DIY in-ground plunge / mini pool WITHOUT using CONCRETE

lukmar

Member
Jun 30, 2021
5
Denmark
I am planning to make small in-ground pool:
Lenght: 4m (13 feet)
Width: 2m (7 feet)
Depth: 1.2m (4 feet)

The project is a `budget` one also for a reason we just want to try it to see if we even like it (we have small backyard)

I was sniffing around this forum and learnt that most of the people prefer to use concrete / gunite for floor base or cinder blocks for walls (of course reinforced with rebars), f.e.x.:

DIY Concrete Block Soaking Pool - In Progress, Advice Welcome!

But ... for a shallow pool like mine, where the max pressure at the bottom of the pool would be just: 12kPa (or: 1.75 psi) - isn't that overkill?
I saw people making structures from plywood or cement boards (of course waterproofed either by epoxy sealer or any other way like bitumen) in wooden frames (f.e.x.: 2x6 inch pressure treated lumber) or reinforced in any other way, f.ex: wiring or epoxy / fibre glass.
It seems much easier to work with and form to own wish (inlet, out, skimmer holes). It is also easier to destroy in case kids are bored with a pool or it turns out not to be good idea.

At the moment my plan is:
1. Dig a pitch deep enough
2. Geotextile + 4 inches of crushed rock (stabilising) at the bottom.
3. 2 or 4 inches of XPS insulation on top of a rock for bottom insulation and at the outside of the walls (idea stolen from: Insulating the pool)
4. Plywood or cement boards (on top of the XPS and outside walls layer) for floor and walls construction (wooden beams, joists, etc..)
5. epoxy / fiberglass liner or tiles (I am already aware of special filler / glue needs for tiles in the pool).
6. Backfil with crushed rock or other material (which would drain well).

Does it make sense / would that work?
It seems (in theory) that this could have some benefits:

1. Better drainage - as having crushed rock at the bottom without concrete base would drain water out better
2. Easier to work with (I am planning to do it as a hobby project alone in a spare time) - no need to order or make lots and lots of concrete, lighter materials, etc...
3. Easier to disassemble

If it has any sense, some other questions:

1. What is the most optimum way to waterproof / treat wooden parts (and plywood), f.ex.: will bitumen be enough?
2. Is it better to insulate inside or outside walls?

Other questions I have is:

1. Many people recommend grounding the pool structure itself (wiring rebars inside concrete structure and grounding them) - what is the point of that, since water inside the pool does not electrically connect with the structure (isolated with liner)?
2. In case of building walls with cinder blocks - is it OK to reinforce with rebar only deeper parts of the walls (where pressure is higher)? This would make destroying / cover it with soil easier in case I decide to bury it?

Enough for a first post - and apologies in case it is out of place as well (not sure if it is the best forum group)?
 
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mas985

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But ... for a shallow pool like mine, where the max pressure at the bottom of the pool would be just: 12kPa (or: 1.75 psi) - isn't that overkill?
It isn't about pressure. Concrete isn't used for strength so much as custom design. It is pretty easy to get any shape or size with concrete but other types such as fiberglass has limits because of shipping and vinyl pools are limited in size because of liner manufacturing process. So the biggest reason people go with concrete is because of customizability.

1. Better drainage - as having crushed rock at the bottom without concrete base would drain water out better
Drainage to were? Anything below grade does not drain very well even with gravel. The gravel is irrelevant since you are basically in a mud bowl either way. Yes water will easily pass through the gravel but stop at the mud boundary. It is the mud that is the limiting factor. But water under the pool only matters when it is empty. A hydrostatic valve can prevent lifting.

2. Is it better to insulate inside or outside walls?
Neither. Heat loss through the walls accounts for less than 10% of the total heat loss in a typical pool so insulating is generally not worth the additional cost. Components of heat loss,

Evaporation: 55%
Radiation: 25%
Convective: 10%
Conductive: 10%

A pool cover is a much better investment as it reduces all but the last component.


2. Easier to work with (I am planning to do it as a hobby project alone in a spare time) - no need to order or make lots and lots of concrete, lighter materials, etc...
3. Easier to disassemble
Based upon this, why not go with an AG pool. Some designs, you can even partially bury it if you want.
 
