hen water freezes, it expands. That’s
why a bottle of soda explodes if
i t ’s put into a freezer to chill quickly
and forg o t t e n .
When water freezes in a pipe, it expands the
same way. If it expands enough, the pipe
bursts, water escapes and serious damage
HY PIPES BURST
S u r p r i s i n g l y, ice forming in a pipe does
n o t
typically cause a break where the ice blockage
occurs. It’s not the radial expansion of
ice against the wall of the pipe that causes
R a t h e r, following a
complete ice blockage
in a pipe, continued
the pipe cause
water pre s s u re to
i n c rease downs
t ream — between
the ice blockage and a closed faucet at the
end. It’s this increase in water pressure, literally
thousands of pounds of pre s s u re, that
leads to pipe failure. Usually the pipe bursts
where little or no ice has formed.
Upstream from the ice blockage the water can
always re t reat back towards its source,
so there is no pre s s u re build-up to cause
Water has to freeze for ice blockages to occur.
Pipes inside a house that are protected adequately
by placement within the building’s
insulation, insulation on the pipe itself, or
heating, are safe.
G e n e r a l l y, houses in nort h e rn climates are
built with the water pipes located on the
of the building insulation, which protects
the pipes from subfreezing weather.
H o w e v e r, extremely cold weather and holes
in the building that allow a flow of cold air to
come into contact with pipes can lead to
freezing and bursting.
Water pipes in houses in southern climates
often are more vulnerable to winter cold
spells. The pipes are more likely to be located
in unprotected areas outside of the building
insulation, and homeowners tend to be less
aware of freezing problems, which may occur
only once or twice a season.
Pipes in attics, crawl spaces and outside
walls are all vulnerable to freezing, especially
if there are cracks or openings that allow
FREEZING AND BURSTING PIPES
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AT U R A L
A Z A R D MI T I G AT I O N
I P E S I N S I D E A H O U S E T H AT A R E
P R O T E C T E D A D E Q U AT E LY B Y P L A C E M E N T
W I T H I N T H E B U I L D I N G
’S I N S U L AT I O N,
I N S U L AT I O N O N T H E P I P E I T S E L F
O R H E AT I N G
, A R E S A F E.
A publication of the Institute for Business and Home Safety
cold, outside air to flow across the pipes.
R e s e a rch at the University of Illinois has
shown that “wind chill,” the cooling effect
of air and wind that causes the human body
to lose heat, can play a major role in accelerating
ice blockage, and thus bursting, in
Holes in an outside wall where television,
cable or telephone lines enter can pro v i d e
access for cold air to reach pipes.
The size of pipes and their composition (e.g.,
copper or PVC) have some bearing on how
fast ice forms, but they are relatively minor
factors in pipe bursting compared with the
absence of heat, pipe insulation and exposure
to a flow of subfreezing air.
H E N IS IT CO L D EN O U G H TO FR E E Z E?
When should homeowners be alert to the
danger of freezing pipes?
That depends, but in southern states and
other areas where freezing weather is the
exception rather than the rule (and where
houses typically do not provide adequate
built-in protection or builders care l e s s l y
place pipes in unprotected areas),
alert threshold” is 20 degrees F.
This threshold is based upon re s e a rch
conducted by the Building Research Council
at the University of Illinois. Field tests of
residential water systems subjected to winter
t e m p e r a t u res demonstrated that, for uninsulated
pipes installed in an unconditioned
attic, the onset of freezing occurred when the
temperature fell to 200 F or below.
This finding was supported by a survey of 71
plumbers practicing in southern states, in
which the consensus was that burst-pipe
problems began to appear when temperatures
fell into the teens.
However, freezing incidents can occur when
the temperature remains above 20
0 F. Pipes
exposed to cold air (especially
flowing a i r,
as on a windy day) because of cracks in
an outside wall or lack of insulation are vulnerable
to freezing at temperatures above
However, the 20
0 F “temperature alert threshold”
should address the majority of potential
burst-pipe incidents in southern states.
I T I G AT I N G T H E PR O B L E M
Water freezes when heat in the water is transf
e rred to subfreezing air. The best way to
keep water in pipes from freezing is to slow
or stop this transfer of heat.
Ideally, it is best not to expose water pipes to
s u b f reezing temperatures, by placing them
only in heated spaces and keeping them out
of attics, crawl spaces and vulnerable outside
walls. In new construction, proper placement
can be designed into the building.
In existing houses, a plumber may be able to
re route at-risk pipes to protected are a s ,
although this may not be a practical solution.
If that’s the case, vulnerable pipes that are
accessible should be fitted with insulation
sleeves or wrapping (which slows the heat
transfer), the more insulation the better. It is
i m p o rtant not to leave gaps that expose the
pipe to cold air.
