5 Best Roof (Attic) Insulation Options: Cost, Pros & Cons, Comparison

Space heating and air conditioning are the top energy expenses in US homes, representing 51% of total consumption according to the US Energy Information Administration.

However, this also means there is a major opportunity for energy efficiency measures that reduce your heating and cooling costs. There are three main ways to achieve this, which can be combined for greater effect:

• Upgrade to newer HVAC equipment with a higher nameplate efficiency and a lower operating cost.
• Use smart controls and thermostat setback to optimize the operation of heating and cooling equipment.
• Improve your insulation level and seal air leaks, which reduces the workload on your space heating and air conditioning system.

As a rule of thumb, your home loses 25% of its heat through the roof during winter, and gains 25% of outdoor heat through the roof during summer. This means you can greatly reduce your annual heating and cooling costs by improving roof insulation.

In this guide, we will discuss the best roof insulation options available for homeowners.

• Foam board or rigid foam insulation
• Loose-fill or blown-in insulation
• Spray foam and foamed-in-place insulation
• Structural insulated panels (SIP)
• Batt and roll insulation

What Is the R-Value in Roof Insulation?

To describe the energy performance of a home or commercial building, you need metrics with a numerical value.

When comparing insulation options for your roof and other structural elements of your home, you will find that their performance is described by the R-Value.

In simple terms, the R-Value can be described as the “thermal resistance” or “heat flow resistance” of a building element such as a wall or roof.

As the R-Value increases, the resulting heat flow through that building element is reduced in the same proportion.

• For example, if the insulation level of a wall is improved from R-8 to R-16, heat flow will be reduced by half.
• If you achieve an even higher insulation level of R-24, heat flow drops by two-thirds.

Like any other physical variable, the R-Value has a measurement unit: K⋅m²/W. The letter K stands for degrees Kelvin (temperature difference), the letter W stands for watts (heat flow), and m² is simply square meters (area).

You can actually calculate the heat flow through a two-dimensional surface if you know the area, temperature difference and R-Value.

As a quick example, assume a wall with R-25 insulation and an area of 100 square meters is exposed to a temperature difference of 20°K. In this case, you only need to multiply the temperature difference and area and divide the result by the R-Value.

• Heat flow = Temperature Difference x Area / R-Value
• Heat flow = (20°K)(100 m2) / (25 K m2/W) = 80 W

In this case, 80W of heat would be moving through the wall section. This is equivalent to a heat loss of 273 BTU/hour. Note how heat transfer increases if the R-Values is reduced to 10:

• Heat flow = (20°K)(100 m2) / (10 K m2/W) = 200 W (or 682 BTU/hour)

In this case, the R-25 wall has 2.5 times more resistance to heat flow, which means it reduces heat loss by 60% compared with the R-10 wall.

Depending on where you live, local building codes may specify minimum R-Values for specific construction elements.

If you are not sure about the level of insulation you need, EnergyStar has a map with the recommended R-value and cost effectiveness across the U.S.

Keep in mind that insulation works both ways, reducing heat loss during winter and heat gain during summer. This means a higher insulation level reduces the energy consumption of both your furnace and air conditioner.

Now that we have discussed the meaning of the R-Value, we can compare some of the most common insulation options.

Pro Tip: Installing insulation with a high R-Value is easier in a new construction since the roof and its underlying structure have not been built yet.

Upgrading insulation in an existing home is also viable, but costs are increased because construction elements must be modified.

Foam Board or Rigid Foam Insulation

Foam board insulation uses materials such as polystyrene, polyisocyanurate, polyurethane and phenolic. This type of insulation is normally used for unvented low slope roofs but can also be used in floors and walls.

Foam board has specific requirements depending on the type of application, which are described by the US Department of Energy:

• Indoor applications: For fire safety, foam board insulation must be covered with 1/2″ gypsum boards or other materials approved by building codes.
• Outdoor applications: Foam board insulation must be covered with weatherproof facing.

Foam board insulation is characterized by offering a high R-Value relative to its thickness. Compared with many other insulation materials, foam board has twice the heat flow resistance per inch of thickness.

Foam board can also be used effectively to cover wood and steel elements, which have a low resistance to heat flow.

The main disadvantage of foam board insulation is the difficulty of using it in existing properties.

Rigid boards can be installed with relative ease in new constructions with unfinished walls and ceilings, but the installation becomes much more difficult when you need to remove existing building elements.

The R-Value of foam board insulation varies depending on the specific product used, but you will generally get between R-4 and R-8 per inch of thickness. Based on this range, a 3-inch layer of foam board will result in an insulation level between R-12 and R-24.

