What's inside your walls? A primer on the best insulation.

Just say “no” to hollow walls

Do you live in a home with hollow walls? A lot of homes built before the 1990s have no insulation in them at all. Most newer construction does include basic insulation. Contractors often use fiberglass batts in the exterior walls for new construction, because they’re easy to get and cheap. If you live in an older home, you’ll have to insulate the walls yourself. Depending on your situation, it might not be that hard to do nor that expensive. So how do you choose the best insulation for your needs?

Two great reasons to insulate your home

Insulation serves two purposes. The first is for energy efficiency. Today’s housing developers often use exterior wall insulation to meet minimum building codes for energy conservation. These requirements often focus on thermal efficiency (the R-value) and not on noise control (STC).

The second purpose, of course, is to reduce the noise that passes through walls. Insulation can be effective against airborne noise like conversation and television. However, on its own, it’s less effective against low frequencies such as:

Music with a lot of bass
Engine noise
Impact noise

You’ll probably need additional solutions to mitigate low-frequency noise.

Higher R-values indicate better climate control, and higher STC ratings indicate more effective noise control. You probably need both. And the results are dependent on multiple factors, including the rest of your wall assembly.

Comparing insulation materials

So how do the different materials stack up? For these purposes, let’s assume a typical 2″x 4″ wall with gypsum affixed on both sides. Results may vary, depending on your source — I’ve seen conflicting data. And of course, how you apply the materials makes a difference, too.

Insulation product	R-value	STC rating	Density	Fire resistance
Fiberglass batts (R11)	2.2-2.7 per inch	38	0.76 lb/ft³	Melts
Blown-in fiberglass	3.2 per inch	38	1.8-2.3 lb/ft³	Melts
Blown-in cellulose	3.2-3.8 per inch	41	1.5-2.0 lb/ft³	Non-combustible (treated with a fire retardant)
Rockwool Safe’n’Sound	3.0-3.3 per inch	45	2.5 lb/ft³	Non-combustible. Made from rock.
Open-cell foam	3.5-3.6 per inch	38	0.5 -0.75 lb/ft³	Combustible, may require an ignition barrier

Your next steps

Unless you’re remodeling your home and removing the drywall, the least-expensive and simplest way to add insulation is to blow it into your existing walls. Find a company that specializes in insulation. They’ll bring pressurized equipment and loose insulation material. First, they cut small, round holes in your drywall. Then they insert a nozzle into each cavity and blow the insulation in. When they’re finished, the technicians will replace the cut pieces of drywall and secure them with drywall compound. You can paint over them.

Best material for blown-in insulation

Your best option is to use blown-in cellulose. Resist any sales pitch from your contractor about using blown-in fiberglass. It’s more profitable for them, but less optimal for your home. As you can see from the above chart, it won’t satisfy all your soundproofing and home security needs.

You can review more information on insulation materials on YouTube — for example, Dr. Energy Saver, This Old House, Bob Vila, and plenty of others.

Photo of holes cut in wall for adding insulation. — When you blow insulation into your walls, you’ll need to cut some holes to insert the nozzle of the blowing machine. They’re easy to patch later.

You can also do the insulation yourself. Stores like the US-based Home Depot rent blower equipment by the hour, and you can buy packs of cellulose from them such as Green Fiber.

Best material for remodeling or for new construction

Finally, if you’re remodeling your home and you plan to replace the drywall anyway, your best insulation option is to use Rockwool batts. It’s denser, it’s fireproof, and it’s the best of all the sound-absorption materials.