Hardware stores and home centers carry the
n e c e s s a ry materials, usually in foam ru b b e r
E S E A R C H AT T H E UN I V E R S I T Y O F IL L I N O I S H A S S H O W N
T H AT
“W I N D C H I L L,” T H E C O O L I N G E F F E C T O F A I R A N D
W I N D T H AT C A U S E S T H E H U M A N B O D Y T O L O S E H E AT
C A N P L AY A M A J O R R O L E I N A C C E L E R AT I N G I C E
B L O C K A G E
, A N D T H U S B U R S T I N G, I N WAT E R P I P E S.
U L N E R A B L E P I P E S T H AT A R E A C C E S S I B L E S H O U L D
B E F I T T E D W I T H I N S U L AT I O N S L E E V E S O R
W R A P P I N G
, T H E M O R E I N S U L AT I O N T H E B E T T E R.
or fiberglass sleeves. Better yet, plumbing
supply stores and insulation dealers carry
pipe sleeves that feature extra-thick insulation,
as much as 1” or 2” thick. The added
protection is worth the extra cost.
Cracks and holes in outside walls and foundations
near water pipes should be sealed with
caulking to keep cold wind away from the pipes.
Kitchen and bathroom cabinets can keep
w a rm inside air from reaching pipes under
sinks and in adjacent outside walls. It’s a
good idea to keep cabinet doors open during
cold spells to let the warm air circ u l a t e
around the pipes.
Homeowners in the south need to be alert
to the danger of freezing and bursting water
pipes when the outdoor temperature thre a tens
to drop to 20 degrees F.
That’s the temperature at which ice is likely
to start forming in water pipes located in an
unheated portion of a house.
Once ice forms into a blockage in a pipe, continued
growth of ice in the pipe can lead to
excessive water pressure. It’s pressure of the
water that has no place to go when ice builds
that causes the pipe to burst, rather than ice
pushing against the wall of a pipe.
Although 20 degrees F is well below the
f reezing temperature of water, two factors
make this the critical outdoor temperature:
1. The temperature of an unheated portion
of a house is almost always at least
a few degrees above the outdoor temp
e r a t u re. For example, an insulated
attic may be at 37
0 or 380 F when the
outdoor temperature is 32 degrees F.
2. Water “supercools” several degre e s
below freezing before any ice begins
In re s e a rch tests at the University of Illinois,
water pipes placed in an unheated, insulated
attic consistently started forming ice
when the outdoor temperature dipped just
The 20 degree F threshold is primarily for
homes in the south and other areas where
freezing may occur only once or twice a season.
Homebuilders in these areas often overlook
the threat of freezing and place pipes in
Builders in the north, where temperatures
in the teens and lower are common, routinely
install more building insulation and usually
do not place pipes in unheated portions
of a building.
There are several precautions homeowners in
the south can take when the temperature is
expected to dip into the low 20s:
Seal all openings where cold air can
get at unprotected water pipes. It’s
especially important to keep cold wind
away from pipes, which speeds up the
Cover pipes with foam or fiberg l a s s
insulation sleeves, the thicker the better.
Install heating tapes or cables on vulnerable
Be sure to follow manu -
facturer instructions carefully to avoid
starting a fire.
Leave cabinet doors open under the
kitchen and bathroom sicks to allow
w a rmer room air to circulate aro u n d
Let faucets drip slowly to keep water
flowing through pipes that are vulnerable
to freezing. Ice might still form in
the pipes, but an open faucet allows
water to escape before the pre s s u re
builds to where a pipe can burst. If the
dripping stops, it may mean that ice is
blocking the pipe;
keep the faucet
open, since the pipe still needs pre ssure
A C K FR O S T NI P S AT WAT E R PI P E S
H E N TE M P E R AT U R E DI P S TO 20 DE G R E E S IN TH E SO U T H
Electric heating tapes and cables are available
to run along pipes to keep the water fro m
These must be used with extre m e
caution; follow the manufactur e r’s instru ctions
carefully to avoid the risk of fire.
Tapes and cables with a built-in therm o s t a t
will turn heat on when needed. Tapes without
a thermostat have to be plugged in each
time heat is needed, and may be forgotten.
E T T I N G T H E WAT E R RU N
Letting a faucet drip during extreme cold
weather can prevent a pipe from bursting. It’s
not that a small flow of water prevents freezing;
this helps, but water can freeze even with
a slow flow.
R a t h e r, opening a faucet will provide re l i e f
f rom the excessive pre s s u re that builds
between the faucet and the ice blockage when
freezing occurs. If there is no excessive water
p re s s u re, there is no burst pipe, even if the
water inside the pipe freezes.
