Insulating interior walls isn’t absolutely necessary, but it does offer a number of benefits. It controls noise, improves energy efficiency, provides fire protection, and reduces the risk of moisture problems.
Insulating interior walls is the next best option if you can’t add insulation to your exterior walls due to local building regulations or restrictions on altering historical homes. While it isn’t as effective at temperature control as exterior insulation, interior insulation does help maintain stable indoor temperatures, which reduces your need for heating and cooling. It’s typically around half the cost to install compared to exterior wall insulation, so you can still get a good return on your investment.
Even if you have good exterior insulation, it’s helpful to insulate interior walls of rooms you don’t use year round, such as guest rooms, storage rooms, and three-season rooms. This helps keep your warm or cooled air where you are.
Noise control is one of the biggest benefits of good interior insulation. Insulation dampens sound waves, particularly high-frequency waves, decreasing the level of noise that passes through the walls from one room to the next. If you have thin walls or a large, active family, adding insulation to the walls throughout your home creates a more relaxing environment for everyone.
Even in smaller households, certain rooms can benefit from better noise control. Your home office, game room, kid’s playroom, and even the bathroom are good places to consider adding interior insulation.
If insulating for noise reduction, also known as acoustic insulation, is your main goal, acoustic (soundproofing) batts and rock wool are good choices. Acoustic batts are usually made from mineral wool, fiberglass or polyester, but are denser than standard thermal insulation batts.
If a fire starts in your home, how fast it spreads can mean the difference between life and death for you and your loved ones. Fire-resistant wall insulation slows the spread of flames, keeping fire contained to a smaller area so you have more time to escape. It also protects your home and belongings from more extensive damage.
Mineral wool is fire-resistant and often used as a fire stop with a 1- to 3-hour fire rating. It doesn’t spread flames or produce smoke. Fiberglass and cellulose treated with fire retardants are both noncombustible, but can still spread a certain amount of flames and produce some smoke. Even so, insulation products made from these materials can achieve a 2- to 3-hour fire rating. Loose-fill (blown-in) is the safer choice. Batts are often faced with paper, which burns easily and can spread flames.
Most residential insulation products meet Class 1 Fire Rating requirements, the highest possible. This means 25 or less for flame spread and 450 or less for smoke development. The lower the number, the less flame spread and smoke.
Activities such as showering and cooking put heat and humidity into your home’s air. When this warm, humid air hits a wall adjoining a cool room, condensation can form inside the wall. That can cause mildew and rot, which damages your walls and can harm your health. Good interior wall insulation reduces the risk of moisture problems by slowing heat transfer and regulating temperatures.
Depending on the wall’s location and your climate, you might benefit from adding a vapor control layer (VCL). These plastic layers are more important in cold climates and mixed-humid climates. By reducing the amount of moisture entering the wall, they ensure only small amounts of condensation form.
Unlike vapor barriers, they don’t completely block moisture. This way any moisture that does form can escape again. The exact type of VCL that will work best and the optimal location in your wall also depend on your climate.
Options for Interior Insulation
If you’re still building your home, both batt and loose-fill insulation will be equally easy to install. If you want to add insulation to your existing interior walls, you’ll need to decide if you’re willing to open up the walls or not. The most common forms of insulation, including fiberglass, cellulose and mineral wool, are available in loose-fill form. This means it can be blown into the wall through a hole of 1/2 to 2 inches with no need to tear the whole wall apart. This form is overall more efficient than batts because it fills in the tiny spaces that batts leave open.
Injection spray foam insulation is another option for insulating the enclosed cavities of your existing interior walls. Even if the wall already has some insulation, it’s usually possible to inject the spray foam in around it to create a more airtight seal. This material is different from standard spray foam insulation, which can only be applied to open walls.
While rigid foam board is highly efficient, it’s more expensive than most other forms and requires opening the wall, so it’s not an ideal choice for insulating existing interior walls.
By regulating temperatures, controlling noise, and reducing your fire risk, interior wall insulation makes your home a more comfortable and safer place. Most common insulation materials work well in interior walls, but researching the strengths and weaknesses of each will help you find the one that best meets your needs.