You can expect to pay between $2,500 and $5,500 when installing rigid insulation boards, depending on project size, scope, and location.

Loose-Fill or Blown-In Insulation

Loose-fill insulation uses materials like cellulose, fiberglass, mineral wool – broken down into small particles that easily reach tight spaces. This type of insulation is an excellent option for enclosed areas and other hard-to-reach locations; there is no need to deal with rigid boards, and the insulation material is simply blown in with specially designed equipment.

Depending on the application, this type of insulation may also be poured in, but the US Department of Energy does not recommend this procedure.

The main advantage of loose-fill insulation is being able to increase the R-Value of existing construction elements with minimal disruption. – This is especially helpful when you need to insulate areas with irregular shapes, where rigid insulation materials would be difficult to install property.

Blown-in insulation is also useful when there are many obstructions limiting the use of rigid boards.

Loose-fill insulation is also an environmentally friendly option, according to the US DOE, since the materials used are often recycled from other applications:

• Cellulose can be obtained from recycled newspapers.
• Fiberglass insulation often contains 40-60% recycled glass.
• Mineral wool often contains around 75% recycled industrial waste.

Like in any design decision, each option has pros and cons. Loose-fill insulation has a low R-Value relative to its thickness, typically below R-4 per inch. This means you can only achieve a high R-Value if there is a wide gap that can be filled. Since this type of insulation is not rigid, its density and R-Value can also change over time, and it can suffer from moisture issues.

You can expect to pay between $1,750 and $4,500 when installing loose-fill insulation, according to RoofingCalc.com. This type of insulation is suitable for DIY projects, but make sure you read the manufacturer’s instructions carefully, to ensure proper use and safety.

Spray Foam Insulation

Spray foam insulation is normally made of cementitious, phenolic, polyisocyanurate or polyurethane. The applications of spray foam insulation are similar to those of loose-fill insulation, since it can be easily used in enclosed spaces.

• Spray foam insulation is a great option for enclosed spaces, and especially areas with irregular shapes and multiple obstructions.
• Spray foam is not only useful as insulation material. You can also use it to seal air leaks around your roof structure and other areas of your home.
• You can also apply spray foam around the edges of window and door frames, where air leaks are very common.

Spray foam insulation can be purchased in different sizes depending on the application. When you only need to insulate a small space, a spray container can be used.

However, spray containers are impractical and wasteful when you need to apply a large amount of insulation. In these cases, insulation is foamed-in-place with high pressure equipment.

The R-Value offered by spray foam insulation can be compared with that of foam board. Open cell foam spray typically offers an insulation level below R-4 per square board foot (1 inch high), but this increases to around R-6 for closed cell foam spray.

The main drawback of spray foam insulation is its high price. The total cost depends on the amount of material applied, but you can expect to pay up to $3,000 to $5,000 when insulating a large attic space, according to RoofingCalc.com.

Structural insulated panels (SIP)

Structural insulated panels (SIP) are more than just insulation. SIPs are prefabricated structural elements that can be used to build roofs, ceilings, walls and even floors.

SIPs are characterized by their superior insulation level, and they can cut your energy bills by 12-14% according to the US Department of Energy. You can find SIPs with insulation levels of over R-40, or even over R-60 in some cases.

• SIPs are made of structural materials with insulation cores that are typically made of foam board, liquid foam, or straw.
• According to the US DOE, the thickness of these insulation cores typically ranges from 4 to 8 inches.

The main drawback of SIPs is their limited application in existing homes. Since they are structural elements with built-in insulation, you can only use them by replacing your existing roof or walls.

Structural insulated panels will typically increase your construction costs by around $12-$14 per square foot of building envelope, according to RoofingCalc.com.

Batt and Roll Insulation

Batt and roll insulation is probably the first option that comes to your mind when someone mentions home insulation. As its name implies, this type of insulation is sold in rolls that are applied to your building envelope. Common materials include fiberglass, mineral wool, plastic fibers, and natural fibers.

Insulation rolls are cut into sheets of adequate size and applied between structural elements like studs, joists, and beams.

Batt and roll insulation is the most common option in US homes, characterized by its low cost. This product is sold in standard sizes and the insulation level is typically below R-4 per inch of thickness.

Batt and roll insulation is easier to apply in new constructions, where the spaces between structural elements are exposed. Using it in existing homes with finished building elements is much more difficult – loose-fill and spray foam are better options.

Another drawback of batt and roll insulation is the low R-value per inch of thickness, which is outclassed by options like SIPs and spray foam.

According to RoofingCalc, the typical cost of batt and roll insulation can range from $750 to $1,500.

Summary of Roof Insulation Options: Comparison Table

The following table summarizes the main features of the five insulation options compared in this guide: