Last winter we were involved in online discussions about freezing pipes and insulation. We stressed that the best way to avoid freezing problems is to bed and slope piping to ensure that air is the only thing left in the pipes between water usage events. However, we recognize — and it was pointed out to us — that in some situations in our northern climate it’s necessary to insulate the pipes to prevent freezing.

Some situations that call for insulation come to mind:

• A shallow house sewer line to a septic tank passing under a sidewalk or path that is kept open for the winter
• Sewer lines between multiple sewage tanks, such as septic tanks in series or septic tank to pump tank or media filter
• Where the supply line runs from a septic tank under a driveway or other areas where topography or soils limit the depth the pipe can be buried
• Sewer connections coming out of mobile homes (How often do you have to crawl under a mobile home to thaw the pipes?)

We regularly discuss three ways to insulate piping in these situations:

• Sleeve the pipe in a larger pipe to create insulating air space
• Use polystyrene sheets over the top of the pipe
• Use pre-insulated pipe with urethane foam surrounding the pipe and encased in a polyethylene sleeve 

EXPLORE YOUR OPTIONS

Our preferred method is using pre-insulated pipe, which may be a bit more expensive but is worth the investment. This is based not only on our experience in Minnesota, but also when we have troubleshot systems in various areas. We have found the sleeve technique lacking in areas of extreme and prolonged cold like northern Minnesota. So while it may be effective in areas to the south or out west, not so much where we live. Also, when insulation sheets are used, we often see the mistake of choosing a material not rated for soil burial.

During the discussion, colleagues from Wisconsin, Colorado and other states provided very good information. While everyone agreed that pre-insulated pipe was probably the best choice, it’s not always available when and where it’s needed, so another method is used. Let’s expand on the discussion of alternative solutions:

When using expanded polystyrene sheets over the top of the piping, 1 inch of high-density, closed-cell, extruded foam equals the insulation value of approximately a foot of soil cover. So check in your state plumbing codes to see the thickness of insulation needed based on frost or freeze protection zones. Also, the sheets should be laid 6 inches above the crown of the pipe. It is projected that the zone of protection extends down and inward from the edge of the insulation, toward the pipe, at approximately a 45-degree angle. This is important when considering the necessary coverage of insulation over the pipe. In Minnesota, where cold spells can be colder and last longer than in other regions, a common error is not insulating a wide enough area over the pipe.

A little different but similar approach is to create a box around the piping using the polystyrene sheets. There should be a minimum of 6 inches of backfill aggregate around the piping between the sheets and the pipe. Thickness of the insulation sheets will depend on the type of original soil, depth of bury and projected frost depth. Check the plumbing codes for water supply or sewer pipes for the ratings in your area.

PLAN AHEAD FOR WINTER

Another suggestion is to use heating cables in troublesome areas such as crossing driveways. This involves using the pipe within a larger pipe, using Fernco or other quick-disconnect couplings so a new heating cable can be slid into the pipe when necessary. To the extent it’s possible — depending on the depth of bury — also consider laying polystyrene sheets over the top. On a personal note, a neighbor of Jim’s in northern Wisconsin successfully used this approach where his sewer pipe crossed from the septic tank next to the house under the driveway to a pump tank. Before this solution, the piping froze periodically.

Another pipe-within-a-pipe suggestion from an installer was to wrap the sewer pipe in bubble wrap and duct tape inside the larger pipe. The issue with this approach is we’re uncertain whether bubble wrap provides more insulation value than simply using a double pipe, and whether all bubble wrap products provide the same insulating value. It’s better to use rated materials for your area that have proven results.

Always remember that the best time to insulate is during the initial installation rather than after the fact. So for problem areas, it’s best to discuss pipe insulation with the homeowner during the project bid process.