Thursday, June 18, 2009

Another insulated pool

Back when I posted about the insulated in-ground pool that I'm building, I asked if anyone else is building such a pool. I've received a few answers:
  • One reader in Melbourne is building such a pool.
  • Several have been built in the United States, but only one of the ones I've heard of is residential. The rest are all commercial facilities.
  • Insulated pools are standard when the pool sits on top of a parking structure. Apparently installations like these are simply impossible to heat if the pool is not insulated, and there are structural isolation benefits as well.
Up until now, though, no pictures! Thankfully, the reader from Melbourne has recently written in to share a few pictures of his insulated pool. Here's the standard picture of the dig:

The pool is 46 feet long, which is exactly the same length as mine. His is skinnier (10 feet wide) and more shallow (max 6 feet), which is appropriate for a lap pool. Below, it looks like they are installing an in-floor cleaning system. Very nice.

Below is a pic of the insulation going in. He is using Dow Highload 100, sold there as Dow HD300, in the same thickness that I used (2 inch). He says:
The insulation I'm using is Dow HD300 in 50mm boards. This product is made for insulating under coolroom floors with trucks driving on top, and is overkill given its compressive strength specs of 2% compression (1mm) after 20 years of 250 kPa or around 25 tons per sq meter. However, the pool contractor and engineers had never seen pool insulation done before and through an abundance of caution over-specified for the highest compressive strength product they could find to be sure it wasn't going to settle. With the loads from this pool of only around 2 tons per square meter, we have more than an order of magnitude margin of safety. In the end, the cost differential between this and lesser rated products was so small that in the interests of getting the pool contractor comfortable with signing off we went with the HD300.

The contractors didn't glue the boards to the soil with foam, instead they used the rather unsubtle method of nailing it through with steel rod. I had two concerns about this:
  • This will mean there's some heat conduction losses through the steel rod from the soil to the concrete, though the total surface area of steel in contract with the cement shell would still be minimal so this probably isn't a big deal.
  • A risk of the rod eventually rusting and applying pressure to the concrete shell, but the foam will (I hope) compress enough to accommodate any rust expansion and prevent concrete spalling off the shell were this ever an issue.
The upside is at least I don't have to worry about the compression issues for the expanded foam glue you'd used and hence avoids the risk you mention in your blog that this may place extra strain on the shell as it settled, and from the photos it seems the contractors have got a good solid base without the rocking problems you'd mentioned.

The steel rod seems like a good idea. I tried to find an equivalent product here and failed, which is why I ended up with the polyurethane foam. One other contractor I've talked with in the U.S. also used foam, but I neglected to ask him if he chose not to use steel nails for some reason.

Here in California we use Dobies to seperate the rebar from the ground/insulation. Dobies are simple 3" x 3" x 3" concrete cubes with a wire in them. Check out the much snazzier looking rebar spacers they use in Australia. The wall does not appear to have a bond beam at the top, but instead is pretty thick the whole way up.

Insulating the piping has been a major effort on my project. It's not clear in these pictures if this pool's piping is insulated.

Gunite going in:

His pool is in basically the same condition as mine right now. Note the clever combination of bench seat and stairs at the right hand side of the pool. Very nice. The pool looks deeper than it is because the lot slopes up to the left, and the left hand side of the pool is a retaining wall (raised bond beam).

It's a nice looking project, and I'm very curious to see how it turns out. Thanks a lot, Melbourne!


  1. Very interesting project. What type of insulation is being used in this process?

  2. Right in the post, Larry:
    "He is using Dow Highload 100, sold there as Dow HD300"

    I used Dow Highload 40.

  3. Have a look at They have been insulating swimming pools for 10 years.
    The new panel system can also be tiled so no need for concrete pools anymore.

