense. Thank you