R15 Insulation Thickness [Full List]

This article lists the thickness required for different insulation materials to reach a heat resistance of R15.

If you already don’t know, each insulation material comes with an R-value rating per inch thickness. To calculate how thick an insulation needs to be to achieve an R-value, say R15, all you have to do is divide the required R-value with the Rvalue of the insulation material. Higher the R-value of the material, the lesser the thickness you need for the insulation.

R15 insulation thickness varies from 2.5 to 5 inches depending on the insulation material used. R15 batt insulation thickness is 4.12 inches, R15 rigid foam insulation thickness is 3 inches, and for cellulose, it is 4.41 inches

Given below is the R15 insulation thickness for different insulation materials;

Insulation MaterialR15 Thickness Required (in inches)
Fiberglass batt4.12 inches
Fiberglass (loose-fill) 5 inches
Spray foam (open cell)4.17 inches
Spray foam (closed cell) 2.50 inches
Cellulose (loose-fill) 4.41 inches
Rockwool batt4.17 inches
Rockwool (loose-fill) 5 inches
Foam board (rigid foam)3 inches
R15 insulation thickness

Several different insulation materials can be used as R15 insulation, each with its advantages and disadvantages. Given below are the popular insulations used along with their R-value and R15 thickness;

Fiberglass batt insulation R15 thickness

It is one of the most common types of insulation used in homes. It’s inexpensive and can provide good sound dampening, as well as good thermal properties. This fiberglass-based insulation is most often used when an existing wall is being re-insulated. The material comes in several thicknesses, but more than one batt can be stacked for even greater energy efficiency.

In addition, fiberglass is a combination of glass and plastic polymers which make it naturally fireproof. However, cellulose, foam board, and rock wool are more durable than fiberglass batt.

R15 thickness for fiberglass batt insulation is 4.12 inches. This is because the Fiberglass batt has an R-value of 3.6 to 5. So, it needs to be 4.12 inches thick to acquire the R15 insulation rating.

Fiberglass (loose-fill) R15 thickness

Loose-fill fiberglass works very well in attics and basements. It is lightweight and cannot get damaged by nails or other sharp objects. However, it does allow more moisture vapor to escape than other types of insulation, so it should be installed on top of some sort of vapor barrier to prevent rot and mildew growth within walls.

Insulating a house with fiberglass insulation is a quick, effective, and cost-efficient way to add R-value. The process involves blowing in loose material, which creates air pockets between fibers. Loose-fill fiberglass is less dense than fiberglass batt, so it needs a thicker layer to achieve the R15 value.

R15 thickness for fiberglass loose-fill is 5 inches. This is because blown-in fiberglass has a lower R-value of 3.2, so it must be 5 inches thick to get R15 insulation. 

Spray Foam (closed-cell) R15 thickness

Spray foam insulation is a very cost-effective way to insulate a house. It’s also one of the most environmentally friendly ways. More importantly, it requires the least thickness to reach the R15 insulation value. It makes closed-cell spray foam ideal for places where you cannot put thick insulation due to limited space. 

Spray foam insulation is made from chemical polyurethane that comes in two liquids that are sprayed onto walls and form a layer on top of them. These liquids quickly harden into solid polyurethane, which provides excellent thermal and acoustic protection and seals up possible air leaks. This insulation is very effective in air-tightening challenging spots and corners.

Closed cell spray foam R15 insulation thickness is 2.5 inches. This is due to the air-tight structure of closed-cell spray foam and its high R-value of 6.5.

Spray Foam (open-cell) R15 thickness

Spray foam insulation is made from two chemicals that, when mixed, create expanding foam. This expanding foam is sprayed onto wall and ceiling cavities, creating an air-tight seal. Once it’s sprayed in place, it creates an effective insulation barrier between your house and outside elements. 

However, compared to open-cell spray foam, closed-cell foam provides better insulation than open-cell spray foam in hot and cold weather. Also, open-cell spray foam is more suitable for roofs and ceilings than walls because the open-cell structure requires more thickness to provide R15 insulation than the closed-cell spray foam.

With an R-value of 3.5, open-cell spray foam takes a thickness of about 4.17 inches to achieve the R15 insulation mark.

Cellulose (loose-fill) R15 thickness

Cellulose insulation is made from recycled newspaper and is widely considered one of the most effective home insulation options. You can effectively blow loose-fill cellulose into any small spaces, such as gaps between framing or between a wall and floor. 

Additionally, unlike other types of insulation, cellulose does not create unwanted moisture build-up, and it’s better than fiberglass for use in cold climates. Also, cellulose tends to be cheaper than other types of insulation. It can be used as an additional layer of insulation over any existing insulation or on its own in joist cavities that lack any insulation.

R15 cellulose insulation thickness is 4.41 inches. This is because loose-fill cellulose insulation has an R-value of only 3.5, so a thickness of 4.41 inches is needed for R15 insulation.

Rockwool batt R15 thickness

Rockwool is fiber insulation with natural origins consisting of crushed stones and minerals. It has a high resistance to heat, fire, mold, and mildew. Batt insulation is made by matting the fibers together into a blanket; it’s an efficient insulator that doesn’t take up much space in your walls.

Rockwool batt has been proven highly efficient at providing insulation. This type of insulation offers both warmth and soundproofing properties so that you can use it in different areas. It can be used to insulate floors, walls, or even ceilings.

With an R-value of 3, the Rockwool batt is 4.17 inches thick to give R15 insulation properties.

Rockwool (loose-fill) R15 thickness

Blown-in or loose-fill Rockwool is made from molten rock and mineral fibers that are then turned into fine dust, blown into an enclosed space, and forced into cracks and crevices using a vacuum system. One of its most significant benefits is that it prevents air infiltration better than any other type of insulation available. 

Blown-in Rockwool does not require a vapor barrier (though installing one can increase energy efficiency), making it easier to install. It’s also less expensive than rigid insulation materials like fiberglass batts.

Loose-fill Rockwool has an R-value of 2.5 to 3.7, meaning having a 5 inches thickness will give you an R15 insulation rating.

Foam board (rigid foam) R15 thickness

If you’re planning on building your own home from scratch, foam board insulation can save you a lot of money and hassle. The two biggest benefits are its low cost and its longevity compared to the batt-style fiberglass. On top of that, because foam board is a rigid material, it’s less susceptible to air infiltration compared with fiberglass batts.

R15 rigid insulation thickness is 3 inches. The Foam board has an R-value of 5, so it needs to be 3 inches thick to get to R15 insulation.

Where to use R15 insulation?

R15 insulation is most commonly used in 2×4 and cavity walls, irrespective of what US climate zone you are in. In addition, you can efficiently use R15 insulation in floors, basements, and crawl spaces if you are in Zone 1 or 2. It is not recommended to use R15 insulation in attics.


All in all, finding which insulation is best for your home depends on your specific climate, building design, and budget. In most cases, you will want a balance between cost and performance. Without research and expert help, it can be challenging to know what types of insulation are ideal, so I suggest using our guide as a starting point or doing further research on your own.

This insulation guide can help you decide which type of insulation you should use in your house. Remember, if you’re wondering how much insulation you need, measure your home and then consult a professional contractor to help design a plan that will work best with your budget and climate. Happy building!

Charles John

Experienced HVAC technician with 8 years of experience in the industry. Capable of handling all sorts of heating and cooling equipment as well as proficient in operational management, construction-related techniques such as preventative maintenance, electrical troubleshooting and AutoCAD

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