Building Energy & Efficiency Workbench

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Engineering reference

Building Energy & Efficiency: theory, method, and sources

This environmental & energy engineering workspace publishes 10 governing equations, 5 stated assumptions, 1 documented boundary, and 2 sources so the numbers it returns can be checked rather than taken on trust.

Calculations run locallyCalculation & source methodology

How this tool works

Ten modules cover envelope heat loss, layered assembly R-values, infiltration, degree-day energy estimates, HVAC electrical demand, lighting energy, solar gain, energy use intensity, heat recovery, and simple payback.

The degree-day and payback modules connect a physical retrofit to an annual energy figure and then to a financial one, which is the sequence used in a preliminary energy audit.

Calculators and topics covered

building energy
efficiency
HVAC
insulation
U-value
R-value
degree days
EUI
heat recovery
lighting energy

Core equations

\text{conductive loss:}\quad Q = U \cdot A \cdot \Delta T

\text{layered assembly:}\quad R_{\mathrm{total}} = \sum \left(\frac{t}{k}\right),\; U = \frac{1}{R_{\mathrm{total}}}

\text{infiltration load:}\quad Q = \frac{\mathrm{ACH} \cdot V \cdot \rho \cdot c_{p} \cdot \Delta T}{3600}

\text{degree-day energy:}\quad E = \frac{UA \cdot HDD \cdot 24}{\eta }

\text{HVAC electrical energy:}\quad E = \frac{Q_{\mathrm{thermal}} \cdot h}{\mathrm{COP}}

\text{lighting energy:}\quad E = LPD \cdot A \cdot h \cdot \mathrm{days}

\text{solar gain:}\quad Q = A \cdot SHGC \cdot I

\text{energy use intensity:}\quad EUI = \frac{\text{annual}\; \mathrm{kWh}}{\operatorname{floor} }\; \text{area}

\text{heat recovery:}\quad Q = \varepsilon \cdot \dot{m} \cdot c_{p} \cdot \Delta T

\text{simple payback:}\quad \mathrm{years} = \frac{\text{cost}}{\text{annual}}\; \text{savings}

Method and assumptions

Assumptions

Steady-state heat transfer with constant U-values; thermal mass and dynamic response are not modelled.
Degree-day estimates assume a fixed balance point temperature and a constant system efficiency across the season.
Infiltration uses a single air change rate rather than a pressure-driven or wind-dependent model.
Solar gain uses a single irradiance value with no shading geometry, orientation, or hourly sun path.
Simple payback ignores discount rate, fuel escalation, maintenance, and equipment replacement.

Limitations and design boundaries

Educational screening calculations only. Building design and retrofit decisions require hourly weather, schedules, controls, moisture, thermal bridges, equipment curves, codes, commissioning data, and qualified analysis.

Sources and references

Primary sources are preferred for ratings, standards, manufacturer data, and externally defined constants.

Source policy

ASHRAE Handbook—Fundamentals
Kreider et al., Heating and Cooling of Buildings