If your basement was finished before about 2009, there is a fair chance the insulation behind the drywall does not meet current code. Depending on how it was built, there is also a chance it is growing mould where nobody can see it.
Neither of those is scaremongering. Both come down to specific changes in how we understand basement walls, and both are worth knowing before you finish a basement or buy a house with one already done.
Why a Basement Is Not Just a Cold Room
An above-grade wall has air on both sides. A basement wall has soil on one side, and that changes everything.
Concrete is porous. Moisture moves through it continuously from the ground, as vapour rather than as visible water, and it does this in basements that have never flooded and never felt damp. A foundation wall can look and feel perfectly dry and still be passing moisture into the room all year.
That single difference is why an assembly that works fine above ground can fail badly below it. It is also why a cold basement is a different problem from one room that never warms up upstairs. Upstairs you are managing heat. Down here you are managing heat and water at the same time, and the water decides what you are allowed to build.
The Two Ways It Goes Wrong
Almost every failed basement comes down to one of these.
Fibreglass batts pressed against concrete. This is the assembly that works perfectly well on walls above ground and fails below it. Vapour coming through the wall saturates the batt. Fibreglass holds that moisture against the concrete, the material loses most of its insulating value once wet, and the cavity becomes an excellent place for mould. Mineral wool behaves the same way in this position. Neither should sit directly against a foundation wall without a rigid moisture-resistant layer in between.
Polyethylene on the wrong side. This one is more common and less understood. If the poly sheet sits between the concrete and the insulation, moisture coming in from the concrete has nowhere to go. It condenses on the plastic and stays there. Mould can follow within months, entirely hidden behind finished drywall.
The vapour barrier belongs between the insulation and the drywall, not against the wall.
The uncomfortable part: batts between studs with poly on the interior was the standard way to finish a basement for decades. Enormous numbers of Ontario basements were built exactly that way by competent builders following the practice of the time. It is not a sign anybody cut corners. It is just a method that building science has since caught up with.
What Changed in 2009
There is a second issue in older basements that has nothing to do with moisture.
The old approach was to insulate only the top portion of the foundation wall, typically to around 600mm below grade, on the reasoning that the soil below that stays relatively stable in temperature.
That has not been code-compliant since 2009. The Ontario Building Code now requires basement wall insulation to run full height, from the underside of the subfloor down to the top of the slab, at a minimum effective R-20 where the wall forms part of the building envelope.
Effective is the word that matters. It means the performance of the whole assembly including the thermal bridging through the studs, not the number printed on the packaging.
So a basement finished in 2004 to the standard of its day can be fully half-height and entirely legal at the time, and simply not meet what is asked now. Existing basements are not retroactively required to comply. But if you are opening it up, redoing it, or finishing it for the first time, the current requirement applies.
What Correct Looks Like
The assembly building scientists recommend for Canadian basements starts with continuous rigid foam, XPS or polyisocyanurate, fastened directly to the concrete.
The mechanism is worth understanding because it explains every rule above. The rigid foam keeps the face of the concrete above the dew point temperature. If the concrete surface stays warmer than the point at which the room's air would condense on it, moisture never turns into liquid water there. No condensation means no wet insulation and nothing for mould to grow in.
Studs, if you want a finished wall, go in front of the foam rather than against the concrete. Any additional batt insulation goes between those studs, where it stays dry. The vapour barrier, if the assembly calls for one, goes on the warm side.
Getting a basement done properly is less about the R-value on the label and more about the order those layers go in. A lower R-value assembly built in the right sequence outperforms a higher one built in the wrong sequence, because the wrong sequence ends up wet.
Where to Spend First
An honest note, because basements are rarely the best first move.
Heat rises. In most houses the ceiling is losing more than the foundation, and if the attic has never been upgraded it is usually cheaper and more effective to start at the top. A basement is the better priority when the space is being used and is uncomfortable, when you are finishing it anyway, or when there is a known moisture problem that needs resolving regardless.
If someone quotes you a basement before they have looked at your attic, ask why.
Older Toronto Basements
Age changes the picture considerably.
Pre-war houses across Toronto's older housing often have rubble or fieldstone foundations rather than poured concrete, with no damp proofing and no drainage layer outside. Those are a different conversation entirely and insulating them incorrectly can do real damage.
Postwar poured concrete foundations, which is most of North York, are more straightforward, though many were never damp proofed to modern standards either.
Anything from the 1980s onward generally has exterior damp proofing and a drainage layer, which makes interior insulation considerably more predictable.
Signs Something Is Wrong Behind the Drywall
You cannot see the assembly without opening it, but there are indications worth taking seriously.
A musty smell that is stronger near the walls, or stronger after rain. Drywall that is soft, stained or discoloured near the floor. Baseboards lifting or paint bubbling low on the wall. Visible mould in a closet or behind furniture on an exterior wall. Air that feels damp in the basement while the rest of the house feels normal.
Any of those on a basement finished before 2009 is worth investigating properly rather than painting over. Painting over a damp patch on a basement wall is one of the more expensive ways to save money, because the assembly behind it carries on doing whatever it was doing.
One caution on checking it yourself. Cutting an inspection hole in finished drywall is easy enough, but if there is mould in the cavity you do not want to open it up and disturb it in an enclosed space without knowing what you are dealing with. A borescope through a small hole tells you most of what you need without releasing anything into the room.