  4. Have a look at they have government grants to insulate the floor and walls of swimming pools in the UK New and existing pool

  5. If you build an in-ground pool in the UK it must be insulated to meet new building regulations from the 1st October 2010.
    Polypool meet these new requirements and have a great technical team who will provide all of the calculations to ensure your pool passes building regulations.

  6. Arch L.P.Santiago / NS, Inc./cnsi_2006@yahoo.com5:15 PM, November 21, 2010

    Looks pretty good. Can you suggest using HD300 for above-ground concrete pools as well?

  7. I see no reason why you couldn't use HD300 above ground as well. You would have to think through how your cladding was connected to your insulation. Would you have the 12" bond beam, then 2" insulation, then wonderboard or some kind of concrete base for whatever your external cladding is? Or are you going to have a wooden external cladding?

    I wish I'd done this on my pool. At first I didn't think I had enough room in the wall. Then we changed the coping on the top of the pool to be wider. Then to trim the wall to the right thickness the masons ended up mortaring 1.5" thick concrete blocks to the wall. That should have made me realize that we could put in insulation instead.

    I've seen a home in Italy that was built with a concrete block and XPS combination, where the XPS was approximately 1" layer between the inner and outer concrete blocks, which were themselves just 3" thick. It wasn't really clear what gave the wall it's structure. Maybe you could use something like this.

  8. Hi, I'm not an engineer !
    ... but am super interested in this subject and want to insulate a new pool I plan on building in the East-Bay Area of California.
    Is there any reason one couldn't hang the panels instead of attaching to the sides. For example: from the top-sides, drape over a thick plastic film/sheeting over the sides (like that visqueen stuff they use for drainage). Then secure it at the top .. maybe wrap the plastic 1-2 times around a 2x6 / etc and stake it into the ground around the perimeter of the pool. Then, use a glue (not the expanding foam) to spot-glue the panels to the plastic sheet (just a few dabs to each panel instead of covering the whole back).
    Would that be easier ? Seems like a lot of work gluing/foaming panels to the dirt sides.
    Obviously the fit would not be as tight .. what about occassionally gluing the plastic-sheet to the earth as you're laying it out ?
    Also, for the spacing/pockets that might develop between the earth and the plastic ... wouldn't nature (a few rains) take care of that ? (for example if I build the pool one year, and do the decking a year later).
    Thank You Ian & bloggers for all the valuable info on this site.

    1. What POol contractor did you end up using in the bay area.

    2. We didn't use a pool contractor. We ended up being the general contractors ourselves, since we couldn't get any of the pool contractors to bid on doing an insulated pool.

  9. I was worried about my pool settling in a way that would cause it to hang on the dirt shelves and break the shell at the edge of the shelf.

    Now, if I were to do it over again, I'd get rid of those dirt shelves and have a simpler shell. That would make it much less important to avoid settling. In that case, you could just stick the insulation to the dirt in spots and then shoot the shotcrete/gunite. Hanging it from sheets seems too loose to me -- you are inviting a lot of motion when you hit it with shotcrete/gunite, and I think that's bad.

    On the bottom, I am not a fan of settling at all. I would do it exactly as I did it (documented in earlier posts).

    I'm an even bigger fan of burying the pipes in the pool shell now that I've learned another problem with having them outside: despite having insulated those pipes, enough heat is leaking from them to retard the growth of my lawn over the pipes. It's not a big problem, but I don't like it. Pipes in the shell is just such an elegant solution... I should have thought of it myself.

  10. I am in the process of building a pool in Ireland with fairly heavy insulation and have numerous comemnts both good and bad. Its a self-build project and I an a engineer (geotechnial) by trade so tend to probably be over confident in what I am doing. Most of the negative comments come from other pool builders and waterproofing suppliers but others have been very helpfull. All sorts of concerns about the integrity of the walls, concern about the floor insulation compressing and "massive" water loads. The comments are all mainly due to ignorance but some are valid. the construction comprises of a 200mm mesh reinforced base slab. Walls of hollow block infilled with concrete and rebar with 100mm high density insulation and then an inner 100mm solid block. this will then be rendered. Ontop of the floor slab, we palcing either 50mm or 100mm (still to decide) of insulation then 100mm mesh reinforced screed to form the floor. Finally the corners will be rounded and the whole pool painted with chlorinated rubber pool paint.
    So if anyone can offer their comments or if this helps others, thats great.

