Underground lines - not the place to skimp!

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The big thing to remember is that water in the ground is the enemy... If your insulation soaks up water, it turns into a thermal conductor and becomes WORTHLESS! There are two basic types of foam, open cell and closed cell. Open cell is like a sponge, it absorbs water readily, and has minimal insulation value when wet. Closed cell is essentially waterproof - it doesn't absorb water to any significant degree.

Open cell foam is good insulation, but ONLY in places where it is essentially kept dry, or where it is important to have a moisture permeable material - it is great in INTERIOR house walls, but it is not going to work properly underground! Don't attempt any sort of "waterproofing layer" on the outside of an open cell block, as any leak will eventually compromise the entire installation.

Closed cell foam is what you want for underground "foam in place" type installs, as it is inherently waterproof, and thus both keeps the PEX from losing heat, and keeps the ground water out.

I am not an expert on their products, but my recollection is that the Tiger Foam stuff is open cell, or not suitable for underground applications (If I'm wrong, feel free to correct me :coolhmm: )

I would suggest going with the foam contractor, and be sure to specify a CLOSED CELL foam! That will solve the volume problem as he just sprays as needed, (and charges accordingly) Rather than spending a lot of money on foam board, I'd consider just partially filling the trench (Maybe after dropping in your data and power conduits in the part you'd fill (Remember, you can't put power and control wiring in the same conduit, and you should keep the two types of conduit as far apart as you comfortably can... The other option might be to see if anyone has a narrower bucket for your digger that you could borrow or possibly rent....

Thanks Gooserider. I just checked the technical data for Tigerfoam and all three of their products products (Quick Cure, Slow Rise, and Fire Resistant) are closed cell. They claim closed cell content of 90% or better. I have no idea what is typical in other products. I hadn't even heard of Tigerfoam until I read a posting on here where someone mentioned it. XPS sheathing (think blue or pink Dow foam) is also a closed cell foam. I believe that the foil-faced polyisocyanurate sheathing typically is an open cell foam. I will check with spray foaming contractors though when I get to the point of doing this though.
Trenching & Foaming : Great idea but you all need to take into consideration the ELEMENTS. The black plastic does deteriorate over time, the foam will crack from the expansion and contraction of the EARTH. The tree roots will weave it's way through the plastic and foam alowing water in. The Blue plastic pipe is good idea if it only had one PEX running through it ( less heat transfer from hot water out to cool water return ) but even then it should be buried four to five feet in order to get below FROST LINE. Also there needs to be a sand bottom in the trench and then sand all around to help with the Earth movement factor. FROM the school of HARD KNOCKS, commonly known as BUBBLE BURSTER.
Pete, I purchased a $600 kit of Tigerfoam with the intent of injecting it into the corregated pipe along with the sleeves I used in my first abysmal underground system. I was impressed with the Tigerfoam, but it came in two tanks about the size of a standard outdoor grill propane tank. When the foamer showed up to do the second time around he came with a 55 gal drum of the stuff. I did about 170'-180' underground for about $800. I'm pretty certain I would have had to buy a bunch of those $600 kits to do my trench. I was impressed with the product, but pricey for large jobs. Regardless, even if the foam contractor was hundreds of dollars more I'd go with that 55 gal approach again just to resist the temptation of going light on the foam.

