What’s wrong with a Passive House?

Why aren’t passive houses mainstream? Why isn’t every new home built in the UK produced to the design specifications of passive houses?

Here I’ll go through what’s bad about passive houses, whether it’s true, and what might be done to fix the problems.

1) Passive houses are boxy and ugly

Passive House in California

Passive houses are designed with “Form Factor” in mind during the design phase. The Form Factor is, honestly, how boxy the house is, as it relates to the total surface area of all of the external walls (the Heat Loss Area) divided by the total floor area. A lower number means there’s less surface area for heat to escape. A normal detached house is about 3, which is good.

The rule is: halve the Form Factor and you can halve the insulation thickness to get the same output.

Here’s an example of an apartment block at 1.75.

A block of flats in London to demonstrate the concept of Form Factors

1.75 is a great form factor – passive house people love that low number, even though it’s an ugly building. Read about how to calculate the Passive House Form Factor here. Now look at this building:

Stunning cubist architecture on the St Lawrence river in Montreal

Stunning cubist architecture on the St Lawrence river in Montreal

It’s got more surface area for heat to escape from (so it will require over twice as much insulation thickness as the block of flats).

As such, Passive House designers will hate it, even though it’s aesthetically beautiful.

Although Passive Houses have to obey stringent design specifications to achieve their energy efficiencies, they can still be beautiful, as this one shows:

2) You can’t open the windows in a passive house

This myth is born from the super-high levels of airtightness that passive houses need in order to retain heat that is lost through draughts in normal homes. Passive houses also use Mechanical Ventilation Heat Recovery Exchangers for ventilation, further fuelling the myth that opening the windows will break them.

Of course you can open the windows in a passive house. Firstly it’s a requirement in all modern homes for windows to be a form of escape in a fire, so they legally have to open. But more than that, sometimes you just want a nice breeze coming in through the house on a hot day.

More importantly, with passive houses the MVHR unit guarantees clean, fresh air in the home without opening the windows – this is great during cold days, when the last thing you want to do is open a window to let cold air in.

3) Passive houses are more expensive than normal houses

These days the homes we see built to passive house standards are only five to ten per cent more expensive than Building Regulations homes (which are the worst-performing buildings you can legally build, remember).

If a Passive House costs an extra $30,000 on your budget initially then you’ll make back the savings within about ten years – and you’ll have lived all of that time in supreme comfort. Past ten years and you’re actually saving money from the cost of the house – a great feeling.

4) Passive houses are airtight, so they’ll get stuffy and moist

The MVHR units in all passive houses guarantee the supply of clean, filtered and warm air (even in winter) at the levels all humans need to be healthy and happy (30 cubic litres an hour). MVHR units remove pollutants, bad smells and moisture from the air automatically. They are super-cheap to run and very quiet, too.

Compare this to normal UK homes which rely on noisy, greasy ventilation fans in the bathroom and kitchen and trickle vents in the window frames. They make air stale and stop moist air from escaping your property, which causes mould and health problems.

Here is the physics behind how Passive House clean air ventilation systems work.

5) No-one can get the MVHR systems to work properly

This is true. Camden Council has failed in their flagship passive house projects to correctly monitor the installation and commissioning of their MVHR units. There are many other examples of MVHR units not working properly.

It is crucial that before any MVHR system is installed it is designed to consider occupancy habits, moisture levels indoors, necessary airflow rates, pressure drops, air resistance and acoustics.

Once it’s designed it is crucial it is installed by HVAC technicians that understand how MVHR units work. The air will leak if they crush the ductwork, use glue to connect the ducting or bend the ducting too much.

Properly installed, MVHR unit will match the design and it will work properly. Hundreds of thousands of MVHR units work perfectly on the European continent. We have problems with MVHR systems here in Passive House flagship problems in the UK. It’s not a problem with the technology, but how it’s installed…

6) There are no builders or architects skilled in passive houses in the UK

There are a handful of really good passive house builders and architects in London, a few more in the north and Scotland.

There are also lots and lots of builders and architects in the UK that tell you they know everything about low energy design and that “passive home thing”. They either greenwash the project or they discredit it and tell you they prefer their way of doing it. And that’s why we have 80 years of leaky, cold homes and high energy bills in the UK.

7) Passive houses don’t suit the UK climate

Passive house in Scotland

In winter the MVHR system will keep the internal temperatures of a Passive House to a warm and comfortable 20DegC with ease. In summer, as long as overheating calculations have been made and there’s not too much south-facing glazing, a passive house in the UK summer will stay cool and comfortable.

