What’s a Passive House?

We get asked this a lot when we tell people we’re building a passive house, and that’s fair enough as it’s not yet common in Australia. Passive House is best explained by the experts which I’ll link to at the end of this post. We’ve had to create an explanation that works for us when talking to friends and family. People often think we’re talking about passive solar design, which is also an awesome set of design principles, and although it shares some similarities it’s quite different to passive house.

I normally say something like this: Passive house a set of design principles to guide creating a high performing energy efficient home. It should mean that we hardly have to use any mechanical heating and cooling to keep the house comfortable all year round (keeping energy use and bills low). The principles are based on preventing hot air coming inside the house during summer and stopping cold air coming inside during winter; this is achieved through the use appropriate insulation, super air-tightness and preventing thermal bridges. It’s very much about building a home suited to the local climate and the particular site and not specific checklist of materials and technology. We should end up with a very comfortable home in terms of temperature and a healthy home in terms of indoor air quality.

So far most of our family and friends have been both supportive and fascinated by what we want to achieve. My explanation of passive house sparks further conversation generally around the need for air-tightness in a passive house, explaining a thermal bridge, a little about insulation, windows and air quality. 

Tighty tight tight!
Passive house is a lot about air-tightness. Most regular house are not airtight – you don’t necessarily see air gaps, but they are there in spaces around windows, places were services have been brought inside from outside, where the roof meets the walls, under doors etc. All these little gaps allow hot air inside during summer and cold air in during winter. They also mean all the cool air you create with your air con slips outside and means your AC has to work harder.Many regular houses have about 15-20 air changes per house, where as a passive house aims for 0.6 (link).

How do you make it air tight?
Air tightness is achieved in a passive house by two main things:

  1. Wrapping the house in an airtight membrane that lets moisture out, but doesn’t let air through. It’s often describes as a Gortex jacket for your house. All the edges of this membrane are then taped. If you follow any passive houses on Instagram the wrapping of the building is a key and proud moment in construction.
  2. Attention to detail! The trades people on site need to be fastidious about sealing up all gaps and holes even the teeny tiny ones. Occupiers then need to be fastidious about not creating new holes as they live in the house.  Careful how you put your pictures up!

What is a thermal bridge?
As far as PassiveHouse is concerned, its a bridge too far 🙂
It is any part of the building that directly connects the outside of the building that is able to transfer heat (or cold), such as a window frame, beam, or a piece of glass, or even where services come into the house!To build a passive house all thermal bridges must either be broken or designed out.  An example from our case is the original house design had a supporting steel beam over the carport that with the engineers approval have replaced with laminated beams, usually called LVL or GluLam.Windows are also a classic thermal bridge and mean passive houses need double or triple glazed windows and thermally broken (in the case of Aluminium) or uPVC, or at the high end timber.

So, you can’t open your windows?
We get asked this a lot. It’s normally the first question after we have raved on about air-tightness. Yes, we can open the windows and we will be opening them a lot. With two kids, a dog, a love of sitting outside for beer at the end of the day and living in a beach side suburb we’re hoping for a lot of indoor-outdoor life.  Opening the doors and windows just means the house isn’t ‘performing’ as optimally as it can, but we will be as humans if we’ve got an indoor/outdoor life and a healthy home! The house will return to high performance when required.

Double glazed windows
Double glazed windows are another key component of passive houses.  In some climates triple glazed windows are needed. Regular single glazed windows are huge thermal bridges (see above) and allow heat and cold to transfer in and out of a building.  Double and triple glazed windows are incredible pieces of engineering that mean you still have a window to let in light and views without transferring heat and cold into or out of the building.Operating them can take a bit of getting used to as they open and close differently and feel very different due to that fact that they are intricate pieces of engineering. A bonus of double glazed windows in they also block out noise. Being near a busy road we hope we will enjoy this feature. There is also a benefit from a security perspective as the lock in up to 6 different points around the frame.  Bear in mind that double glazed doesn’t necessarily mean air-tight!  Trap for young players right there and we’ve needed to do our research and specify this.

Appropriate insulation is key to stopping or slowing heat (energy when it comes down to it), moving from outside or inside or vice versa depending on the season.

Fresh clear air
The air-tightness required to keep a passive house thermally comfortable could pose a problem for fresh air inside the house, if it wasn’t for a mechanical heat recovery ventilation system (MHRV). The system allows fresh air to come inside and stale air to go outside without transferring heat, nor does it let dust or pollen inside. I can’t wait! We can have the windows closed and fresh filtered air inside. I’m allergic to a lot of pollen’s – they will stay outside, but my inside air will be fresh. I hope we never have bush fires and smoke haze like we did in summer 2019/2020 ever again, nor another dust storm, but if we do, it won’t get inside our passive house.

Thermal comfort
Apparently a passive house should stay at around 22 degrees all year round without needing additional heating and cooling (link). There might be a few extreme days either hot or cold, but we hope a small split system AC unit will help us out on those days. As Gavin loves to say: A friend of mine once said of Passivehouses in Europe “Fart in Autumn and it keeps you warm until Spring”🙂 It’s not strictly true, as no system is 100% efficient, but suffice to say that the need for heating or cooling  is seriously reduced.  Typically passive houses require 10% of the energy of a Australian Building Standard house.

Further details
My explanation in this post is the over a beer intro version of what a passive house is for someone who hasn’t heard of it before. It’s the very basics, to actually design and build it requires a lot of science – modelling, research and understanding of material properties – it’s all physics. We have engaged a passive house architect to help us work out how to build our house as a passive house; more on this in future posts as the work she is putting into working out how to make sure our house is high performing is enormous and deserves a post of it’s own. In the meantime if you’re interested to know more about passive house check out the Australian Passive House Association or the International Passive House Association.

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