How to Insulate Underground Water Pipes

Even underground water pipes can freeze. In colder climates, it’s important to insulate the underground water pipes to stop this from happening. However, this does present problems. What can you use for insulation? Remember, too, that it has to be strong enough to withstand the weight of the earth on top of it.

Step 1 - Types Of Insulation

There are two different ways of insulating underground water pipes. The first is watertight, and the second is intended to prevent water from soaking into the insulation. The first is undoubtedly better, although it takes more work.

Step 2 - Watertight Insulation

To create watertight insulation for water pipes, the water pipes needs to be inside a larger pipe. Use a 9 inch sewer pipe to hold the smaller water pipe. It can either be made of concrete or plastic. This will be the major cost of the project. Beyond that, you need as much expanded polystyrene as you can obtain. In many instances, people will be happy to give this away. You will need several pieces that are large enough to be cut so they fit in the sewer pipe as discs, with the centers cut out to hold the water pipe.

Step 3 - Construction

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 insulate the water pipes and also support them. It’s a very cheap and efficient method for insulating water pipes and has the bonus of being completely watertight.

Step 4 - Non Watertight

Another method that isn’t watertight is even simpler. Put 3 inches of gravel at the bottom of a 30 inch trench and then line the trench with heavy plastic sheeting. 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, then another 4 inches on top.

Fold the plastic sheeting over, so the sides completely cover the insulation and tape in place using heavy tape. Now, take 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. This will prevent most moisture entering, although it won’t be watertight. Fill in the trench with soil. In the event of water entering the trench, it will seep through, and the insulation will become wet and ineffective.