Should I use plywood or OSB for subfloor replacement in a NB home and which performs better in Maritime humidity?

For subfloor replacement in a New Brunswick home, plywood is the better choice — especially given Maritime humidity conditions. OSB has its place in wall and roof sheathing, but the continuous moisture cycling that NB homes experience makes plywood the more reliable subfloor material for flooring installations.

Why Plywood Outperforms OSB in NB Conditions

Plywood and OSB are both engineered wood panels, but they respond to moisture very differently. Plywood is made from cross-laminated wood veneers bonded with waterproof adhesive — when it gets wet, it swells somewhat but returns close to its original dimensions when it dries. OSB (oriented strand board) is made from compressed wood strands and resin, and it has a critical weakness: when the edges absorb moisture, they swell permanently and don't fully recover. That edge swelling is called edge swell, and it creates raised ridges at panel seams that telegraph through your finished flooring as visible bumps and accelerated wear.

In NB's Maritime climate, subfloors are exposed to spring snowmelt moisture, summer humidity pushing through crawl spaces, and the dramatic seasonal swings between 20% winter RH and 65%+ summer RH. Even in a well-maintained home, subfloor panels see more moisture movement than in drier inland provinces. Plywood handles this cycling far better than OSB.

Specifying the Right Plywood for NB

Not all plywood is equal for subfloor use. Specify tongue-and-groove (T&G) plywood rated for subfloor application — in NB you'll typically see this as 3/4 inch (19mm) T&G spruce-pine-fir (SPF) plywood graded CSA O325 or equivalent. The tongue-and-groove edges eliminate the need for blocking between joists and prevent differential movement between panels. Look for panels stamped with an Exposure 1 or Exterior bond classification — this means the adhesive is waterproof and the panel can handle moisture exposure during construction without delaminating.

For joist spacing of 16 inches on centre (the most common framing in NB homes), 3/4 inch T&G plywood is the standard. If your joists are 19.2 or 24 inches on centre, step up to 7/8 inch or 1 inch panels to prevent floor flex and bounce.

When OSB Might Be Acceptable

If budget is a hard constraint, OSB rated Exposure 1 with a sealed or sanded face is used in subfloor applications across NB and performs adequately when kept dry during and after installation. The key is protecting it from prolonged moisture exposure — cover it quickly after installation, never let it sit wet through a NB spring rain, and ensure your crawl space or basement has proper vapour control before installing OSB subfloor panels. Even then, plywood is worth the modest price premium.

Practical Tips for NB Subfloor Replacement

Use construction adhesive (PL Premium or equivalent) in the joist grooves before fastening panels — this eliminates squeaks and dramatically stiffens the floor assembly. Fasten with 3-inch ring-shank nails or coarse-thread screws at 6-inch spacing on edges and 8-inch spacing in the field. Leave a 3mm expansion gap between panels to allow for seasonal movement — NB's humidity swings make this gap non-negotiable.

If you're replacing subfloor over a crawl space or basement, address the moisture source before laying new panels. A proper vapour barrier (6-mil poly minimum) on the crawl space ground, adequate ventilation, and ideally a dehumidifier will extend the life of your subfloor by decades.

When to Hire a Pro

Subfloor replacement that involves cutting or sistering floor joists, repairing rot, or assessing structural damage requires professional assessment — this crosses from flooring work into structural renovation and may require a building permit from your local municipality or Rural Service Commission. Surface-level panel replacement over sound joists is a capable DIY project, but anything structural warrants a professional eye.

Need help finding a flooring professional who can assess your subfloor conditions? New Brunswick Flooring can match you with a local installer for free — browse contractors through the New Brunswick Construction Network directory.