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Dew Point will happen differently for the various climate zones we live in - cold, temperate, arid or tropical. For each zone, planning the correct insulation and air tightness strategy is critical because dew point is affected by many factors - seasonal temperature change, sunlight and air pressure. Your Energy Advisor can help determine the best strategy.

When you plan a retrofit for air tightness and insulation to the exterior, considerations for dew point and drying potential must be part of designing the most appropriate layering.

Two important points to remember:

1. Do as much as you can to prevent water from entering the assemblies.
2. When water does enter the assembly, use materials and methods that deal with water effectively. Allow for drying potential, inside and out.

HOW DO YOU DO THIS?

• Keep impermeable materials warm. Vapour barriers and hard elements like plywood or OSB sheathing must be placed inside the thermal boundary.
• Use effective mechanical ventilation to keep humidity levels balanced. And HRV or ERV can help keep air fresh and moving around all year long.
• Use interior and exterior back vented finishes. Give moisture and humidity a chance to dry out.
• Use smart air and vapour barrier materials.
• Use durable and long-lasting materials for flashing and moisture membranes.

In Canada's 2020 National Building Code - Division B, Section 9.25.5.2, there are instructions for how a building envelope assembly should treat low permeance materials - these are layers that don't allow air or moisture to pass through. The following diagrams from the 2020 NBC Illustrated User's Guide show the requirements for assemblies with low permeance layers in different locations: