Ignoring best practices when rendering in the winter months can be costly – particularly when applying renders, thin coat systems or paint finishes that rely solely on dry conditions for the water within the product to evaporate and allow curing to take place. While most contractors are mindful of temperature and rainfall, humidity levels are often overlooked, and this can lead to serious failures on site.

If humidity levels are high, the water within the applied render has little or no ability to evaporate. This means that even after several days, the coating may appear dry but still be uncured. Should rainfall occur, the water can soak into the render, causing it to re-emulsify and resulting in the coating running off the wall and staining anything below.

Far from ideal, right? To prevent this, we recommend reading our Winter Working Guidance, alongside the additional tips from our experts below.

Winter Working: Thin Coat and Paint Coatings

To avoid thin coat or paint wash-off during winter months, it’s essential that:

• The substrate is dry before application
• Humidity levels are checked at the project location

At this time of year, humidity levels are often extremely high, with readings in the 90% range not uncommon. This means there’s very limited dry air available for the water in thin coat or paint products to evaporate into.

High humidity drastically slows the drying process and may result in the product taking several days or even longer to dry. This risk is that if rainfall occurs during this period, wash-off may occur.

To reduce this risk:

• Wait until humidity levels have dropped and there is suitable drying weather
• Sheet the scaffold to the sides and top to prevent water ingress
• Use drying fans (see guidance below)

Tips to Assist with Drying

To assist with drying the render, a heavy-duty variable-speed 30” floor-mounted air circulation drum fan can be used. The fan should have:

• A minimum rating of 10,000 CFM
• The ability to rotate 360° on its vertical axis

Please note: We do not recommend that you use a heating fan, as this can cause localised early drying of the render.

Once the render has initially set:

• Position the fan near the wall at an angle
• Do not face the fan directly at the wall
• Allow air to flow across the render surface

The fan should be left running during the day until the render reaches the required moisture content for finishing coats to be applied.

What Constitutes a Dry Substrate?

The accepted guidance for a dry substrate suitable for covering is as follows:

• Thin coat finishes: moisture content must not exceed 20–22%
• Paint finishes: moisture content must not exceed 12–14%

Moisture levels should be checked using a standard moisture meter.  During winter months, surface moisture or dew can form on the render overnight. This must be allowed to dry fully before any decoration or finishing work commences. An air circulation fan can be used to assist with this process.

Always:

• Take moisture readings in several locations
• Record each reading by taking a photograph
• Confirm acceptable moisture levels before applying primer or topcoat

Bagged Renders

Bagged render products will also take longer to dry and cure during the winter months. As with thin coat products, high humidity levels mean there is limited dry air available for evaporation.

This slows the chemical reaction that occurs in hydraulic renders and may result in curing times of 36 hours or more before the render hardens sufficiently. There is no detrimental effect caused by this delayed curing process; however, additional care must be taken in poor weather conditions.

To reduce risk:

• Monitor humidity levels
• Sheet scaffolding to the sides and top
• Use air circulation fans as detailed above

For any more information, please don’t hesitate to get in touch.