Soundproofing between floors in two- and three-family homes: what actually works

Owners of two-families, triple-deckers and condos in Somerville and Cambridge ask me about soundproofing between floors. Old wood-framed floors carry footsteps and voices straight down. The good news: with the right mix of fixes, you can make a big difference without tearing the whole building apart.

The key is knowing which kind of noise you are fighting, because the fixes are different.

Impact noise vs airborne noise

• Airborne noise travels through the air first: voices, music, TV. It leaks through thin ceilings, gaps and holes.
• Impact noise starts as a hit on the floor: footsteps, dropped objects, chairs sliding. It travels through the framing itself, which is why the thump from upstairs is so hard to stop.

Most complaints I hear in Greater Boston multi-families are about impact noise, so a good plan treats both.

STC and IIC: what the code numbers mean

The 2021 International Building Code, Section 1206 sets sound rules for walls and floor-ceiling assemblies between dwelling units:

Higher numbers mean better isolation. The same section says penetrations for pipes, electrical boxes, recessed fixtures and ducts must be sealed, lined or insulated to keep the rating.

Massachusetts made no amendments to this chapter. Keep in mind that the state's residential code covers detached one- and two-family homes, so the IBC numbers apply to buildings with three or more units, like most triple-deckers. On two-families, I still use STC 50 and IIC 50 as a good target.

Soundproofing between floors: methods that work

These are the pieces I combine, from the ceiling below to the floor above:

1. Mineral wool in the joist bays. It absorbs sound inside the cavity. On its own it helps a little; combined with the next steps it helps a lot.
2. Resilient channel or isolation clips. They hang the new ceiling so it is not screwed tight to the joists. This breaks the path that carries footsteps.
3. Double drywall with damping compound. Two layers add mass, and a damping layer between them turns vibration into tiny amounts of heat.
4. Sealing every gap. Acoustic sealant at the edges and around lights and boxes. One open recessed light can undo a lot of work.
5. A floating floor or acoustic underlayment upstairs. A cushioned layer under hardwood or tile is one of the best fixes for impact noise.
6. Rugs with thick pads. The cheapest fix of all, and it works on footsteps right away.

From the ceiling below or the floor above?

• Ceiling below is usually easier when the upstairs unit is occupied or belongs to someone else. You control the work.
• Floor above is where impact noise starts, so treating it there is very effective, especially during a kitchen or flooring project.
• Both gives the best result when a gut renovation opens everything up.

In a two-family, the residential code requires wall and floor assemblies between units with at least a 1-hour fire-resistance rating (IRC R302.3). When I open a ceiling between units, I plan the new assembly so it keeps the fire separation while improving sound.

Condo association rules

If you own a condo in Somerville, Cambridge or Boston, read your master deed, bylaws and rules before you change flooring. Many associations set rules for underlayment under hard floors because of exactly this problem. On the permit side, Somerville's building permit submittal checklist includes an approval letter from the condominium association for the proposed work. My condo renovation rules guide covers what to ask your board.

What drives the cost

I don't quote soundproofing by the square foot without seeing the space. The price depends on:

• Whether we remove old plaster ceilings or build below them (which costs headroom).
• Lead-safe work. Under the EPA's RRP rule, anyone paid to disturb paint in a pre-1978 home must be certified and use lead-safe practices. My lead paint guide explains what that adds.
• Lights, sprinkler heads, ducts and pipes in the ceiling that need to be moved or sealed.
• Flooring removal and replacement upstairs.
• Access, parking and stair carries on tight Somerville streets.

Every building is different, so I give a written line-item budget after a free walkthrough.

Need a hand?

I quiet down floors in two-families, triple-deckers and condos across Somerville, Cambridge and Greater Boston. Book a free walkthrough and I will look at your ceiling and floor build-up, then send a written line-item budget, usually within 48 hours.

Want someone on your side? See what I do as an owner's representative and how I price work.

About the author

Mateus Magno founded Magno & Co in 2020. He is a licensed Massachusetts Construction Supervisor (CS-123658), registered Home Improvement Contractor (HIC #220484) and certified Lead-Safe Renovator Supervisor who has renovated and managed over $5 million in projects across Greater Boston.

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