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Typical copper water supply lines to washing machine, but they are on an exterior wall. Not sure if insulated inside the wall or not, assuming they are not since house is in TX.
This last winter was rough and this coming winter might be as well. Had one period where it was below zero for over 48 hrs and the line froze. This has happened before. Any suggestions to keep the lines from freezing?
I would cut out the section of drywall in front of the pipes. Then I would try to slide some foam board insulation behind the pipes (against the outside wall).
You could also try to cut out a lengthwise section of foam pipe insulation, and wrap it on the pipe with the open end of the insulation towards the inside (warm) of the wall, like this:
The idea is to insulate the pipes from the cold outside wall, but expose them to the warm inside wall.
If you want to go one step further, fill the rest of the cavity with fiberglass or Rockwool insulation, leaving the space between the pipes and the inside wall open, like this:
Would be best to move the lines into the heated space.
Second best would be to remove the section of drywall from above floor to just below valves and replace with a door(open when it gets cold out) or better a vented door or cover to let heat in the wall.
Can also check and/or add insulation at time when adding the door/cover.
Another option is to wrap the pipes with heat tape.
To install cut open the drywall and wrap the pipe(s) with the heat tape per instructions with the tape. You can plug it into a wall outlet near the location. The tapes are thermostatically controlled but I would not rely solely on the thermostat. The best approach is to monitor the weather and when arctic temps are predicted just plug it in.
I would suggest that after you open the wall and wrap the pipe install a vent cover as recommended by @crip659 to allow as much heat as possible to circulate into the wall. Make sure that insulation is in place between the pipe and the exterior wall.
An option that doesn't include opening the wall, yet still increases insulation, is to pop off the square/rectangle faceplate and spray some expanding foam into the wall. Just poke the straw into a gap around the box and spray. You might want to move the straw around so it doesn't all go in one spot. If you don't have any insulation, it should all fall to the bottom anyway. Just don't over fill, since that could cause the foam to expand back out around the box.
Most of these foams are difficult to clean up, so make sure you have the correct solvent handy (according to the can) to clean up any mistakes. Wear gloves, old clothes, and maybe even a face shield and hat. This stuff is usually really very extremely sticky and will not come out of fabric, and it smears on (rather than wipes off) everything else.
It might take 1 or several cans of foam, depending on the type you use. Some foams don't expand as much as others, but you would want a dense foam to increase insulation more, so a foam that doesn't expand as much might be what you want.
For example (not suggesting brand or retailer):
https://www.amazon.com/Loctite-TITEFOAM-Insulating-Sealant-1988753/dp/B01N21KQ4I/?th=1
https://www.amazon.com/Red-Devil-0912-Expanding-Polyurethane/dp/B000UOCEUA
There are many more types of foam than this. If you also have critters, there's foam to handle that. There's also fire retardant foam, but you don't need that for this application.
There are also 2 part expanding foams, but these you have to mix and guess at how much you need. And it would be more difficult to get into the wall without making a big hole or getting just the right funnel.
Again, not suggesting brand or retailer:
https://www.amazon.com/TotalBoat-Urethane-Density-Flotation-Reinforcement/dp/B01AAWRZ0A?th=1
If you want to put more insulation in your whole house, there are professionals that an do spray, blow-in, or other insulations that can guide you as to what you need and want versus cost and mess. Insulating a house after it's built isn't an easy thing, as you are finding out.
Let them run a little.
If they are running, just a little, they will not freeze. Of course you will need to disconnect them from the washer if you are letting them run. You could tuck them down into the washer drain hole or rig something to let them run into the floor flood drain.
Physics stack has got my back on this! Woo! Hah!
If you don't want to open the walls to reroute or insulate the pipes and you don't want to waste water, but you don't mind slightly increasing your hot water energy bill, you can set up a hot water recirculating pump at your water heater and a thermostatic bypass valve at the washing machine water supply.
The circulating pump (example) is usually installed inline with the hot water flow near the water heater, and it is usually stainless steel. It must be running during freezing weather.
The thermostatic bypass valve (example) should be installed between the hot and cold water supply at the washing machine, and you must leave the red and blue laundry valves open in freezing weather.
In operation, when the bypass valve body is less than 90 degrees F, it allows water to flow from the hot water pipe into the cold water pipe. The stainless pump provides a slight pressure differential between hot and cold, so warm water flows through the bypass valve into the cold water pipe and returns to the water heater. No water is wasted. As soon as the bypass valve body reaches 90 degrees F, the valve closes and water flow stops. This valve is designed for use under a sink, so you will have to make an adapter to work with laundry connectors.
In my experience with this system, a little water circulates about every 15-20 minutes with a barely audible hiss. The hot water pipe stays lukewarm all the time and the cold water pipe stays tepid, not warm, but definitely not cold enough to freeze.
I have this setup in my house where a previous owner ran pipes to reach a second floor bathroom through an unheated space. Those pipes would freeze starting in November. Since I installed this system, there have been no freezes, even though the temperature went as low as -18F. The system works.
If you are ambitious, you can hook up an outdoor temperature sensor to control the pump so that it only runs during freezing weather. If you are concerned about leaving the laundry valves open, buy new steel mesh laundry hoses.
To prevent copper pipes freezing in the slab foundation in our house in Dallas TX, I adhered a 3 ft long strip of closed cell foam against the exposed edge of the slab where our kitchen faucet supply passes through the slab, i.e. outside. The strip entirely covers the exposed edge of the slab. If you have a slab, this is the first thing to do and it alone might be sufficient to prevent freezing.
Our kitchen sink supply copper pipes are the only pipes we have in an exterior wall. These pipes froze in a cold snap in about 1980. I was able to thaw them by aiming a hair dryer at the exposed edge of the slab opposite the sink.
Over time this insulation detached and fell away and in the record cold snap of 2021 the pipes froze again. Luckily I detected the frozen pipes at about 11 pm and thawed them with a hair dryer before any evident damage was done. I put another strip of closed cell foam (cut from an old camping pad) against the slab and covered that with a 24"x72" camping pad held to the slab and siding with patio furniture.
In a kitchen renovation about 20 years ago the supply pipes above the slab were covered with foam insulation.
