Building Physics & Energy Calculators
12 free building physics & energy calculators — standards-based, with formulas, worked examples, and no sign-up.
Heat, air, and moisture move through a building envelope whether anyone calculates them or not — these tools make the movement visible before it becomes an energy bill or a mould problem. The backbone is the fabric chain: assembly U-values layer by layer per ISO 6946 with thermal-bridging corrections, junction psi-values against a 59-junction catalog, and whole-room heat loss and gain per EN 12831 or ASHRAE Manual J across multiple rooms. Degree days turn those loss coefficients into seasonal kWh, and the energy efficiency tool extends to consumption modelling and payback.
Moisture is the second thread, and the tools cover it at both the surface and inside the assembly. The condensation risk calculator runs the Glaser method (EN ISO 13788) with monthly vapour-pressure profiles through 122 materials; moisture & mould risk works the surface side via the temperature factor fRsi per BS 5250, predicting mould growth at cold spots; and the psychrometric calculator supplies the humidity ratio, dew point, and enthalpy numbers the other two lean on. Summer gets its own treatment: overheating risk per CIBSE TM59/TM52 with hourly simulation and adaptive comfort criteria — increasingly the binding constraint in well-insulated homes.
Two practical tools translate the physics into purchases: the insulation R-value calculator checks your assembly against IECC 2021 climate-zone minimums and counts batt bags or blown-in depth, and the blown-in insulation tool works a real attic top-up from the manufacturers' coverage charts, converting existing depth to R and reading bag counts off the published tables.
All Building Physics & Energy Tools
Energy Efficiency Calculator
Building thermal performance, energy consumption analysis, and cost savings projections.
Thermal Movement Calculator
Thermal expansion, joint widths, and sealant selection for facade elements.
Condensation Risk Calculator
Interstitial condensation analysis using the Glaser method with vapour pressure profiles.
Thermal Bridge Calculator
Thermal junction analysis with psi-values, temperature factors, and standards compliance.
Heat Loss & Gain Calculator
Room-by-room heating and cooling load analysis with fabric loss, ventilation, solar gains, and internal gains.
U-Value Calculator
Assembly thermal transmittance with layer-by-layer resistance calculation and thermal bridging corrections.
Overheating Risk Assessment
Summer comfort analysis with hourly simulation per CIBSE TM59/TM52, EN 15251, and ASHRAE 55.
Moisture & Mould Risk Calculator
Surface temperature factor and mould growth prediction for wall junctions and assemblies.
Psychrometric Calculator
Moist-air properties: humidity ratio, enthalpy, dew point, and wet-bulb at any atmospheric pressure.
Degree Days & Energy Calculator
Heating and cooling degree-days with a simplified building energy estimate per ASHRAE & CIBSE.
Insulation R-Value Calculator (IECC Climate Zones)
Prescriptive minimum insulation R-values from IECC 2021 Table R402.1.3 for ceilings, wood-frame walls, floors, basement walls, and crawl space walls across climate zones 1-8, with the full compound code option text (cavity and continuous-insulation alternatives), a fiberglass batt purchase plan with layers and bag counts, and blown-in fiberglass or cellulose depth in whole inches.
Blown-In Insulation Calculator (Bags & Settled Depth)
How many bags of blown-in attic insulation a top-up needs, priced from verified manufacturer coverage charts: existing insulation converted to R-value from its measured depth at representative R-per-inch figures (fiberglass batt 3.1, loose-fill fiberglass 2.5, cellulose 3.7 per inch), the shortfall ΔR = target − existing read against the Greenfiber INS541LD cellulose chart (19.05 lb bags, settled and initial installed depth columns) or the Owens Corning AttiCat expanding fiberglass chart (27.5 lb bags, settling negligible) with linear interpolation between published rows, whole-bag purchase counts, the added settled depth to reach, and an already-at-target check that prices nothing when the attic already meets the target.
Frequently Asked Questions
What order should I run the fabric tools in?
U-value first — it needs only the assembly build-up. Feed the resulting U-values into heat loss & gain for room loads, and if junctions are significant (they are, in well-insulated buildings) add psi-values from the thermal bridge tool. Then degree days converts the total heat-loss coefficient into annual energy. Skipping straight to an energy number without the U-value step is where most estimates go wrong.
Condensation risk or moisture & mould — which moisture tool do I need?
They answer different questions. Moisture & mould checks the internal surface: is any spot cold enough (fRsi below the BS 5250 threshold) to grow mould? Condensation risk checks inside the construction: does vapour diffusing through the layers hit dew point within the assembly (Glaser, EN ISO 13788)? A wall can pass one and fail the other — surface mould at an uninsulated junction, or hidden interstitial condensation behind an internally insulated solid wall.