How to install cladding 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 the control layers – like your vapour barrier, air barriers and insulation to do their jobs without being assaulted by wind, precipitation and UV rays.

By Mike Reynolds

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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 the winter it’s cold and dry outside, in summer it’s hot and humid outside, so walls should be designed to dry in either direction as needed.

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 behind exterior cladding is an important part of the strategy needed to allow walls to dry, particularly in the warmer months.

Drainage planes [building paper, weather barrier, etc.] allow any moisture that does accumulate behind cladding a chance to drain away harmlessly, and the air space allows humid air to escape. In order for both of these actions to take place, there must be a continuous space behind cladding where water can drain and air can flow.

Common practices that are best avoided:

    • • Sealing the top cladding board to trim boards with caulking: this creates a dead end that traps humidity in walls.
      • Cutting boards short and relying on caulking to fill the gaps at the ends: 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], first install a layer of vertical furring to allow a drainage cavity, then the horizontal as a second layer. Horizontal furring strips used alone 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.
  • Mike Reynolds is a former home builder, a LEED for Homes Green Rater and the editor of Ecohome.net.