Heat Loss Calculator
Calculate building heat loss through walls, windows, ceiling, floor, and air infiltration. Size HVAC systems accurately with R-value analysis.
Building Dimensions
Insulation R-Values
Heat Loss Rate
30,011 BTU/hr
Recommended R-Values by Climate Zone
| Component | Zone 1-2 | Zone 3-4 | Zone 5-7 |
|---|---|---|---|
| Attic | R-30 to R-49 | R-38 to R-60 | R-49 to R-60 |
| Walls | R-13 | R-13 to R-15 | R-13 to R-21 |
| Floor | R-13 | R-19 to R-25 | R-25 to R-30 |
- Windows are often the biggest source of heat loss - consider upgrading
- Air sealing can reduce infiltration losses by 25-40%
- Adding attic insulation is usually the most cost-effective upgrade
- Use design temperature from ASHRAE data for accurate sizing
- For precise sizing, have a professional perform a Manual J calculation
Related Calculators
About This Calculator
Understanding where heat escapes from your building is essential for proper HVAC sizing, energy efficiency improvements, and reducing heating costs. Our comprehensive Heat Loss Calculator analyzes heat transfer through walls, ceilings, floors, windows, doors, and air infiltration to determine your total heating requirements and identify the biggest opportunities for energy savings.
Heat loss calculations use the fundamental thermal transfer formula (Q = A × ΔT ÷ R) to quantify how much heat escapes through each building component per hour. In 2026, with natural gas averaging $1.20-1.50/therm and electricity at $0.12-0.25/kWh, reducing heat loss by 20-30% through strategic insulation and air sealing typically saves $200-600 annually—with many improvements paying back in 2-5 years.
This calculator provides both total heat loss (for HVAC sizing) and a component-by-component breakdown showing where your building loses the most heat. Whether youre sizing a new heating system, planning energy upgrades, or diagnosing comfort problems, understanding your buildings thermal performance is the essential first step.
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How to Use the Heat Loss Calculator
- 1Enter your building`s floor area and average ceiling height to calculate volume.
- 2Specify total window area (in square feet) and number of exterior doors.
- 3Select R-values for walls, ceiling/attic, floor/foundation, and windows.
- 4Toggle Advanced Mode for custom air tightness, design temperature, and indoor setpoint.
- 5Adjust ACH (air changes per hour) based on building age and condition.
- 6Set your outdoor design temperature for proper HVAC sizing.
- 7Review the heat loss breakdown by component (BTU/hour).
- 8Examine percentage distribution to identify highest-impact upgrade opportunities.
- 9Use total heat loss to size heating equipment with 10-20% safety margin.
Formula
Q = A × ΔT ÷ RHeat loss (Q) in BTU/hour equals the area (A) in square feet multiplied by the temperature difference (ΔT) between inside and outside, divided by the R-value (thermal resistance). Lower R-values mean faster heat transfer and greater losses. Air infiltration is calculated separately using air changes per hour (ACH), volume, and temperature differential.
Understanding R-Values and Thermal Resistance
R-value measures a material`s resistance to heat flow—higher values mean better insulation and lower heat loss:
How R-Value Works:
Heat Loss = Area × Temperature Difference ÷ R-Value
Doubling the R-value cuts heat loss in half. However, there are diminishing returns: going from R-5 to R-19 saves more than going from R-30 to R-60.
Common Building Component R-Values:
| Component | Poor | Average | Good | Excellent |
|---|---|---|---|---|
| Walls (2×4) | R-4 (uninsulated) | R-13 (fiberglass) | R-15 (spray foam) | R-21 (2×6 + foam) |
| Walls (2×6) | R-5 | R-19 | R-21 | R-25+ |
| Attic/Ceiling | R-5 | R-30 | R-49 | R-60+ |
| Floor (over crawl) | R-3 | R-19 | R-25 | R-38 |
| Basement walls | R-0 | R-10 | R-15 | R-20 |
| Windows (single) | R-0.9 | - | - | - |
| Windows (double) | - | R-2.0 | R-3.0 (Low-E) | R-5.0 (triple) |
| Doors (solid wood) | R-2.0 | R-3.0 | R-5.0 | R-7.0 |
Whole-Wall R-Value vs. Cavity R-Value: The R-value on insulation packaging is the cavity R-value. Wood studs (R-1.25 per inch) reduce effective wall R-value by 15-25% due to thermal bridging.