A dripping faucet wastes some water, so only
pipes vulnerable to freezing (ones that ru n
t h rough an unheated or unpro t e c t e d s p a c e )
should be left with the water flowing. The
drip can be very slight. Even the slowest drip
at normal p re s s u re will provide pre s s u re
relief when needed.
W h e re both hot and cold l ines serve
a spigot, make sure each one contributes
to the drip, since both are subjected
If the dripping stops, leave the faucet(s)
open, since a pipe may have frozen and will
still need pre s s u re relief.
F YO U SU S P E C T A FR O Z E N PI P E
If you open a faucet and no water comes out,
don’t take chances. Call a plumber.
If a water pipe bursts, turn off the water at the
main shut-off valve; leave the faucet(s) open
until repairs are completed.
Don’t try to thaw a frozen pipe with an open
flame; you may start a building fire.
You might be able to thaw a pipe with a
hand-held hair dry e r.
Slowly apply heat,
s t a rting close to the faucet end of the pipe,
with the faucet
open. Work toward the
D o n ’t use electrical appliances
while standing in water; you could
O I N G O N A TR I P
When away from the house for an extended
period during the winter, be careful how
much you lower the heat. A lower temperat
u re may save on the heating bill, but there
could be a disaster if a cold spell strikes and
pipes that normally would be safe, freeze and
A solution is to drain the water system. This
is the best safeguard. With no water in the
pipes, there is no freezing. This re m e d y
should be considered even when the homeowner
is not leaving but is concerned about a
serious overnight freeze.
To drain the system, shut off the main valve
(usually at the water meter or where the main
line enters the house) and turn on every
water fixture (both hot and cold lines) until
water stops running. It’s not necessary to
leave the fixtures open, since the system is
filled mostly with air at that point and not
subject to freezing.
When re t u rning to the house, turn on the
main valve and let each fixture run until the
pipes are full again.
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F T H E R E I S N O E X C E S S I V E WAT E R
P R E S S U R E
, T H E R E I S N O B U R S T P I P E,
E V E N I F T H E WAT E R I N S I D E
T H E P I P E F R E E Z E S
Pipes in attics and crawl spaces should
be protected with insulation or heat.
Pipe insulation is available in fiberglass
or foam sleeves. Home centers and
hardware stores have sleeves providing
1/8 to 5/8 inches of insulation; specialty
dealers have products that pro v i d e
up to 2 inches of insulation. (Check the
Yellow Pages under “Insulation” or
“Plumbing Supplies” for sources.) The
extra thickness is worth the price and
can save a pipe that would freeze with
Heating cables and tapes are eff e c t i v e
in freeze protection. Select a heating
cable with the UL label and a built-in
thermostat that turns the heat on when
needed (without a thermostat, the cable
has to be plugged in each time and
might be forgotten). Follow the manufacturer’s
Doors on cabinets under kitchen
and bathroom sinks should be left open
during cold spells to allow the warmer
air of the room to circulate around the
Exterior pipes should be drained or
enclosed in 2” fiberglass insulation
Pipes leading to the exterior should be
shut off and drained at the start of the
w i n t e r. If these exterior faucets do not
have a shut-off valve inside the house,
have one installed by a plumber.
Hoses should be removed and store d
inside during the winter.
O U T H E R N HO M E O W N E R S FA C E
H R E AT O F FR O Z E N PI P E S
Water damage from frozen pipes that burst can be a major problem for homeowners in southern
states, maybe even a bigger problem than in the colder north.
Home builders in the south often do not consider the threat of freezing weather and thus place
water pipes in vulnerable locations.
Houses built on slab foundations, common in the south, frequently have water pipes ru n n i n g
through the attic, an especially vulnerable location.
In addition, southern homeowners experience relatively few severe cold spells, so they don’t
consider freezing a problem and don’t bother checking the condition or location of pipes in their
In the north, by contrast, home builders know freezing is a threat, and they usually do not place
water pipes in unheated portions of a building or outside of insulated areas.
Southern homeowners should be aware that pipes in attics, crawl spaces and outside walls are all
subject to freezing and bursting. If these pipes don’t have insulation or heat to protect them, a
strong overnight freeze can cause trouble.
Changes in building codes will require homes built in the future to have adequate protection for
pipes, but many plumbing installers are not aware of the changes. Houses already built re m a i n
These suggestions for homeowners in southern states will help them prevent freezing pipe damage:
73 Tremont Street, Suite 510, Boston, Massachusetts 02108-3910
N S T I T U T E F O R BU S I N E S S A N D HO M E SA F E T Y
The Institute for Business and Home Safety (IBHS) is an independent, nonprofit
o rganization created by the pro p e rty-casualty insurance industry
to reduce deaths, injuries and pro p e rty damage through natural hazard
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