Airtightness: Friend or Foe?

18 May 2017

 

Jon is a qualified teacher and Regional Manager for pro clima, focussed on technical and educational support for the design community. 

The so-called 'affordable' home is only affordable to buy; when what we really need (for little extra capital cost) is a home that is 'affordable to run' — meaning a home with very predictable performance such as a Certified Passive House. Houses that can be easily and cheaply heated across every room have good air quality and are comfortable simply because they have been designed for their specific climate — by optimising solar gain, but not relying on it. Measurable airtightness is a key factor. 

Recently, I was asked this question at a conference: "You and the previous speaker seem to differ in your opinions about ventilation, draught-stopping and airtightness. This is confusing, so what is the message we should be giving our clients?"

I answered: "Draught stopping IS airtightness — just at different points on the continuum, from very leaky to very  airtight". The previous blog post on blower door testing discussed this in more detail. 

The next part of the answer regarding ventilation was "You need to ventilate. Please realise airtightness is not the enemy. Lack of ventilation is the enemy."

This message is the same if you have a deliberate strategy for creating airtightness (through the use of specialist products) or achieve moderate levels of airtightness accidentally. Every home needs deliberate ventilation.

One of the significant hurdles to overcome is a misquote of a BRANZ airtightness statement where an entire industry only ever heard the first few words and think airtightness is a bad thing. The complete quote: "Overall, NZ newer homes are on the border of becoming too airtight, if infiltration is to be relied on to supply fresh air. It would also appear that a ventilation scheme that relies on open windows is too unreliable." Ensuring the New Zealand Building Stock is Moisture Tolerant — G Overton, M Bassett, S McNeil, 2009.

We are very used to hearing about airtightness up until that comma, and before we get to the ventilation part.

Other common questions and emphatic statements about airtightness are along these lines:

  • "I couldn’t live in an airtight box" — Me neither, that sounds like a coffin.
  • "I want to open my windows" — Who told you that you couldn't?
  • "Will I suffocate in a power cut?" — If you can move to open a door within 21 days of the power going off, you'll be okay.

So let's clear up the message.

Draught stopping and airtightness are the same thing: control. If you contain the air volume (by deliberate airtightness measures), you can control the indoor environment (by deliberate ventilation strategies). Suitable ventilation strategies might include opening multiple windows on a regular basis provided it is always windy, or using a balanced-pressure ventilation unit.

To steal from Alvin Toffler's famous quote that literacy is being able to "learn, unlearn, and relearn", prepare your homeowner for the cost of building, unbuilding, and rebuilding if you neglect 'optional' heating and ventilation as part of a combined internal-environment control strategy.

We must rethink our thinking to remain literate.

We cannot afford to build houses that are only affordable to buy.

If you'd like to know more, please contact pro clima

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