Short answer: Fix the cause of any moisture before you insulate or frame. Natural Resources Canada is blunt: "Do not insulate a basement with moisture problems from the inside," or "your new walls will rot." Most basement moisture comes from outside water (grading, downspouts, cracks), from the ground through the concrete, or from indoor humidity condensing on cool walls. Each has a different fix.
Signs to look for
Natural Resources Canada and CMHC list these warning signs:
- Damp spots or staining on walls or floor
- White, powdery deposits on concrete (efflorescence)
- Peeling or blistering paint
- Mould on joists, stored items or in corners
- A musty, damp smell
Find the source: the plastic sheet test
NRCan's Keeping the Heat In suggests taping a sheet of plastic about 1 m square to the wall and floor, and checking it after a day or two:
- Moisture on the room side: indoor humidity is too high and condensing on cool concrete.
- Moisture underneath: water is moving through the concrete from the ground.
Fixes by cause
| Cause | Typical fix |
|---|---|
| Surface water from outside | Slope grading away from the house; extend downspouts at least 2 m (6 ft) from the foundation |
| Cracks and leaks | Repair cracks; exterior waterproofing and drainage for major leaks |
| Water through the slab or walls | Address outside water first; vapour-control strategy matched to the insulation |
| Indoor humidity | Dehumidify in summer; keep winter humidity at or below about 45% (CMHC) |
| Sump or backup problems | Repair the sump, fit a tight cover, address backup risk |
CMHC adds that a damp basement "may generate much more moisture than all other sources combined", so fixing it helps the whole house.
Two easy-to-miss points
New concrete needs time. NRCan advises allowing about a year of drying before insulating new concrete.
Vapour barriers. NRCan cautions against adding polyethylene over rigid foam board, which can create a double vapour barrier, and recommends polyethylene only in basements proven dry. Your insulation and vapour-control approach also has to meet the Building Code, which is checked through your permit inspections.
What this means for your project
Moisture work is usually cheaper before finishing than after. Once walls are closed, as NRCan notes, "interior finishes hide or obscure moisture problems as they develop." If your basement shows any of the signs above, address them first, then plan the finish. Our basement renovation page covers the full process, and our basement finishing cost guide covers budgeting.
Not sure if your basement is ready to finish? We will look for signs of moisture during your free estimate and explain what needs attention before framing starts.
REQUEST A FREE ESTIMATESources: Natural Resources Canada: Keeping the Heat In, Chapter 6, Basement insulation (PDF) · CMHC: Moisture and Air (PDF)