Sep 27, 2015: Build Tight, Vent Right

I got a nice email this week from the Alternative Technology Association (ATA) thanking us for participating in Sustainable House Day (which they organised).

‘A big thank you to everyone who turned out for Sustainable House Day on September 13! It was a wonderful day with over 15,000 visitors at the 150 homes and gardens. Initial survey feedback shows that the day has had an impact, with over 30% of attendees having already taken action to make their homes more sustainable.’

One of the people who came to the Greeny Flat on the day also sent me a link to this interesting article about the need for a mandatory air-tightness standard for buildings in Australia. The article makes some very valid points about the draftiness of Australian buildings, including the following:

‘There is a definite need to update Australian mentality around air leakage, as we are stuck in the last century. I am disappointed by how often in Australia I’ve heard the old adage, “Buildings need to breathe.” This is not true.

People need to breathe, and buildings need to control moisture. We need to replace the old sayings with “build tight and ventilate right”.’

Build Tight, Vent Right

Regular readers or our Newsletter and website will be familiar with this adage as I use it often. It succinctly sums up the two important criteria for creating buildings that are not only energy efficient but also safe, healthy, comfortable, and durable.

‘Built Tight’ means excellent air-sealing and insulation.  These are the obvious things to do to make a building energy efficient. Clearly a drafty building with little or no insulation is likely to require a LOT of energy to heat and cool. What many people (including most architects and builders in Australia) don’t realise is that these two have to work together. You can have all the insulation in the world but if your building is drafty the insulation won’t do any good. (Imagine being out in a blizzard wrapped in a thick woolen blanket with lots of holes in it… it’s not going to keep you warm). Similarly, the best air-sealing is no good without insulation (image being in a blizzard wrapped only in a tightly sealed garbage bag… it won’t keep you warm either and you’ll likely die of asphyxiation before you die of cold). So you need BOTH good insulation and good air-sealing for energy efficiency.

Testing the air-tightness of the Greeny Flat with a Blower Door Test

Testing the air-tightness of the Greeny Flat with a Blower Door Test

BUT (and it’s a big but) we have to remember to ‘Vent Right’ as well. In a well-sealed building it becomes essential to provide adequate ventilation in order to control humidity, reduce the risk of mould, provide fresh air and remove pollutants and odours. I go into this in more detail in our section on ventilation which is one of a set of principles that make for excellent Passive Solar Design. The point I want to make here is that, if you only focus on air-sealing and insulation without providing good ventilation, you can easily create an indoor environment that is unsafe, unhealthy, uncomfortable and quite possibly toxic. This is what has come to be known as ‘Sick Building Syndrome‘.

Which brings me back to the problem I have with the article mentioned above… it’s all very well to advocate for an air-tightness standard for Australian buildings and for mandatory air-tightness testing, but ONLY if we advocate for a mandatory ventilation standard as well.

When I was living in Montana a few years ago the state government passed a law requiring that all new residential buildings had to be Blower Door Tested for air-tightness and could not exceed 4 ACH50. (ACH50 is a common measurement of building air-tightness and stands for ‘Air Changes Per House at Minus 50 Pascals’. An extremely air-tight building might have an ACH50 of less than one and an extremely drafty building might have an ACH50 of over 20). Suffice to say that 4ACH50 is fairly air tight… The problem was that the Montana government didn’t pass any law requiring that buildings had to also be properly ventilated. So we had a situation where builders were creating very air-tight buildings (in some cases much tighter than the required 4 ACH50) and were not providing any controlled ventilation. I left Montana soon after this took effect so I don’t know what the outcome has been but it seemed to me that the government had created a huge time-bomb with the very real potential to make a lot of people sick and even to kill people.

So Why Not Just Open a Window?

A lot of people think that the way to ventilate a house is to simply open a window. It’s true that you can get plenty of fresh air by doing that and it can go a long way towards keeping a building healthy, safe and durable. Unfortunately it can also negate any effort to make the building energy efficient and comfortable. If you open windows in the winter you let the warm air out and cold air in and the building will require a lot more energy to keep the interior comfortable.

So How Can We Get Fresh Air Without Losing Heat?

The best way is by using a Heat Recovery Ventilation (HRV) System. We didn’t do that in the Greeny Flat because HRV’s were pretty hard to find in Australia when we were planning the project. Instead, we use exhaust ventilation in combination with an earth-tempering tube on the fresh air intake. You can read more about this system and about HRV’s on our ventilation page. I’ve recently learned of a business in Sydney which sells and installs HRV’s and you can visit their website at


If our goal is to create buildings that are safe, healthy, comfortable, durable and energy efficient we must adhere to the old saying ‘Build Tight, Vent Right’.

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