Attic vapour barriers go on the warm side of insulation — meaning the ceiling side, between the drywall and the attic insulation. The barrier prevents warm interior moist air from rising into the cold attic and condensing in the insulation. Without proper vapour control, attic insulation gets wet, R-value drops, and mould grows on framing.
This guide covers attic vapour barrier installation for a typical renovation: ceiling drywall removal, vapour barrier install, then drywall and finishing.
Before you start
Three things to settle:
1. Vapour barrier on warm side (ceiling side). In Canadian climate, warm interior air carries moisture. The vapour barrier on the warm side blocks this moisture from migrating into the cold attic. ALWAYS on warm side — wrong side traps moisture in the wall.
2. Continuous and sealed. Gaps in vapour barrier defeat its purpose. Tape every seam (Tuck Tape), seal at penetrations (recessed lights, bathroom fans, electrical), and continue at edges (where ceiling meets exterior wall).
3. Ventilation must be maintained. Attic ventilation (soffit vents in, ridge vent out) prevents moisture buildup in attic itself. Vapour barrier blocks moisture entry; ventilation removes any that does enter. Both must work together.
What you'll need
Tools
- Stapler
- Utility knife
- Tape measure
- Caulk gun
Materials
- 6-mil polyethylene vapour barrier (sheets or rolls)
- Tuck Tape (sheathing tape)
- Acoustic sealant (Tremco)
- Spray foam (Great Stuff) — for sealing penetrations
- Plastic 'gaskets' for electrical boxes — code-required vapour-tight gaskets
Recessed lights need IC-rated for insulated ceilings
Standard recessed cans heat up; combustible insulation around them is fire risk. IC-rated cans (Insulation Contact rated) are designed for direct insulation contact.
Verify all recessed cans in attic ceiling are IC-rated before insulating. Replace non-IC-rated.
Step-by-step instructions
Remove existing ceiling drywall (if renovation)
If renovating: remove old drywall to access framing.
If new construction: framing is exposed already.
Inspect framing for damage and existing wiring.
Install IC-rated recessed lights and other ceiling fixtures
Install fixtures BEFORE vapour barrier.
IC-rated cans for recessed lights.
Boxes for ceiling lights and fans.
Bathroom fan housings.
Install vapour barrier sheet over framing
Cut polyethylene sheet to size of room.
Stretch tight; staple to bottom edge of joists.
Sheets overlap 6-12 inches at seams.
Cover entire ceiling area.
Tape every seam with Tuck Tape
Tuck Tape (red sheathing tape) over every seam between sheets.
Pressure-roll the tape — full adhesion.
Check continuous coverage.
Seal at electrical boxes and recessed lights
Cut barrier carefully around each box.
Vapour-tight gasket (plastic ring) on the box; barrier sealed to gasket with sealant.
Or: tape the cut edges of barrier directly to the box housing.
No gaps.
Seal at bathroom fans
Bathroom fan housing should have a vapour-tight gasket.
Cut barrier around housing, seal with tape and sealant.
Fan must vent to outside (through roof or soffit), not into attic.
Seal at edges (top plate of walls)
Where vapour barrier meets the top plate of exterior walls: acoustic sealant bead.
Continuous barrier around perimeter.
Wall vapour barrier (if any) connects to ceiling barrier — air-tight assembly.
Add insulation in attic
After vapour barrier, install attic insulation.
Blown-in cellulose or fibreglass: R-50-60 typical.
Batts: less common in attics; if used, R-50+ in 12+ inch deep system.
Vapour barrier on warm side, insulation cold side.
Maintain attic ventilation
Soffit vents not blocked by insulation (use rafter chutes/baffles).
Ridge vent or roof vents allow air out.
Continuous airflow through attic prevents moisture and heat buildup.
Install drywall over vapour barrier
1/2 inch (or 5/8 inch fire-rated for garages/code-compliant areas) drywall.
Screw to joists per drywall practice.
Tape, mud, prime, paint.
Common mistakes that cost you
- Vapour barrier on cold side. Reverses moisture flow, condensation in wall. Always warm side (interior).
- Untaped seams or unsealed penetrations. Vapour leaks through gaps. Tape and seal everything.
- Non-IC-rated cans in insulated ceiling. Fire risk. Replace with IC-rated.
- Bathroom fan venting into attic. Moisture in attic = mould. Vent to outside.
- Insulation blocking soffit vents. Attic ventilation fails. Use rafter baffles.
Frequently asked questions
Polyethylene or smart vapour retarder?
Polyethylene: traditional, 6-mil, full vapour barrier. Cheap.
SmartVapor (CertainTeed MemBrain, Tyvek SmartVapor): variable permeability — closes when wet, opens when dry. More forgiving in case of leaks.
For most residential, polyethylene with proper installation is enough.
Can I add a vapour barrier to existing attic without removing drywall?
Difficult. Vapour barrier should be on warm side (under insulation, on top of drywall — but you can't reach there from above).
Practical: install above existing insulation (cold side) — but that's wrong side.
Alternative: use vapour-retarding paint on ceiling drywall (BIN, Zinsser DIF) — provides modest vapour control without major work.
What about cathedral ceilings?
Cathedral ceilings: vapour barrier same approach (warm side, between drywall and insulation).
Closed-cell spray foam in cathedral: provides vapour control as part of the assembly. No separate poly needed.
See cathedral ceiling article for full details.
When to call us instead
Attic vapour control and insulation are part of every full-home renovation. We coordinate insulation, ventilation, and air sealing as one system.
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