R38 Insulation Thickness For Different Insulation Materials: A Brief Guide

R38 insulation has an R-value of 38, which is a measure of its thermal resistance. Using R38 insulation can reduce thermal conductivity by 97 percent.

The thickness of R38 insulation ranges from 5.38 inches to 15.7 inches, with closed-cell spray foam requiring the thinnest and blown-in fiberglass requiring the thickest.

Insulation Material.R38 Thickness Required (Inch)
Fiberglass Batt Insulation10.45”
Open Cell Spray-Foam11.4”
Closed Cell Spray-Foam5.38”-7.6” (Thinnest R38 insulation)
Foam Board (Rigid Foam) Insulation9.5”-10.5”
Blown-In Fiberglass15.7”
Blown-In Cellulose11.98”
Blown-In Rockwool13.8”
Rock Wool Batt11.25”
R38 insulation thickness for different materials

1. Fibreglass Batt insulation R38 thickness

R38 batt insulation is typically 10.45 inches thick. However, after manufacturing, it might be thicker to compensate for a fair amount of compression during handling and installation. Compression of the batt will decrease the R-value of the compressed area. 

Fiberglass insulation is the fluffy pink batts that we usually see in home improvement stores, and people would generally relate to that as typical insulation. However, fiberglass insulation is made out of 80% recycled glass and is naturally soundproof as well. Therefore, these fiberglass batts come in various R-values to insulate different parts of homes in all climate zones across the USA and Canada.

The fiberglass insulation is available in 16″ and 24″ width batts. This allows the batts to snugly fit into the open spaces between the framing members with a measurement a little less than that of the batts. When installing the batt’s care should be taken not to compress the batts to fit inside the framing members. Compressing the batts will lower the R-value for the compressed area because the pores inside the material are reduced.

Measuring the spaces between framing members before even ordering the insulation is essential. If smaller sizes are needed, you can easily cut the batts in the required size by using a sharp utility knife.

2. Spray Foam Insulation R38 thickness

Spray foam insulation is a robust, easy, and accurate method whereby homes can be insulated in a much shorter time than other insulation methods. There are two types available, each one of which fulfills a unique purpose, and care must be taken to use the right kind of spray foam for the purpose it is fabricated. Each type of spray foam has different depth or thickness requirements to reach an R-value of R38.

Closed Cell Spray Foam

Closed-cell spray foam is the thinnest R38 insulation on my list. It should be applied to a depth or thickness of 5.38″ to 7.6″ to ensure adequate resistance to conduction as required of R38 insulation.

Closed-cell polyurethane spray forms a closed-cell when sprayed that eradicates the penetration and build-up of moisture inside the foam that will cause mold growth and reduce the R-value. It also creates an airtight barrier that reduces convection as well. In addition, due to the high resistance of closed-cell spray foam moisture build-up, mold growth and bacterial growth are eliminated, allowing insulation to last longer.

Closed-cell spray foam is dense and is used effectively as insulation in buildings and even appliances. The most significant advantage of closed-cell spray foam is that a solid vapor barrier is created between the two spaces on either side of it. Because no moisture can penetrate the cells, mold growth is eliminated, and the strength of the closed-cell spray foam will even strengthen the structure of the building. However, because a chemical compound is used for the blow action, it might leave a smell inside the insulated room for a while, although it will rapidly diminish.

Open Cell Spray Foam

As previously stated, each type of spray foam has a different purpose, and that can be easily seen with open-cell spray foam. The blow action is attained with a water base that allows more expansion, leading to thinner membranes and more porous foam.

Because of this expansion open-cell spray foam can reach hard-to-reach places like nooks and crannies and still provide an adequate cover inside the hard-to-reach space. The expansion of open-cell spray foam will give us cover in hard-to-reach places, but we are left with less dense insulation that is not air and watertight due to the expansion. Also, because of its decreased density, we will need a deeper or thicker layer of foam to attain an R-value of R38 and, an 11.4″ deep or thick cover of open-cell spray foam must be applied.

The lighter and less dense foam is not insulated against moisture and air, and therefore it doesn’t support the structure, is less soundproof, and can be penetrated by air and moisture. In the presence of air and moisture, mold growth can take place, and any odors will reach the surrounding spaces.

When insulating using spray foam, it is therefore recommended to use each type for its distinctive purpose. For example, use closed-cell spray foam in all open spaces to create strong, durable, water and airtight insulation that enhances soundproofing. Use the open-cell spray foam afterward to reach all the hard-to-reach spaces to get the whole structure insulated to a lesser degree.

3. Foam Board (Rigid foam) Insulation R38 thickness

To achieve an R38 rating with rigid foam board insulation, you will typically need a thickness or depth of foam board between 9.5 to 10.5 inches.

