Wall Assembly R-Value & Heat Loss Calculator: Total R, U-Factor and Heat Loss
Total R-value and U-factor of a wall, roof or floor assembly from its layers (US R or metric RSI), with air films and an optional stud framing fraction, heat loss in watts and BTU/h, annual energy and cost from degree-days, the temperature through the wall and the payback of adding insulation.
How it works
R_total = Σ layer R + inside film + outside film (wall 0.68 + 0.17, US R) · with framing: 1/R = f ÷ R_framing path + (1 − f) ÷ R_cavity path · RSI = R ÷ 5.678 · U = 1 ÷ RSI (W/m²·K) · Q = U × A × ΔT · annual kWh = U × A × HDD (°C·days) × 24 ÷ 1,000
Thermal resistances add in series. The R-value of a wall is the sum of every layer’s resistance plus the thin films of air on the inside and outside surfaces (ASHRAE values: R-0.68 for still air on a wall, R-0.61 under a ceiling with heat flowing up, R-0.17 outside in a 15 mph winter wind). The U-factor, the heat flow per square metre per degree, is the reciprocal of the total. Multiply U by the area and the indoor-outdoor temperature difference for the steady heat loss at design conditions, the figure a heating system has to replace; multiply by heating degree-days instead for a whole season. Studs, plates and headers are a parallel path: in a 2×4 wall at 16 in on centre they cover about 25% of the area and conduct three times as much heat as the batts between them. With a framing fraction, the calculator replaces the highest-R layer with the framing’s R-value for that share of the wall and averages the two U-factors (the parallel-path method), which gives a whole-wall R below the label on the insulation. Because U is a reciprocal, each added R helps less: going from R-5 to R-15 cuts the loss by two-thirds, from R-20 to R-30 by one-third. Continuous insulation over the studs adds to both paths, so it is the most effective upgrade. Windows, air leakage and thermal bridges at corners and rim joists are not included and often dominate.
Worked example
Vinyl siding (R-0.6) + 7/16 in OSB (R-0.6) + an R-13 batt + 1/2 in drywall (R-0.45) + air films (0.68 + 0.17 = R-0.85) = R-15.5, or RSI 2.73 and U = 0.366 W/m²·K. Sixty square metres at 21 °C inside and −10 °C outside lose 0.366 × 60 × 31 = 681 W (2,325 BTU/h). Over a 3,500 °C·day season that is 0.366 × 60 × 3,500 × 24 ÷ 1,000 = 1,846 kWh, about $185 a year at $0.10 per delivered kWh. Adding R-10 of continuous insulation makes it R-25.5 and saves 39%, about $72 a year.
Sources: ColoradoENERGY: R-value table (air films, materials) and 2×4 wall parallel-path example (R-12.03) · Engineering ToolBox: heat loss by transmission, U = 1 ÷ ΣR, Q = U·A·ΔT (concrete wall example U ≈ 3.13 W/m²K)
Frequently asked questions
How do I calculate the R-value of a wall?
Add the R-values of every layer (siding, sheathing, insulation, drywall) and the two air films. For a stud wall, also account for the framing: R-13 batts in a 2×4 wall give about R-15.5 through the cavity but only about R-12 for the whole wall once studs are included.
What is the difference between R-value and U-factor?
R-value is resistance to heat flow (higher is better); U-factor is how much heat flows per unit area per degree (lower is better). U = 1 ÷ total R. Codes often list maximum U-factors for whole assemblies, which include framing and air films.
How do I convert between US R-value and RSI?
RSI (m²·K/W) = US R ÷ 5.678, and US R = RSI × 5.678. An R-13 batt is RSI 2.29; an RSI 3.5 product is about R-20. Choose the unit your layer values use with the "Layer R-values are in" switch.
What are typical R-values per inch?
Fiberglass and cellulose about R-3.2–3.8, mineral wool R-3.1–4.0, EPS about R-4, XPS R-5, foil-faced polyiso R-5.0–5.6, softwood R-1.25, concrete block R-1.11 for 8 in and a 4 in face brick about R-0.44 in total.
How much heat goes through a wall?
Heat loss = U × area × temperature difference. An R-15.5 wall (U = 0.366 W/m²·K) of 60 m² with 31 °C between inside and outside loses about 680 W, or 2,300 BTU/h. Over a heating season, multiply U × area by degree-days × 24 to get kWh.
Is adding more insulation worth it?
The savings shrink as R grows because heat flow is 1 ÷ R. Going from R-11 to R-21 cuts wall loss by almost half, while going from R-30 to R-40 saves only a quarter of an already small loss. Compare the yearly saving shown with the installed cost.
Related calculators
Building-physics estimate using steady-state formulas and typical material values; real assemblies, workmanship, air leakage and climate data can move results significantly. Confirm with a qualified designer for code compliance.