Foam makes a room quieter. It won’t make it silent.

No, and no insulation will. Low frequency carries enormous energy and long wavelengths, and it passes through cavity insulation almost unaffected. Controlling bass takes mass and decoupling — double drywall, sound clips, a separate stud wall — and even then it’s reduced rather than eliminated. Foam will cut the voices and the television around it, which for many people is most of the annoyance, but we won’t sell it to you as a bass solution.

Barely. Footsteps are impact noise — the vibration goes into the joists and travels through the structure, so the air gap you just filled was never the path. Filling the floor cavity does reduce airborne sound between levels, so voices and a television upstairs get quieter, but the thumping stays. Real solutions live on the floor above: underlayment, carpet, or a decoupled ceiling below.

Yes, for interior partitions. Open-cell is soft and porous, so it dissipates sound energy that enters it. Closed-cell is rigid and dense — excellent thermally, poor acoustically — and being stiff it can transmit vibration through the framing it’s bonded to. It’s also roughly double the price. This is the one application on our site where the cheaper material is the correct one.

It depends entirely on your target. For an office where you want calls to stay private and the hallway to stop intruding, cavity fill plus sealing is usually enough. For a drum room, a rental unit party wall, or a nursery next to a highway, you need mass and decoupling too — and that’s your drywall contractor’s work, not ours. We’ll tell you which category you’re in, and we’re happy to coordinate our part around theirs.

Some, but check your windows first. In most houses the exterior wall already outperforms the windows by a wide margin, so filling the wall improves the weakest link by very little. Sealing the wall does help with the higher-frequency hiss of traffic, and if the wall is open for other reasons it’s worth doing. If exterior noise is the whole reason you’re calling, spend the money on the glass.

Seal the gaps, then fill the cavity. Sound behaves like water — a small opening undoes a large amount of good construction. Back-to-back electrical outlets, the gap under a hollow door, an unsealed top plate, a duct running between two rooms. Several of those you can fix yourself in an afternoon for very little, and we’ll happily point them out during the assessment even though there’s no work in it for us.

On an interior wall, not really — both sides are already heated, so there’s no meaningful temperature difference to insulate against. What you do get is more even room-to-room temperature and less noise transfer through shared ducts. On an exterior wall or a wall against a garage, you get both benefits at once, and that’s the case where a single job pays off twice.