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R38 Insulation Thickness Guide (Table for All Types)

According to the US Department of Energy, heating and cooling are responsible for about 49% of a household’s energy bill. In most cases, a reasonable percentage of the energy expenses is caused by air leaks and heat loss. This is why insulation is a worthwhile investment for all households. 

If installed properly, insulation can save up to 10% on your total energy costs. Using the right insulation thickness for your project and climate is crucial if you want a significant increase in your energy savings.

R38 insulation is required in ceilings in hotter regions and floors in colder regions. The thinnest insulation layer achieving the R38 rating is with closed cell spray foam polyurethane (5.38-7.6") and the thickest insulation layer achieving the R38 rating is with loose-fill fiberglass (15.7").

Insulation Capacity of R38

The R-value of an insulation material measures its level of resistance to conductive heat flow. 

The R-value required by a project would be determined by your climate and the installation location.

Depending on your locality, R38 can be used to insulate floors, attics, and cathedral ceilings

When installed correctly, it can reduce heat transfer by about 97%! 

R38 can reduce heat transfer by 97 percent when installed correctly, heat bouncing from a house

This means a significant reduction in your energy expenses, especially if you previously did not have any insulation in place. 

R38 Insulation for Ceilings and Attics

Ceilings and attics are arguably the parts of a building that requires the most insulation.

The use of R38 for ceiling and attic insulation is dependent on climate zones, as you can see in Table N1102.1.3 of the International Residential Code (IRC).

You also need to check your local building code to confirm the recommended R-value before proceeding with your project.

The R-values presented in the table are minimums, so R38 can be used in ceilings in zones 0 and 1, which are classified as extremely hot humid (0A)*, extremely hot dry (0B)*, and very hot humid (1A).

R38 is sufficient in these zones because you are not often trying to keep the heat in. Insulation is still important, though, because you also need it to keep heat out.

According to energy star, R38 should be added to uninsulated attics in zones 1-4. These include those mentioned above but also climates that are hot humid (2A), hot dry (2B), warm humid (3A), warm dry (3B), warm marine (3C), mixed humid (4A), mixed dry (4B), and mixed marine (4C).

Examples of states that fall into these zones are:

  • *0A and 0B-no states
  • Florida (1A and 2A)
  • Alabama (2A and 3A)
  • Mississippi (3A)
  • New Mexico (3B-not everywhere)
  • California (3C-not everywhere)
  • Most parts of Missouri (4A)
  • Nevada (4B-not everywhere)
  • Oregon (4C-not everywhere)

If we go back to the IRC, we can see in Section N1102.2.1 that R38 insulation can be used where R49 is required if the R38 insulation is uncompressed, installed over 100% of the ceiling or attic area, extending up over the top plates at the eaves of the ceiling. 

Floor R38 Insulation

Looking again at Table N1102.1.3, we can see that, in general, floors do not require as much insulation as ceilings. This is evidenced by the fact that R38 minimum insulation is only required in floors in climate zones 7 and 8.

Zones 7 and 8 are classified as very cold and subarctic/arctic, respectively.

States that have climate zones 7 or 8 and which require R38 for floor insulation include parts of Alaska, Colorado, Wisconsin, Virginia, Washington, Texas, Tennessee, Michigan, Minnesota, North Dakota, Wisconsin, and Wyoming.   

States that have climate zones 7 or 8 and which require R38 for floor insulation

To avoid mistakes, check your local building code for your climate zone and the recommended R-ratings for insulating various locations in your home.

R38 Insulation Thickness Guide: Table

Insulation TypeInherent R-value (per inch thickness)Thickness to achieve R38Best-selling option
Loose-fill fiberglassR3.115.7″Owens Corning Atticat Expanding Fiberglass Insulation
Loose-fill mineral woolR3.113.8″ROCKWOOL Stone Wool  Insulation Granulate 
Loose-fill celluloseR3.411.98″ThermoFloc Loose Fill Cellulose Insulation
Fiberglass battR2.510.45″Owens Corning Kraft Faced Fiberglass Insulation Batt
Mineral wool battR3.411.25″Rockwool 
Plastic fiber battR3.89″Supersoft Recycled Plastic Insulation
Natural fiber battR3.410″UltraTouch Natural Cotton Fiber Batt Insulation
Open cell polyurethane spray foamR3.611.4″Versi Foam Open Cell Spray Foam Kit
Closed cell polyurethane spray foamR65.38-7.6″Heatlok HFO Pro Spray Foam Insulation
Foam boardsR59.5-10.5″Owens Corning FOAMULAR Rigid Foam Board Insulation
Inches required to achieve R38 for different types of insulation


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