How to Insulate An Underground Sprinkler Vault

What is the best way to go about this?
I have some great insulation that I want to cut and size to fill a small underground area that holds my sprinkler cut off handles.
I call it a vault, but it is simply a dirt hole with a piece of metal around it to keep the dirt from falling inside, and a plastic lid.
Can I just put the insulation directly into the hole, or do I need to put plastic over the lid, on top of the dirt, to keep it dry? My feeling is that when the snow melts, it will dampen the insulation and freeze it.
Also, could I just put the insulation inside of a plastic bag to keep it dry, or will this defeat the purpose.
I know this is such a small thing, but it boggles my mind.
Many thanks!
Kate
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Kate wrote:

To protect our cold sensitive plants we loosely wad up newspapers, put in trash bags and tie around plants. It works fine so I see no reason you couldn't put your insulation in plastic bags.
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dadiOH
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On 11/5/2010 1:34 PM, dadiOH wrote:

put them inside a plastic bag.
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You do need to keep the insulation dry. It will work just as well inside a plastic bag. How wll it works depends on your location. The heat source in this case is the ground. If the ground under your sprinkler valves freezes anyway then the insulation won't help you. Most people drain their sprinkler system if it is in danger of freezing.
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On 11/5/2010 12:49 PM, jamesgangnc wrote:

It is only a foot deep. I tend to be overly cautious.
Thank you.
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Unless the insulation extends down past the frost line it will do little good. You mention snow and freeze. This requires you to winterize the system. Turn off the water inside the house and blow out the pipes with a compressor.
R
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+1...
Why on earth you would ever need to "insulate" a winterized lawn sprinkler system boggles the mind...
If you are concerned about snow melt/thaw cycles filling the "vault" with water and doing some kind of damage then you need to rebuild your "vault" so that it doesn't trap and pool water... Some anally retentive people will dig the valve boxes deeper than needed and place some gravel under where the valves will be installed to allow for some drainage...
Adding insulation to such a vault will not prevent damage from water getting in and freezing...
~~ Evan
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On 11/5/2010 1:56 PM, Evan wrote:

I believe the previous owners froze up one year, and hired a plumber to extend the shut-off valve underneath the house. I don't worry about that part freezing, but I worry about the short piece of pipe on the outside leading to the inside.
It is hard to describe.
I did put a plastic bag underneath the lid so hopefully snow won't melt and get inside.
It has been fine for five years without insulation, but a plumber recently looked at it and suggested adding insulation.
Thanks.
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The plumber is misguided and/or optimistic. Insulation will not prevent pipes from freezing unless the insulation is trapping heat from somewhere. You didn't mention where you're located, so it's tough to know what sort of climate you have. What's your estimate of the typical bad-cold-snap low temperature in your area?
It doesn't sound like you have a basement. If not, there is little possibility that the foundation would lose enough heat that the insulation could trap. Even if you did have a basement, you'd have to have uninsulated foundation walls, the valve box would have to be right next to the house, and the insulation would have to extend down past the frost line on the exposed three sides to have much hope of the insulation actually preventing a freeze.
A safer bet would be to install electrical heat trace tape with a thermostatic control, but that obviously involves more work and expense than throwing some insulation on top of the pipes. BTW, what exactly do you mean when you said "I have some great insulation that I want to cut and size"? Is it fiberglass or rigid foam insulation?
Frankly, if your pipes have been blown out it's unlikely that there will be a problem. A pipe won't rupture unless water is filling the pipe when it freezes. If the pipe isn't full, it's very unlikely that freezing would damage it.
R
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On 11/5/2010 1:45 PM, RicodJour wrote:

worry about though. The plumber said it may be a good idea to insulate it. It has been five years without a problem, but I tend to be on the cautious side.
Thanks.
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It isn't rocket science to correctly drain a sprinkler system and once done there is no need to insulate anything. I agree with those that have said any attempt at insulating the valve box is a waste of time. Insulation only slows the heat transfer, it doesn't stop it. If the frost line extends down a foot or more in your area, then it's going to freeze with or without insulation.
The solution is to blow it out correctly. It sounds like the plumber might be concerned about the one section of pipe that heads back to the house and is on the other side of the blow out port from the sprinklers. If so, I'd put a T with a drain plug in the line. After blowing it out, you can open the drain plug. That will work if the line slopes in the right direction, which it should if the install was done correctly. If it doesn't then I'd fix it by repitching that piece of pipe. Or wait and see if it does freeze and bust and then just replace it once correctly and be done with it. But I don't see what the issue is that's specific to the valve box freezing which should just be blown out.
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