R49 Insulation Thickness Guide: How Many Inches Needed | Airflow Academy (2024)

The R value of an insulating material is basically the temperature difference needed between its two sides to conduct 1 BTU of heat per hour through an area of one square foot per inch thickness.

R49 insulation rating means that it takes 49 Fahrenheit of temperature difference between its two sides to conduct one BTU of heat per square feet area of that insulation material.

The R-value of a material increases with an increase in its thickness. Therefore, different types of insulating materials and their combinations can achieve R49, given they are thick enough to provide adequate thermal resistance.

How thick is R49 insulation?

Simply put, the thickness of R49 insulation varies with the type of insulation material. Given below is a table listing the thickness required for different insulating materials to be R49.

Due to their high resistance to heat conduction, R49 insulations are used in attics irrespective of what climate zone the houses are in. However, they are more widely used in US climate zones 5 to 8.

As you can see from the above table, closed cell spray foam insulation needs the least thickness to be R49. This means that this material offers more resistance to heat flow through it than other insulations.

Let us discuss each insulation material above and the thickness they need to reach R49 level insulation.

Spray foam insulation thickness

There are two types of spray foam insulations; closed cell and open cell.

R49 thickness of closed cell insulation is 7 to 8 inches, whereas the same for open cell spray foam is 14.5 inches.

Closed cell spray foam

Closed-cell spray foam insulation is made up of tiny bubbles which trap air inside them. The trapped air provides additional insulation by preventing the transfer of heat into the structure. Closed-cell spray foams have very low thermal conductivity compared to fiberglass, mineral wool, cellulose fibers. It also has excellent fire-resistant properties. These characteristics make it ideal as building insulation.

Like I said before, this insulation material needs the least thickness to achieve R49. This is due to the closely packed, air-tight structure of this insulation material. The R-value of closed-cell spray foam is around 6.5 per inch of its thickness. Therefore, closed cell spray foam insulation needs to be 7 to 8 inches thick to achieve R49 insulation rating (49/6=7.537 inches).

The advantage of having higher thermal resistance is that this material can be used in places where insulation thickness is a problem. For example, closed cell foam insulations are a good choice if your attic or walls do not have room for thicker insulations.

Open cell spray foam

Open cell spray foam insulation is similar to closed cell insulation, but its cells are not encapsulated. This means that it conducts heat faster than its closed cell version.

The R-value of open cell spray foam is 3.5 per inch. Therefore, it takes around 14.5 inches of this insulation to reach the R49 level of thermal resistance.

The advantage of open-cell spray foam is that it is more flexible, so this can be used in places like ceilings and roofs. Also, because there is no tightness between the individual cells, it does not require any special tools to install.

R49 Batt insulation thickness

Batt insulation has more R value than blown-in type insulations. The most popular batt insulations are fiberglass and rock wool.

R49 thickness for fiberglass batt ranges from 9.5 to 14 inches and that for rock wool insulation is 14.5 inches.

Fiberglass batt

Fiberglass is a widely used insulating material. They come as batts and rolls to make their installations easier.

Fiberglass batt insulations have a decent R value of 3.6 to 5 per inch. Therefore, fiberglass batt insulation needs to be 9.5 to 14 inches thick to achieve R49 insulation rating.

Fiberglass batts can be used pretty much anywhere. However, R49 ones are used only on attic ceilings to reduce air conditioning loss in extreme climates.

Further reading: Roll vs batt insulation

Rock wool Batt

Rock wool insulation is made from rock wool which is crushed stone. Rock wool insulation comes in different densities depending upon how finely they are crushed.

Rock wool batts are very similar to fiberglass batts. As a result, they also have similar R-values.

R-values of rock wool insulation varies from 2.8 to 4.4 per inch. Therefore, it takes around 14.5-inch thickness of rock wool insulation to reach R49 insulation rating.

Further reading: Rockwool R value

Blown-in fiberglass

Blown-in fiberglass insulation is basically fiberglass blow in using a hose. This is an excellent option to insulate sneaky spots in an attic.

Since fiberglass is blown in, the resultant density of the insulating material is less than that of batt insulation. This considerably affects its R-value. As a result, blown-in fiberglass insulation has an R value of 3.2 only. Therefore, it needs to be at least 16.25 inches to R49 insulation.

Blown-in rock wool insulation

This type of insulation is usually installed with a vacuum system. Vacuum systems suck out all the air inside the wall cavity and then fill up the space with the insulation material. This makes sure that there is absolutely no air left behind after installation.

However, blown-in rock wool insulation is less dense than rock wool batts. So, they are likely to conduct slightly more heat. The average R-value of blown-in rock wool insulation is 3 per inch. Therefore, it will take 16 inches of thickness for this insulation material to be rated R49.

If you want to know how much blown insulation you need here is a blown insulation calculator.

Foam Board (Rigid Foam) Insulation

Foamboard, aka polystyrene, is another type of insulating material that surprisingly has more heat resistance than fiberglass and rock wool. It is also a material widely used in packing.

With an R-value of 5 per inch, foam board or polystyrene can achieve R49 insulation with 10-inch thickness.

You can also use my insulation thickness calculator to find out the thickness required for different insulation materials for your desired R-value.