Understanding Rain Screen Wood Siding Systems
Rain Screen Wood Siding Principles
Rain screen siding technology has been in existence since as early as the 12th century A.D. Some wooden stave churches in Norway that were built in the late 1100's using early rain screen wood siding technology are still in existence today. Many Japanese temples were also built about the same time using similar rain screen siding principles.
The rain screen siding principle is straightforward: first build an outside wall and weatherproof it (building envelope), secondly construct an outer layer of siding (rain screen) to keep the weather (rain, snow, sleet,wind and sun) away from the inner wall. The air space in the wall cavity between the building envelope and the rain screen allows moisture to escape and also creates a passive insulation layer.
Unlike masonry curtain wall construction where moisture is nearly trapped in the wall cavity, a proper wood rain screen siding system should be an open-joint rain screen system or a ventilated wood siding system. With a wood rain screen system, moisture from inside the structure passes to the outside and water that penetrates through the joints in the rain screen can also escape.
The University of Alaska Fairbanks Cooperative Extension Service published an excellent research paper on the subject of exterior ventilated cladding systems. This research titled, "Exterior Ventilated Cladding" explains the building science behind rain screen systems and rain screen siding principles.
The rain screen siding is the first layer of defense from the elements including wind driven rain. An open joint ventilated rain screen is considered a pressure-equalized system. This means that when winds are blowing against the surface of the rain screen, some of the pressure passes though creating back-pressure. In theory this pressure is exactly equal. In reality there are hardly ever ever "static" pressures; the wind changes directions and pressure (dynamic pressure). The best designed wood rain screen systems have large amounts of unrestricted air flow behind the rain screen and are firmly fastened to the building envelope.
The Institute for Research in Construction,
National Research Council of Canada published an excellent research paper titled, "Pressure Equalization in Rain Screen Wall Systems". Click on the icon to the right to view this whitepaper.
In the early 1970's, the first "energy crisis" spawned a new generation of energy efficient weather tight home and building designs. These new super-insulated designs did an excellent job of reducing the energy costs for heating and cooling. They were often so tightly sealed, however, that moisture was trapped within. The need for effective rain screen siding systems was now needed for both residential and light commercial construction.
The US Department of Energy published an extensive study of the effects of moisture and proper methods of dealing with trapped moisture. This 273 page study offers a comprehensive understanding of moisture issues and several methods of mitigating these problems. This building science research paper is titled, "Moisture Control Handbook" and explores a multitude of topics including:
- Mold, mildew and condensation
- Thermal bridging
- Effective surface drainage
- Wetting and drying of building assemblies
- Moisture Control for warm, cold and mixed climates
Effective rain screen siding systems solve a myriad of issues addressed in these research studies. While rain screen siding systems do not resolve all moisture issues in a building design, properly designed rain screen systems create an excellent method for moisture to escape through the building envelope while promoting a healthier environment within the structure.
A common misconception about rain screen wood siding systems is that they are only effective in keeping rain off the structure. It is true that properly designed rain screen wall systems keep bulk rain water away from the building envelope. It is also equally true that the proper spacing in the wall cavity of a well designed rain screen system is the key to effective moisture management from within the structure.
The Canadian Wood Council has published an excellent research paper about key design concerns that should be considered prior to building a project. The paper is titled, "Wood Specification: Durability". This 2 page paper is a "must read" article for architects, builders or owners before designing your project. It deals with topics such as; why a durable high performance wood building envelope adds value, how to include durability concerns into your design, life cycle cost considerations, how to minimize the premature deterioration of wall systems and lots more.
Advantages of rain screen wood siding technology
This "layered" rain screen siding system creates a multitude of benefits:
- The inner wall is first weatherproofed to keep the weather from entering into the interior of the structure (building envelope).
- The outer wood siding keeps the weather away from the structure (this siding serves as the "rain screen").
- The open space between the layers of the rain screen system allows trapped moisture to escape and also creates an energy-efficient air space that acts as an insulator. This layer helps buffer the transmission of both cold and heat.
- Removing trapped moisture creates a healthier environment both inside and outside the building envelope.
- Rain screen wood siding systems are naturally beautiful!
Until recently, the only rain screen wood siding system option were rain screen designs that required furring strips. Using furring strips in a rain screen siding system creates the proper 3/4' spacing in the wall cavity to allow for adequate moisture elimination. Rain screen systems that use furring strips are considerably better than a non-ventilated wall system.
Typical rain screen wood siding detail using furring strips:
The main disadvantages of a typical rain screen application using furring strips include:
1.) Air flow and moisture is restricted to individual wall cavity chambers created between the furring strips. Cross flow of air between the chambers is not possible.
2.) The furring strips provide thermal bridging to the building envelope. At 16" on center spacing, there is typically upwards of 16% of the building envelope surface area in direct thermal contact with the furring strips.
3.) The furring strips themselves trap moisture behind them and become a viable food source for molds and mildews. Mold and mildew promote rot and decay and can create an unhealthy environment and structural damage outside the building envelope.
4.) Additionally the furring strips themselves inhibit the free flow of moisture from inside the building to escape. Since this occurs over nearly 17% of the building envelope, a considerable amount of moisture is trapped inside the home or building. This creates additional opportunities for mold and mildew to develop-inside the structure. This can create an unhealthy environment inside the home.
5.) The labor costs and material costs of a rain screen application using furring strips are considerably higher than alternative rain screen system options.
6.) In vertical siding applications, complex furring strip designs are required, often using multiple layers of furring strips.
The Climate-Shield Rain Screen Wood Siding System creates a paradigm shift for rain screen wood siding system design.
While there are a few rain screen hidden fastener clip systems on the market, none perform as well as the Climate-Shield Rain Screen Wood Siding System. The Climate Shield Rain Screen Clip and the system's patented technology allows architects, builders and owners the ability to design rain screen siding systems for horizontal, vertical or diagonal rain screen siding installations. There is no need to be concerned with designing complicated furring strip layouts. The Climate-Shield rain Screen Clip acts as its own spacer, uniformly, each time and every time.
When you use the Climate-Shield Rain Screen Wood Siding System:
There is no need to worry about water being trapped behind traditional furring strips because no furring strip is needed! The Climate Shield rain screen clip provides the proper spacing off the side wall and can be screwed directly into suitable exterior sheathing. This means that the cost of the furring strips and the cost to install them is eliminated from the building budget.