Painting house exterior during autumn/fall

It’s fairly easy to decide not to paint during an extreme heat wave or when it’s freezing outside. Temperature fluctuations (highs and lows) is an important factor to consider when painting your house exterior it affects paint dry time. Autumn/early fall, is a good time of the year in Vancouver to paint. Daytime and night-time temperatures fluctuations are often closer in this time of the year. In Vancouver, for example, average low temperatures are 48 F in October and 38 F in November. This makes mid-November a cut-off month for exterior painting season.

How does temperature Affects Painting?

As a Thumb rule, do not paint when the RH is above 85% or when the Dew Point is closer than 3 degrees to the ambient air temperature. Here below we can see how they inter-relate.

What is water vapour?

Water vapour is water in its gaseous state-instead of liquid or solid (ice). Water vapor is totally invisible.

What is Relative Humidity(RH)?

RH measures water vapour but relative to the temperature of the air. It is expressed as the amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature.

 How to measure RH%?

There are several electronic instruments available and these vary in accuracy and may use a Whirling Pschycrometer –it is one of the most reliable instrument.

What is Dew Point?

The dew point is the temperature at which air is saturated with water vapor. When air has reached the dew-point temperature at a particular pressure, the water vapor in the air is in equilibrium with liquid water, meaning water vapor is condensing at the same rate at which liquid water is evaporating. Below the dew point, liquid water will begin to condense on solid surfaces (such as blades of grass)

Click here to calculate the dew point using combination of temperature and relative humidity

How the applied paint dries?

When a coat of paint is applied on to a surface, the paint starts to absorb heat (energy), then solvents begin to evaporate from the paint film. You feel the same effect if you spill petrol, solvent or alcohol on your skin, as it evaporates it cools your skin down.

Please note that higher humidity can interfere with the supply of oxygen necessary for the curing of alkyd paints and delays the tack-free time from hours to days. With latex paints it is possible for the evaporation of water to be slowed as the paint dries. If the coalescing solvent (film forming glycol) evaporates before the water leaves the film, you will end up with a paint film that is weak or can crack and/or peel.

Relationship between Relative Humidity and Dew Point?

The following chart shows a range of ambient temperatures from 15 down to 5.

painting house exterior during autumn

You can see that as the Relative Humidity falls so does the Dew Point. If you now look at the green highlighted squares you will see that they all have a Dew Point around 3 degrees below the ambient temperature and they all have a Relative Humidity around the 81 to 84% level.

Let us see typical painting scenario. The air temperature is 10°C and we measure the RH as being 75%. The temperature of substrate (sidings/trims) that we want to paint is 8°C. Question is can we paint it? With air temperature 10°C and RH 75%,the dew point is reached at 6°C.which means the temperature of substrate (sidings/trims) should be 3°C higher than dew point i.e.9°C whereas it is 8°C.Answer is we should not paint in this case

The Best Paints for Cold Weather

Many major paint manufacturers like Sherwin Williams and Benjamin moore offers specially formulated paints that for cold weather applications. Most of these are rated for applications as low as 35.6°F(2°C) It’s best to use these paints which is designed for lower curing temperatures are more reliable than standard paints mixed with additives for freeze-resistance .

So when can we paint?

  • First rule, the surface temperature of what we want to paint should be 3 degrees above the Dew Point.
  • Do not apply paint above 85% RH.
  • Keep within the temperature guidelines for application as stated on paint manufacturer’s product data sheets remembering that the best and easiest application temperatures are generally those in the middle of the recommended ranges i.e. temperate conditions rather than extremes.
  • Applying the paint when temperatures are rising to avoid getting caught by Dew Point.

If you are looking to fully repaint your house exterior or repaint only the exterior trims &doors Please click here to contact us or call us.



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