What are the most common DIY mistakes homeowners make when adding attic insulation in a Saint John home?

The most common DIY attic insulation mistake in Saint John is adding insulation without air sealing first — and it single-handedly wastes 30-50% of the insulation's potential benefit. But there are several other mistakes that are just as predictable in NB homes, and Saint John's coastal conditions make some of them worse than they'd be elsewhere.

Skipping Air Sealing Before You Insulate

This is the big one. Insulation slows heat transfer — it does not stop air movement. When you blow or lay batts over an attic floor that still has open pot light housings, unsealed plumbing stacks, bathroom fan penetrations, and attic hatch gaps, warm interior air keeps flowing straight through those holes into the cold attic. That air carries moisture with it, and in Saint John's Maritime climate — where humidity is persistent year-round — that moisture condenses on cold roof sheathing and framing. The result is mould and rot that hides above your new insulation where you can't see it.

The fix is straightforward: before you add a single bag of insulation, go into the attic with a can of spray foam and acoustical sealant and seal every penetration you can find. Pot light housings need to be covered with an airtight box (or replaced with IC-rated airtight fixtures). The attic hatch needs weatherstripping and an insulated cover. Plumbing stacks, electrical wires, and bathroom fan ducts all need to be sealed at the attic floor. This step costs $100-$300 in materials and delivers more than half the total energy benefit of the project.

Burying the Soffit Vents

Saint John homes — especially the older stock in the North End, East Saint John, and the West Side — often have shallow roof pitches and tight soffit-to-ridge clearances. When homeowners blow cellulose or fibreglass into the attic without first installing ventilation baffles (also called chutes or rafter vents) at every soffit bay, the insulation piles up against the eaves and blocks the soffit vents entirely.

This is a serious problem. Blocked soffit vents eliminate the intake side of your attic's ventilation system, trapping moisture in the attic space and creating exactly the conditions that produce ice dams — which Saint John already sees plenty of. The NB Building Code requires 1 square foot of net free ventilation area for every 300 square feet of attic floor area, balanced between soffit intake and ridge exhaust. Foam or cardboard baffles stapled to the roof sheathing at each rafter bay before insulating maintain that critical airflow channel. Don't skip them.

Not Going Deep Enough

Most Saint John homes built before 1990 have R-20 to R-28 in the attic — sometimes less in older houses. The NB Building Code minimum for new construction is R-50, and R-60 is the recommended target for optimal performance in our climate. Saint John accumulates roughly 4,800 heating degree days per year, and that gap between R-20 and R-50 represents a significant amount of heat escaping through your ceiling every single winter.

A common mistake is adding just enough insulation to cover the joists and calling it done. At R-3.5 per inch for cellulose, reaching R-50 from scratch requires about 14 inches of insulation. Topping up from R-20 to R-50 still requires adding roughly 8-9 inches. Many DIYers underestimate how much material they need and end up with R-35 or R-40 — better than before, but leaving real money on the table.

Compressing Batts or Leaving Voids

If you're laying batts rather than blowing in loose-fill, fit matters enormously. A batt with even a 5% void area — a gap behind a wire, a corner that isn't quite filled — loses up to 25% of its rated R-value. Batts must be cut precisely, split around wires rather than bent over them, and never compressed to fit a shallower space. An R-20 batt crammed into a space that's only 4 inches deep gives you something closer to R-13.

Missing the Rebate Window

This one isn't a technical mistake, but it costs Saint John homeowners real money. NB Power's Total Home Energy Savings Program and the Canada Greener Homes Grant both require a pre-retrofit EnerGuide evaluation before any work begins. Homeowners who insulate first and ask about rebates later are ineligible — full stop. Combined, these programs can put $5,000-$10,000 back in your pocket on a comprehensive attic upgrade. Book the energy evaluation first, then do the work.

If you're planning an attic project and want a professional to handle the air sealing and blowing, New Brunswick Insulation can match you with a local contractor at no cost. Browse insulation professionals in the Saint John area through the New Brunswick Construction Network directory at newbrunswickconstructionnetwork.com/directory?category=insulation.