How does spray foam insulation compare to blown-in cellulose for soundproofing interior walls in a Saint John rowhouse?

For soundproofing interior walls in a Saint John rowhouse, open-cell spray foam provides superior noise reduction compared to blown-in cellulose, but cellulose offers better value and easier installation for moderate soundproofing needs.

Open-cell spray foam delivers exceptional sound dampening because its cellular structure absorbs sound waves across a wide frequency range. At 3.5 inches thick in a standard 2x4 interior wall, open-cell foam can achieve an STC (Sound Transmission Class) rating of 39-42, which represents a noticeable reduction in conversational noise, television sound, and general household activity between rowhouse units. The foam completely fills the cavity with no gaps or voids, eliminating the sound flanking paths that reduce the effectiveness of other insulation types.

Blown-in cellulose, while not as acoustically effective as open-cell foam, still provides meaningful sound reduction at a much lower cost. Dense-packed cellulose at 3.5 pounds per cubic foot in the same 2x4 wall achieves an STC rating of 35-38 — a solid improvement over an empty wall cavity (STC 33-35) but not quite matching spray foam's performance. Cellulose's recycled newspaper fibers do absorb sound energy, and the dense-pack installation method ensures complete cavity fill without the air gaps that plague poorly installed batts.

Cost considerations are significant in Saint John's competitive housing market. Open-cell spray foam for interior walls runs $1.50-$3.50 per square foot installed, while dense-pack cellulose costs $1.25-$2.25 per square foot. For a typical rowhouse party wall (8 feet high, 20-30 feet long), you're looking at $240-$840 for cellulose versus $360-$1,260 for spray foam. The 15-25% sound reduction improvement with spray foam may not justify the 40-60% cost premium for many Saint John homeowners.

Installation timing and disruption matter in occupied rowhouses. Cellulose can be blown into existing walls through small holes drilled from either the interior or exterior, with holes patched and painted in a single day. Spray foam requires removing drywall to access the full cavity, spraying the foam, allowing cure time, trimming excess foam, and installing new drywall — a multi-day project that displaces residents and creates more dust and disruption.

For Saint John's older rowhouse construction (many built in the 1960s-1980s), moisture management requires careful attention. Interior walls don't typically need vapour barriers, but if you're treating a party wall that's partially exterior (common in end units), ensure the spray foam contractor understands which walls are interior-only versus those with exterior exposure. Open-cell foam is vapour-permeable and won't trap moisture, while cellulose naturally buffers humidity fluctuations.

Practical recommendations: If noise from adjacent units is a serious quality-of-life issue and budget allows, choose open-cell spray foam for maximum sound reduction. If you want noticeable improvement at reasonable cost, dense-pack cellulose delivers solid performance. For the best of both worlds, consider cellulose in most interior walls with spray foam only in the most critical noise barrier walls (bedroom party walls, home office walls adjacent to neighbor's family room).

When to hire a professional: Both applications require professional installation. Dense-pack cellulose needs specialized blowing equipment and experience achieving proper density without over-packing and bowing drywall. Open-cell spray foam requires heated spray equipment, proper mixing ratios, and respiratory protection during application.

Need help finding a professional insulation contractor experienced with rowhouse soundproofing? New Brunswick Insulation can match you with local specialists who understand Saint John's housing stock and acoustic challenges.