| Wall Type | Cavity Insulation | Whole-Wall R-Value |
|---|---|---|
| 2×4 with R-13 batts | R-13 | R-10 to R-11 |
| 2×4 with R-15 spray foam | R-15 | R-12 to R-13 |
| 2×6 with R-19 batts | R-19 | R-14 to R-16 |
| 2×6 with R-21 spray foam | R-21 | R-17 to R-18 |
| 2×6 + 1" foam sheathing | R-19 + R-5 | R-20 to R-22 |
Heat Loss Through Building Components
A typical building loses heat through multiple pathways. Understanding the distribution helps prioritize improvements:
Typical Heat Loss Distribution:
| Component | Poorly Insulated | Average | Well Insulated |
|---|---|---|---|
| Walls | 25-35% | 20-25% | 10-15% |
| Windows/Doors | 25-35% | 20-25% | 15-20% |
| Ceiling/Attic | 20-30% | 10-15% | 5-10% |
| Air Infiltration | 15-25% | 15-20% | 10-15% |
| Floor/Foundation | 5-15% | 5-10% | 5-10% |
Heat Loss Calculation Examples:
Wall Heat Loss:
Wall area: 1,500 SF
R-value: R-13
Temperature difference: 70°F indoor, -5°F outdoor = 75°F
Heat Loss = 1,500 × 75 ÷ 13 = 8,654 BTU/hr
Window Heat Loss:
Window area: 200 SF
R-value: R-2 (double-pane)
Temperature difference: 75°F
Heat Loss = 200 × 75 ÷ 2 = 7,500 BTU/hr
Key Insight: Windows often lose as much heat as walls despite being 10× smaller because their R-value is 5-10× lower.
Impact of Improvements:
| Upgrade | Before | After | Heat Loss Reduction |
|---|---|---|---|
| Attic: R-11 → R-49 | 12,500 BTU/hr | 2,800 BTU/hr | 78% |
| Windows: Single → Double Low-E | 15,000 BTU/hr | 4,500 BTU/hr | 70% |
| Air sealing: 1.0 ACH → 0.35 ACH | 18,000 BTU/hr | 6,300 BTU/hr | 65% |
| Walls: R-4 → R-13 | 27,000 BTU/hr | 8,300 BTU/hr | 69% |
Air Infiltration and ACH
Air infiltration—uncontrolled air leakage through gaps, cracks, and penetrations—is often the largest source of heat loss in older buildings:
Air Changes per Hour (ACH): ACH measures how many times the entire air volume of your building is replaced per hour through air leakage.
Infiltration Heat Loss = Volume × ACH × 0.018 × ΔT
Where:
- Volume = Floor Area × Ceiling Height (cubic feet)
- 0.018 = Air heat capacity factor (BTU per cubic foot per °F)
- ΔT = Indoor temperature - Outdoor design temperature
ACH by Building Type:
| Building Condition | ACH50* | Natural ACH | Description |
|---|---|---|---|
| Very Tight | 1-2 | 0.10-0.15 | Passive House, new with air barrier |
| Tight | 3-5 | 0.15-0.35 | New construction, blower door tested |
| Average | 5-10 | 0.35-0.65 | Standard modern construction |
| Loose | 10-15 | 0.65-1.0 | Older home with some weatherization |
| Very Loose | 15-25 | 1.0-1.5+ | Drafty old home, minimal sealing |
*ACH50 = air changes at 50 Pascals pressure (blower door test) Natural ACH ≈ ACH50 ÷ 20 (rule of thumb)
Infiltration Heat Loss Example:
2,000 SF home, 8` ceilings = 16,000 cubic feet
Natural ACH: 0.75 (older home)
Temperature difference: 75°F
Infiltration Loss = 16,000 × 0.75 × 0.018 × 75 = 16,200 BTU/hr
Common Air Leak Locations:
| Location | % of Total Leakage | Fix Cost |
|---|---|---|
| Attic penetrations | 20-30% | $50-200 |
| Ductwork (in uncond. space) | 15-25% | $200-500 |
| Windows/doors | 10-20% | $50-300 |
| Electrical outlets/switches | 5-10% | $20-50 |
| Foundation/sill plate | 5-15% | $100-400 |
| Recessed lights | 3-8% | $50-150 |
| Plumbing/wire penetrations | 3-8% | $25-100 |
Design Temperature and Climate
The outdoor design temperature determines how much heating capacity your system needs on the coldest expected days:
What Is Design Temperature? Design temperature is the outdoor temperature that your heating system must overcome to maintain indoor comfort. ASHRAE defines it as the temperature that is exceeded 99% of the hours in a typical year (99% design temperature).
