What kind of vapor barrier do I need in my walls for New Brunswick's climate?

For New Brunswick's climate, you need a Class I vapor barrier (polyethylene sheeting) on the warm side of your insulation, which means on the interior face of your wall assembly before drywall installation.

The National Building Code of Canada 2020 (adopted by New Brunswick, effective May 1, 2025 for new applications) requires a vapor barrier with a permeance rating of 60 ng/(Pa·s·m²) or less. The most common and code-compliant option is 6-mil polyethylene sheeting, which provides excellent vapor control for our cold climate conditions.

Why New Brunswick Needs Interior Vapor Barriers

New Brunswick's harsh winters create significant temperature differentials between heated indoor air and frigid exterior conditions. Without proper vapor control, warm, humid indoor air can migrate through wall cavities and condense when it hits cold surfaces, leading to moisture damage, mold growth, and structural deterioration. Our climate zone requires vapor barriers on the warm (interior) side to prevent this moisture migration.

Installation requirements are critical for effectiveness. The polyethylene must be continuous with all seams overlapped by at least 150mm (6 inches) and sealed with acoustical sealant or vapor barrier tape. Penetrations for electrical boxes, plumbing, and other services must be carefully sealed. Many contractors use vapor barrier boxes around electrical outlets or apply expanding foam around penetrations.

For heritage homes built before 1950 (common in Saint John's Uptown, Fredericton's downtown, and rural Loyalist-era properties), retrofitting vapor barriers can be tricky since these homes were designed to "breathe." Adding vapor barriers without addressing air leakage can sometimes trap moisture. A building envelope specialist should assess older homes before major vapor barrier work.

Modern alternatives include vapor barrier paint for minor renovations where installing poly sheeting isn't practical. However, these paints typically don't achieve the same low permeance ratings as 6-mil poly and aren't suitable for new construction or major renovations.

If you're planning new construction or a major renovation requiring building permits, contact your local authority - municipalities like Moncton (856-4375) or Fredericton (460-2020) in incorporated areas, or your Regional Service Commission in rural areas. The vapor barrier installation will be inspected before drywall installation, and proper installation is essential for passing inspection and maintaining your home warranty coverage.

For complex wall assemblies or heritage retrofits, consult with a building envelope specialist who understands New Brunswick's specific climate challenges and code requirements.