How Thick Should Your Roof Insulation Be? A Comprehensive Guide

Every home needs insulation. Well-insulated foundations, walls, doors, and roofs increase comfort levels by keeping your home cool in the summer months and warm in the winter. As a consequence, they significantly lower energy costs.

Insulation made with high-quality materials also keeps pollutants, harmful toxins, and allergens out, drastically improving the air quality in your home. These layers are also water resistant, preventing the growth of mold and mildew, which harm the respiratory system.

Roof insulation thickness depends on a few factors, such as the climate you live in, the type of insulation you opt for, and a numerical figure known as the R-value.

So, what kind of insulation do you need? How thick should insulation be? How does it impact residential roofing installation? Let’s take a closer look.

Understanding R-Values

Before we get into roof insulation thickness and how it impacts the roof installation process, we need to look at the concept of R-values.

An R-value is given to insulation materials, where the “R” stands for resistance to heat flow.

It is measured per inch of thickness and describes how effectively insulation can prevent heat from leaving or entering your home. R-values depend on insulation thickness, density of materials, or type of insulation.

Higher R-values correspond to better climate control and higher energy efficiency. But they can be more expensive. Materials over R60 provide top-notch insulation, while those lower than R30 will be cheaper but less effective.

Some states recommend an R-value figure—in Minnesota, the minimum roof insulation thickness should be calculated on an R-value of at least R49. This figure can change depending on whether you’re installing a new insulation barrier or repairing an existing one.

To arrive at the R-value of a particular material, you need to divide the thickness of the material by its thermal conductivity. Thicker materials have a higher R-value, while low thermal conductivity is more effective at conditioning air.

Breaking Down Climate Zones

The second factor that impacts the thickness of roof insulation is the climate zone. The United States’ program Energy Star categorizes states into several zones: 1 being the warmest and 8 being the coldest.

Areas that fall in climate zone 1 include Hawaii and Guam. Those in category 8 include specific boroughs in Alaska, such as the North Slope and Northwest Arctic. The rest of Alaska is in category 7.

Energy Star recommends specific R-values that you need to insulate your attic depending on your state’s climate zone. States that fall in climate zone 1 will not need as much insulation as those in zone 8.

Refer to the chart below for the recommended R-values per climate zone:

Zone                    Uninsulated Roof                    Roofs with 3 to 4 inches of pre-existing insulation

1                           R30 - R49                                R25-R30

2                           R30 - R60                                R25 - R38

3                           R30 - R60                                R25 - R38

4                           R38 - R60                                R38

5 to 8                    R49 - R60                                R38 - R49

All figures are from Energy Star, a program by the US EPA and the Department of Energy.

Choosing the Type of Material

The third factor that impacts the thickness of insulation has to do with the types of roofing materials you can use to trap heat.

Here are some of the most popular and effective options:

Spray Foam Insulation

A more expensive form of insulation, this involves adding a polyurethane or latex spray layer under the roof. It provides significantly higher R-values and is moisture-resistant. However, it comes at a pretty high cost—installing one that meets the minimum insulation thickness for a flat roof can run you up to $10,000.

Batt Insulation

Made from the popular material, fiberglass, batt blanket insulation comes in various R-value options. It is more cost-effective than spray foam insulation but will require a lot of layers to achieve the ideal thickness needed by the structure.

Foam Board Insulation

Made of materials like polyurethane or polystyrene, this type of insulation has a fairly high R-value. But at the same time, it may require an additional coating of a fire-resistant material, such as gypsum sheathing.

To meet energy-saving requirements, the roof insulation thickness need not be as high as that of batt insulation.

Blown-in Insulation

This type of insulation can be made up of different materials, from fiberglass to cellulose. The name refers to the process by which it is filled: the material is blown into a space through a tube, making it possible to insulate hard-to-reach areas.

While fiberglass requires less insulation thickness to be effective, it isn’t the best option for structures in colder climates. On the other hand, cellulose needs a thinner layer to be effective but is considerably more flammable than fiberglass.

There are many other procedures and materials that can be used to insulate a roof. Which brings us to the final question: What is the recommended roof insulation thickness based on the R-value of materials used?

Determining Insulation Thickness

Each material has a different R-value per inch of thickness.

For example, foam board insulations range from R3.6 - R5.8 per inch. In this case, 10 inches of insulation thickness will give you an R-value of R38 to R58.

To determine the ideal insulation thickness, you need to identify your climate zone and the recommended R-value. According to Energy Star, given that the material has an R-value of at least R3 per inch, a roof in a warmer climate zone must be between 13 and 14 inches.

Similarly, a roof in a colder zone should be between 16 and 18 inches.

To get a better idea of how much this will cost, you can take a look at this roofing cost calculator.

The Bottom Line

At the end of the day, figuring out the right roof insulation thickness can be very complicated and is a job best suited for a professional roofing company.

For more specific questions related to flat roof thickness, or for more general roof installation tips, we recommend getting in touch with AMECO. With almost 50 years of experience offering energy-efficient and high-quality roof installation services, we’re happy to assist you with your roof insulation needs.