Frozen Pipes in a Crawl Space: How to Prevent and Thaw Them

Why do pipes freeze in a crawl space?

Because a vented crawl space is, by design, connected to the outdoors. The foundation vents let outside air through so the space can dry, and in winter that air brings the outdoor temperature with it. A U.S. Department of Energy Building America study of sealed crawl spaces put it plainly: outdoor air brought into a crawl space keeps it cold in winter and may lead to frozen pipes.

The American Red Cross lists the pipes that freeze most often, and a crawl space holds several of them: water supply lines in unheated interior areas such as crawl spaces and basements, pipes running along exterior walls with little or no insulation, and the outdoor hose bibs fed from them. Seattle Public Utilities points to the same places: pipes against exterior walls and where the water service comes in through the foundation. Under a typical house that means the supply line where it enters, the branches that run along the rim joist to an outside faucet, and any run that sits close to a vent opening.

One more thing decides which pipe goes first: what is between it and the cold. Floor insulation that has sagged or fallen away from the subfloor no longer wraps the pipes that run through the joist bays, and a crawl space whose batts are on the soil is a crawl space whose pipes are in open air. That is also the most common cause of cold floors, which is why the two complaints so often arrive together (why floors over a crawl space are cold has the diagnosis).

At what temperature do crawl space pipes freeze?

Water freezes at 32 °F. What there is not is a single outdoor temperature at which a crawl space pipe is guaranteed to freeze, because the pipe does not feel the forecast. It feels the air around it, and that depends on:

• How long the cold lasts. One frosty night chills the crawl space far less than three days that never warm above freezing.
• How open the space is. Open vents, a missing vent screen or a gap where a pipe passes through the foundation let the coldest air reach the pipes directly.
• Insulation. A sleeve on the pipe and batts tight to the subfloor slow the loss of what little heat the pipe has.
• Whether the water is moving. The Red Cross notes that running water through a pipe, even a trickle, helps keep it from freezing.

You will see 20 °F quoted online as the temperature at which pipes start to freeze. It is usually credited to old university research, but we could not trace it to a study we could read, so this page does not use it as a rule. The working rule is the one the utilities and the Red Cross give: once the forecast is at or below freezing, protect the pipes.

How often does the Seattle area get cold enough to freeze pipes?

More often than the region's mild reputation suggests, and much more often toward the foothills than near the Sound. NOAA’s 1991–2020 climate normals count the average number of days a year on which the low reaches 32 °F or below. At Seattle-Tacoma International Airport that is 25.2 days a year, and almost all of them fall between November and February: 3.0 in November, 7.2 in December, 7.0 in January and 5.8 in February. Across the 8 long-record stations around the metro the count runs from about 25 days to about 70.

* NOAA marks these as “representative” normals for the 32 °F count, built from 17 to 22 years of the 30; the others are “standard” normals with 24 or more years.

Three things are worth taking from those numbers. The stations out toward the Cascade foothills, Landsburg, Monroe and Snoqualmie Falls, see two to nearly three times as many frosty nights as the stations near the water. Nights at 20 °F or below happen only a few times a year anywhere on the list, and about once a year at the airport and Sand Point; the highest average is 5.6 at Monroe. And the last column, days that never warm above freezing, averages 3 a year or fewer at every station listed: those rare all-day freezes are the stretches when a vented crawl space has no daytime warmth to recover with, and they are when most pipes go. A station measures one spot, not your crawl space. The rain and housing figures for each city are in the climate and housing data.

How do you keep pipes from freezing in a crawl space?

Most of it is done before the cold arrives, and most of it is cheap. In the order Seattle Public Utilities and the Red Cross give it:

Before the first freeze

1. Find the main shutoff and try it. Seattle Public Utilities asks households to know where their water, gas and electricity shutoffs are, and says the main water shutoff is usually in a basement, crawl space or garage. If yours is under the house, make sure you can reach it in the dark.
2. Deal with the outdoor faucets. Drain and remove the hoses. Where there is a separate shutoff for the outdoor faucets, close it and open each spigot to drain the line; where there is not, fit insulating covers. Shut off and drain an in-ground sprinkler system to its maker’s instructions.
3. Insulate the exposed pipes. SPU names unheated or exposed areas such as basements, attics and garages, and a crawl space is the same case; the Red Cross says to insulate hot and cold lines alike. Foam or fiber pipe sleeves are the usual product, fitted to the maker’s instructions.
4. Close the gaps where pipes come in. SPU asks for caulk around pipes where they enter the house, so cold air does not run along the pipe into the space.
5. Check the floor insulation above the pipes. Batts that have fallen away from the subfloor leave the pipes in the open. Batts that are wet or droppings-soiled are a replacement job, not a re-hang, which crawl space insulation replacement covers.

When freezing weather is forecast

1. Close the foundation vents. SPU’s advice is to close foundation vents to limit the cold air coming in. This is the single biggest step for crawl space pipes, because it stops outdoor air flowing past them.
2. Let a faucet drip. In severe cold SPU asks for a slow drip of cold water from one indoor faucet, and the Red Cross says to choose a faucet served by the exposed pipes. SPU adds that water should not be left running in an unoccupied building.
3. Open the cabinet doors under kitchen and bathroom sinks, so the pipes there get the room’s heat.
4. Hold the thermostat steady. SPU says to keep it at 55 °F or higher, day and night, even when you are away; the Red Cross suggests the same setting day and night during a cold spell.

Open the vents again when the cold is over

SPU’s advice carries its own warning: open the foundation vents again in spring to prevent dry rot. A vented crawl space is built to exchange air. Washington’s residential code makes vents the default and allows a crawl space to go without them only when it has a continuous Class I liner sealed to the stem walls with a radon system, and a continuously running exhaust fan (section R408.3). A vent closed for a cold snap is a freeze precaution; the longer it stays closed after the freezes have passed, the more it becomes a moisture problem. Why the year-round answer is “open” for most houses, and when it is not, is the subject of whether crawl space vents should be open or closed.

