Foam in ditch, plan of attack

[trehugr]

Foam It Green or Tiger Foam, both are probably the same thing, they cost the same. I will be using one or the other. Question. Anyone who has experience with the foam in the ditch method, how did you proceed to get an even layer of foam all the way around the pex ?

 

[JrCRXHF]

I was going to wire the pex off the ground on some plastic tent spikes so it sat about 4" off the ground the fill with foam above the tubes another 4"

but this was just my idea how i was going to go at it.

 

[deerefanatic]

Lay a bed, then lay another, and another, etc..... Insert pipes at desired height somewhere in here..........

That's how my foam guy did it..... Too much foam at once can melt your pipes

 

[trehugr]

I was thinking of putting down a layer of blue styrofoam for something clean an level to work on...

 

[Jim K in PA]

I just got off the phone with my local spray foam guy. It is only going to cost me about $300 for him to do my 55' trench. Given my short run, he says I only need a 2-3" layer under the pipes, and a 2-3" layer above. He only sprays in 2-3" lifts to keep the heat down and avoid damaging the PEX. The sides will have 3-4" because of the spread pattern.

I plan on running two 1.25" lines for the heat S/R and two .75" lines for potential solar DHW S/R lines. I might as well drop the extra lines in now.

For locating, I will have all the lines cut to length and in the trench. I also plan to use some rigid foam and/or zip ties to keep all both pairs of runs fairly close together.

 

[MrEd]

Do we know for a fact that all spray foams will hold up in direct contact with the heat, dirt and water? If I call a local company, do I need to specify precisely what compound or formulation I want used? Call me cynical, but I have little faith that the person on the other end of the phone is going to tell me the particular product s/he uses won't do what I need it to.

 

[Willman]

Do we know for a fact that all spray foams will hold up in direct contact with the heat, dirt and water? If I call a local company, do I need to specify precisely what compound or formulation I want used? Call me cynical, but I have little faith that the person on the other end of the phone is going to tell me the particular product s/he uses won’t do what I need it to.

Check out BASF web site. They make the chemicals for a lot of companies.

(broken link removed)

 

[Jim K in PA]

Yes, by definition, CLOSED CELL foam does not allow water to penetrate it through absorption or transpiration. The foam itself is very good at keeping water and air out, which is why it is such a good insulator. The trick is to make sure the applicator does a good, careful job of ensuring that the lower and upper layers of foam have a sound, continuous bond between them, and that the piping is reasonably well centered in the foam. It is not hard to do, but a sloppy contractor can leave thin spots or gaps that could allow water to penetrate between the layers of foam.

With respect to the foam board, when you say "styro", are you talking about expanded poly-styrene? If so, that is NOT what is used or desired under slab. Blue/pink board is a closed cell polystyrene which is a homogenous product, unlike EPS which has a grain structure and lower ultimate strength. If you want to put a layer of Blue Board under/over your pipes, you will have some challenges. You better make sure the foam board is lying flat on a flat bottomed trench, otherwise when you back-fill it, you will undoubtedly have some deflection of the board/pipe/foam/board and may crack it, allowing the very thing you do not want - water infiltration paths. Sprayed in foam allows for an imperfect trench bottom profile, and it will adhere to itself perfectly. Many forms of closed cell foam boards also have a thin sealing layer that will prevent the foam from sticking to it properly.

Either spray foam the pipe in place, or pay the $$ for the quality pre-insulated stuff.

 

[SE Iowa]

Just an idea from a farmer. It seems that the largest concern tends to be water infiltration around the pex which causes high heat loss due to the ease of heat transfer in liquids. Farmers spend lots of money tiling the ground to dry it quicker. We have noticed that when you dig a trench, that water from the above surrounding areas tends to accumulate at the bottom (duh, this is why tiling works). Has anyone thought about putting a 3" tile in the bottom of the trench, then spray foam (or use extruded polystyrene sheets), then lay your pex tubes, then foam overtop again?? 3" tile from Menards cost $28 per 100 foot. This would eliminate any potential problems as any water would go down to the tile and could be directed away from the trench.

 

[Jim K in PA]

closed cell is critical, eh? now to figure whats what, foamwise. suppose next q= how closed do the foamcells get?

Pook - check out http://www.foam-tech.com/products/urethane_foam/open_closed_cell.htm

Here is the text of that page if the link does not work:

What is the Difference Between Open-cell and Closed-cell Urethane Foams?

Open-cell foam is soft - like a cushion or the packaging material molded inside a plastic bag to fit a fragile object being shipped. The cell walls, or surfaces of the bubbles, are broken and air fills all of the spaces in the material. This makes the foam soft or weak, as if it were made of broken balloons or soft toy rubber balls. The insulation value of this foam is related to the insulation value of the calm air inside the matrix of broken cells. The densities of open-cell foams are around 1/2 to 3/4 of a pound per cubic foot.

Closed-cell foam has varying degrees of hardness, depending its density. A normal, closed-cell insulation or flotation urethane is between 2 and 3 pounds per cubic foot. It is strong enough to walk on without major distortion. Most of the cells or bubbles in the foam are not broken; they resemble inflated balloons or soccer balls, piled together in a compact configuration. This makes it strong or rigid because the bubbles are strong enough to take a lot of pressure, like the inflated tires that hold up an automobile. The cells are full of a special gas, selected to make the insulation value of the foam as high as possible.

