Built tight, vent right

I had another engaging discussion with Canadian building scientist, Gord Cooke this morning. We were talking about the moisture problems that we are experiencing in Australia; particularly regarding moistures getting into our building fabric, (wall & roof systems). Click here to look at my Water, Air, Vapour Thermal blog.

I liked Gord’s summary.

“If you insulate the building fabric it needs to be airtight. If it’s airtight the building needs to be ventilated. If it’s ventilated it needs to be airtight.”

The theme here is that airtightness and ventilation go hand in hand. We insulate our houses to improve thermal efficiency but this comes at a cost. Air entering the building fabric is now impeded by the insulation, and if that air carries water vapour it will condense and cause structural damage and health problems.

The main solution to this problem is to make the building structure airtight and not allow the moisture laden air in. But now that we’ve prevented the moisture laden air from entering the building fabric it still remains inside the rooms of the house.

So, we now need to remove this moisture from the house. The solution here is ventilation. An exhaust fan and an open window will do the trick but now we’re introducing cold air back into the house which seems counterintuitive when the original aim was to insulate the house and keep it warm.

The logical conclusion is to install an whole-house ventilation sysyem. This can be an exhaust system, a supply system or a balanced system such as an ERV (Energy Recovery Ventilation) unit. Essentially these ERV air handling units are a balanced system that introduces fresh air and expels stale air. The two air paths cross through a central polymer core which transfers the heat from one air stream to the other. In winter the moisture laden internal air is removed and the cool fresh outside air is heated before it enters the conditioned space. In summer the principle is the same, fresh outside air is cooled on the way in. (I will blog in more detail about these units in another post).

So what’s the lesson? Build tight and vent right. Remember that these two things are inseparable and are a consequence of insulating a building.

Typical ERV unit below:


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