These rigid sheets of polystyrene or polyurethane are seldom used in residential buildings but are instead used in commercial buildings. They are used in conjunction with external cladding systems where continuous insulation code is required.

Cutting these foam boards can be a challenge using a heated knife. It usually leads to a tedious and time-consuming job that needs to be done. Employing foam board for residential purposes might seem to be feasible but are not recommended. There is, however, no R38 foam board available.

4. Blown-in Fiberglass and Cellulose Insulation

Fiberglass and cellulose insulation are blown in using a pneumatic sprayer, and care must be taken to ensure no compaction occurs. To attain an R-value of R38, a depth or thickness of the blown-in insulation of 15,7″ is needed for fiberglass and 11,98″ for cellulose.

Cellulose is seen as a more sustainable means of insulation, with the recycled paper being used as insulation. Insulation with both materials, however, provides a way to reach the hard-to-reach spaces, especially in an attic.

Blown-in fiberglass and cellulose insulation can be used in new buildings and is an easy way to upgrade existing insulation in an attic or floor.

Working with blown-in fiberglass insulation can be dangerous if the applicator does not stay within safety regulations. Some of the effects might be irritations of the eyes, respiratory system, and itching of not covered skin. It is recommended to wear a face mask, respiratory mask with long-sleeved shirts,  long pants, and safety boots. An excellent tip is to cover any exposed skin with baby powder to limit irritations. Bathing afterward is a superb way to remove fiberglass, open pores of the skin so that you can get rid of irritations.

5. Insulation With Blown-in Rock Wool

Rockwool is a mineral wool substance that is inorganic and can be used as thermal insulation in attics, wall cavities, and ceiling cavities. Rock wool applied to a depth or thickness of 13.8″ will attain an R-value of R38. It is usually made of rock, furnace slag, and various organic materials. 

This porous material that reduces airflow is fire resistant and improves soundproofing. All the advantages make it one of the most sought-after insulation materials.  It can be used in various construction types and all climate zones. 

6. Insulation with Rock Wool Batts

Easy to handle and pre-fabricated batts with a thickness or depth of 11.25″ will be needed to attain an R-value of R38.  It is used in floors of crawlspaces and attics, supplying water-resistant insulation with no forming of mildew, fungi, or mold. Additional uses include the use of batts in the outer walls of steel and wooden framed buildings.  

The rock wool batts provide noise reduction and fire resistance that attribute to safer and quieter working environments that increase production. These batts are easy to install and transport because of their lightweight.

Further reading: Rockwool R value

R 38 insulation chart
R 38 insulation thickness chart

What is The R-value, and What Does it Mean?

The significance of the R-value is that it describes the thermal resistance of the insulation in square meter kelvin per watt. Therefore, it indicates the material’s resistance to thermal conduction through the thickness or depth of the material. A higher R-value will indicate more resistance to thermal conductivity, with a lower R-value showing less resistance to thermal conductivity. Therefore, the R-value is inversely proportional to thermal conductivity. Thus,  insulation with a lower R-value will insulate your home less than insulation with an elevated R-value.

Therefore, the logical conclusion to be drawn from this information is that insulation of the correct R-value should be used in each climate zone to maximize the home’s energy efficiency. Therefore, optimal insulation will ensure minimal heat loss due to thermal conductivity. The optimized insulation will adequately insulate your home from thermal conduction from the outside, resulting in less energy used for heating and a lower energy bill.

The R-value is computed considering only thermal conductivity, while thermal movement through convection and radiation is not considered. Therefore, alternative measures like adding foil-faced kraft paper to your insulation with the foil face facing the warmer environment might also decrease convection and radiation. The addition will not change the R-value but will reduce thermal convection and radiation.

Where To Use R38 Insulation and Its Thermal Resistance.

Attic insulation in climate zones 1 -4 can be done with R38 insulation as well as ceilings that can be insulated with R38 insulation in climate zones 4 and 5, while floors can be insulated with R38 insulation in climate zones 7 and 8 for improved comfort and energy saving.

An insulation having an R-value of R38 is rated to minimize thermal conductivity by 97 percent. Therefore the overall heat loss due to conductivity will be 97 percent less than without insulation. As a result, you will need less energy to keep a constant temperature in your house, leading to savings on your energy bill.

To Conclude

The thickness of insulation to reach an R-value of R38 for Spray foam, Fiberglass, Cellulose, and Rock Wool in various forms has been quantified, tabulated, and discussed to select the appropriate materials and forms of insulation. 

Some of the materials used for insulation can be subdivided, like spray foam can be either closed-cell spray foam or open-cell spray foam. The role of each of them has been discussed because they have different purposes.

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

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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