Understanding Dew Point on Site: Why Condensation Matters

Dew point is one of those concepts that sounds technical, but it affects almost every waterproofing, tiling and flooring job – especially when temperatures drop or humidity spikes.

In this article, ARDEX Technical Services Advisor Chris Brady explains what dew point is, why it matters for screeds, adhesives and membranes, and how installers can avoid condensation-related failures on site.

What is dew point?

Dew point is the temperature at which moisture in the air condenses and turns into liquid water on a surface.

You see it every day in simple situations:

– A cold can of drink that quickly becomes wet on the outside.
– Steam fogging up bathroom mirrors and shower glass.
– Moisture appearing on cold windows on winter mornings.

None of those surfaces are generating water themselves – they’ve simply dropped to a temperature where the water vapour in the surrounding air condenses on them.

Why dew point matters in construction

In tiling, flooring and waterproofing, extra water turning up where you don’t expect it can cause real problems. Dew point is important because:

– Cement-based products (screeds, tile adhesives, repair mortars, levelling compounds) can become weaker or more brittle if they are saturated or if curing is disrupted by cold, wet conditions.
– Epoxy systems can have their chemical reaction interfered with if free water is present on the surface or trapped between coats.
– Membranes and coatings can lose adhesion if condensation is present when they are applied or between coats.

In short, unexpected condensation means you are no longer working over the dry, prepared surface you thought you had.

When do dew point problems occur?

Dew point issues are driven by a combination of temperature and humidity.

Common risk scenarios include:

– Late-afternoon or evening applications outdoors – As the sun goes down, surface temperatures fall. A slab that was warm and dry at 3 pm can drop below dew point after sunset, and condensation may appear on fresh membranes or coatings.
– Cold, still winter mornings – Surfaces may already be at or below dew point before work starts, leaving a film of moisture that is easy to miss, especially on darker substrates.
– Poorly ventilated internal spaces – High humidity from other trades (plastering, painting, hot water, etc.) can create conditions where moisture condenses on cooler surfaces, especially overnight.

The problem is not always obvious. A surface can look visually “dry” but still carry a fine film of moisture that affects adhesion and curing.

Where is dew point a major concern?

Dew point can cause trouble both inside and outside, but it is especially critical when working with:

– Exterior facades, balconies and retaining walls – Rapid temperature swings and shade changes can cause surfaces to cross the dew point during or soon after application.
– Below-grade work – Basements and retaining walls may have cooler internal faces combined with high humidity.
– Internal wet areas – Steamy environments, such as bathrooms, can see surfaces reach dew point quickly if ventilation is poor.

Any situation where the surface temperature can unexpectedly drop, or where humidity is elevated, should trigger a dew point check.

Practical impacts on membranes and coatings

Condensation at or between coats can lead to:

– Poor intercoat adhesion – Water trapped between membrane coats can prevent them bonding correctly.
– Blistering or pinholing – Moisture trying to escape through a curing layer can create bubbles or voids.
– Soft or under-cured films – Epoxies and some other systems may never reach their designed hardness if water interferes with the chemistry.

For two-coat or multi-coat systems, it’s especially important to ensure that any overnight dew has fully dried off the previous coat before recoating.

How installers can manage dew point risk

A few simple planning steps can dramatically reduce dew point-related problems:

– Choose application times carefully – Aim to apply membranes and coatings when temperatures are stable or rising, rather than when they’re about to fall. Mornings are often better than late afternoon.
– Check for condensation before recoating – On cool or humid days, expect overnight dew on exterior surfaces. If moisture is present on a first coat, allow it to dry fully before applying the next.
– Improve air movement and humidity control indoors – Use dehumidifiers, air-conditioning in “dry” mode, or gentle air circulation to reduce humidity. Avoid blowing cold air directly across wet coatings, as this can cool the surface and promote condensation.
– Monitor both air and surface temperatures – Remember that surfaces can be cooler than ambient air, especially in shaded or exposed locations.

Being aware of how quickly conditions can change – particularly in the late afternoon and overnight – is half the battle.

Key takeaways on dew point

To keep projects on track:

– Recognise that dew point is about when and where water condenses, not just how humid the air feels.
– Understand that extra moisture at the wrong time can damage screeds, adhesives, epoxies and membranes.
– Plan application and recoat times to avoid periods when surfaces are likely to fall below dew point.
– Take advantage of ventilation and drying equipment in internal areas.

If you’re unsure whether conditions are suitable for a particular product, contact ARDEX Technical Services for project-specific advice before you start.

ARDEX Technical Hotline: 1800 224 070

Subscribe to our newsletter