R-Value

R-value is a measure of a material's resistance to heat flow, expressed in units of square foot times degree Fahrenheit times hours per BTU. The higher a material's R-value, the more effectively it slows the transfer of heat through it, which is why fiberglass batts, spray foam, and rigid foam board are all rated and compared by this single number. Builders and homeowners use R-value to decide how much insulation a wall, attic, floor, or roof needs to stay energy efficient in a given climate.

R-value matters directly for comfort and utility costs because a home's walls and roof are constantly losing heat in winter and gaining heat in summer. The Insulation Calculator uses recommended R-value targets by climate zone to tell you how thick a layer of insulation needs to be, while the Heat Loss Calculator uses the R-values of a home's walls, windows, and roof to estimate total heat loss in BTUs per hour during cold weather.

Because R-value directly determines how many BTU of heat pass through a surface, it is one of the first specs to check when sizing a furnace or air conditioner. A well-insulated home with high R-value walls needs a smaller heating or cooling system than a poorly insulated one of the same size, which is why insulation upgrades and equipment sizing are often evaluated together.

R-value is defined through its relationship to heat flow:

R = ΔT / (Q / A)

Where:

• R = thermal resistance (R-value), in ft²·°F·hr/BTU
• ΔT = temperature difference across the material, in °F
• Q = rate of heat flow, in BTU per hour
• A = surface area, in square feet

Rearranged for practical use, heat loss through a surface is:

Q = A × ΔT / R

R-values of stacked layers are additive:

R_total = R₁ + R₂ + R₃ + ...

Suppose a wall section has an area of 100 square feet, an R-value of R-13, and the temperature difference between inside and outside is 40°F.

Using Q = A × ΔT / R:

Q = 100 ft² × 40°F ÷ 13 = ~308 BTU per hour lost through that wall section.

If you added a second layer of R-13 insulation, the wall's total R-value becomes R-26, cutting the heat loss roughly in half to about 154 BTU per hour, which the Heat Loss Calculator would reflect in a lower total home heat loss estimate.

• Higher R-value means better insulation. The number describes resistance to heat flow, so it should be maximized within the cost-effective range for your climate zone, not minimized.
• R-values stack additively. Adding insulation layers combines their R-values directly, which the Insulation Calculator uses when recommending how much more material to add over existing insulation.
• Climate zone sets the target R-value. Colder regions need higher attic and wall R-values than milder ones, and the Insulation Calculator adjusts recommendations accordingly.
• R-value directly feeds heat loss estimates. The Heat Loss Calculator multiplies surface area by the temperature difference and divides by R-value to estimate the BTUs lost through each wall, window, and roof section.
• Installation quality affects real-world performance. Gaps, compression, or moisture in insulation can reduce its effective R-value well below the rated number, even when the correct material was used.