Other than higher cost over what we consider snake-oil exterior siding products (cement-board, vinyl, aluminum), the biggest problem with wood siding is the potential for mold growth and rot. Rot, mold, and mildew are all caused by fungi which flourish pretty much wherever there is water, food, and oxygen.
The “food” that fungi metabolize is any organic material, and in the case of wood, hemi-cellulose. It is important for wood siding to be able to dry out quickly to prevent fungal growth, especially in regions with wetter climates.
Our shou sugi ban is naturally resistant to fungal growth due to the traditional heat treatment that burns off hemi-cellulose content, but same as all wood siding it is still critically important to install it by best practices to promote rapid drying and in turn maximize wood longevity. The best way to prevent fungal growth is by preventing moisture build-up in the wall assembly with a rainscreen.
Before the North American post-war construction boom and development of engineered and resin building materials, exterior siding was installed over solid-sawn shiplap sheathing and then later asphalt-impregnated felt paper. This construction allowed the wood to dry out quickly, and together with a higher grade of available wood at the time, is the reason old structures have siding in good condition for 100 years or more.
Modern engineered materials, such as plastic builder’s wrap (weather-resistive barrier or WRB), trap moisture behind the siding and do not allow it to dry out. This is bad. Softwood siding longevity theory is premised on using a porous softwood for expedient moisture transfer, and installing it in a way that allows it to dry out as quickly as possible.
What is a Rainscreen?
A rainscreen is an exterior skin system made up of siding, an air gap via furring strips, WRB layer, and structural sheathing substrate (nowadays OSB or plywood). The siding sheds 90% or more of precipitation, but not all of it. Counter-intuitively, the primary function of siding is not to shed water, it is to protect the WRB/substrate from UV degradation and to act as a cosmetic facade. The ventilated air gap made by furring between the siding and WRB in a rain screen allows the moisture that penetrates, or is absorbed by the siding from rain and dew, to drain and evaporate quickly. The air gap also allows any water vapor that’s expelled from the structure’s interior to escape instead of condensing and becoming trapped between the WRB and siding, creating an environment where fungi can grow.
Lightweight porous softwoods with high tannin content work best for exterior siding since they absorb and release moisture quickly (which also improves thermal performance in all seasons), and since the tannin is poisonous to fungi as another benefit. Therefore the prevalence of redwood, cedars, cypresses, and other similar species in siding applications. Note also that when these species absorb moisture the tannins can bleed out, and when installed in direct contact with resin barriers can chemically rot the WRB.
How Difficult is it To Install a Rainscreen?
Fairly simple if you know how. For horizontal siding, install vertical furring strips on top of the weather barrier every 16” on center since siding always moves and 24” on-center may not keep a flat plane over decades. If installing vertically, some municipalities may require you to attach another set of furring strips horizontally over the first set so that water can still drain downward and vapor to exhaust upward. Horizontally installed furring layout can also be vented laterally out the penetrations and corners. We also see diagonal furring layouts on vertical siding installs for proper ventilation without the more expensive criss-cross grid layout.
It’s difficult for us to give guidance on the thickness of your furring and air gap since the building science is not conclusive and local codes vary. What we have heard is that low-rise construction does not need the same air gap as mid-rise, since increased vertical height makes for more square footage of WRB and siding that need vapor exhausted to stay dry. Additionally we are not sure of how solid-sawn furring, ripped plywood strips, or resin furring will compare in performance in the long run. Then there is also the question of whether chemically-treated furring is preferred and what type of chemical treatments are acceptable or which are not, what width is required for each furring spec, and how various species compare for solid-sawn furring. However, from what we have seen our customers use, we know that non-chemically-treated solid-sawn Douglas fir or equivalent species, in #2 and better grade, 3/4″ or more in thickness, and 3.5″ or more in width, works great. We recommend consulting with your architect and builder for the best solution since each project is different.
Rainscreen Installation Considerations
It is extremely important to note that the dimple, drain, grid, and mesh wraps available on the market nowadays are designed to minimize cost, and though better than installation directly over WRB, are insufficient for ventilation due to volume of air movement allowed as well as capillary action. Tangentially, in our opinion these modern wraps are unacceptable since they do not offer a stable and flat substrate to keep siding in a flat plane as it tries to move dimensionally from exposure to moisture and temperature cycles over time. Basically they allow wood siding to rot and cup.
Rainscreen Siding Cost
Plan for an additional construction cost of $0.75 – $2.00 per square foot depending on spec and region. Keep in mind, remediation can cost over $60 per square foot in aggregate, not to mention litigation, medical bills from mold allergies, disappointment, lost time, and stress.
Health Implications Related to Not Installing a Rainscreen
In addition to the wood siding longevity discussion, if not allowed to dry out quickly fungi will grow within any wall system and often spread into the structure itself. Mold and fungal spores can contaminate the interior air and cause allergies, asthma, respiratory infection, and even rheumatic diseases. These illnesses can be extremely debilitating, and following improved construction practices can prevent them.
Rain screens have been a standard envelope spec in Western Europe and Japan for decades and are finally catching on in North America as well. The trend is your friend.
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