How do I calculate heat loss for my house?

Heat loss is calculated using the formula Q = U × A × ΔT for each building component (walls, ceiling, floor, windows, doors), plus infiltration losses. U is the thermal transmittance (1/R-value), A is the surface area, and ΔT is the temperature difference between indoors and the outdoor design temperature. Sum all components for the total heating load in BTU/hr.

What is the ASHRAE 99% heating design temperature?

The ASHRAE 99% heating design temperature is the outdoor temperature that is exceeded (warmer) 99% of the hours during the heating season. It represents a near-worst-case condition for sizing heating equipment. For example, if your design temperature is 5°F, the outdoor temperature is above 5°F for 99% of winter hours.

What is the difference between R-value and U-factor?

R-value measures resistance to heat flow — higher is better insulation. U-factor measures the rate of heat transfer — lower means less heat loss. They are reciprocals: U = 1/R. Walls and ceilings are typically rated by R-value (e.g., R-13 batt insulation), while windows and doors are rated by U-factor (e.g., U=0.30 for double-pane low-e).

How much of my heat loss is from infiltration?

Infiltration (air leakage through gaps, cracks, and penetrations) typically accounts for 25-40% of residential heating loads. In older, drafty homes it can be even higher. Air sealing is often the most cost-effective way to reduce heating costs because it addresses the single largest component of heat loss.

Heat Loss Calculator

Calculate heating load in BTU/hr with ASHRAE design temperatures by ZIP code. Component-level breakdown for walls, ceiling, floor, windows, and infiltration using real R-values from an assembly library.

Location & Conditions

ZIP Code

Indoor Setpoint(°F)

Room Dimensions

Length(ft)

Width(ft)

Ceiling Height(ft)

Exterior Walls

Construction

Wall Construction

Ceiling/Roof

Floor Type

Infiltration

Windows & Doors

Window Type

Window Area(sq ft)

Door Type

Exterior Doors

Understanding Residential Heat Loss

Why Design Temperature Matters

The ASHRAE 99% heating design temperature is the outdoor temperature that is exceeded 99% of winter hours. Using the wrong design temperature can swing your result by 30% or more. This calculator auto-loads the correct value for your ZIP code — always verify it matches your local conditions.

Component-Level Thinking

Heat escapes through every surface and every air leak. The breakdown chart shows exactly where: walls, ceiling, floor, windows, doors, and infiltration. Infiltration (air leakage) alone accounts for 25-40% of residential heating loads — it is often the single largest component and the most cost-effective to address.

R-Value vs. U-Factor

R-value measures resistance to heat flow (higher is better insulation). U-factor measures heat transmittance (lower is better). They are reciprocals: U = 1/R. Walls and ceilings are rated by R-value; windows and doors by U-factor. This calculator uses proper assembly R-values that include air films — not just insulation R-values.

This Tool vs. Full Manual J

This calculator handles single-room heating loads. A full ACCA Manual J adds room-by-room analysis, cooling loads, latent loads, solar gain by window orientation, duct losses, and ventilation per ASHRAE 62.2. Use this tool for quick estimates and envelope diagnostics — use Manual J for equipment selection and permit submittals.

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This tool provides calculations for informational purposes only and does not constitute professional engineering, construction, or safety advice. Verify all results independently before use in design, construction, or compliance decisions. See our Terms of Service.

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