Pete, I must have misunderstood the multiple trench stuff. I can't speak to Batman's comments regarding sand etc. At our latitude the frost line is much less. I just blew foam on the dirt. We'll see. So far it's amazing this season.
How is the spray foam install going to put up with ground heaving due to frost or settling? I'm only going to be able to put my lines 2' or so in the ground because of the water table in a 50' area. I'd like to save a little and do it my self but I don't want to have to do it twice, I might be better using the store bought lines.
PC, One of the main purposes of using closed cell foam is that it is imprevious to water. If I was worried about heaving I'd go deeper to avoid soil movement and maybe blow a little bigger block of foam. The cross section dimensions of my lines are in a foam block about 12" wide by about 10-11" deep. Maybe even more depth in some locations since the process is not precise. BUT, given the heat transfer properties of the foam I'd be worried less about energy loss by going deeper (into the water table) than of foam cracking by staying above the water table. Look at the thickness of foam around the commercial products. It's probably in the neighborhood of 1" thick surrounding the PEX. Because I did it in the trench I have at least 3" all around and between my two lines. I'm not making a recommendations to you just stating that the "energy retention" capability is a function of two factors: 1) the coefficient of thermal conductivity of the insulation material and 2) the thickness of the insulation. As best I can tell, most of the commercial piping uses the same polyurethane closed cell stuff as we used in the trench. So that leaves insulation thickness as the driving factor as to which process retains the most energy. Bottomline, from what I read here the folks that purchase the GOOD commercial piping stuff do not see much loss and since your run is only 50' using the commercial stuff probably makes sense for you. But my real point is, if done right, I don't see why water table should be a factor. This is where I add my usual disclaimer asking Heaterman to chime in to correct me if I'm missing something since I've done all of ONE install, he's done many.
I have to run 170' in total, 50' of it is the problem area. The line set will cost a little over $3000 for my total run if I did it in store bought line set and being a plumber I can get 500' rolls of pex way cheaper than that. I guess that I should talk with the insulation guy to find out how much deflection the foam will take before it splits, I'm just thinking of that wonderful guy Murphy and his law.
I would point out that the PEX isn't going to be overly happy about flexing and ground movement either. While it would probably take more than the foam would, I don't really think it's a good idea to be installing any kind of line in an environment where ground movement is a concern.

Thanks of this thread I am re-assessing my Pex Runs. I WAS going to throw 2 1-1/4" lines & a 3/4" in a 4" PVC conduit which runs under the length of my house, and then spray foam it--or try--along the length..... The house will keep it dry, but it still gets damn cold under that run. NOW....however I am thinking differently. There is no room under the house to dig a trench. Barely even room to crawl. I can really do the tench, plastic & foam because of the potential of rodents burrowing into it..... But....I am thinking of a wooden box, running the 80'ft I need to insulate. I can put wooden keepers every so often to keep the pipes up 3" and then be able to spray foam it easily. Once the foam is on I can screw on a lid. This would be a 6 x 6" ID box. The wood is "free" from my mill, so no worries there..... Crawling and feeding the pipe under the house is the biggest nightmare as 1) there is no room & 2) I've fought with claustrophobia my whole life..... used to drive me crazy when I was working construction under houses.......especially remodels. UGH! Guess it's time to Cowboy Up and "get er done!"
HF, If it is a long run in a very confined space, you could purchase a commercial foamed product like Logstor for that tough to work space. Then use a pex connector to do foamed in trench once outside. Your idea of building a wood form to "cast" the foam around the pex certainly could work... IF you can get all the spray equipment under there, hold the pex to assure spacing, and see what you're doing. Under our house in the tightest crawl space it would be nearly impossible to make all that happen. You can see from the pics we took doing our place the gear my contractor put on just to blow foam outdoors. He probably would have declined to work in my space that is very restrictive. If you plan on foaming in the trench, ask your foam contractor what he thinks. If he can do it in your form, I'd just let the foam grow out of the top like baking a muffin and not worry about a top. Best wishes and if it's that tight, I'd probably use short sections of the pre-foamed product then transition to foam in trench outside.
Tennman, thanks for the advice. I was wondering if I could get it all to work..... After spending around 5hrs crawling around under my house I have abandoned all plans for using that as an access route. Outdoors isn't an option either.... Check out this thread: https://www.hearth.com/econtent/index.php/forums/viewthread/86076/
Larry, a question about your underground install: From the pictures it looks like the pex unrolled nicely—I’ve heard it has a lot of memory and is very difficult to work with, particularly the larger sizes.

I’m going back and forth between foam in ditch using 1 1/2 inch pex and using preinsulated thermapex. I’d prefer the larger pipe, but the cost to foam in my area is $12.50/ft—that is the same price as the thermapex. Of course, the thermapex is slightly less than 1 inch ID, so that is a tradeoff. Plus, I can use a trencher for he thermapex; I’d have to use a backhoe for any other option so that is going to add about another $350 to the job. I can live with the additional expense, but right now I don’t have much of a window for foam because of the temp, so the other option is to insulate the pex and run it through conduit—thus my question about how stubborn the pipe is.

