Exteriors and weather

Reading dew point before an exterior coat goes on

A morning can look perfectly dry and still be wrong for painting. If the substrate is within a few degrees of the dew point, moisture will condense back onto it as the sun drops, whatever the forecast said at eight o'clock. We measure both figures rather than guessing from how the wall feels.

A decorator holding an infrared thermometer against a render wall before starting exterior coating

What the two numbers actually tell you

Surface temperature is measured directly on the wall with a contact thermometer or infrared gun, not the air temperature from a phone app. Dew point comes from humidity and air temperature together. When the gap between the two closes to within three degrees, moisture is likely to form on that surface again before the coating has had a chance to skin over.

Contact thermometer readings taken on the actual elevation being worked
Humidity checked at the wall, not from a general weather site
Readings repeated through the day as conditions shift

Where the failures show up later

Coatings applied too close to dew point often look fine on the day and then bloom, flatten in patches or lose adhesion within weeks. Render and masonry paints are particularly prone because the film needs to breathe while curing, and trapped moisture underneath the surface disrupts that process invisibly.

Shaded and north-facing elevations behave differently

A south wall in full sun can be safely above dew point by mid-morning while the north gable of the same building is still damp to the touch. We treat each elevation of a building as its own weather zone rather than applying one blanket start time to the whole job.

North and shaded faces checked separately from sunlit elevations
Later starts scheduled for the slower-drying sides of a building
Work sequenced to follow the sun round the building where the programme allows

What we do when conditions are marginal

If the gap is too tight we hold off rather than push on, and we would rather explain a half-day delay honestly than hand over a coating that is already compromised. Where the programme is tight we look at moving to a different elevation or a different task that day instead of losing the day entirely.

How we work on this

We carry contact thermometers and hygrometers to site rather than relying on forecast apps
We record readings per elevation, not once for the whole building
We reschedule specific faces of a building independently of the rest of the programme

Questions we are asked about this

Can you paint if it rained the night before?

Usually yes, once the surface has had time to dry and the dew point gap is wide enough. We check the wall itself rather than assuming an overnight shower rules out the whole day.

Does this apply to all exterior coatings?

It matters most for masonry paints, render systems and anything water-based applied outdoors, since these rely on evaporation to form a proper film. Oil-based joinery work is less sensitive but still affected in damp autumn conditions.

Why did our last contractor not mention this?

It is easy to skip if a crew is working to a fixed daily target rather than to the conditions on the wall. We would rather flag a delay than apply over a compromised surface.