Insulation R-value guide: recommendations by climate zone

R-value is the single most important number to understand when planning an insulation project. It tells you how well insulation resists heat flow, and getting the right R-value for your climate zone is the difference between an insulation upgrade that pays for itself and one that underperforms.

This guide explains what R-value means, breaks down recommended R-values by climate zone, compares R-values across insulation materials, and helps you calculate exactly what your home needs.

What R-value is

R-value measures insulation’s resistance to conductive heat flow. The “R” stands for thermal resistance. The higher the R-value, the better the material insulates.

R-value is measured per inch of thickness. When you see a material rated at “R-3.5 per inch,” that means each inch of that material provides 3.5 units of thermal resistance. A 10-inch layer of that same material provides R-35 total.

R-values are additive. If you have R-19 fiberglass batts in your attic and add 8 inches of blown-in cellulose (R-3.5 per inch = R-28), your total attic R-value becomes R-47. This matters because it means you do not need to remove existing insulation to reach a higher target. You can build on what is already there.

Heat always moves from warmer spaces to cooler spaces. In winter, heat moves from your heated rooms through the ceiling, walls, and floor toward the cold outdoors. In summer, outdoor heat pushes inward. Insulation slows this heat transfer. Higher R-value means more resistance and less heat loss.

Recommended R-values by climate zone

The Department of Energy divides the country into seven climate zones and provides R-value recommendations for each. These represent the insulation levels that deliver the best return on investment for most homes.

Understanding the ranges

The ranges reflect that different home designs and situations call for different insulation levels within the same climate zone. A well-sealed newer home may perform adequately at the lower end of the range. An older, leakier home benefits from the higher end.

When in doubt, aim for the upper end of the recommended range. The marginal cost of adding a few more inches of insulation during installation is small compared to the cost of hiring a crew to come back later and top it off.

Attic R-values are the highest priority

Notice that attic R-values are the highest for every climate zone. Heat rises, and the attic is the primary pathway for heat loss in winter and heat gain in summer. Insulating the attic to the recommended R-value delivers the biggest energy savings per dollar spent.

If your budget limits how much insulation you can install, prioritize the attic first, then floors over unheated spaces (crawl spaces, basements), and then exterior walls. For attic-specific advice, see our attic insulation guide.

R-values by insulation material

Different insulation materials deliver different R-values per inch. This determines how much material thickness you need to reach your target R-value.

Fiberglass (batts and blown-in)

Fiberglass is the most widely used insulation material. At R-2.2 to R-2.7 per inch, it requires the most thickness to reach a given R-value. In attics where space is not limited, this is not a problem. In walls with fixed cavity depths, fiberglass may not provide enough R-value.

Cost: $0.50 to $1.50 per square foot for batts, $1.00 to $2.50 per square foot for blown-in.

Cellulose (blown-in)

Cellulose delivers R-3.2 to R-3.8 per inch, roughly 40% more R-value per inch than fiberglass. This means less material depth to reach the same R-value. Cellulose also resists air movement through the material better than fiberglass, which improves real-world performance.

Cost: $1.00 to $2.50 per square foot. See our blown-in insulation cost guide for detailed pricing.

Open-cell spray foam

Open-cell spray foam provides R-3.5 to R-3.7 per inch, similar to cellulose. Its main advantage is not R-value per inch but its ability to seal air leaks completely. It expands to fill every crack and gap, creating a continuous thermal and air barrier.

Cost: $0.75 to $2.00 per square foot.

Closed-cell spray foam

Closed-cell spray foam is the highest-performing common insulation material at R-6.0 to R-7.0 per inch. It requires less than half the thickness of fiberglass to reach the same R-value. It also acts as a vapor barrier and adds structural rigidity.

Cost: $1.50 to $3.50 per square foot.

Rigid foam board

Rigid foam panels vary widely in R-value depending on the specific material. Expanded polystyrene (EPS) provides R-3.8 to R-4.4 per inch. Extruded polystyrene (XPS) delivers R-5.0 per inch. Polyisocyanurate (polyiso) offers R-5.7 to R-6.5 per inch.

Cost: $1.25 to $3.00 per square foot.

For a detailed comparison of spray foam and fiberglass, read our spray foam vs. fiberglass guide.

How to calculate what R-value you need

Follow these steps to determine the right R-value for your insulation project.

Step 1: Identify your climate zone

Find your location on the Department of Energy’s climate zone map. Your zone number (1 through 7) determines the baseline R-value recommendations for your home.

Step 2: Determine what you have now

Check your existing insulation. In the attic, measure the depth of the current insulation and identify the material. Multiply the depth (in inches) by the material’s R-value per inch to calculate your current R-value.