The advantages of the closed-cell foam compared to open-cell foam include its strength, higher R-value, and greater resistance to the leakage of air or water vapor. The disadvantage of the closed-cell foam is that it is more dense, requiring more material, and therefore, more expense. Even though it has a better R-value, the cost per R is still higher than open-cell foam. The choice of foam should be based on the requirements for the other characteristics - strength, vapor control, available space, etc.

Both types of foam are commonly used in most building applications. Some are inappropriate in specific applications. For example, you typically would not use open-cell foam below grade where it could absorb water; this would negate its thermal performance because water is a poor insulator compared to air. Closed-cell foam would be a good choice where small framing sizes need the greatest R-value per inch possible. Basically, the choice depends on the conditions of each installation.

 

[Jim K in PA]

Just an idea from a farmer. It seems that the largest concern tends to be water infiltration around the pex which causes high heat loss due to the ease of heat transfer in liquids. Farmers spend lots of money tiling the ground to dry it quicker. We have noticed that when you dig a trench, that water from the above surrounding areas tends to accumulate at the bottom (duh, this is why tiling works). Has anyone thought about putting a 3" tile in the bottom of the trench, then spray foam (or use extruded polystyrene sheets), then lay your pex tubes, then foam overtop again?? 3" tile from Menards cost $28 per 100 foot. This would eliminate any potential problems as any water would go down to the tile and could be directed away from the trench.

Hey Farmer. Yep, if you have concerns about water collecting/ponding in your trench, then stone and/or drainage pipe would be wise to include in your install. However, properly applied foam or good quality pre-insulated pipe will do fine running through a lake if need be. My trench is only going to be 20-30 inches below grade, and my soils drain fairly well (good farm land). I don't expect to have any water stagnation issues in the piping trench.

 

[Jim K in PA]

holy dyslexia!

Mine or yours . . . ;-)

anyway your script details urethane & doesnt mention styrofoam. i wonder about water permeability in both? underground

Re-read the article, as it points out the difference in the structure between open and closed cell foam. You can google "styrofoam" and see what it is compared to urethane. Let us know what you find.

The issue is that closed cell foam, be it urethane based, EPS (styrofoam) based, iso-cyanurate based, or rice krispie treat based, it will not allow water to penetrate the insulating material.

 

[Jim K in PA]

nothing perfect = permeability, i think. ashrae has permeability values but i get crosseyed.
skin of closed cell may be critical factor

I am having a little trouble understanding your english. If you mean that nothing is 100% impermeable, then you are mostly correct, but I think you would agree that many people have used polyethylene pipe without water permeating out it!

The degree and/or rate of permeability of a strucutre like foam is dependent on a lot of factors, including properties of the material, skin thickness, sample thickness, temperature, hydraulic pressure, temperature cycling, mechanical factors, etc. A good subject for a thesis paper, I would think. Of course, the analyses have all been done.

Bottom line? If you feel foam in the trench is not for you, then don't do it.

 

[trehugr]

Now I'm worried... maybe there is something you can paint on the foam to help reduce the risk of permeation.

 

[deerefanatic]

I think you're all worried over nothing........

As for the blue-board stuff..... DONT DO IT!! my old lines (put in by previous owner) were boxed in with blue styro board..... That stuff soaked up water like a sponge and weighed up like a brick! (Which is why they sold their OWB, by the third winter they went through 5 semi loads of wood and had a completely thawed driveway and cold house....)

Urethane insulation has a long track record of success underground..... It's been used on pipelines for years.... A good product like Corbond will last as long as you could desire....

 

[foxt]

When you go with the foam-in-trench method, what happens when that tubing starts to expand? Given the expansion rates of pex relative to delta-T, I'd imagine that the tubing will be trying to grow when that hot water hits it (a big delta-T vs the ambient temp when it was first foamed). For any sizeable length trench, it seems like something is gonna have to give, no? Or do you need to select al-pex when you use this method to limit the expansion factor?

 

[Jim K in PA]

When you go with the foam-in-trench method, what happens when that tubing starts to expand? Given the expansion rates of pex relative to delta-T, I'd imagine that the tubing will be trying to grow when that hot water hits it (a big delta-T vs the ambient temp when it was first foamed). For any sizeable length trench, it seems like something is gonna have to give, no? Or do you need to select al-pex when you use this method to limit the expansion factor?

The coefficient of expansion for just about any tubular material is far greater axially than radially. Therefore, the pipe length will grow much more substantially than its diameter. With respect to the closed cell foam, it has a good degree of compressibility, and is not very brittle, so can/does accomodate the small dimensional changes in the PEX diameter. As for axial movement, I intend to make sure that the foam extends up above grade line or into a termination that will prevent water from migrating down the length of the pipe.

 

[foxt]

Right, it was the axial expansion I was thinking about. If pex expands 1.1" (wirsbo) per 100' per 10*F change, then over a 100' trench you could potentially see over 10" of expansion over that run when the boiler comes up to operating temps. Then as the boiler moves withing its temp range, the pex/foam assembly will undergo recurring stress. That foam is sticky stuff, so I was expecting that the tubing would be glued to the foam. I was wondering what those who have done this before have seen - does the foam give enough to stretch with the tubing (this would be my guess)? Does the tubing slide within the foam (doubtful, I can't even get the stuff off my hands much less expect it to allow pex to slide within it!)? Does the foam develop stress cracks along the way (that would open the door to water infiltration)? Since the reports here indicate good performance over a number of years, I am curious as to what is actually happening in the ground ...