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lukmar

Member
Jun 30, 2021
5
Denmark
Thank you for taking time an sharing your knowledge. Let me reply to this a little bit thou:
It isn't about pressure. Concrete isn't used for strength so much as custom design.
There may be many other reasons aside strength to choose specific construction material but this not change the fact that construction must be strong enough.
I guess all I want to know if using concrete boards, potentially reinforced with net or fibreglass will do (especially if backfilled properly)?

Heat loss through the walls accounts for less than 10% of the total heat loss in a typical pool so insulating is generally not worth the additional cost.
I am aware this is commonly shared opinion but here is very interesting article trying to prove otherwise:
Insulating the pool
Excerpt:
Most contractors tell me that the dirt under the pool is a fine insulator, but I think they really don't know what they're talking about. Houses use under-slab insulation to insulate their 70 degree interiors from the 55 degree earth heat sink. The pool will be at 85 degrees, which is twice that temperature gradient, so I think insulation will matter even more for the pool. In particular, I'm most concerned about the water table contacting the bottom of the pool in the spring and sucking all the pool's heat into an underground plume...

Let's suppose that concrete conducts 1.7 W/m-K. To convert that to more familiar units, 8 inches of concrete would be R-0.68 (which is terrible -- single-paned windows are better). I'll guess that the dirt, even when wet, insulates a bit as well, so that the bottom of the pool is about R-2. My pool will have an average depth of 7 feet (it has a 10.5 foot deep diving area), so it'll have an exposed area of about 1790 feet. If the pool temp is 85 degrees, and the ground temp is 60 degrees, that's 537,000 BTU/day, about three times my expected loss through the top in the spring. That's why I'm insulating the pool.

I agree that the most projects I saw did not bother insulating pools but still curious about theory, especially when isolation is actually quite cheap (except maybe floor, where XPS boards are needed).
 

poolnoobgrandma

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I am planning to make small in-ground pool:
Lenght: 4m (13 feet)
Width: 2m (7 feet)
Depth: 1.2m (4 feet)

The project is a `budget` one also for a reason we just want to try it to see if we even like it (we have small backyard)

I was sniffing around this forum and learnt that most of the people prefer to use concrete / gunite for floor base or cinder blocks for walls (of course reinforced with rebars), f.e.x.:

DIY Concrete Block Soaking Pool - In Progress, Advice Welcome!

But ... for a shallow pool like mine, where the max pressure at the bottom of the pool would be just: 12kPa (or: 1.75 psi) - isn't that overkill?
I saw people making structures from plywood or cement boards (of course waterproofed either by epoxy sealer or any other way like bitumen) in wooden frames (f.e.x.: 2x6 inch pressure treated lumber) or reinforced in any other way, f.ex: wiring or epoxy / fibre glass.
It seems much easier to work with and form to own wish (inlet, out, skimmer holes). It is also easier to destroy in case kids are bored with a pool or it turns out not to be good idea.

At the moment my plan is:
1. Dig a pitch deep enough
2. Geotextile + 4 inches of crushed rock (stabilising) at the bottom.
3. 2 or 4 inches of XPS insulation on top of a rock for bottom insulation and at the outside of the walls (idea stolen from: Insulating the pool)
4. Plywood or cement boards (on top of the XPS and outside walls layer) for floor and walls construction (wooden beams, joists, etc..)
5. epoxy / fiberglass liner or tiles (I am already aware of special filler / glue needs for tiles in the pool).
6. Backfil with crushed rock or other material (which would drain well).

Does it make sense / would that work?
It seems (in theory) that this could have some benefits:

1. Better drainage - as having crushed rock at the bottom without concrete base would drain water out better
2. Easier to work with (I am planning to do it as a hobby project alone in a spare time) - no need to order or make lots and lots of concrete, lighter materials, etc...
3. Easier to disassemble

If it has any sense, some other questions:

1. What is the most optimum way to waterproof / treat wooden parts (and plywood), f.ex.: will bitumen be enough?
2. Is it better to insulate inside or outside walls?