Example: You have 6 inches of fiberglass batts in your attic. At R-2.5 per inch, that is 6 x 2.5 = R-15 currently installed.

Step 3: Calculate the gap

Subtract your current R-value from the recommended R-value for your climate zone. That gap is the additional R-value you need.

Example: Your climate zone recommends R-49 for the attic. You have R-15 currently. You need to add R-34 worth of insulation.

Step 4: Choose your material and calculate thickness

Divide the R-value gap by the material’s R-value per inch to determine the required thickness.

Example: R-34 needed / R-3.5 per inch (cellulose) = approximately 10 inches of blown-in cellulose to add on top of your existing batts.

Step 5: Factor in real-world performance

The calculation above gives you the minimum thickness. In practice, consider adding 10% to 15% more material to account for settling (cellulose) and to ensure full coverage around obstructions. A good contractor will factor this into their quote.

Adding R-value to existing insulation

One of the most common and cost-effective insulation upgrades is adding new material on top of what is already there. Here is how to approach it.

When you can add on top

You can add new insulation over existing material when:

• The existing insulation is dry and free of mold or pest contamination
• The existing insulation is evenly distributed (no major thin spots or bare areas)
• There is no sign of roof leaks or moisture problems in the attic
• The existing material is not vermiculite (which may contain asbestos)

When you should remove first

Remove existing insulation before adding new material when:

• Moisture damage or mold is visible
• Pest droppings or nesting material is present
• The insulation contains vermiculite (have it tested for asbestos before disturbing it)
• Animal urine contamination has occurred
• The insulation is severely deteriorated or compressed beyond usefulness

Material compatibility

R-values are additive regardless of material type. These combinations work well:

• Blown-in cellulose over fiberglass batts. The most common top-off scenario. The blown-in material fills gaps in and around the batts, improving overall performance.
• Blown-in fiberglass over existing blown-in. Adding more of the same material works perfectly.
• Blown-in cellulose over blown-in fiberglass. The cellulose layer adds R-value and improves air resistance on top.

Do not install a second vapor barrier over existing insulation that already has one. This can trap moisture between the layers. If existing batts have kraft-paper facing, either remove the facing or ensure it faces the warm side of the assembly.

Common R-value mistakes

Avoid these errors that reduce insulation performance and waste money.

Compressing insulation to fit

Compressing fiberglass batts reduces their R-value. A batt rated R-19 at 6.25 inches thick will deliver significantly less R-value if compressed into a 3.5-inch wall cavity. Use the correct batt thickness for the cavity depth, or choose a material better suited to the available space.

Ignoring air sealing

R-value measures resistance to conductive heat transfer. It does not account for air leaks. Air moving through or around insulation bypasses it entirely. Sealing gaps around wiring, plumbing, recessed lights, and ductwork penetrations before insulating is critical to achieving the rated R-value in practice.

Using the wrong R-value for your zone

Installing less insulation than recommended for your climate zone means you will never achieve optimal energy savings. The initial cost savings from using less material is offset by higher energy bills every month for the life of the insulation.

Neglecting walls and floors

Many homeowners focus exclusively on attic insulation because it is the most accessible. But uninsulated walls and floors over crawl spaces also contribute significantly to heat loss. A whole-home approach delivers better results than over-insulating one area while neglecting others.

Overlooking existing insulation condition

Adding new insulation over wet, moldy, or pest-damaged material traps the problem and can make it worse. Always inspect existing insulation before adding to it. If there is any sign of moisture, address the source of the water before insulating.

Blocking attic ventilation

Insulation pushed against the underside of the roof deck at the eaves can block soffit vents, trapping moisture in the attic. Use rafter baffles (also called vent chutes) to maintain airflow from the soffits to the ridge vent while keeping insulation in place.

Get R-value recommendations for your home

Every home is different. A qualified insulation contractor can assess your current insulation levels, identify the biggest opportunities for improvement, and recommend the right R-value targets for your climate zone. Find contractors in your area:

• Utah | Salt Lake City
• Idaho | Boise
• Colorado | Denver
• Oregon | Portland
• North Carolina | Charlotte | Raleigh
• Tennessee | Nashville
• Florida | Tampa
• Arizona | Phoenix
• Ohio | Columbus

Frequently asked questions

For more on insulation types and pricing, explore our blown-in insulation cost guide, our spray foam vs. fiberglass comparison, or our attic insulation guide. For wall-specific projects, see our wall insulation retrofit guide. Visit our insulation contractor hub to connect with qualified professionals near you.