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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