Design Temperatures by City:
| City | 99% Design Temp | Zone | Heating Degree Days |
|---|---|---|---|
| Miami, FL | 47°F | 1 | 200 |
| Houston, TX | 29°F | 2 | 1,600 |
| Atlanta, GA | 22°F | 3 | 2,800 |
| Washington, DC | 17°F | 4 | 4,200 |
| Chicago, IL | -4°F | 5 | 6,500 |
| Minneapolis, MN | -16°F | 6 | 8,000 |
| Fairbanks, AK | -47°F | 7 | 14,000 |
Temperature Differential (ΔT):
ΔT = Indoor Setpoint - Outdoor Design Temperature
Example: Chicago with 70°F setpoint
ΔT = 70 - (-4) = 74°F
Impact of Design Temperature on Heat Loss:
| City | ΔT | Relative Heat Loss |
|---|---|---|
| Miami (ΔT = 23) | 23°F | 31% |
| Atlanta (ΔT = 48) | 48°F | 65% |
| Chicago (ΔT = 74) | 74°F | 100% (baseline) |
| Minneapolis (ΔT = 86) | 86°F | 116% |
Finding Your Design Temperature:
- ASHRAE Fundamentals Handbook (professional resource)
- ACCA Manual J software
- Local HVAC contractors
- State energy codes
Window and Door Heat Loss
Windows and doors are the weakest thermal points in any building envelope:
Window R-Values and U-Factors:
| Window Type | R-Value | U-Factor | Heat Loss/SF* |
|---|---|---|---|
| Single-pane, clear | R-0.9 | 1.1 | 83 BTU |
| Single + storm | R-1.8 | 0.56 | 42 BTU |
| Double-pane, clear | R-2.0 | 0.50 | 38 BTU |
| Double, Low-E (hard coat) | R-2.7 | 0.37 | 28 BTU |
| Double, Low-E (soft coat) | R-3.2 | 0.31 | 23 BTU |
| Triple-pane, Low-E | R-5.0 | 0.20 | 15 BTU |
| Quad-pane, Low-E + argon | R-8.0+ | 0.12 | 9 BTU |
*At 75°F temperature differential
Window vs. Wall Comparison:
15 SF window (double-pane, R-2): 15 × 75 ÷ 2 = 563 BTU/hr
15 SF wall (R-13): 15 × 75 ÷ 13 = 87 BTU/hr
Window loses 6.5× more heat per square foot
Door R-Values:
| Door Type | R-Value | Notes |
|---|---|---|
| Hollow wood (old) | R-1.5 | Minimal insulation |
| Solid wood (1-3/4") | R-2.5 | Better but still poor |
| Steel, polyurethane core | R-5 to R-7 | Good performance |
| Fiberglass, foam core | R-5 to R-8 | Best insulating |
| Storm door addition | +R-1 to R-2 | Cost-effective upgrade |
Total Window/Door Heat Loss Example:
Windows: 250 SF at R-2.5 = 250 × 75 ÷ 2.5 = 7,500 BTU/hr
Doors: 2 at 20 SF each, R-4 = 40 × 75 ÷ 4 = 750 BTU/hr
Total: 8,250 BTU/hr (often 20-30% of total heat loss)
Foundation and Floor Heat Loss
Heat loss through floors and foundations depends heavily on construction type:
Floor/Foundation Types:
| Type | Heat Loss Path | Typical R-Value |
|---|---|---|
| Slab-on-grade | Edge of slab to ground | R-0 to R-10 (perimeter) |
| Crawlspace (vented) | Floor to cold air | R-0 to R-30 (floor insulation) |
| Crawlspace (conditioned) | Walls to ground | R-5 to R-15 (wall insulation) |
| Basement (unheated) | Floor to cold basement | R-0 to R-25 (floor insulation) |
| Basement (heated) | Walls to ground | R-5 to R-20 (wall insulation) |
| Over garage | Floor to unheated garage | R-19 to R-38 (floor insulation) |
