Insulation Calculator

Insulation slows heat flow through a building assembly. Its thermal resistance, usually called R-value, depends on both material thickness and thermal conductivity. This calculator uses the SI relationship R = d divided by k, where thickness d is converted from millimeters to meters and conductivity k is measured in watts per meter-kelvin. A thicker layer or a material with lower conductivity produces greater resistance. Because products vary with density, temperature, moisture, and installation quality, use the conductivity listed on the current manufacturer data sheet whenever possible. A real wall, roof, or floor contains more than one layer. Interior and exterior air films, sheathing, gypsum, masonry, cavities, and finishes each contribute resistance. Resistances in a simple series assembly add together, so this calculator combines the insulation R-value with the entered air-film and additional-layer values. The reciprocal of that total is the U-value. A lower U-value indicates less heat passing through each square meter for each degree of temperature difference, while a higher total R-value indicates stronger resistance to heat flow. The estimated heat loss multiplies U-value by assembly area and the indoor-to-outdoor temperature difference. The result is a steady-state load in watts. It is useful for comparing insulation options and making an early heating-load estimate, but it is not a complete building energy model. Air leakage, thermal bridges through framing and fasteners, solar gains, ventilation, ground coupling, changing weather, and equipment performance can materially change actual energy use. For example, 100 mm of fiberglass with conductivity 0.040 W/m·K has an insulation resistance of 2.5 m²·K/W. Adding 0.8 for air films and 0.5 for other layers gives 3.8 m²·K/W overall and a U-value near 0.263 W/m²·K. Closed-cell spray foam at 150 mm and conductivity 0.025 reaches an insulation resistance of 6.0 before other layers are counted. Treat the output as a transparent design estimate rather than a code-compliance certificate. Confirm required R-values or U-values under the applicable energy code, account for framing fractions and repeated thermal bridges, and check whether published product values are aged or initial ratings. Correct installation matters as much as the nominal material value: gaps, compression, moisture, and poorly fitted boards can reduce field performance. For final envelope design, have a qualified professional evaluate the complete assembly and local climate conditions.