Vapour barrier installation in a basement is more nuanced than other walls. The wrong approach traps moisture and causes mould; the right approach lets the wall manage moisture appropriately. The key is understanding that modern Canadian basement insulation systems use rigid foam against concrete AS the vapour barrier — and adding polyethylene under drywall creates a moisture-trapping double barrier.
This guide explains where vapour barriers belong in a typical Ottawa basement and the right products and installation method.
Before you start
Three things to settle:
1. Foam = vapour barrier in modern Ontario practice. 2+ inches of rigid XPS foam (or 1.5+ inches polyiso) against concrete provides Class II vapour retarder per Building Science best practice and Ontario Code. No additional polyethylene needed when using foam.
2. Don't double-up vapour barriers. Two vapour barriers (foam against concrete + poly under drywall) trap moisture between them. Mould grows there. Old code allowed both; modern best practice is one only.
3. Seal seams and penetrations carefully. Vapour barrier only works if continuous. Tape every seam in the foam, seal around plumbing/electrical with Tuck Tape or specialty foam sealant. Gaps make the barrier useless.
What you'll need
Tools
- Utility knife
- Tape measure
- Caulk gun
- Stapler
Materials
- Rigid XPS foam (2-2.5 inches, R-10 to R-12.5)
- Foam adhesive (PL Premium or foam-specific)
- Tuck Tape (red sheathing tape) — for sealing foam seams
- Spray foam can (Great Stuff Pro Series) — for gap sealing
- Acoustic sealant (Tremco)
- Optional: 6-mil polyethylene (for older approach — only if not using continuous foam)
If using rigid foam against concrete, skip polyethylene
Code in Ontario allows the foam-only approach when meeting specific R-value and thickness requirements.
Adding poly creates a double vapour barrier — moisture trap. Don't combine.
If your basement insulation is fibreglass batts only (older approach), polyethylene is required.
Step-by-step instructions
Confirm wall assembly approach
Modern: rigid foam against concrete + framing + batts. No poly.
Older: framing + fibreglass batts + 6-mil poly. Less effective but acceptable in some retrofits.
Decide approach before purchasing materials.
Modern approach: install rigid foam against concrete
Cut foam to fit wall.
Apply construction adhesive to back of foam.
Press against concrete wall, hold until adhesive sets.
Cover entire wall — no gaps.
Tape every foam seam
Tuck Tape (red sheathing tape) over every seam.
Includes corners and where foam meets sill plate above.
Pressure-roll the tape — full adhesion required.
Tape penetrations (pipe holes, etc.) carefully.
Seal foam-to-floor and foam-to-rim joist
Bottom edge of foam where it meets concrete floor: spray foam (Great Stuff) bead.
Top edge where foam meets rim joist: continue foam up to rim joist (insulate rim joist with foam too).
Air seal at floor + ceiling = no air leakage path.
Frame in front of foam (for batt insulation cavity)
2×4 framing with 1/2 inch gap between framing and foam.
Or framing tight against foam (alternative).
Pressure-treated bottom plate on concrete.
Install batts in framing cavity
Mineral wool or fibreglass batts (R-13 to R-15).
Friction-fit; no compression.
Cut around plumbing, electrical.
Modern approach: skip the poly
Don't install 6-mil polyethylene under drywall.
Foam against concrete already provides vapour control.
Drywall directly over framing.
Older approach (no foam against concrete)
If you didn't use foam: install 6-mil polyethylene over framing INSIDE the basement.
Stretch tight, staple to studs.
Tape seams with Tuck Tape.
Acoustic sealant where poly meets ceiling, floor, and other surfaces.
Then drywall.
Polyethylene approach: seal at boundaries
Where poly meets ceiling: acoustic sealant bead.
Where poly meets floor: acoustic sealant or trim with foam backing.
Where poly meets electrical/plumbing: tape and sealant.
Continuous = airtight.
Inspect for completeness
Walk the wall. Look for: gaps in foam, untaped seams, damaged poly (rip and patch), unsealed penetrations.
Vapour barrier integrity is binary — works or doesn't.
Fix any issues before drywall.
Common mistakes that cost you
- Two vapour barriers (foam + poly). Trapped moisture, mould risk. Pick one approach.
- Unsealed seams or penetrations. Vapour barrier with gaps doesn't work. Tape and seal everything.
- Poly behind insulation, not in front. Vapour barrier always on warm side (interior). Reversed = condensation in wall.
- Acoustic sealant skipped at boundaries. Air leaks through tiniest gaps. Sealant continuous around perimeter.
- Insulating wet basement. Mould guaranteed. Fix moisture issues first.
Frequently asked questions
Why are basement vapour barriers different from above-grade walls?
Above-grade walls: vapour drives outward in winter (warm interior, cold exterior). Vapour barrier on interior side.
Basements: concrete is cold; vapour can drive INWARD from concrete during certain conditions. Foam against concrete is the modern solution.
Different building science — different approach.
Can I use SmartVapor (variable permeability membrane)?
Yes — Tyvek SmartVapor or CertainTeed MemBrain.
Permeability varies with humidity: closes when wet (vapour barrier), opens when dry (lets wall dry inward).
More forgiving than poly but not commonly used in residential basements.
What about my old basement with poly under drywall?
If basement is dry and no mould: leave alone.
If signs of moisture or mould: more complex remediation required.
Consider energy audit and rebuild during renovation.
When to call us instead
Basement insulation and vapour management are core to our basement renovations. We use modern foam-based assemblies that don't trap moisture.
Book a free 60-minute site visit →Related Ottawa services
If you're finishing a basement, our team handles insulation and vapour control as part of fixed-price scope. Book a free 60-minute consultation.