Slab Heat Loss Calculation: Slab heat loss occurs primarily at the perimeter (edge), not through the center:
Perimeter Heat Loss = Perimeter (feet) × F-Factor × ΔT
F-Factors (BTU/hr per linear foot per °F):
| Edge Insulation | F-Factor |
|---|---|
| Uninsulated | 0.90 |
| R-5 perimeter | 0.70 |
| R-10 perimeter | 0.55 |
| R-15 perimeter | 0.45 |
Floor Over Crawlspace/Basement:
Standard formula: Q = A × ΔT ÷ R
But ΔT is modified because crawlspace/basement is warmer than outdoors:
Crawlspace: ΔT = Indoor - Crawlspace temp (typically 50-55°F)
Basement: ΔT = Indoor - Basement temp (typically 55-60°F)
Over-Garage Floor (Critical Area): Bonus rooms over unheated garages are notorious for comfort problems:
Garage temperature: Often near outdoor temp (especially with car doors opening)
Required insulation: R-30 to R-38 minimum
Common problem: Only R-11 installed (built to minimum code)
Energy Improvement Priorities
Strategic improvements focus on areas with highest heat loss and best ROI:
Priority 1: Air Sealing (Best ROI)
| Improvement | Cost | Annual Savings | Payback |
|---|---|---|---|
| Weatherstripping doors | $20-50 | $30-80 | <1 year |
| Outlet/switch gaskets | $20-30 | $20-40 | <1 year |
| Attic penetrations (foam/caulk) | $50-200 | $100-250 | 1-2 years |
| Duct sealing | $200-500 | $150-400 | 1-3 years |
| Full house air sealing | $500-1,500 | $200-500 | 2-4 years |
Priority 2: Attic Insulation
| Current | Upgrade To | Cost/1,000 SF | Annual Savings | Payback |
|---|---|---|---|---|
| R-11 | R-49 | $800-1,500 | $150-350 | 3-6 years |
| R-19 | R-49 | $600-1,200 | $100-250 | 4-6 years |
| R-30 | R-60 | $500-1,000 | $50-150 | 5-10 years |
Priority 3: Windows (Expensive but Impactful)
| Current | Upgrade To | Cost/Window | Annual Savings | Payback |
|---|---|---|---|---|
| Single | Double Low-E | $400-800 | $25-75/window | 8-15 years |
| Old Double | Triple Low-E | $600-1,200 | $15-40/window | 15-30 years |
Priority 4: Wall Insulation Most expensive unless walls are open for renovation:
| Method | Cost | When Appropriate |
|---|---|---|
| Blown-in retrofit | $1.50-3/SF | Existing walls, minimal disruption |
| Spray foam (new construction) | $1.50-4/SF | New walls, renovation |
| Continuous exterior foam | $3-6/SF | Deep energy retrofits |
2026 Cost-Effectiveness Ranking:
- Air sealing (always do first)
- Attic insulation to R-49+
- Duct sealing/insulation
- Basement/crawlspace insulation
- Window improvements or storm windows
- Wall insulation (if accessible)
Blower Door Testing
Professional blower door testing provides accurate air infiltration measurement:
What Is a Blower Door Test? A calibrated fan is installed in an exterior door, depressurizing the building to 50 Pascals. The airflow required to maintain this pressure indicates total air leakage.