Other questions I have is:

1. Many people recommend grounding the pool structure itself (wiring rebars inside concrete structure and grounding them) - what is the point of that, since water inside the pool does not electrically connect with the structure (isolated with liner)?
2. In case of building walls with cinder blocks - is it OK to reinforce with rebar only deeper parts of the walls (where pressure is higher)? This would make destroying / cover it with soil easier in case I decide to bury it?

Enough for a first post - and apologies in case it is out of place as well (not sure if it is the best forum group)?
It sounds like you aren't 100% invested in a pool, given your thoughts on maybe refilling it if you don't like it. What you've described sounds like a LOT of work. In your position I would investigate DIY liner pools. If you love the pool you can replace the liner in a few years, but if not, I think filling the hole will be easier, and you won't have invested as much blood, sweat, and tears.
 
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Newdude

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+1. I’d go wood and spend the bulk of the project money on a liner. If you love it you can reconsider replacing it when the liner goes in 7-12 years. If you don’t use it, it’s easy to remove.

We have seen several plywood pools threads over the years here if you search for them. This one was framed with pallets.
 
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mas985

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I am aware this is commonly shared opinion but here is very interesting article trying to prove otherwise:
Insulating the pool
First, the ground temps near the pool walls is nowhere close to 55 degrees. That is the temperature at about 30 feet of depth. Above that, the temperature increases with decreasing distance to the surface.
amplitude-vs-depth.gif


So at a depth of about 6 ft down, the soil temperature is only about 10 degrees below that of the surface temperature which on average should be a bit above the average daily air temperature. So if the average air temp is 75F, at 6ft down the temperature is 65F and the average pool wall surface temperature is only about 70F. Also, I don't think he calculated the surface area of that pool correctly. Your pool would have a wall surface area of about 250 sq-ft by my estimate.

But my main point was that when you calculate the heat loss for all the components, conduction is a small part of the total so the improvement is small.
 

Bperry

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First, the ground temps near the pool walls is nowhere close to 55 degrees. That is the temperature at about 30 feet of depth. Above that, the temperature increases with decreasing distance to the surface.
amplitude-vs-depth.gif


So at a depth of about 6 ft down, the soil temperature is only about 10 degrees below that of the surface temperature which on average should be a bit above the average daily air temperature. So if the average air temp is 75F, at 6ft down the temperature is 65F and the average pool wall surface temperature is only about 70F. Also, I don't think he calculated the surface area of that pool correctly. Your pool would have a wall surface area of about 250 sq-ft by my estimate.

But my main point was that when you calculate the heat loss for all the components, conduction is a small part of the total so the improvement is small.
That was going to be my point as well. The evaporation heat loss is so much higher than conduction through the ground it makes sense to attack the bigger fish more than the small potatoes.

The house analogy doesn’t work. A more appropriate analogy would be opening all the windows in the house and removing the roof, then adding slab insulation to try and make up for the heat loss.
 
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lukmar

Member
Jun 30, 2021
5
Denmark
Thank you so much for the answers, I really appreciate how active / helpful this forum is.
I understand the point of evaporation and importance of minimising it (having the cover) and this is not questionable.
I am not arguing for the sake of it - just want to understand exactly the heat loss components (as a lamer).
You say:

Heat loss through the walls accounts for less than 10% of the total heat loss in a typical pool so insulating is generally not worth the additional cost. Components of heat loss:

Evaporation: 55%
Radiation: 25%
Convective: 10%
Conductive: 10%
My questions are (maybe because I am not an expert in thermodynamics):
1. Does it mean that walls / floor insulation works on conductive component only? What about radiation and convective parts listed but not included in your wall insulation part?
2. Aren't those example numbers based on average pool sizes, which are much bigger than mine? In my case surface to volume ratio is much higher, therefore conductive component part is higher (smaller containers cool down faster)?

Real life example we all could related to (and please forgive me if it is a naive one):
Coffee ;-) ... - if it is served in non-isolated cup (f.ex.: tin stainless steel) it will cool down much faster than isolated one (independent if they are both covered or not)?

Also, just found this calculation here:
Heat Loss from Open Water Tanks

Screenshot 2021-11-13 at 19.06.49.png

It looks like for 'typical' water temperature in pool - first row - heat loss through uninsulated walls is more significant than suggested in the thread. I know it uses bare steel here, but isn't concrete also as poor insulator?
It looks like It can also be significantly reduced by adding insulation (f.ex: 10 times lower when 2 inch insulation is applied).
Unless I misunderstood something....
 
