Why Do Pipes Freeze in Walls of Unheated Spaces?
Last updated October 31, 2026
The short answer
The Role of Insulation and Air Gaps
Exterior walls in unheated spaces like crawl spaces, garages, or attics often have significant air gaps. Cold outdoor air infiltrates these cavities, directly chilling the pipes running through them. If the wall lacks proper insulation, there is no thermal barrier to protect the plumbing from freezing temperatures. This makes pipes in these locations the most vulnerable during a cold snap. Over time, even small drafts can create a consistent zone of below-freezing air around the pipe, especially on nights with little cloud cover or high winds that increase the rate of heat loss from the structure.
How Stagnant Water Loses Heat
Water inside a pipe constantly loses heat to its surroundings through conduction. In an unheated space, the surrounding wall materials and air are often near the outdoor temperature. When faucets are not used, water sits motionless in the pipe, allowing its temperature to drop steadily. This process continues until the water reaches its freezing point, at which point ice begins to form. The rate of cooling is faster for pipes that are closer to the outside sheathing of the wall or that are made of conductive metals like copper, which readily transfer thermal energy to the cold environment.
The Physics of Ice Expansion
Water is unique because it expands when it freezes. This expansion creates tremendous pressure inside a confined pipe. Since metal and plastic pipes are rigid, they cannot contain this force indefinitely. The ice blockage itself also increases pressure upstream. The combined stress often causes the pipe to split or burst at its weakest point, leading to a major leak when the ice thaws.
Why Preventive Measures Fail
Many common tips, like letting a faucet drip, may not protect pipes in unheated walls. The drip only helps water move in the main supply line, not in isolated branches inside cold cavities. Similarly, insulating pipes is less effective if the surrounding air space itself is frigid. The most reliable prevention addresses the space's temperature, not just the pipe itself. For a long-term solution, you may need to add insulation to the wall cavity, seal air leaks, or reroute plumbing away from exterior walls. In extreme climates, installing a thermostatically-controlled heat cable or insulating the entire unheated space might be necessary.
- Inadequate or missing wall insulation.
- Vents or cracks letting in cold outdoor air.
- Pipes located on north-facing exterior walls.
- Thermostats set too low during extreme cold.
- Closed interior doors blocking warm airflow.
- Power outages disabling space heaters.
What to Do After a Pipe Freezes
If you suspect a frozen pipe, turn off the main water supply immediately to prevent flooding when it thaws. Open faucets to relieve pressure. You can attempt gentle thawing with a hair dryer, starting from the faucet end and working backward. Never use an open flame. If you cannot locate the freeze or access the pipe, or if a leak starts, you need professional help. A plumber can safely thaw the line and make repairs. If water has already leaked into the structure, local water damage pros can extract it and begin drying to prevent secondary damage like mold growth or wood rot.
Water damage does not wait
Call (410) 237-0394 any time, day or night, and describe what happened.
Call Now (410) 237-0394Frequently asked questions
At what temperature do pipes typically freeze?
Pipes are at risk of freezing when surrounding air temperatures drop below 20F for an extended period. However, pipes in completely uninsulated exterior walls can freeze when temperatures are sustained below 32F, especially during windy conditions that increase heat loss.
Can pipes freeze even if the heat is on?
Yes. If pipes run through an unconditioned space like a garage or crawl space, the local temperature there can still fall below freezing. Interior heat does not reach these isolated areas. Poor insulation or drafts in exterior walls also leave pipes vulnerable despite a warm home.
Who should I call for a burst pipe?
First, call a plumber to stop the water flow and repair the pipe. Then, contact local water damage pros for cleanup and drying. Water inside walls and floors needs rapid extraction to prevent mold and structural damage. Policies vary, and your insurer decides what is covered.