Shelter Heat Loss Estimator

How heat loss is calculated

The formula is: Heat Loss (BTU/hr) = (Surface Area / R-value) x Temperature Difference. Each surface, walls, ceiling, floor, and windows, is calculated separately, then summed. A higher R-value means less heat escapes. Doubling R-value from R-11 to R-22 cuts that surface's heat loss in half. This tool adds 25% for air infiltration, which accounts for gaps around doors, outlets, and penetrations.

Tips for success

• Ceiling first, always. Heat rises, so the ceiling is usually the biggest loss surface. In a poorly insulated space, 40% or more of your heat escapes upward. A thermal blanket or extra attic insulation pays back faster than wall improvements.
• Windows lose 5x more heat per square foot than walls. A double-pane window is only R-2 vs R-11 for a standard wall. Hanging heavy blankets over windows during a power outage can cut window losses by more than half.
• Consolidate to one small room. Heating a 150 sq ft bedroom uses far less fuel than a 1,200 sq ft home. An interior room with fewer exterior walls and no windows cuts heat loss dramatically.
• Body heat matters. Two people in a well-insulated small room each add roughly 300 BTU/hr of heat, which can meaningfully offset a small deficit in mild conditions.
• Never use outdoor gas heaters indoors. Grills, torpedo heaters, and unvented combustion appliances produce CO that can incapacitate you within minutes. Only use heaters with an oxygen depletion sensor rated for indoor use.

Practical note

This is an approximation using the standard BTU/hr = (Area / R-value) x delta-T formula plus a 25% infiltration factor. Real heat loss is also affected by thermal bridging, door sealing quality, and foundation type. Add another 10–20% buffer for worst-case planning. This tool is designed for ballpark sizing, not engineering design.

R-value quick reference

• R-1: Single-pane glass, thin plywood, tent fabric. Essentially uninsulated.
• R-3.5: 2x4 stud cavity with no insulation fill, just dead air.
• R-11: 3.5 in fiberglass batt, the standard 2x4 wall fill in pre-1980 homes.
• R-19: 5.5 in fiberglass batt, standard 2x6 wall or floor insulation.
• R-30/R-38: Attic blown-in cellulose or fiberglass, the minimum recommended for cold climates.
• R-2 (window): Double-pane low-E glass. Roughly 6x better than single pane but still the weakest point in most walls.

Common mistakes

• Using zero-head pump ratings for heat. Heater ratings on the box are often at ideal conditions. Propane efficiency drops in cold weather and at low tank pressure. Budget 10–15% less than the rated output.
• Forgetting infiltration. The 25% infiltration factor in this tool is a minimum estimate. Older homes with loose siding, gaps around outlets, and poor door sweeps can add 40–50% over conduction alone.
• Sizing for average cold, not the coldest night. Your shelter will be fine during mild nights. The moment that matters is a 10°F night with a 60°F indoor target and a failing heat source. Size for the worst case.
• Ignoring window count. A room with 4 large windows and R-11 walls can lose more heat through glass than through all four walls combined. Count windows accurately.
• No CO detector when using combustion heat. Even indoor-rated propane heaters should run with a window cracked and a battery-operated CO detector nearby. This is not optional.

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