A few winters ago an outdoor supply pipe split on a vacant unit during a three-day cold snap, and by the time a neighbour noticed the water running, the repair plus the wasted-water charge came to about 128,000 yen. That was an expensive lesson in something I now do every November: walk every property and check the insulation on every pipe that sees outside air. Freeze protection is cheap. A burst pipe behind a wall is not.
Start with the freeze depth for your area
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Japanese municipalities publish a design freeze depth, the touketsu-shindo, which is how far frost reaches into the ground in a hard winter. In the milder parts of the country it is near zero. Where my properties sit it runs about 30cm, and the local water bureau uses that same figure to set how deep the service line has to be buried. Any buried pipe shallower than that number is at risk, and every pipe above ground is at risk regardless. The rule I follow is simple: buried water lines should sit below the local freeze depth, and where they cannot, they get both insulation and heat. On one property the garden tap line ran only 12cm down along a concrete path, and rather than dig a proper trench I re-routed that 3m section along the foundation wall where it could be lagged and heat-traced, which took a Sunday and about 4,000 yen.
Insulation thickness, matched to exposure
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For pipe running through an unheated but enclosed space, such as a pipe chase or under a raised floor, 20mm-wall polyethylene foam tube is enough. It costs me about 250 yen per metre. For pipe fully exposed to the wind, an outdoor faucet riser or a meter box, 20mm foam alone will not hold overnight below minus 4 degrees, so I add self-regulating heat cable under the insulation. Elbows, valves, and the union at the water heater are where splits actually start, because the foam tube never wraps a fitting cleanly, so I wrap those in self-amalgamating tape first, then foam, then weatherproof tape over the seam. I also tape every longitudinal slit in the foam tube closed, because an open slit facing the wind lets cold air sit right against the pipe and the insulation does almost nothing. Where two lengths of tube meet I butt them tight and tape the joint rather than leaving a 10mm gap that becomes the weak point. On metal pipe I choose the closed-cell foam rated for outdoor UV, since the cheap grey tube crumbles after two or three summers and then it is just decoration.
Self-regulating cable versus constant wattage
I only buy self-regulating heat cable now. It varies its output with pipe temperature, so it draws almost nothing at 10 degrees and ramps up near freezing, and you can cut it to length and even overlap it without a hot spot. It costs around 1,800 yen per metre installed. The older constant-wattage cable is cheaper per metre but burns power all winter and fails if it overlaps itself. On a typical outdoor faucet I use about 1m of cable and it adds only a few yen a day to the bill during the weeks it actually runs.
The points that actually burst
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After years of this I have a fixed checklist for each property:
- Outdoor faucet risers and garden taps, the single most common failure
- The cold inlet union at the gas water heater, especially wall-hung units on a north wall
- Water meter boxes, which act like a cold sink even when buried
- Any pipe crossing an unheated bathroom or laundry space on an exterior wall
- Pipe runs under a raised first floor with open foundation vents
Draining down a vacant unit
The pipe that split on me was in an empty unit, and an empty unit is the worst case because no one runs a tap and no heat is on. Now, any unit that will sit vacant through winter gets a full drain-down: I shut the main stop, open every tap high and low, drain the water heater from its bottom valve, flush the toilet and pour a cup of non-toxic plumbing antifreeze into the bowl and each trap so the water seal cannot freeze and crack the porcelain. It takes about 30 minutes and it removes the risk entirely rather than just reducing it. I tag the main stop so whoever turns the water back on knows to close the heater drain first.
The cold-snap card I give tenants
Hardware only goes so far, so every tenant in a freeze-prone unit gets a laminated card each December. It says: when the overnight forecast is minus 4 or colder, leave one tap running a pencil-thick trickle overnight, open the vanity cabinet doors so room heat reaches the pipes, disconnect and drain any garden hose, and never pour boiling water on a frozen pipe because the thermal shock is what actually bursts it. If a pipe does freeze with no visible split, close the main stop, open a tap, and call me before thawing it slowly with warm towels or a hair dryer. Since I started handing those out, tenant-caused freeze damage has dropped to one minor case in five years.
Winterizing a whole property takes me about half a day and maybe 6,000 yen in materials per unit, and against a five-figure burst-pipe bill plus a water charge for thousands of litres that ran unnoticed, that is the easiest math I do all year.
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