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Bperry

Gold Supporter
Aug 20, 2020
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Knoxville, TN
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Thank you so much for the answers, I really appreciate how active / helpful this forum is.
I understand the point of evaporation and importance of minimising it (having the cover) and this is not questionable.
I am not arguing for the sake of it - just want to understand exactly the heat loss components (as a lamer).
You say:


My questions are (maybe because I am not an expert in thermodynamics):
1. Does it mean that walls / floor insulation works on conductive component only? What about radiation and convective parts listed but not included in your wall insulation part?
2. Aren't those example numbers based on average pool sizes, which are much bigger than mine? In my case surface to volume ratio is much higher, therefore conductive component part is higher (smaller containers cool down faster)?

Real life example we all could related to (and please forgive me if it is a naive one):
Coffee ;-) ... - if it is served in non-isolated cup (f.ex.: tin stainless steel) it will cool down much faster than isolated one (independent if they are both covered or not)?
If your pool is above ground and uses metal walls, then insulating the walls in addition to the cover might be worthwhile. Or if your pool water surface is very small and your pool extremely deep with metal walls (like your cup example) some insulation might help a little. But pools are generally more wide and shallow rather than drinking cup shaped.
 
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mas985

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My questions are (maybe because I am not an expert in thermodynamics):
1. Does it mean that walls / floor insulation works on conductive component only? What about radiation and convective parts listed but not included in your wall insulation part?
Yes, primarily. Convection only applies to fluids (water and air). Radiation only occurs between material that is different in temperature. The pool walls will be in contact with the earth so there is a continuous temperature gradient from the water through the concrete into the earth for some distance. But there really is not a fundamental temperature difference between objects for radiation to be significant so it can be ignored.

2. Aren't those example numbers based on average pool sizes, which are much bigger than mine? In my case surface to volume ratio is much higher, therefore conductive component part is higher (smaller containers cool down faster)?
There will be some differences but I do have a thermal transfer calculator in my signature that can be used to establish estimates for your particular case.

Real life example we all could related to (and please forgive me if it is a naive one):
Coffee ;-) ... - if it is served in non-isolated cup (f.ex.: tin stainless steel) it will cool down much faster than isolated one (independent if they are both covered or not)?

Also, just found this calculation here:
Heat Loss from Open Water Tanks

View attachment 381808

It looks like for 'typical' water temperature in pool - first row - heat loss through uninsulated walls is more significant than suggested in the thread. I know it uses bare steel here, but isn't concrete also as poor insulator?
It looks like It can also be significantly reduced by adding insulation (f.ex: 10 times lower when 2 inch insulation is applied).
Unless I misunderstood something....
They don't even include convection and conduction so that tells you the significance. Also, metal has a very high thermal conductivity. Are you using metal walls exposed to the air? If not, then those numbers do not apply.

The heat transfer for an IG pool is different than an AG pool because there is air next to the metal walls. Based upon your description, you have an IG pool that is using concrete/fiber walls. That is a lot different than an exposed metal tank it terms of heat loss.

One thing to be aware of for heat transfer is that all the materials contribute to the rate of heat transfer.
For materials in series, the thermal conductivity is calculated by the inverted sum of the inverses. Kt = 1 / (1/K1 +1/K2 + 1/K3 +.....)

So the lowest thermal conductivity material will dominate. Conductivity is about the same for both concrete and dry earth. The rule of thumb for dry dirt is an R-Value of about 1 per foot. Plus they are about 10x better than metal. And yes, fiber insulating board is about 30-40x that of dirt but you also need to take into account thickness. So 2" of fiberboard can be the equivalent of 60" of dirt. But technically, the dirt can be much thicker than that. It is a very complex situation because it depends on surface temperatures and water tables as to how the dirt will conduct heat from the pool. In some cases, it will actually conduct heat into a pool (when the earth is warmer than the pool).

I would consider these organizations the leading experts in heat loss for swimming pools. This is what I based my thermal model on.

 
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