U-Value and R-Value Heat Transfer Calculator

Conduction through a wall, window, roof or floor follows a single governing equation. The U-value tells you how much heat crosses one square foot per degree of temperature difference; multiply by area and the temperature difference across the surface to get the hourly heat flow. You can enter either the whole-assembly U-factor or the total R-value, and switching modes keeps the conductance consistent by taking the reciprocal. Inputs are imperial; SI W/(m²·K) values must be converted first.

This is the same conduction term that sits at the core of a full envelope calculation. To combine it with infiltration, internal and solar loads, move on to the heat load calculator, and for a walk-through of where each term comes from, see the HVAC load calculation formula guide.

ΔT = 70 − 20 = 50 °F. Q = 0.3 × 150 × 50 = 2,250 BTU/h, flowing outward because it is colder outside. In total-R mode the equivalent input is R = 1 ÷ 0.3 ≈ 3.33 h·ft²·°F/BTU and the answer is the same. These defaults are example assumptions, not universal design conditions.

Real walls are not one number. The series layers add up, but the studs create a second, lower-resistance path that must be weighted by area. The following example uses approximate textbook-style layer values (check the ASHRAE Handbook - Fundamentals or the product data for your materials) for a 2×4 wall with R-13 batts and about 25% framing area.

Area-weighted U = 0.75 ÷ 15.4 + 0.25 ÷ 6.8 ≈ 0.049 + 0.037 = 0.086 BTU/(h·ft²·°F), a whole-wall R of about 11.7. That is lower than the R-13 on the batt label, and it ignores extra studs at corners, headers and plates, which push the real figure down further. Insulation continuous across the framing, such as exterior foam, reduces this bridging.

The table below shows the hourly heat transfer through 100 square feet of various surfaces with a 50°F temperature difference. The U-values are approximate whole-assembly orientation values, not product data, and the insulated wall rows use nominal cavity R. Notice how the load falls as insulation improves.

The last three rows use the insulation label as if it were the whole assembly, to show the arithmetic. Real framed assemblies have a higher U-value because of studs, joists and air films, as the wall example above shows (U ≈ 0.086 for a nominal R-13 wall).

• Confusing U-value and R-value — remember U = 1 ÷ R, so a low U-value (good) corresponds to a high R-value.
• Adding U-values for layered assemblies — add resistances (R-values), then take the reciprocal of the total.
• Calling R-13 cavity insulation an R-13 wall — framing, films and thermal bridges change the whole-assembly R.
• Forgetting surface air films, which add resistance and belong in the assembly total.
• Using gross wall area without subtracting window and door openings, which have very different U-factors.
• Using a center-of-glass window value instead of the whole-product NFRC U-factor.
• Treating this conductive result as the whole load — it excludes solar through glass, air leakage and moisture.
• Mixing units — a U-value in SI (W/m²·K) is about 5.68 times the imperial value used here; do not interchange them.

A single application of Q = U × A × ΔT tells you the conduction through one surface, but a real building is a patchwork of surfaces, each with its own U-value and area. The practical workflow is to take off the area of every exterior assembly from your plans, subtract the openings, and run the conduction term for each one separately before summing. A typical single-story house might break down into framed walls, an attic ceiling, a slab or floor over a crawlspace, and a dozen windows and two doors, each carrying its own U-factor. Conduction through those fabric elements is only one line item; the procedures in ACCA Manual J add air infiltration, duct losses, solar heat gain through glazing and internal gains on top of it to arrive at design heating and cooling loads.

Because the result is area-weighted, the surface that dominates is rarely the one with the worst U-value. An attic ceiling at a low U may have 1,500 ft² of area while windows at a far higher U total only 200 ft². Multiplying U by A reveals the true contribution of each, which is why a load calculation cannot be done by intuition; every assembly must be weighted by its actual square footage.

The right-hand column is the UA value, the product of U and A, summed across the envelope. Total UA for this example is about 212 BTU/(h·°F), so at a 70 °F design temperature difference the fabric conduction alone is about 14,800 BTU/h before infiltration and other terms are added. Notice that the walls dominate even though the windows have a much worse U-value. The heat load calculator lets you fold in the remaining loads and convert the result into tons, where one ton equals 12,000 BTU/h.

Lowering a U-value always reduces conduction, but the returns are not linear. Because U = 1 ÷ R, the first increments of insulation remove far more heat flow than the last. Going from an uninsulated wall to a filled cavity cuts conduction a lot, while adding continuous exterior foam on top of that trims a smaller amount per 100 ft² at a 50 °F ΔT. Past a certain depth the marginal insulation saves so little that the money is better spent on glazing, air sealing or duct improvements.

Two factors complicate the simple Q = U × A picture. The first is thermal bridging: wood or steel framing conducts around the insulation, so the U-value of a real wall is higher than the center-of-cavity number suggests. ASHRAE Handbook - Fundamentals describes parallel-path and isothermal-plane methods, and steel-framed walls in particular can lose a large share of their nominal R-value unless continuous insulation breaks the path. The second is windows, where the NFRC label reports a whole-unit U-factor that already blends the center-of-glass, edge and frame. Always use that assembly figure rather than the more flattering center-of-glass number.

• Attack the highest UA first — usually large wall or ceiling areas or single-pane windows, not the assembly with the technically worst U-value.
• Expect diminishing returns: doubling R from 19 to 38 only halves an already small conduction term.
• Break thermal bridges with continuous insulation so the as-built U-value approaches the design intent.
• Compare whole-unit NFRC U-factors between windows, not center-of-glass values.
• Confirm any new assembly meets the IECC requirements for your climate zone before relying on the savings.

U-values are not just an engineering convenience; they are written into building energy code. The International Energy Conservation Code (IECC) sets maximum fenestration U-factors and minimum or maximum assembly values for each climate zone, so a window legal in a warm zone may fail the prescriptive path in a cold one. Builders can comply either by meeting those component values or by showing an equivalent total UA under the code's trade-off path, which is the area-weighted sum described above. The U-factors on the window stickers that demonstrate compliance are certified by the NFRC, and the material R-values behind opaque assemblies trace back to ASHRAE data and manufacturer testing. The IECC climate zones guide explains how the zones are assigned, and the design temperature guide covers choosing the ΔT.

A final caution on units. The imperial U-value used here is in BTU/(h·ft²·°F), while most of the world quotes thermal transmittance in metric W/(m²·K). A metric U-value is about 5.68 times the imperial one for the same assembly, so a window listed at 1.4 W/m²·K corresponds to about U-0.25 in imperial terms. Mixing the systems is one of the most common ways a load calculation goes silently wrong, so confirm which convention a datasheet or label uses, and use the unit converter where needed, before plugging the number into Q = U × A × ΔT.

Have a designer verify assemblies and thermal bridges before using the result for code or equipment sizing. ASHRAE Handbook - Fundamentals provides heat-transfer, air-property and load-calculation background. Example inputs here are stated scenarios, not prescribed handbook values. For residential design, ACCA Manual J, S and D address load calculation, equipment selection and duct design. This calculator does not certify a complete design or code compliance.