Rain Screen Basics: Moisture Control
80% of all construction-related claims in the United States are due to water damage. Some Rain screen siding basics could mitigate the vast majority of these claims before they occur. Rain screen wood siding systems minimize water infiltration, while providing an effective, durable and aesthetically pleasing option. Building codes require rain screen installation in various locations. Oregon now requires a minimum 1/8-inch-deep gap between siding and the water resistant barriers for all new homes.
Rain Screen Basics: Siding Principles
The contemporary wood panel rain screen design is based on technology found in 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. Asphalt-impregnated building paper is one common product.) and an outer rain screen. An air space gap, usually about ¾”, separates the rain screen from the moisture-resistant surface of the exterior wall.
Furring strips created the gap by overlaying 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.
Many various profiles create a rain screen via face fastener. Standard S4S, E4E planks, or a double bevel profile can be used to create an open-jointed rain screen.
Rain Screen Siding Basics: A Moisture Guard
The rain screen shields the exterior wall from direct exposure to rain events. The air gap prevents moisture stagnation on the framed and sheathed superstructure. The gap promotes a further rapid drying of the rain screen siding by accelerating 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.
Water vapor is further removed from rain screen systems by convection. As sunlight warms the rain screen siding, air temperatures increase inside the cavity, and air rises. Convection currents flow through the gaps, between siding courses and into the cavity. Consequently, these currents work up and out through the upper vent corridor.
Air pressure is equalized throughout the rain screen system’s 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 can not stagnate on these vulnerable surfaces in an open jointed system.
An additional rain screen siding basic 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” and have shown it an important mechanism to remove moisture from the system.
The airspace lacks chimneys in a non-furring strip system. The wall surface is one continuous airspace and the stacking effect is lost in non-furring systems. The benefits are also lost, and these are considerable.
Rain Screen Siding Basics: The Diffusion-Evaporation-Redistribution Cycle
The rain screen wall dries quickly with these chimneys, while increasing the rate of moisture transfer away from the building envelope. Moisture leaches to 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.
This cycle cannot occur in systems without furring strips, therefore the efficiency of the system is significantly reduced.
Furring strip systems are more effective in reducing pressure gradients
Trans-wall pressure gradients reduce as the result of these chimneys in rain screens. This reduction increases when air space is properly compartmentalized. Chimneys don’t exist in some non-furring strip systems. Therefore, the system is less efficient than those properly compartmentalized. The idea is simple: Split up the big problem to make a bunch of little solutions.
These two articles discuss compartmentalization in greater detail:
- Pressure Equalization in Rain Screen Wall Systems. The Institute for Research in Construction, National Research Council of Canada.
- Rain Penetration and its Control, Canadian Building Digest No. 40, Division of Building Research, National Research Council of Canada, 1963.
Furring strips decrease thermal bridging to the building envelope
The thermal conductivity of a substance indicates its ability to conduct heat. The wooden furring strips conduct heat essentially the same as studs and wall cladding. Rain screen fastening clips are composed of metal, usually aluminum and stainless steel. The thermal conductivity of these materials are shown below:
- Wood: 0.09-0.16 Wm/1K
- Aluminum: 204-250 Wm/1K
- Stainless steel: 36-52 Wm/1K
Wood furring strips conduct heat at rates 360 to 2500 times lower than metal rain screen clips. Furring strips decrease thermal bridging to the structure’s interior.
Thermal conduction and bridging are lowest with furring strip installation. Furring Strips constitute up to 16% of overall wall area. Therefore, the gain in effect is significant.
Rain Screen Siding Basics: Furring Strips Protect the Framed Wall
The Viking Clip attaches to the furring strips. The screws fasten through these strips, then through the vapor barrier, cladding, and stud.
Some suppliers advise that their clips may mount anywhere on the wall cladding. This practice won’t work well. Therefore, we advise against such installation for customers located where wind and rain may occur.
Rain Screen Siding Basics: Furring Strips v. Non-furring Strip Systems.
The Viking Clip rain screen system employs furring strips. We’ve seen the labor and material costs of each system. A quality crew installs furring strips at very nearly the time allotted for other methods.
High profile clips create the air gap in non-furring strip systems. These perform dual tasks and use more fasteners. These are therefore also more expensive than the flush mount Viking Clip. These other clips require 2-3 times more fasteners per clip compared to our Viking Clip™. Non-furring strip systems also may require a metal starter rail or additional hardware. In summary, the labor expense will be a wash at worst. We’ve concluded through experience that the furring strip is the best value and the highest quality installation.
Rain Screen Siding Basics: Furring Strips Help Clear the Air
Some companies claim the furring strips trap water and conclude these are prone to mildew damage. We protest such fake news with some further proof:
The US Department of Energy published an extensive study of the effects of moisture. They further 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 actually improve the efficiency of moisture removal. The reason is that moisture cycles within the furred airspace cavity, while undivided airspace lacks the same benefit.
Keyword searches of academic, architectural and consumer complaint databases show no reports of furring strip water trapping or rotting in rain screen systems. This claim is a red herring created by companies with a vested interest in promoting their non-furring rains screen systems over furring strip systems.
In conclusion, rain screen systems that use furring strips are more effective in moisture control and often less expensive than non-furring strip systems.