thans for any thoughts you have


Steve, The spray polyurethane firms up literally in seconds (not cured, but firmed). Yes the pex has a lot of memory but your typically dealing with very long lengths if you plan on foaming in the trench. I’ve told other folks that I’d guess the logical trade-off point of foam-in-trench vs thermopex would be 30-50’. That would be a relatively short run and the energy benefits of foam in trench would be marginal to just buying thermopex. So it’s a cost and energy trade problem. For my relatively long run of approaching a 400’ round trip (170' under ground), when my house is not calling for the fan I routinely see 1.5-2 deg F loss of temp round trip from boiler and back. That’s amazing evidence of thermal efficiency (loss of energy), which ultimately translates into less wood and time in the woods. For a much shorter run a 1-2 deg loss is still great so thermopex could make sense. The energy loss/ft length is linear so you do the math on our application. On my system I’m losing .005 degF/ft length including going thru my HX and manifold in my root cellar. I'd bet I'm losing half of that energy in my root cellar, but that loss is warming the living space above. But for a short run to get a foamer to come out and if he has minimum charges, thermopex could be the right choice for you. But from an energy efficiency standpoint it can never match my lines in a block of foam 10†x 12†cross section. I wouldn’t worry about the pex memory. The foam sets so fast it’s not hard to work with as it’s held by the foam block as you move down the trench.

I will add that for me, Lord willing, this is the last house I will ever own. So 5-10 years from now if I want to go to a Froling or other latest wiz bang nuclear boiler, it will be plug and play with my underground. What I got underground just can't get any better.

Best wishes.
I have the Logstor pipe running 90 feet each way and have a 1-1.5 degree loss, round trip. Even when we put the pipe in the ground with the "bend", it straightens out after a bit. I've used other piping systems in the past and the pipe you use will make or break your system's savings over oil. I went from 21 cord to 14 cord, and I expecr to do even better next year with real seasoned wood.
If I was to use a pre-foamed product, Logstor would be at the top of my candidates. I think I priced it and for my application (170' trench) foaming in trench was about 50% less expensive when you include the pex cost. What we're learning from this sticky is that foaming in trench bids vary dramatically depending local contractor rates and competition. If foam-in-trench wasn't viable from cost or availability, this looks like the next best thing. I'm going to guess your system is similar to mine with about 15-20' of pex between the foundation and the HX. That puts your round trip at roughly 240-260' round trip for a total system energy loss of about .006 degF/ft. What I really need to do is move my in-house AZEL sensor from the HX fittings to where the pex enters my root cellar. That would allow me to measure the underground transit loss. Then we can start comparing purely the underground performance of various methods. But the physics doesn't change. If the insulating material is the same between the products compared, then the only other variable in retaining energy is insulation thickness. Thanks for the input MH. Your B&B is older than our place by about 20 years. When your place was being built settlers were just entering our area building log cabins. Love history.
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Just thought I would add some hopefully useful information to this thread, as it was very useful when i was planning my lines.
I did 2 runs from a central location to heat 2 houses on my property. The first run was up to a 3400 sq ft log house at 200' from the boiler/storage. I did not get any pictures of that install as we were trying to beat the weather, but we did the foam in trench by laying plastic in the trench and foaming the tubes right in on top of the plastic, 2 guys 1 holding up the tubes and the other foaming, we used the Foam it green kits and it took 2 602 kits to do the 200 foot run. I felt this worked out good and would say if you have 200 ft to go 2 of those diy kits work really well, its easy to do and pretty user friendly.
The second run was 100 ft down to a 1500 sq ft house, this time we did something different, still using the Diy foam it green kits, but Instead of the plastic in the trench, (that can be kinda a pain) we took 1" thick XPS foam board and made troughs out of it, you can see it in the pictures below, This worked really good, it was easy to make using a table saw and some 3M super 77 spray adhesive. Took 1 guy about 3 hrs to make all the troughs needed (if you cut your sheet into 6 equal strips it gives you 16' of trough per sheet) The spray foam bonds to this stuff great so no need to worry about connecting the end joints, just put it in the trench end to end.
It took 1 602 Foam kit to do this 100' trough and worked perfect. So if you are thinking of doing this yourself that should give you a good estimate of materials needed. The foam troughs helped control foam usage to a pretty exact amount even witth some extra use at the ends to seal up to the house and shop it worked out perfect.