Test Results Explained:
| Metric | What It Measures | Target |
|---|---|---|
| CFM50 | Cubic feet per minute at 50 Pa | Lower is better |
| ACH50 | Air changes per hour at 50 Pa | <3 for new construction |
| ACHnatural | Estimated natural infiltration | 0.35 minimum for health |
Converting ACH50 to Natural ACH:
ACHnatural ≈ ACH50 ÷ N-factor
N-factor varies by climate and building height:
- 1-story, cold climate: N = 16
- 2-story, moderate climate: N = 20
- Multi-story, mild climate: N = 25
ACH50 Standards:
| Standard | ACH50 Requirement |
|---|---|
| ENERGY STAR Homes | ≤3-5 (varies by climate) |
| DOE Zero Energy Ready | ≤3 |
| Passive House | ≤0.6 |
| Typical new construction | 3-7 |
| Older homes (pre-1980) | 10-25 |
Cost and Availability:
| Service | Cost | When to Use |
|---|---|---|
| Basic blower door test | $150-300 | Before/after improvements |
| With thermal imaging | $300-500 | Identify leak locations |
| Full energy audit | $300-800 | Comprehensive assessment |
Finding the Leaks: During a blower door test, technicians use:
- Thermal imaging (shows cold air infiltration)
- Smoke pencils (visualize air movement)
- Hand feel (detect drafts around penetrations)
Using Heat Loss for HVAC Sizing
Total heat loss determines heating equipment capacity:
From Heat Loss to Equipment Size:
Heating Capacity Needed = Total Heat Loss × Safety Factor
Safety Factor: 1.10-1.25 (10-25% margin)
Example:
Total heat loss: 65,000 BTU/hr
With 20% safety: 65,000 × 1.20 = 78,000 BTU/hr
Select: 80,000 BTU furnace (standard size)
Don`t Oversize:
| Sizing | Result | Problems |
|---|---|---|
| 10-25% over heat loss | Correct | Handles design conditions + margin |
| 25-50% over | Mild oversize | Some short-cycling, acceptable |
| 50-100% over | Moderate oversize | Short-cycling, comfort issues |
| 100%+ over | Severe oversize | Constant short-cycling, poor humidity |
Equipment Adjustment for Efficiency: Heat loss = OUTPUT BTU needed. Input BTU depends on efficiency:
Required Input = Required Output ÷ AFUE
Example: 80,000 BTU output at 95% AFUE
Input = 80,000 ÷ 0.95 = 84,200 BTU input
Duct Loss Consideration: Add 20-40% if ducts run through unconditioned spaces:
Ducts in attic: Add 25-35%
Ducts in crawlspace: Add 20-30%
Ducts fully in conditioned space: No addition
Heat Loss Calculator vs. Manual J:
| Method | Accuracy | Use Case |
|---|---|---|
| This calculator | ±15-25% | Budget planning, improvement prioritization |
| Manual J | ±5-10% | Final equipment selection, new construction |
Always get Manual J calculation before equipment purchase.
Pro Tips
- 💡Always address air leaks before adding insulation—insulation doesn`t stop air movement, only slows heat conduction.
- 💡Windows and doors are the weakest thermal links—even modest upgrades like storm windows provide significant benefit.
- 💡Attic insulation is usually the most cost-effective improvement—aim for R-49 or higher in cold climates.
- 💡Thermal bridging through studs reduces effective wall R-value by 15-25%—consider continuous exterior insulation.
- 💡Bonus rooms over garages often have inadequate insulation—check these spaces first for comfort problems.
- 💡A blower door test ($150-300) identifies air leakage and measures actual ACH—essential before major improvements.
- 💡Duct losses in unconditioned spaces can add 25-40% to heat loss—seal and insulate ductwork.
- 💡Use design temperature for your area, not average winter temperature—systems must work on the coldest days.
- 💡Basement walls below grade lose less heat than above-grade sections—focus insulation where walls are exposed.
- 💡Single-pane windows can lose 5-8× more heat per square foot than insulated walls—prioritize these areas.
- 💡Recessed lights in insulated ceilings are major air leakage points—use IC-rated, airtight fixtures or covers.
- 💡Calculate payback period before major improvements: Cost ÷ Annual Savings = Years to payback.
Frequently Asked Questions
R-value measures resistance to heat flow (higher is better). U-value measures heat transfer rate (lower is better). They are mathematical inverses: U = 1/R. Windows typically use U-value because they have low R-values where small decimal differences are significant. A U-0.30 window equals R-3.3.
