Current building codes require enhanced levels of thermal performance when renovation permits are applied for. That can have a large cost impact on a renovation project. It can also create a genuinely large improvement in how your house performs.
A wall or roof gets its thermal performance from three things working together, not from insulation alone.
The vapour retarder
Interior moisture is created simply by living: breathing, sweating, cooking, cleaning, drying wet clothes. From simple physics, moisture moves from warm to cold. So during the cooler parts of the year, interior vapour is constantly trying to push out through your exterior walls and roof, from the warm inside toward the cold outside.
If too much of it gets through, it condenses when it reaches a surface cold enough, such as the framing on the outside of the wall. Now there is bulk moisture sitting in the wood framing and in the insulation, and that leads to mould, structural damage, and insulation that no longer insulates.
The vapour retarder slows the passage of water vapour diffusing through the wall surface into the assembly. It should keep vapour low enough that the wall can naturally dry out before it builds up to damaging bulk water.
The rule that gets broken most often: the vapour retarder must be on the warm side of the insulation to be effective. Put it on the cold side and you have built a condensing surface inside your own wall.
It does not have to be polyethylene film. It can also be a primer, a paint or a wall finish that retards vapour passage.
The air barrier
If interior air can escape to the exterior, the warmth you paid to create is out there heating the neighbourhood. That is the obvious cost.
The less obvious cost is that air travelling freely through a wall, in either direction, carries moisture with it, and that causes damage inside the assembly. Mechanical heating and cooling systems also rely on a properly functioning air barrier to do what they were sized to do.
For a wall to perform thermally, the air barrier has to be:
- Strong and durable
- Continuous, sealing every area of the wall and every penetration through it
- Properly supported
It can sit anywhere within the exterior wall assembly as long as it is continuous. It can be the same material as the vapour barrier, such as polyethylene film installed on the warm side. It can be an airtight foam insulation inside the wall. It can be a membrane on the exterior of the sheathing.
The insulation itself
Insulation is the part everyone thinks about first and it is the part that does the least on its own. Batts in a stud bay with a leaky air barrier and a vapour retarder in the wrong place will underperform their rating and can make the assembly wetter than it was before.
Get the sequence right and the same insulation transforms an older Okanagan house: quieter, cheaper to heat and cool, and no longer accumulating moisture where nobody can see it.
What this means for your project. When a renovation triggers a thermal upgrade, the cost is real. So is the benefit, but only if the three layers are detailed as one system. That detailing is the difference between a wall that performs and a wall that just has insulation in it.
