moisture management explained for cold-climate builders and renovators

Most people think moisture problems only come from leaks. In reality, moisture moves through buildings in several different ways. Understanding these mechanisms is the foundation of building science.

'Bulk' water in the form of rain (or melting snow) is often the largest moisture load a building experiences. Water gets behind siding or roofing at windows, doors, corners, or junctions. Damaged/old materials that need to be replaced are a second (but sometimes way more obvious) cause of leaks. The challenge is when an assembly cannot drain or dry properly, materials begin to deteriorate.

Air leakage moves huge amounts of moisture. Pressure differences, the wind and the stack effect all contribute to moving air. Heat and moisture tag along with air movement. Warm, moist air leaking into cold assemblies can deposit large amounts of moisture through condensation. This is one of the biggest reasons:

• attics get moldy
• roof sheating rots
• insulation performance drops
• walls develop hidden damage

Water vapour is pushed around by pressure differences. It naturally moves from areas of high concentration to low concentration. The movement is much slower than air leakage, but it's still very important.

Vapour control strategies depend on several factors, so there's no one-size-fits-all solution. In cold climate construction and renovation, you need to take into account things like:

• climate zone
• insulation placement
• material permeability
• assembly design
• drying potential
• envelope air tightness (now and in the future)

The phrase 'houses need to breathe' confuses controlled air leakage with drying potential with uncontrolled ventilation. They are definitely not the same thing.

Read more: '5 Rules for Comfortable and Healthy Homes'

People generate a surprising amount of moisture indoors, especially in cold Canadian winters when the windows are always closed and households spend a lot of time indoors. Without proper ventilation, this moisture accumulates inside the building.

This can contribute to symptoms of moisture problems like condensation and mold growth that cause poor indoor air quality. These are some of the common problems that lead to comfort complaints.

How we build houses keeps evolving. Since the 1980s, more insulation, lower air tightness targets, better windows, exterior insulation, and heat pumps have become more common in both Canada and the US.

We focus on Canadian code regulations because we're Canadian, eh? But there are similar requirements in the US under the IRC (International Residential Code) and the ASHRAE 62.2 ventilation standard.

In Canada, the difference between how we built prior to 2020 version of the National Building Code of Canada, or the BC Energy Step Code, or Ontario's S.B.-10 is this: tiered energy performance targets. We're now in an era where 'nice to have' is the legal, code compliant requirement

These change to energy performance targets will improve comfort and effeiciency, but they also change how buildings handle moisture. Details that may have worked before can now create searious moisture risks in tighter assemblies in both new homes and retrofit projects.

For builders and renovators, understanding how air leakage rates and material choices affect condensation risk and drying potential is becoming an essential jobsite skill. Control layer continuity can solve things like thermal bridging but we need to look at how the whole assembly is going to perform.

For renovators, we know the learning curve is always steeper, because when you open up a wall, you always find all sorts of surprises. You have always needed to be prepared to solve puzzles and Part 10 of the National Building Code of Canada (Alterations to Existing Buildings) adds another dimension. Don't let Part 10 requirements surprise you!