Rainscreen Wall Systems for Wet Climates
Apr 20, 2023
The April rains are here! I can recite in my head the well-known saying that ‘April showers bring May flowers.’ After a brief streak of unusually hot days in the second week of April, we are now experiencing more typical spring weather with some overcast and rainy days amidst the warmer sunny ones. Since I am commuting regularly between the Catskills of New York and Philadelphia by train, I have been spending time pondering the rain and its effects on buildings. This is the perfect time of year to assess the exterior siding and roofing materials of your home following any harsh winter weather and determine the maintenance that might be needed before summer’s heat. Evaluating areas prone to water damage or leaks is utmost importance to prevent further deterioration or mold development.
Having grown up in the Pacific Northwest, I am quite familiar with frequent rain that persists for days. Our homes serve as perfect shelters to keep us dry, drinking tea or hot cocoa, while watching and listening to the rainfall outside. I’ve also experienced the intense but brief afternoon monsoon rains when living in the Southwest, pattering loudly upon the clay tile and metal roofing of homes, and creating an amazing petrichor from creosote plants when the rain subsides. I also recall rainy days at summer camps in Oregon that were not so pleasant since even though there were cabin shelters for our bunks, these were not fully enclosed, and things inside would get wet!
The designs of houses and shelter originate from a need to protect us from the elements, but the science of water and moisture interactions with building materials and systems is one that has evolved in more modern times. The walls and roof of a house are described as the building enclosure system, a series of different materials layered together to create the ideal weather barrier for the given climate. Building enclosure systems vary for different climates since the temperature fluctuations and humidity, rainfall, snowfall, and freeze-thaw conditions will require different solutions. Insulation thickness, vapor barrier placement, air-gaps, and exterior sheathing and siding materials will be unique for each climate type. For rainy and wet climates, it is important to keep moisture away from any primary structural framing and also important to allow water a way out from any layers of the enclosure system it might find its way into.
Waterproofing is a necessary layer in the building enclosure system, giving a secured seal to the walls and roof to keep water outside and away from the building’s structural elements. Waterproofing serves as the direct underlayment between exterior cladding, siding, and roofing materials. Just as our raincoats are the last layer to put on before we put up our umbrella, waterproofing is the last layer before the finish materials on the enclosure system. Think of the finish materials as the hat, gloves, scarf, and umbrella! In wet climates, where annual rainfall approaches 40-inches or more accumulatively, rainscreen wall systems are a necessity.
A rainscreen wall system is one that creates and air-gap between the waterproofing membrane and the exterior finish materials. This is accomplished with the use of furring strips, which are smaller wood or sometimes plastic strips that are attached to the framing system (studs or masonry) or exterior sheathing. The final finish materials, which are those that are visible from the exterior and create the visual look of the house, can be made of numerous material options. One of the most common is either wood or vinyl siding, but stone, metal, and composite materials are also used. My preference is to use exterior cladding materials that are healthy for workers and the environment from their sourcing, manufacturing, transport, installation, and end-of-life recyclability or re-use potential. This would generally exclude any vinyl or plastic-based composite materials.
The exterior finish materials of a rainscreen system will be taking the brunt of the weatherization, from UV-rays and wind-driven rain, these materials should be durable and low-maintenance. Water- and UV-resistant coatings are important for the exterior finish materials. Behind the finish materials, there should be a minimum of a ¼” air gap, which is created naturally by the furring strips. It is best to choose furring strips that are ½” so that the ¼” clearance is secured. The surface on the other side of the air-gap is the waterproofing membrane, so any moisture that drives into the cavity, by wind or vapor pressure, will be stopped from going further into the wall system and prevents moisture from penetrating to the insulation or framing system. For water that settles inside the cavity, it will either evaporate or drip downwards following the flow of gravity.
Weep holes are created at regular intervals along the height and width of the wall to allow for water to drip back to the outside of the wall. Weep holes are small openings that have a piece of flashing (break metal) that is formed to drip the moisture to the outer part of the wall with an angled lip. Weep flashing is also installed along the entire base of a rainscreen wall system. In wet climates, the rainscreen system is the best way to ensure proper drying of exterior wall materials and prevent moisture from penetrating to important insulation and structural elements of the wall. Deep roof overhangs with adequate gutters will provide additional protection of exterior walls in super rainy climates. These regions tend to occur along coastlines, and include the Pacific Northwest, parts of the Northeast, and southern Florida.
Older structures in these locations may not have been designed with the current building science knowledge of proper rainscreen layering. If you are planning any exterior siding replacements for your house and want to know the best selection for your location and climate, reach out to AIDA, LLC today for a consultation. You can always find more information and healthy home resources at Aletheia Ida Design and Architecture, LLC (AIDA, LLC) at www.aletheiaida.com.