Architects memo no. 69: February 2002
Ventilation? no sweat
The second paragraph of Architects Memo No. 52 is famous in that it contained the only error ever detected (or admitted to!) over the two decades that these memos have been written. Apart from this ego-shattering slap, the paragraph did contain some useful information about relative humidities.

It taught that, at any given temperature, air can only hold a specific amount of water and, when that limit is reached, no further drying can take place in such a saturated atmosphere.

The air in a room of 36 cubic metres volume holds about 455 millilitres of water at 20°C and 70% relative humidity. It has 'room' for only another 200mls before it is completely saturated. A coat of paint to walls and ceiling adds about 3,200mls of water to the environment - 16 times more than it can cope with.

The answer to this problem is simple - ventilate; get some air moving through the place. In older homes this occurs regardless but modern homes are so 'tightly' built that ventilation has to be a conscious effort.

Older paints, particularly those with high solvent levels and other odiferous materials, demanded conscious ventilation simply to keep the environment habitable.

Some modern paints have odour levels so low that ventilation is no longer a pre-requisite for enjoying a freshly painted room.

Lack of ventilation at 20°C is not catastrophic but does lead to the slower development of the ultimate properties of the film, especially toughness and abrasion resistance.

Lower temperatures, however, add another dimension.

The same room as described earlier holds 227mls of water at 10°C and 70% relative humidity with room for only 100mls more. Not only is the water burden of painting doubled at these conditions but other factors can also come into play, as mentioned in (Painting Tips Resene News Winter '98).

At cooler temperatures, the binder of the paint becomes a little harder and is a less hospitable place for the surfactants and thickeners all waterborne paints contain. If this is coupled with very slow drying times due to poor ventilation, some of these incipient materials can infuriatingly find their way to the surface of the paint producing a 'smeary' surface.

This can be removed by washing with warm water, but its removal is often accompanied by post-painting hyperventilating.