Drainage planes and air spaces: Siding installation tips so your walls can dry.
The walls of your home are an environmental separator. Their job is to keep the inside in and the outside out. Exterior cladding is your first line of defense against weather elements, and its job is to allow interior control layers to do their jobs without being assaulted by wind, precipitation and UV rays.
Walls in heating/cooling climates like Canada need to be designed to dry during all seasons, not just winter, which is how we most often build them now.
Moisture will always flow from areas of high concentration to low, and the side of the wall those different conditions are on will reverse between seasons due to insulation and/or air conditioning. In winter it is cold and dry outside, in summer it is hot and humid outside, hence the need to dry in both directions.
In order for something to dry, an exchange of energy needs to take place. When we heat a house, it dries outwards, when we cool a house, it dries inwards (or at least it should). A well-ventilated air space between sheathing and cladding is an important part of the strategy needed to allow walls to shed moisture in all seasons and conditions.
Drainage planes (building paper, house wrap, etc.) allow any moisture that does accumulate behind cladding a chance to drain away harmlessly. The air space between cladding and sheathing should also allow humid air to escape. In order for both of these actions to take place, there needs to be a continuous space behind cladding where water can drain and air can flow.
Common siding installation mistakes to avoid:
- Caulking the top cladding board to trim boards: This creates a dead end that traps humidity in walls.
- Cutting boards short and relying on caulking to fill the gap: Caulking will eventually fail, letting water in but also potentially keeping it in. Boards are best installed tight, a certain amount of shrinking will occur but any water that gets in will be able to dry. Be sure to seal all cut ends with paint.
- Installing horizontal furring to attach siding: If your siding requires horizontal furring (such as board and batten), install a first layer of vertical furring to allow a drainage cavity, then a second layer. Horizontal furring strips can stop air flow and prevent water from draining out. Diagonally installed furring is an option that can also work without needing a second layer.
- Double furring at the corners: corners are often needlessly fortified with furring strips (or strapping), where they should ideally be left open and able to dry. If water is going to leak into walls it will be at joints and junctions like corners.
Air spaces behind siding allows water to drain and moisture to escape. © Ecohome
How much of an air space is needed behind cladding?
The air space between cladding and sheathing doesn't need to be great but it needs to allow continuous drainage without creating pockets where water can be trapped inside wall assemblies (referred to as perched water). It should also allow air to flow through from top to bottom, taking advantage of natural convection to remove moisture.
1x4 and 1x3 furring are most commonly used as they are easily available and affordable, but that doesn't mean you can't find even cheaper alternatives that could save some wood and maybe some money for someone that likes a DIY project. If you have enough material to meet your needs, you could use old flooring, even old plywood ripped into strips on a table saw.
An air space of even 1/8th of an inch will do the job, so what you use as furring is really only a spacer and isn't too important as long as your siding can grab it, or grab the sheathing behind with longer nails. The most important thing is that the material be consistent in size to avoid walls looking warped.
Air Air space behind siding for drying and drainage © Ecohome
Can you keep water and moisture out of walls?
Some of the things we do that were originally intended to keep walls dry (polyethylene vapour barriers, for example) are now more commonly keeping them wet. We have grown to rely heavily on caulking - often referred to as 'liquid carpenter' - as a a cure-all for every mistake, and to allow for quicker installation by cutting less precisely.
Caulking always looks great at first, but it will break down over time (more quickly on south facing walls), and at some point may actually hold water in instead of keeping it out.
Setting out on a mission to stop all water and moisture from ever getting into your walls will more likely ensure that they will get wet and stay wet. The walls that will last the longest are the ones that have been designed to realistically manage moisture and dry out, not those designed to achieve the unachievable. With a few material exceptions of course, for the most part it is okay for walls to get a litte wet as long as they can also dry out.
An analogy on this topic credited to Professor John Straube of the University of Waterloo may help you accept this defeat graciously: with a price tag of several billion dollars, a Trident ballistic nuclear missile submarine is one of the most sophisticated and expensive machines ever made. With unlimited funds and access to the most advanced technologies, in the end they still assume water will leak in and they install a pump. Conclusion: you can't build something that doesn't leak, so focus on making sure it can dry when it does.
- Building Science Corporation: Drainage Planes and Air Spaces