Insulation helps to keep interior temperatures comfortable when it’s unbearably hot or cold outside. Sure, we rely on furnaces and air conditioning equipment to heat and cool the indoors, but insulation plays a major role, too—by impeding heat flow. In the winter, insulation helps to keep heat inside. On hot summer days, the same insulation makes it more difficult for outdoor heat to move indoors. An added bonus is that insulation also makes a building more energy-efficient and less expensive to heat and cool.
Many of us envision fluffy pink fiberglass when we think of insulation. But insulation actually comes in many forms. Understanding the advantages and limitations of different insulation types will enable you to choose the right products, whether you’re dealing with new construction or planning an insulation upgrade.
How It Works
All types of insulation have an R-value rating, and the “R” stands for resistance to heat flow. Heat transfer can occur through conduction (the way a frying pan gets hot on the stove), convection (the natural process of hot air rising above cooler air), and radiation (objects absorbing heat from a heat source like the sun or a radiator). Fibrous, permeable types of insulation, like fiberglass, delay heat transfer by slowing down conduction. Rigid foam and spray foam not only slow down conduction; they can also limit convective heat transfer by closing off air leakage between indoor and outdoor space.
Types of Insulation
Deciding what type of insulation to use depends on a several factors: where it’s installed, R-value, and whether you plan to do the installation yourself or hire a pro. Green value can also influence your decision. Insulation gains green value if it’s made from recycled material.
Made from different materials, these fluffy, fibrous rolls of insulation work just like your favorite winter sweater-by creating tiny air pockets that make it hard for heat to escape. But to work effectively, batt insulation needs to stay dry and free of outside air penetration. Air-sealing prior to installing batt insulation will provide this protection, just like wearing a windbreaker over your sweater.
Batt insulation is typically made to fit between framing members (wall studs, floor joists) spaced on standard 16″ or 24″ centers. Different thicknesses are available.
Fiberglass batts (R-value: 2.9-3.8 per in.) have been around since the 1940s and remain popular due to their low cost and DIY-friendly characteristics (batts can be cut with a utility knife). Unfaced batts are typically used as attic insulation; faced batts are often used to insulate walls.
Mineral wool batts (R-value: 4-4.2 per in.) are made by melting reclaimed slag (a waste product from steel mills) and basalt stone, then spinning the molten material into fibers. This type of fibrous insulation is available in batt and rigid panel form. Although mineral wool (also known as rock wool) is more expensive than fiberglass, it offers some notable advantages like greater R-value, for starters. Mineral wool is denser than fiberglass and much less likely to settle or compress, even when wet. This extra density makes installation challenging. The material is typically cut with a hand saw rather than a utility knife, and it takes more time to fit around common obstacles like electric cable and outlet boxes.
Cotton batts (R-value: 3.0-3.7 per in.) are made from a combination of recycled cotton clothing and reclaimed cotton waste. During the manufacturing process, batts are treated with a borate solution that improves fire resistance, while also making the material repellent to insects and rodents. Insulation batts made from natural fibers (wool and hemp-based batts are also available) have similar thermal properties to fiberglass batts (R-value, air-sealing requirements), and are installed the same way. But they’re much more expensive than fiberglass, and you have fewer choices in terms of batt thickness and width.
This fibrous insulation is made to be blown into attics or building cavities. It slows heat transfer just like batt insulation, although there are some notable differences in R-value.
Loose-fill fiberglass (R-value: 2.2-2.9 per in.) is most often used as attic insulation. The material comes compressed and packaged in bags that are fed into a specially designed blower that shreds the material and blows it through a long, flexible hose that the installer can aim into an installation space. Loose-fill fiberglass can be blown into enclosed stud and joist cavities through access holes that are later patched, but this kind of retrofit requires a skilled contractor.
Cellulose (R-value: 3.6-3.8 per in.) has a high green value because it’s made almost entirely from recycled paper. The insulation undergoes a borate treatment to resist fire, insects and rodents; then it’s compressed into large bales. The insulation is blown into place, just like loose-fill fiberglass. Cellulose can also be blown into enclosed wall and ceiling cavities—called dense-pack cellulose. A third installation method-wet-spray cellulose—involves mixing cellulose insulation with a liquid binder so that it will stay in place when sprayed into open stud bays.
While cellulose and fiberglass insulation perform similarly, cellulose is better at absorbing and releasing moisture without degrading and settling like fiberglass can. This can be an advantage for locations where insulation may get wet.
Rigid foam (R-value: 3.8-6.8 per in.) comes in panel form. 4′ x 8′ is the most common size, and thicknesses range from ½” to 5-1/2″ or more. Some rigid foam panels are made with a foil facing that can serve as a radiant barrier. Rigid foam is also glued to drywall and exterior sheathing to create structural insulated panels (SIPs) and other composite panels that enable builders to simplify construction by achieving sheathing and insulation in a single step. Rigid foam is often used as a supplementary insulation, layered over a wood-framed wall or roof that has been insulated with batts or blown insulation. Foundation insulation is another common use for rigid foam. It can be placed beneath a concrete floor before the slab is poured. It can also be glued or fastened against a masonry wall. Rigid foam can be used in moist locations, but it should be protected from direct sunlight and impact damage. Like spray foam insulation, rigid foam can provide an air barrier as well as a thermal barrier, as long as seams between panels are sealed with special tape.