8) Passive houses don’t raise the price of the house when it’s re-sold

As the eco movement grows it’s likely that people will start to target homes that have good airtightness, proper insulation and are built solidly – as all passive houses are.

One of the reasons that people don’t buy passive houses in the UK is because the people that build them usually live in them for the rest of their lives.

In the UK we’ve been conditioned to believe that spending thousands of pounds a year on heating and electricity is normal. Passive houses can break this trend.

9) Passive houses are too complicated to build

Builders and architects might convince you that passive houses are too complicated to build. They require prior design on areas such as insulations levels, airtightness barriers, windproof membranes and windows positioning and potential overheating issues in the summer.

But shouldn’t your architect and builder be considering this anyway? Isn’t that what you want from a professional home builder; an idea that they’ve considered all the potential issues that might arise when you live in the home.

Passive House design provides a natural framework for self-builders throughout the design and build process. The level of detail in the design process follows naturally into the construction period – airtightness strategy is mapped and identified, wall thicknesses and services penetrations are already finalised.

On Grand Designs, most of the projects break the budget when they over-run. With a passive house design you map out all of the dependencies ahead of time, so the construction process is smoother – and cheaper.

Tiny eco home in Spain

Patrick

4 Comments

  1. You’ve mentioned passive houses in the UK and how it helps with heating and electricity bills over the long run – giving the impression that it is only suitable for buildings in colder climates. What about tropical climates?

    • That’s an interesting question, Michelle, thank you.

      Passive Houses in hot countries – for example, Malaysia – would work in the same fundamental way. High levels of insulation and airtightness would keep cool air inside the home, much like an ice box you might use to store cans of Coca-Cola during a picnic in the park.

      An air-source heat pump could be reversed to become an air-conditioning unit delivering cooling air to the home, and an MVHR unit can provide fresh, filtered air into the property with a cooling coil to make sure you have a healthy supply of air. The MVHR system could also monitor moisture levels in the home to ensure it doesn’t get too sticky. You would also need a significant amount of external shading over your windows to ensure the solar gains weren’t too strong.

      In a hot country such as Malaysia or China with poor air quality, Passive Houses could be very successful, especially as they filter the air that’s coming into the home so it’s purified and clean.

      There are examples of warm-climate passive houses in the south of France. If you would like to visit one please let me know and I will send you the details.

  2. I have built a passive house. It was not expensive and was easy to build. The right architect and there are more of them that can do this are out there.

    Pros
    My house is incredibly warm. You will not need radiators. In fact I only have downstairs UFH that I hardly turn on.
    Cons
    It can be hot in summer. I do have MVHR but this will not cool your house in summer. To be fair I do have a lot of south facing windows and solar gain. The house stores the heat. You need to think about the total design of your house. If you live in the south then do think about how your house will fair all season round and get energy calcs. forget about needing fancy wood burners. You will never use it in a passive house. I have the windows open in winter. I live in Surrey

    • Hi Jane,

      It’s good to hear that your Passivhaus build was straightforward and not expensive – it goes to show that a low energy home can be built without a premium.

      In the UK the focus during the initial planning of the home can be on the heating season, but as you say, we need to also consider the risk of overheating in summer (especially with the 38 DegC days). This is especially so for a home if there’s lot of south-facing glazing.

      The key tactic to preventing summer overheating is also the simplest and cheapest to implement – don’t let the sun hit the windows in the first place! We can do that by fitting external shading, canopies or parasols, or having deeper inset windows into the reveals. Trees also help a lot in shading and cooling the home, and if they’re deciduous you’ll still get the benefit of solar gains in winter when the leaves fall off.

      The next Passivhaus strategy to mitigate summer overheating is passive ventilation – opening windows on each side of the home to allow air to flow through and purge the warm air. The problem here is that hot days tend to be very still, so there’s not much wind about, plus it means that the indoor temperature will typically match the outdoor temperature.

      Another thing to consider is thermal mass, particularly with basements. The ground tends to be quite cool a metre or two down, so basements will always be cooler in summer than above-ground floors. That can allow respite from the heat on the hottest summer days.

      Anyway, I’m really glad to hear you’re enjoying your home, and I agree that fancy wood burners are totally unnecessary in a Passivhaus. I’ve always thought a fire pit in the garden is a good alternative, as you can use it all year round pretty much.

      Best wishes,

      Patrick

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