How Capillary Action Affects the Building Envelope — and How Drainable Housewrap or Rainscreen Systems Help

Moisture management is one of the most important parts of building envelope design. In many wall assemblies, water intrusion is not only caused by bulk water leakage or rain exposure, but also by a less visible mechanism: capillary action.

Capillary action can hold and move liquid water through very small gaps between building materials. When exterior cladding, WRB, sheathing or other wall components are pressed closely together, water can be retained in the interface instead of draining freely. Over time, this trapped moisture may increase the risk of sheathing deterioration, mold growth, corrosion, staining, reduced insulation performance and long-term wall durability problems.

What Is Capillary Action?

Capillary action is the movement of liquid water through narrow spaces, pores or contact gaps due to surface tension and adhesion between water and the surrounding material surfaces.

In building envelopes, capillary action may occur in areas such as:

Small gaps between cladding and WRB
Interfaces between sheathing panels and exterior membranes
Joints, laps, fastener penetrations and rough openings
Porous materials such as wood, fiberboard, masonry or cement-based substrates
Wall assemblies where materials are installed in direct contact without a drainage space

When the gap is very small, water may be held between the surfaces. Instead of flowing downward by gravity, it can spread laterally or remain trapped. This is why a wall can stay wet even when there is no obvious leak.

Why Capillary Moisture Matters in Wall Assemblies

A modern WRB or housewrap is designed to resist liquid water while allowing water vapor to pass through. However, vapor permeability alone does not solve every moisture problem. If liquid water is trapped in direct contact between the cladding and the WRB, the wall may not be able to drain and dry efficiently.

This risk becomes more important in high-humidity climates, coastal areas, rainy regions, shaded walls, and wall systems with absorbent or tightly installed exterior cladding. In these conditions, repeated wetting and slow drying can create a long-term moisture load inside the wall assembly.

Common Problems Caused by Trapped Capillary Water

Slow drying: Water held between close-contact layers has limited airflow and limited drainage path.
Wood decay risk: Sheathing and framing materials can be exposed to elevated moisture for a longer time.
Mold and mildew risk: Persistent moisture can create conditions for biological growth.
Corrosion: Fasteners, metal accessories and metal cladding components may be affected by repeated wetting.
Reduced thermal performance: Wet materials and wet insulation can reduce wall performance.
Exterior finish damage: Moisture accumulation may contribute to staining, blistering or coating failure.

Solution 1: Drainable Housewrap

A drainable housewrap is designed to provide water resistance, vapor permeability and drainage in one material layer. Unlike a flat WRB, a drainable housewrap includes an integrated drainage structure, textured surface or drainage path that helps create a small space between the WRB and the exterior cladding.

This drainage structure helps interrupt capillary contact and allows incidental water to move downward by gravity. Instead of being trapped between two flat surfaces, water has a defined path to drain out of the wall assembly.

How Drainable Housewrap Helps

Reduces direct capillary contact between cladding and WRB
Creates a drainage path behind the exterior cladding
Allows bulk water to drain downward instead of remaining trapped
Supports drying by allowing vapor to pass outward
Improves moisture management without requiring a full furring-strip rainscreen system in every application

For many residential and light commercial wall assemblies, drainable housewrap can be a practical way to improve drainage performance while keeping installation relatively simple.

Solution 2: Rainscreen System

A rainscreen system creates a physical air gap between the exterior cladding and the WRB or sheathing layer. This gap separates the cladding from the water-resistive barrier and provides a dedicated space for drainage and ventilation.

In a rainscreen wall, any water that enters behind the cladding can drain downward through the cavity. At the same time, airflow within the cavity can help remove moisture and accelerate drying. This approach is especially useful for high-performance buildings, wet climates, absorbent cladding materials and wall assemblies where long-term durability is a priority.

How Rainscreen Systems Help

Create a continuous drainage cavity behind the cladding
Break capillary contact between cladding and WRB
Improve drying through ventilation and air movement
Reduce the risk of trapped moisture in the wall assembly
Provide strong protection for demanding climate conditions and high-value buildings

Drainable Housewrap vs. Rainscreen: What Is the Difference?

Both drainable housewrap and rainscreen systems are used to reduce moisture problems behind exterior cladding, but they solve the problem in different ways. A drainable housewrap creates drainage paths within or on the surface of the WRB, while a rainscreen creates a larger physical cavity behind the cladding.

Flat WRB versus drainable housewrap comparison showing capillary water retention and drainage path >>

Compared with a flat WRB, a drainable housewrap helps reduce capillary water retention by providing a drainage path behind the exterior cladding.

Item	Drainable Housewrap	Rainscreen System
Main Function	Water resistance, vapor permeability and integrated drainage	Drainage, ventilation and separation behind cladding
Drainage Method	Drainage channels or textured structure on the WRB surface	Physical air cavity created by furring strips, battens or drainage mats
Capillary Break	Reduces close-contact water retention	Creates a stronger physical separation
Installation	Similar to standard housewrap installation	Requires additional components and detailing
Typical Use	Residential and light commercial wall systems	High-performance walls, wet climates and demanding cladding systems
Cost and Complexity	Lower cost and simpler installation	Higher cost but stronger drainage and drying performance

Design and Installation Considerations

Whether using a drainable housewrap or a rainscreen system, good detailing is critical. A drainage product can only perform properly when water is given a clear path to exit the wall assembly.

Install the WRB with proper shingle-style laps so water sheds outward and downward.
Seal critical joints, seams, penetrations and rough openings with compatible flashing tapes.
Provide drainage outlets at the bottom of the wall, such as weeps or open drainage edges.
Keep drainage paths continuous and avoid blocking them with sealant, mortar, insulation or trim components.
Use proper flashing at windows, doors, transitions, corners and penetrations.
For rainscreen systems, maintain a continuous cavity and ensure both drainage and ventilation are considered.

Conclusion

Capillary action is a real and important moisture mechanism in building envelope design. When wall layers are installed in direct contact, water can be held between materials and may not drain effectively. This can increase the risk of long-term moisture problems inside the wall assembly.

Drainable housewrap and rainscreen systems both help address this issue by creating a capillary break and providing a path for water to drain. Drainable housewrap offers a simple and efficient solution for many wall assemblies, while rainscreen systems provide a more robust drainage and ventilation strategy for demanding applications.

For best long-term performance, these products should be used as part of a complete building envelope moisture management system, including WRB / housewrap, flashing tapes, seam sealing, drainage or rainscreen layers, and critical-detail protection around openings and penetrations.