Pipe Heat Loss Calculator

Hot water pipes in unconditioned spaces (basements, crawl spaces, attics) radiate heat to the surrounding air continuously. This costs money even when no one is using hot water — the water heater cycles on to maintain tank temperature because heat is leaking out through the pipes.

The heat loss formula

For an uninsulated pipe, heat loss per meter per hour is:

Q = U x pi x D x L x (T_water - T_ambient)

Where U is the convective heat transfer coefficient (approximately 10 W/m2K for a still-air environment), D is the outer pipe diameter in meters, L is length in meters, and the temperature difference is in Kelvin (same as Celsius difference).

Run that formula at a 45 K difference (60C water in a 15C space) and common pipe sizes come out at:

• 15mm (1/2 in) copper: ~21 W/m
• 22mm (3/4 in): ~31 W/m
• 35mm (1.25 in): ~49 W/m

Standard 13mm foam lagging cuts that by roughly 75%, and thicker 19-25mm wall gets you to about 87%. What matters is the thickness, not the brand of material: foam and fiberglass have nearly identical conductivity (about 0.035 and 0.040 W/m·K), so a thick foam beats a thin fiberglass wrap every time. The calculator applies a flat 75% for the foam case.

Payback period

Foam lagging runs somewhere around $1-3 per meter and electricity or gas lands near $0.10-0.15/kWh, though both move with your region and the year, so put your own figures in the fields above. On those rough numbers a run in an unheated basement pays for itself in 3-18 months.

In conditioned spaces (inside the heated home), uninsulated pipes contribute heat to the room. That is not wasted in winter, but in summer the air conditioning (AC) has to remove it again. Year-round savings are smaller in climate-controlled spaces.