Rain Screen Basics: Moisture Control
80% of all construction-related claims in the United States are water related. Just a few rain screen siding basics can mitigate most of these issues before they ever occur. Rain screen wood siding systems lessen water damage, providing an effective, durable and beautiful design. Building codes now require rain screen installation in various locations. Oregon requires a minimum 1/8-inch-deep gap between siding and the water-resistant barriers for all new homes.
Rain Screen Basics: Siding Principles
Contemporary wood panel rain screen design is technology inspired by Scandinavian and Japanese structures dating to the 12th century. Many of these buildings remain standing in remarkable condition to the present day.
Details of Wood Screen Siding Systems
A rain screen system consists of a building envelope (A moisture resistant barrier wraps the exterior wall. Felt paper is one commonly installed option.) and an outer rain screen from finished stock. An air space gap, usually about ¾”, separates the rain screen envelope from the moisture-resistant surface of the exterior wall.
Furring strips create this gap by over the exterior wall framing and vapor barrier. The Viking Clip™ mounts flush atop the furring strips with stainless steel screws. Finish siding planks rest on the clip in milled grooves. The thickness of the furring boards provides the desired depth of the air cavity. Our VC2 No-batten clip is also available, providing the bulk and ¾” depth without furring strip installation.
Rain Screen Siding Basics: A Moisture Guard
The rain screen shields the building envelope from direct exposure to the elements. The air gap prevents moisture stagnation on the framed sheathing. This promotes a rapid drying of the rain screen siding, promoting evaporation from the inner siding surface. The vapor barrier functions as a water channel. Water vapor condenses onto the vapor barrier material and drains out at the cavities of the wall base.
As sunlight warms the rain screen siding, this cavity heats up, and air rises. Convection currents flow through the gaps, between siding courses and into the cavity. These currents work this warmed air up and out through the upper vent corridor.
Air pressure is equalized throughout the rain screen system’s open-jointed design. High pressure gradients drive water through the wall to the framed structure in closed or non-ventilated systems. These pressure differentials are greatest during high-wind events. Water won’t stagnate on these vulnerable surfaces in an open jointed system.
An additional benefit is that the air cavity acts as an insulator, thereby reducing the heating and cooling costs of the building.
Advantages of Furring Strip Installation
Furring strip systems increase convectional moisture removal
Convectional drying increases by dividing the airspace into vertical compartments (cavities or chimneys). Convection current velocities generate slightly lower air pressure inside the chimneys than outside. This pressure differential increases airflow through the system. Researchers named this the “stacking effect” mechanism to remove moisture.
Rain Screen Siding Basics: The Diffusion-Evaporation-Redistribution Cycle
The rain screen wall dries quickly with these chimneys, increasing the rate of moisture transfer away from the building envelope. Moisture beads on the furring strip, which then diffuses through the rain screen. Then vapor evaporates into the cavity. Convection currents remove this moisture when it re-diffuses and evaporates from the furring strips. This mechanism is the diffusion-evaporation-redistribution cycle.
Rain Screen Siding Basics: Protecting the Framed Wall
The Viking Clip attaches to the furring strips. The US Department of Energy published an extensive study of the effects of moisture. They studied proper methods of dealing with trapped moisture. This 273-page study concludes that mildew and wood rot occur almost exclusively in a CLOSED airspace. This report cites no evidence that furring strips in a wood screen system “trap water” much less be prone to develop mildew or rot.
To the contrary, the report indicates that wood furring strips improve the efficiency of moisture removal. Moisture cycles properly within the furred airspace cavity, while undivided airspace lacks the same benefit.
The VC2 No-Batten clip is a great alternative where battens cannot be used, providing the essential distance and strength to mount siding directly to the substrate.
In conclusion, Viking Clip and The VC2 No-Batten rain screen systems are more effective in moisture control and often less expensive than closed-ventilation systems.