How to Insulate Underground Water Pipes

blue, underground water pipeline
What You'll Need
9-inch sewer pipe
Expanded polystyrene
Plastic sheeting
Rigid foam-insulation board
Heavy-duty tape

In colder climates, it’s not only important to insulate aboveground water pipes, but also underground ones. To prevent underground water pipes from freezing, use the guide below to install insulation that can withstand the heavy earth on top of it.

Understanding Types of Insulation

There are two different ways to insulate underground water pipes. The first way is watertight, and the second is only intended to prevent water from soaking into the insulation. While the first is better, it takes more work to install.

Installing Non-Watertight Insulation

Step 1 – Install Gravel and Plastic Sheeting

First, put 3 inches of gravel at the bottom of a 30-inch trench. Then, line the trench with heavy plastic sheeting.

Step 2 – Install Foam-Board Insulation

Cut rigid foam-board insulation into strips that are 12-inches wide. Lay several of these strips, to a depth of 4 inches, on top of the plastic sheeting. Then, lay the water pipes on the insulation. Add insulation, also to a depth of 4 inches, on either side of the water pipes. Add another 4 inches of insulation on top.

Step 3 – Tape the Sheeting

Fold the plastic sheeting over, so that the sides completely cover the insulation, and tape it in place. Next, use another piece of plastic sheeting that’s 2-feet wide, and drape it over the top of this package. Tape the edges in place along the side of the insulated package to prevent moisture from entering it.

Step 4 – Fill Trench With Soil

Finally, fill in the trench with soil. While this insulation method works well, remember that if water enters the trench, it will seep through and make the insulation wet and ineffective.

Installing Watertight Insulation

Step 1 – Encase the Pipes

To create watertight insulation for water pipes, the pipes need to be inside of a larger pipe. Use a 9-inch sewer pipe, made of either concrete or plastic, to hold cover the smaller water pipes.

Step 2 – Obtain Polystyrene

Many people give polystyrene away for free, so you should check with neighbors or online classifieds before purchasing any. You will need several pieces that are large enough to be cut, so that they fit in the sewer pipe as discs, with the centers cut out to hold the water pipes.

Step 3 – Install the Polystyrene

Place one disc of expanded polystyrene in the end of the sewer pipe, and then feed the water pipe through it. Surround the water pipe with smaller pieces of expanded polystyrene within the water pipe for about 2 feet of its length, and then insert another disc.

By alternating the small pieces and discs in this manner for the full length of the sewer pipe, you will insulate the water pipes and also support them. This method is completely watertight and efficient.