What If External Rainscreen Insulation Gets Wet?

When it comes to external insulation within a ventilated rainscreen cavity, one of the most common concerns is moisture:

• What if the insulation gets wet?
• Will it degrade or decay?
• Will it grow mould or start to smell?
• Will it get soggy and fall off its fixings?
• Will its thermal performance be reduced?

These are legitimate questions, and they often arise when discussing the use of continuous external insulation in a ventilated cavity. Let’s dive into the details to provide some clarity.

Hydrophilic vs. Hydrophobic Insulation

First, it’s essential to understand the two main types of mineral wool insulation:

• Hydrophilic: Absorbs water.
• Hydrophobic: Repels water.

Many traditional mineral wool products are hydrophilic because they are designed for use in dry interior walls and ceilings, where moisture isn’t a concern. Hydrophilic insulation acts like a sponge, absorbing significant amounts of water, which can lead to several issues.

Problems with Hydrophilic Insulation in Wet Conditions

1. Reduced Thermal Performance: When water replaces air in the insulation’s pores, heat transfer increases significantly because water conducts heat more efficiently than air. As a result, wet insulation can lose up to 20% of its insulating capacity.
2. Structural Damage: Prolonged exposure to moisture can lead to rot, corrosion, or other damage to the building’s structure.
3. Mould Growth: Damp conditions create an ideal environment for mould and mildew, which can negatively affect indoor air quality and occupant health.
4. Increased Energy Costs: As insulation loses effectiveness, more energy is needed to maintain comfortable indoor temperatures, leading to higher HVAC costs.

The Solution: Hydrophobic Mineral Wool Insulation

Modern hydrophobic mineral wool products, like ROCKWOOL RAINSCREEN insulation, provide a much better solution for ventilated cavities in external facades.

Hydrophobic mineral wool acts like a rain jacket rather than a sponge. While untreated mineral wool absorbs water, hydrophobic mineral wool repels it, causing water to bead up and roll off its surface. This resistance is achieved by treating the insulation with mineral oil, making it water-repellent.

How Hydrophobic Insulation Performs

When tested using standard immersion methods, hydrophobic mineral wool absorbs only about 0.3% moisture, even after long-term exposure. This minimal absorption is key to maintaining both the insulation’s integrity and its thermal performance.

In reality, this tiny amount of water penetration (approximately 0.5mm) has little to no effect on the insulation’s performance.

Here we show ROCKWOOL that has been submerged for more than 24hours. It looks saturated, but it is dry inside. 

The Impact on Thermal Performance

For example, a 50mm layer of hydrophobic mineral wool insulation has the following performance characteristics:

• Dry: R-value of 1.47 m²K/W.
• With 20% moisture (hydrophilic insulation): R-value drops to around 1.14 m²K/W.
• With 0.3% moisture (hydrophobic insulation): R-value remains almost unchanged at 1.45 m²K/W.

This minimal drop in thermal performance demonstrates how well hydrophobic insulation maintains its insulating properties, even when exposed to moisture.

The Importance of Drying

Another critical element in rainscreen facade systems is ensuring that any moisture that does enter the cavity can dry out. This is where the design of the rainscreen cavity becomes essential. With proper ventilation, any water on the surface of the insulation will evaporate and be dispersed through the natural airflow within the cavity.

This principle is a core feature of Velo Facades, which uses an open-joint rainscreen design to ensure adequate airflow and drying capabilities.

Key Takeaways for Moisture-Resistant Rainscreen Systems

When designing or specifying externally insulated facade systems, there are two key strategies to prevent moisture-related issues:

1. Product Choice: Ensure you select high-quality, hydrophobic insulation that is designed for external use and can repel water effectively.
2. Proper Ventilation: Design the rainscreen cavity with sufficient airflow to allow moisture to dry and prevent long-term accumulation.

By adhering to these strategies, the insulation will retain its thermal performance, avoid decay, and steer clear of mould, unpleasant odours, or structural damage. Issues in the past have arisen when these principles were not followed, highlighting the importance of thoughtful product selection and design.