Rigid foam is typically classified by type. Polyisocyanurate (“polyiso”) foam has the highest R-value per inch (R-6.5 to R-6.8) of any rigid insulation. Extruded polystyrene (XPS) comes next, with a rating of R-5 per in. XPS foam is easily identified by its color (blue, pink, or purple) and shiny surface. Expanded polystyrene (EPS) is the least-expensive type of rigid foam and has the lowest R-value (around R-3.8 per in.). It’s also more easily damaged than the other types of rigid foam.
Most of us know how good spray foam is at filling gaps and sealing open seams because we’ve used the pressurized cans of spray foam available at hardware stores and home centers. Insulation contractors also use this single-component spray foam to air-seal a house prior to insulating with fiberglass, cellulose or other fibrous insulation. It’s also possible to use spray foam as whole-house insulation, completely filling the spaces between studs, joists and rafters. This two-component spray foam is applied in liquid form. The two compounds are mixed right at the application nozzle, resulting in a chemical reaction that transforms the liquid into foam that is highly expansive and adhesive.
As whole-house insulation, spray foam offers higher R-value and better air sealing than batt or loose-fill insulation. Closed-cell spray foam is the most commonly used type of spray foam insulation. It has a higher R-value (R-6 or more per in.) than open-cell spray foam (R3.6 to R4.3 per in.), and unlike open-cell spray foam, it won’t absorb water.
The main disadvantage of SPF is price. Depending on your location, installing spray foam insulation can add $3,000-$5,000 to the price of insulating an average-size house. Installation requires a highly trained crew, special safety gear, costly chemical compounds and expensive application equipment.
Unlike other types of insulation that are designed to resist conduction and limit convection, a radiant barrier’s purpose is to reflect heat back toward the heat source. If you’ve placed a shiny reflective shield beneath your car’s windshield to keep it from overheating, you know how radiant barriers work. Radiant barriers are found on some rigid insulation panels, in the form of a foil facing. Another type consists of a thin plastic sheet that is coated on one or both sides with aluminum foil. In hot, sunny climates, a radiant barrier can be installed in the attic to reflect roof heat away from living space. This type of installation has been shown to reduce cooling costs by 5%-10%.
Batt and loose-fill insulation are the most user-friendly types of insulation for DIYers, as long as basic safety procedures are followed (see SAFETY ALERT, below). A home center or building supply outlet can help you estimate the size and quantity of insulation you need for a specific application. If you plan to install loose-fill insulation, blowing equipment is usually available for rental where the insulation is sold. To make the most of any insulation installation (in new construction or an existing house), make sure to include air sealing in your plan.
Rigid foam is also easy to work with, but it’s not that useful in retrofit applications. Contractors and DIYers have found that installing rigid foam against basement walls is a good way to make basements more comfortable. But foam used in this way must be covered with gypsum board to meet fire safety requirements.
SAFETY ALERT: Make sure to protect yourself from adverse health effects when working with any type of fibrous insulation (fiberglass, cotton batts, cellulose, mineral wool). Health hazards range from mild skin and eye irritation to severe respiratory problems. Wear a dust mask, goggles, gloves and a long-sleeve shirt.
The R-values given for insulation assume that the material is installed correctly. Although batt-type insulation has a user-friendly reputation, it takes time and skill to install batts correctly. For example, instead of simply stuffing a batt under or over an electrical cable that runs through a stud, the installer should split the batt and slide it over the wire, so that the stud bay can be uniformly filled.
Improper installation of spray polyurethane foam insulation can, in some situations, create enough heat to cause spontaneous combustion. While this is an extreme example, it drives home the importance of hiring an experienced contractor.
Gaps or air spaces where insulation is missing can create comfort and energy problems. These voids can occur during installation, or when a fibrous insulation (fiberglass, cotton batts, cellulose) settles in a wall cavity.
Buildings in hot climates don’t have significant problems with air leakage compared to leaky buildings in cold climates. During cold weather, leaks around windows, doors and elsewhere allow warm air to escape through upper areas of a building. This convective action draws an equal volume of cold outdoor air into the structure through lower leaks. This process, called the Stack Effect, causes the cold drafts and high heating costs that old houses are famous for. Any insulating strategy should include air sealing to reduce the Stack Effect.
Rigid foam insulation and closed-cell spray foam insulation won’t be damaged by moisture and can even serve as a moisture barrier in some applications. But all types of fibrous insulation must be protected from moisture. When these materials get wet, they lose R-value, settle, and contribute to mold growth.