Understanding Rainscreen Design for Home Exteriors
A rainscreen is essentially an extra layer of skin attached to the building to protect the main wall from harsh weather. This adds durability and protection to your home’s multi-layered wall structure. Rainscreens can either be visible or invisible. The invisible variety is part of an overall wall system, where the rainscreen layer is built into the new wall construction. Whereas visible rainscreens are usually add-ons and come in all shapes, sizes, and designs, and can be a great addition to your exterior home design for improved feature, pattern and performance efficiencies.
Understanding rainscreen design can also help your home thermally, as they keep direct sun off the main wall and the air gap between two wall materials acts as insulation. These screens also help in winter acting as an extra layer to reduce energy losses from inside the home.
Double Layer & Air gap
Rainscreens are a type of double wall construction. The inner wall is usually the structural wall with insulation and a waterproof skin (house-wrap or asphalt), followed by an air gap, then the rainscreen skin to the exterior. It’s important that the air gap remains open and vented, otherwise, water can get trapped and mold, mildew and/or wood root can build up.
Air gaps should be a minimum of one-quarter inch wide, but three-quarter inches is recommended for more ventilation and accelerated drying when excess water gets trapped between the layers of the wall. Various products are available to hold the double wall apart and create the air gap. These materials are referred to as furring channels, which often have a C-shaped profile, battens, or plastic 3D dimensional meshes. Battens are usually installed vertically at the same spacing as the wall studs, and provide fixing points for the wood siding panels or any type of exterior finishing. Many wall systems come with proprietary batten designs, where the board and batten system is designed in one so everything easily clips together at the correct spacing.
Installing vertical sidings is a great option in rainscreen design. Excess water has a tendency to build up and pond in joints of horizontal siding, unlike vertical siding where water would normally run down the joints. Any excess water that makes it past the rainscreen into the air gap is filtered down and out the weep holes by gravity and evaporation.
Weep Holes & Ventilation Gaps
The outer skin needs regular weep holes at the bottom of each level to drain any moisture that gets past the exterior skin and drains down, similar to breaks in cavity brick wall designs. Adding ventilation holes to the exterior at the top of your wall is not necessary but also a great idea as it enhances ventilation through the air gap by using the stack effect. This is where opposing pressures draw air up through the air cavity and it gets sucked out the ventilation holes at the top. Ventilation is one of the most effective drying methods for the air cavity, so it's a great idea to have these extra gaps at the top. To prevent unwanted insects, ants and bugs getting into the wall many builders shield these holes from the inside with meshes or products such as ‘ridge-vent’.
Orientation & Residential Roof Design
Frequently residential roof designs under-calculate the width of eaves and overhangs, which are important particularly in rainy and very hot environments. Package homes also often land on sites with little attention to prevailing wind and rain directions and solar orientation. In these cases adding rainscreens to your home can help with protection from the rain, drainage, and insulation. They are also essential in some modern home designs, where the roof wraps directly into wall with no eaves or protection from the elements.
More and more contemporary home designs are building rainscreens into their specifications as standard. The extra cost up front (estimated at around 30% more than standard siding) is proving to pay off in the homes long-term running costs, maintenance, durability, and energy efficiency performance. If your builder is keen to skip this step, because of the extra details and costs, it's important to demand they investigate rainscreens as part of the overall wall system for their long-term benefits and cost savings. States and cities are also becoming aware of the environmental benefits of rainscreens, such as Oregon, mandating a ⅛ inch deep air-gap for all new homes back in 2010.
Rainscreen helps manage moisture between the outside layer of your wall and its inside skin. Understanding rainscreen design for both new and existing construction can help your home’s durability, weather protection, and energy efficiency.