This winter, stay warm… and dry!

When we mention winter’s cold and how to keep our homes cozy, the first thing that likely comes to mind is good insulation.
And we are right, of course! Poor insulation will make your heating bill explode in the dead of winter.

But is that the only criterion needed to evaluate the performance of a wall? Not according to the National Building Code.

In addition to the insulation aspect (the well-known R-value) and strength, the National Building Code pays particular attention to moisture control.

Moisture: the #1 Enemy

Moisture control means preventing condensation, water, and moisture from accumulating inside your walls (be it in the wall cavity or on the interior surfaces).

On top of potentially reducing the energy efficiency of your wall insulation, moisture can cause rot and mould to form in your walls. Rot will attack your home’s structural integrity, and mould poses a serious health risk.1

We cannot see what is going on inside the walls once they are finished. That is why prevention and control are so important.

Where does the moisture in your walls come from?


During the cold season, the difference in temperature between outside and inside your home is greater. The water vapour contained in warm air transforms to liquid water upon contact with cold air: this is called condensation.

Condensation occurs where hot and cold meet… in your walls!

The higher the moisture content of the air, the higher the temperature at which condensation takes place. Therefore, moisture content plays an important role, because in a humid environment, air condenses over a longer period of time and will lead to more moisture inside your walls.

The other components in our home

When you build a house, the materials all contain a certain quantity of water.

For example, a concrete foundation is left to dry until it reaches the desired strength and durability, but it will not yet be totally moisture free when the walls are built. The same goes for the framing wood: a wood structure can contain up to 19% moisture at the time of installation, as prescribed by the National Building Code.

Materials with such high moisture content will continue to release water particles into the air inside the walls once construction is finished.

Solutions to reduce moisture in your walls

Move the condensation point

To move the condensation point (where the water vapour turns to liquid) outside the wall cavity, we have to keep the wall cavity from getting too cold. Condensation will take place where warm air meets cold. We want this condensation point to be located outside the wall cavity, where there is little moisture (and therefore little opportunity for condensation to occur)!

The key is insulating the outside of your walls. With enough exterior insulation, your walls will stay warmer, and the condensation point will move towards the exterior cladding. If condensation happens there, water will then simply flow outside.

Remember that several factors cause the condensation point to vary. These include the temperature and humidity rate, to name only a few. So it is almost impossible to guarantee that the condensation point is located outside the wall assembly at all times (if you want walls of a reasonable thickness!) That brings us to the next point.


Let your walls breathe

After everything we just saw, you might be tempted to wrap your home in an airtight liner to keep the cold and moisture from touching your walls. But keeping your walls from breathing could have the opposite effect! Excessive air tightness traps the air moisture inside your walls, worsening the situation.

Instead, it is important to have appropriate wall ventilation so your walls can expel excess moisture.

Can you imagine your bathroom without any ventilation? The same goes for the rest of your home: getting the moisture out of your walls is the healthy solution for inside your home.

What can I do before building to control moisture?

The National Building Code has provisions covering moisture control in its sections on thermal insulation (, the air barrier system ( and the vapour barrier ( When you comply with the Code, you are sure to build a healthy home. You can also choose assemblies that exceed the standards, such as using R-5 XP panels from BP Canada.

BP Canada has prepared a document presenting a whole range of wall assemblies designed specifically for the Quebec Construction Code. You will find it at


1 To find out more about the health effects of mould, please see the Government of Canada’s site.