Heat tape on exposed runs

For a pipe that cannot be insulated well enough, the Red Cross suggests UL-listed heat tape or heat cable. The Consumer Product Safety Commission’s long-standing advice on heat tape is about fire, and it is worth following to the letter:

• use tape labeled for the pipe’s diameter and length;
• wrap it on the pipe, not over insulation, and do not lap it over itself unless the instructions allow it;
• if the maker recommends insulation over the tape, use a non-flammable kind such as fiberglass, not foam;
• inspect it every fall, and replace it at once if the covering is cracked or charred or any wire shows.

The manufacturer’s instructions govern any installation. A tape that is plugged in under a house and forgotten for a decade is the case the commission was warning about.

Can you heat a crawl space to keep pipes from freezing?

Heating the pipes is better supported than heating the space. The prevention steps in the SPU and Red Cross guidance are all about keeping the house’s own heat near the pipes and the outdoor air away from them: insulation, closed vents, a dripping faucet, cabinet doors open, a steady thermostat and, for exposed runs, listed heat cable. A portable space heater appears in the Red Cross guidance only as a way to thaw a pipe that has already frozen, kept away from anything flammable. None of the guidance read for this page recommends running one unattended under a house as a winter routine.

If a crawl space is cold enough that its pipes need a heater every winter, the useful question is why. The usual answers are missing or fallen insulation, open gaps and vents, or both, and those are things that can be fixed for good.

Does crawl space encapsulation keep pipes from freezing?

It can help, and it is still rarely the reason to do it here. A sealed crawl space stops exchanging air with the outdoors through its vents. The Department of Energy’s Building America program lists fewer frozen pipes among the benefits of unvented, conditioned crawl spaces built to its recommendations, and in its cold-climate study in Ithaca, New York, the sealed crawl space of the occupied test house ran between 55 and 70 °F.

Two things keep that in proportion. The same Building America summary reports that in the marine climate of the Pacific Northwest, vented and closed crawl spaces both performed adequately for humidity and energy use, as long as the ducts were not in the crawl space. And Washington’s sealed crawl space is a liner, a radon system and a continuously running exhaust fan; the state struck the model code’s conditioned-supply-air option, so the code does not ask for house heat to be delivered to the space. With 25 to 70 frosty nights a year depending on the station, and a handful of hard freezes, pipe insulation and closed vents during the cold protect most Seattle-area pipes for a fraction of the cost.

Encapsulation earns its place on moisture, comfort and the condition of the framing, and freeze protection comes along with it. If you are already weighing it for those reasons, whether encapsulation is worth it in Washington sets out the trade, and crawl space encapsulation covers the scope and cost.

How do you thaw a frozen pipe in a crawl space safely?

The sign is usually a faucet that gives only a trickle, or nothing at all. SPU and the Red Cross say the likely places are against exterior walls and where the water service comes in through the foundation, which in a crawl space is often exactly where the pipe is coldest. Then:

1. Check the space is safe to enter. If there is water on the ground near wiring, stop and read the safety checks in what to do in a flooded crawl space first. SPU says not to use electrical devices where there is standing water.
2. Open the faucet the pipe serves. SPU and the Red Cross both say to keep it open: as the ice melts, the water needs somewhere to flow.
3. Use gentle electric heat. An electric hair dryer, an electric heating pad wrapped around the pipe, or towels soaked in hot water. Keep any heater clear of insulation, paper facings and the plastic ground cover.
4. Never use a flame. SPU and the Red Cross rule out a blowtorch, a kerosene or propane heater, a charcoal stove or any other open-flame device; SPU warns that a blowtorch can boil the water in the pipe and make it burst, on top of the fire and carbon monoxide risk. Under a wooden floor, surrounded by dry framing and paper-faced insulation, that rule matters more than anywhere else in the house.
5. Keep going until full pressure returns, then check the other faucets: if one pipe froze, others may have too.

If you cannot find the frozen section, cannot reach it or cannot thaw it, SPU and the Red Cross both say to call a licensed plumber.

Will frozen pipes thaw on their own?

Eventually, once the air around them climbs back above freezing. Waiting is not the risk; the ice is. The Red Cross explains that water expands as it freezes and that the pressure can break metal or plastic pipe however strong it is. So when the water comes back, whether you thawed the pipe or the weather did, go under the house and look along the run for drips, a wet ground cover or soaked insulation.

What to do if a pipe bursts under the house

Water first, crawl space second. SPU’s steps are to close the main shutoff valve at once to stop the flooding and to call a plumber as soon as possible to repair or replace the damaged section. If you cannot close the valve, SPU customers can call the utility’s 24-hour line, listed on its frozen-pipes page, and a crew will turn the water off at the meter. If your water comes from another utility, find its emergency number before winter.

Once the leak is stopped, what is left is a wet crawl space, and that is a separate job:

• Soaked insulation. Batt that has been wet does not recover its insulating value, and fallen wet batt does not go back up. It is usually replaced.
• Water under the ground cover. Water trapped beneath a plastic sheet cannot evaporate, so a soaked section usually comes up.
• Photographs before anything moves, if there may be a claim. What to photograph, and what comes out first, is in the flooded crawl space checklist; whether a policy pays is in what homeowners insurance covers.

Who to call for which part

In Washington, check any contractor’s registration with L&I. How to verify a Washington contractor walks through what the record shows and what it does not prove. If your pipes freeze every cold snap, or the space under the house is still wet from the last one, you can request a free in-home inspection; the written quote should name what is letting the cold in, not just the pipe that froze.