Heat runs downhill
Heat always moves from warm to cold. It never moves the other way on its own. In winter, your warm house is the hill, and the cold outdoors is the valley.
A heater does not “make a house warm” once. It replaces heat that keeps leaking out. The faster heat leaks, the bigger the heater and the bigger the fuel bill.
The bigger the temperature difference between inside and outside, the faster heat flows. A 70 °F (21 °C) room loses heat about twice as fast on a 30 °F night as on a 50 °F night.
Conduction: through solid things
Conduction is heat moving through a solid material, from molecule to molecule. It is why a metal mug gets hot when you pour in coffee.
In a house, heat conducts through walls, roofs, floors and window glass. Some materials slow it down well. Others let it through fast.
- Wood framing and glass conduct heat fairly easily.
- Fluffy or foamy materials with trapped air, like fiberglass, cellulose and foam board, slow it down.
Insulation is any material chosen to slow conduction. Its resistance to heat flow is its R-value. A higher R-value means slower heat loss (DOE).
Convection: carried by air
Convection is heat carried by a moving fluid, such as air or water. Warm air rises, cool air sinks, and the moving air takes heat with it.
In a house, the costly kind of convection is air leakage: warm indoor air escaping through gaps while cold outdoor air sneaks in. Leaks hide around doors, windows, outlets, attic hatches and where pipes and wires pass through walls (DOE).
Air leaks can undo good insulation. A thick wall with gaps around it still loses heat. That is why air sealing usually comes before, or with, more insulation.
Radiation: straight across a room
Radiation is heat moving as invisible infrared light. It travels straight across open space. You feel it from a wood stove across the room, or from the sun on your face on a cold day.
Radiation explains a common puzzle. A room can read 70 °F on the thermostat and still feel chilly. Your body is radiating heat to cold window glass and cold walls nearby.
Better windows, curtains at night and warmer wall surfaces make a room feel warmer at the same air temperature.
Text description
A cross-section of a small insulated house on a cold night, inside about 68 °F (20 °C) and outside about 20 °F (minus 7 °C). Heat flows out by conduction through the insulated walls, the ceiling insulation and the floor. Air leaks: warm air escapes through gaps near the ceiling while cold air is drawn in through gaps near the floor, which is called the stack effect. Radiation: the window loses heat toward the cold night sky. On a sunny day, sunlight through the south window adds heat back.
Which way is the heat moving?
Is each example mostly conduction, convection or radiation?
- Warm air leaking out through a gap around the attic hatch
- A wall stud that is colder than the insulation beside it
- A cold draft under the front door
- Feeling chilly while sitting next to a big window at night
- Heat passing through a single-pane window
- Feeling the warmth of a wood stove from across the room
The heat-loss estimate
A heat-loss estimate is a rough count of how fast heat leaves a home. Heat is often counted in British thermal units (BTU). One BTU warms one pound of water by 1 °F. One watt for an hour is about 3.4 BTU.
For each wall, window, roof and floor:
heat loss (BTU per hour) = area (sq ft) × temperature difference (°F) ÷ R-value
Add up every surface, then add the heat carried out by air leaks. The total is the heat your system must replace.
Change the R-value. Watch what doubling it does to heat loss.
Heat loss: about 2,462 BTU per hour (721 W).
Doubling the R-value halves this number.
Assumptions: Heat loss = area × temperature difference ÷ R. Real walls lose more through framing, windows and air leaks.
The coldest night and the whole winter
Two numbers matter, and they size different things.
- Design temperature is a cold outdoor temperature your area drops below for only a small share of hours each year. Heating designers often use the “99% value”: it is colder than this only about 1% of the hours in a year (ENERGY STAR). Heat loss at that temperature sets how big the heater must be.
- Heating degree days (HDD) add up how cold a whole season is. Each day, HDD is how far the average outdoor temperature falls below 65 °F. NOAA publishes normals for thousands of US stations.
Winter heat roughly equals hourly heat loss per °F × HDD × 24. A cabin losing 300 BTU per hour for each °F, in a place with 6,000 HDD, needs about 43 million BTU a winter.
Climate zone changes these answers a lot. A mild southern or coastal site may have under 2,000 HDD. The coldest zones in the lower 48 states pass 9,000, and parts of Alaska go well past 12,000.
Text description
Left: a bar chart of one sample week. The 65 °F base temperature is a dashed line near the top. Each day's average temperature is a dot, and a bar hangs from the base line down to it. Monday averages 45 °F, giving 20 heating degree days; Tuesday 40 °F, 25; Wednesday 35 °F, 30; Thursday 50 °F, 15; Friday 55 °F, 10; Saturday 30 °F, 35; Sunday 38 °F, 27. The week adds up to 162 heating degree days. Right: whole-winter totals compared on bars. A mild southern or coastal site has under 2,000. The lesson's cabin example has 6,000. The coldest zones in the lower 48 states pass 9,000, and parts of Alaska go well past 12,000. A note gives the rule: winter heat is about hourly heat loss per °F times HDD times 24, so 300 BTU per hour per °F times 6,000 HDD times 24 is about 43 million BTU.
Estimate your own home
Use the estimator below with your own floor plan, or with the plan from lesson 13. It saves to My plan, and later lessons use the result to size heating and firewood.
Check
A 400 sq ft wall is R-10. Inside is 70 °F and outside is 20 °F. About how much heat does it lose each hour?
Summary
- Heat moves from warm to cold by conduction, convection and radiation.
- Insulation slows conduction. Air sealing stops the costly kind of convection.
- Heat loss = area × temperature difference ÷ R-value, added up for every surface, plus air leaks.
- The design temperature sizes the heater. Degree days estimate a winter of fuel.
Next, you will compare heating options against the number you just found.
Go deeper: U-factor, framing and what this estimate leaves out
Windows are usually rated by U-factor, which is 1 ÷ R. A window with a U-factor of 0.30 is about R-3.3. Wood studs cut through the insulation in a wall, so a wall’s whole-wall R-value is lower than the label on the insulation. Professionals use detailed methods, such as Manual J in the US, that also count solar gains, heat from people and appliances, and ground contact. This estimate is for planning only. A heating contractor should size the actual system.