How to insulate a shallow water line 2

Insulation won't prevent freezing, only make the process take longer. Freezing would cause two issues: loss of supply and potential pipe rupture. Since this line cannot be allowed to freeze, I would think two agencies would need to weigh in on the line depth issue: the local fire code official and the building's insurance carrier.

Either increase the bury depth to below frost or heat trace the line. As stated by DMcGrath, insulation only slows the progress of achieving equilibrium. Perhaps a dry pipe system could be designed.

I disagree that insulation only slows process of equilibrium. This may be true with a steady-state "cold" source. Insulation does the same thing as soil cover does, so by placing insulation the frost depth/penetration is raised locally. The insulation affects the freezing front progression and maximum extent, as the freezing temperatures are transient (seasonal) and the heat from the earth is constant. Insulation is used extensively in t he arctic & subarctic for this purpose. And in warmer climes under ice rinks and freezers. Not to say that heat tracing wouldn't be a bad idea, and in a manner it can be tested & replaced if necessary.

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Agree with CarlB, it is accepted practice to insulate to prevent freezing.

Maybe I am missing something, but I don't see how insulating this pipe will prevent the freezing if it is above the frost line.

As I understand it, the frost line is the point underground below which where piping, subject to freezing, should be installed. It is due to the heat transfer from the "earth" to the above ground atmosphere.

Depending on location the "earth" is considered to be a set temperature - say 50 degrees. As you head toward the sky and colder temperatures, the temperature progressively drops due to conductive heat transfer. So at 6 feet, it is 50 degrees, at 5.5 feet it is 45 degrees, etc. So the frost depth is where the temperature will not be lower than 32 degrees.

If the soil temperature is less than 32, eventually the water in a stagnant pipe (fire water) will match the temperature of its surroundings, i.e. less than 32 degrees.

What am I missing?

You're missing that it's not a steady state problem. There is a certain amount of heat energy that will be removed from the ground in the seasonal cold cycle, expressed as "freezing degree days". Lay down a sheet of insulation, the frost will not go as deep beneath it. There wil be a steeper temperature gradient above it.

Or, create a pool of water on the surface in an area. The "cooling energy" necessary to freeze the water will reduce the frost penetration below the pool. The "frost depth" varies a lot from place to place, depending on soil conditions, groundwater levels, surface type, whether cleared of snow, wind effect, etc.

I understand that it is not a steady state problem in that the above ground temperature will have swings so the heat transfer upward will not be steady. Therefore the temperature profile underground will shift accordingly.

So the depth of the 32 degree soil temperature line may shift depending on the temperature above the ground. And isulation will slow down how much the piping loses heat.

But I thought that the frost line was defined as the depth underground that is always above freezing - based upon the prevalent conditions.

Adding insulation will help, but may not be enough - depending on how deep you are and how far from the defined frost line you are. And with a life safety system such as a fire water main, I would not want to be the engineer that accepted an installation above the published frost line and just used insulation. I might consider it on a domestic water pipe, but not fire suppression.

i haven't seen insulating the pipe, but insulating the soil above the pipe is not an atypical fix in vermont usa where frost depth in some areas i worked was up to 4.5 feet.

the basic concept being that the natural freezing of the ground uses the travel path from the edge of the insulation board to the pipe plus the vertical distance from the edge of the board to the surface. i'm not saying simple distance math but there is a relationship. the R-value of your total insulation cross-section and the distance that the blueboard extends away from the centerline are both important factors. there are other factors that can help but i would not be consider (1.if it is a yard area, you may get some insulation value from the snow 2. if it is a main line, the water may not have the opportunity to freeze. 3.anything to do with soil mechanics)

"As I understand it, the frost line is the point underground below which where piping, subject to freezing, should be installed. It is due to the heat transfer from the "earth" to the above ground atmosphere."

It should be obvious that if you build a structure on the ground, there is no longer a "frost line". The frost line is also just a rule of thumb. Soil conditions will the depth of frost. In the same manner, putting insulation on the ground will raise the frost line.

I wasn't considering the piping that is within the footprint of the building. It is much less likely that pipe would freeze due to the structure above.

I was considering the piping outside the footprint. I agree that the "frost line" is subjective in that you could be below this depth and have an incredibly long and cold winter and all bets are off.

But to bury a stagant water pipe above the frost line with only insulation is risky.

But as I thought about it more last night, I would accept it if I calculated/approximated the heat transfer resistance of the 6" of the soil above the pipe and compared it to the heat transfer resistance of the insulation (considering the thickness and material) and the insulation was greater. All other things being equal, I could accept that.

go back to the first post by DMcGrath - you need to get this approved by the fire marshal and by the insurance carrier. they will be the deciding factor whether insulation is enough.

We have successfully used board styrofoam in a couple of locations with shallow waterlines (36" deep). Our typical frost penetration is to 48 here."

While it will not provide an exact answer, there are some standards for shallow insulation to prevent frost heaving/freezing. This could give some insight to the general problem.

I beleive the best example may be in the IRC of all places. - Shallow foundation insulation may be the subject. Sorry, I do not have the code handy to provide a section to refer to.

Basically, it is the horizontal and diagonal use of rigid insulation to prvent the ground from feezing.

Dick

I pulled up the IRC and it is in section R403.3.1 which is about foundations adjoining frost protected shallow foundations.

Being that I am not the most experienced at this, I brought it up with some of the more experienced engineers at my office. After getting the scared deer in the headlights when I brought up not burying the pipe below the frost line, the following two suggestions were brought up.

1. For the length of the pipe, dig into the stone until you are below the frost line. Now depending on the type of stone and the distance of the pipe run, this might not be too easy.

2. Raise the grade elevation in the area so you have the 42" of cover. This will depend on what is going on at grade and if you are able to have more soil in that location.

As was stated before, talk to the insurance carrier. If they are willing to risk freezing by not burying the pipe at the frost line, then do it. Just get it in writing.