  • Underground lines - not the place to skimp!
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Your approach used considerably less foam than what I did just filling the trench. Your trench looked to be about 12" so you saved a bunch of foam. I have no regrets just using lots of foam since I had a contractor show up with a 55 gal drum and foam thickness is one of the factors in energy retention. But if I was to go the DIY route this looks good. I'm not a blue foam guy for any underground, but using it to make a trough makes sense. The only thing you didn't mention was whether the 602 foam kit is closed cell polyurethane. I presuming and hoping it is. Once you get up and running be great to get some loss/ft numbers. My measurements are during conditions when the house demand is not high (maybe high 30's to mid-40's) so the circulation temps are fairly stabilized.
Yes the foam kits are closed cell foam. the trench is actually 24", the trough is 8" wide and about 7" tall so the tubes are surrounded by quite a bit of foam. I tried to get a contractor to come do it, and they all would come, some for more than others, all quoted more than the diy kits cost me, but the big problem was scheduling, They are so busy around here they were scheduling 1 month out and if you missed your day due to weather you needed to re schedule and wait another month. I just could not afford to have weather play that big of a role in getting this done.
maple1 said:
Any worries about things shifting a bit, leading to cracking of insulation & water getting in?
Not at all, that foam is pretty tuff after it cures, I walked on it and it didn't even make a sound. We back filled by hand some around the foam and the rest with the back hoe.
Ok, you guys have sold me on using closed cell foam; I was planning on a 150mm rigid PVC outer tube with single component squirt foam sprayed in through strategic holes.
I'll have to get the foam kits from England; Here in southern Portugal they don't use that sort of thing. They don't insulate or heat warehouses or barns.
The locals don't even heat their houses; the old folks think central heat is bad for the health! We get down to about the frost point in winter.

I like CJ Dave's system with the rigid foam plates. They're much cheaper than the foam kits, and can be a large part of the insulation. A froth-pak kit will cost me $950 out here, double what you would pay in the US.
Foam has some flex in it in my experience; I really don't think you have to worry about ground movements unless you live on a fault line.
Tree roots will break concrete, so all bets are off if you have trees around.

Mark Holden
Odeceixe, Portugal
I included a picture of the stuff I used last time. I actually used the double wrap. I will tell you this, regardless of the "wrap", if there is a pin hole in that black drainage pipe, water will get in and you will be done. You will hate the day you put that crap in the ground. The black pipe I used had holes in it when we put it in the ground and I thought nothing of it. Pipe was on a 4 week back-order and I had to get it in the ground that day. I will never use that stuff again, ever. You get what you pay for. I bought three hundred feet of the double wrap @ $9.00 foot, when I could have paid $12.00 a foot 4 years ago for the decent stuff. Now I paid $18.00 / foot for this new material and frankly I don't care. Ig I can burn 6 or 8 cords, it'll be better than 20. That really takes the pleasure out of using wood. This is the right stuff for my application. If people can foam their own, then more power to ya. The new pipe I laid in the ground will lose 0.5 degrees @ GPM running 100 foot.

i am currently building shed to house my d.s. machine boiler .had boiler in basement for 3 yrs worked great just want wood out next to unit plus feel safer and never any smoke smell in home.i had guy buy that pipe in black corrugated pipe ,then it has pex with insulation around it.very poor product black pipe brittle and damages easy his was full of water steam was coming out of pipe in his shed.i had person tell my about guy selling pipe 6 $ft what a great deal .i went looked at pipe same black pipe that fills with water looks like footing drain pipe from home depo,when i really looked close holes every 15ft ,he said i can get new must be defect no thanks.i ended up buying central boiler under ground pipe 17$ft but totally filled with dense insulation plus outer skin smooth rubber .if i did not see thats guys pipe filled with water i prbaly would of bought cheap junk from listening to people that are cheap or have no clue of quality.i am posting my project on you tube to help others like they helped me in past.you tube go type in bodyshop18336
I've been planning my wood boiler installation for next season and have been paying attention to this thread for awhile. I did some measuring yesterday and found that I can really save some underground distance depending on where I enter my basement to connect to the existing system and again where I enter the shed for the IWB. So I guess the question is...if I can save substantial length of buried pipe by entering a building at the closest point and then run the less expensive pex along the wall or ceiling to the boilers, is it a better option. I can always insulate the indoor pipe to reduce heat loss, and it will be in a heated space anyway. The difference in underground pipe length ( and cost) is significant. 115 feet vs. 170 feet. give or take a few feet either way.
Thanks for your thoughts on this.
I'm going to buy a boiler this spring and its 120 ft from where I want my boiler to my point where I will enter my basement, Has anyone heard of Zsupply pex pipe. The guy I'm buying my boiler from said its good stuff. I talked to one other guy thats been using it for 2 years and its doing great for him. Jus.t was wondering if anyone used it Its made in Michigan