    1. I am building insulated in ground pool 20 Ft X 60 Ft (6m X 18M) depth 3-1/2 Ft (1.07M) to 4-1/2 Ft (1.37M) having reinforced concrete shell monolithic pour in India.Concrete mix is M30.

      50 mm Extruded Polystyrene 32 Kg/Cu.M density board (sheet size 0.6M X 1.25m with ship lap joints) compressive strength of 330 Kpa/Sq. m (43 psi) at 10% compression is placed on compacted soil. We do not have access to Dow HD.

      The concrete slab is 250 mm thick on floor and walls. Steel reinforcement is deformed bar in double cage having 12 mm bars placed at 250 mm centers. The bottom cage is 32 mm above EXP insulation. The concrete cover on the water sides is 75 mm. The rebar cage is identical on floor and walls.

      Tied to rebar cage on top side is 0.75 (3/4) inch PEX tubing at 350 mm centers, on floor and walls to heat pool.

      The wall insulation is same as the floor insulation except is glued to the outside of concrete water after walls cure for 7 days. The wall insulation is covered with polyester mesh and plastered 2 inch thick before adding backfill.

      Any comments will be appreciated.

  11. That's a LOT of insulation! You haven't mentioned how you will insulate the top of the pool. That's actually the most important thing to insulate (but it's also the easiest to change, so you can figure it out later).

    I have not yet built the insulating cover for the top of my pool. I think I know how to do it, but have not yet found a manufacturer. I basically want to make a 9 by 2 array of panels, each 4 feet by 9 feet by 2 inches thick, which can be individually removed and stacked at the end of the pool.

    These need to be tough enough to survive lots of handling, light enough for my wife to pick up easily, heavy and snug enough to stay put when the wind blows, and UV resistant. I'm thinking of vinyl covered extruded polystyrene panels with magnetic strips in the sides that cause them to lock together -- very similar to the construction of standard spa covers, only without the complicated skirt around the edge.

    One interesting thing is they have to deal with rain. My strategy will be to let the rain flow through the edges between the panels into the layer between the insulating panels and the automatic cover underneath. When I pump off the rainwater, I will be losing some heat, but not much. The main thing is to keep the top surface from endlessly evaporating water once the rain stops. This is the major heat loss my pool has right now during the late fall.

  12. (previous poster)
    Iain (& others), what's your take (gut feel) on how a pool enclosure would do in comparison to your 2" thick top-cover array ? (10% as efficient, 50%, 80% ?, .. etc.)

    We’re planning a new pool-build … here’s some experience/info:
    We currently have an auto-cover pool in the SF-East-Bay valley. We use solar panels (w/ pump) only in May & Sep/Oct (we have made it to Thanksgiving a few times at a price). Our panels face south.
    We’ll be building a home & pool in a similar location, but panels will face southwest. We would like to use your pool-shell insulating technique, and of course the auto-cover ... but not sure about how to add top-insulation without making it difficult to put on/off (spouse, kids), which is why I thought that we should also consider enclosures. Obviously enclosures are not the prettiest things to have in a backyard, but there are low-profile ones that can be taken on/off (without permanent tracks). For example: The idea would be to put them on in Oct and take them off in April.

    What we've learned over 6 years at our current site is that in Oct/Nov we lose a lot of heat into the night-air. Also, when swimming (after-work/after-dark) the air is uncomfortably cold in Nov. In the spring there are many windy days which seem to suck out heat off the top of the pool even though it's covered, and also make swimming less-enjoyable. We also go through periods without sun so the panels are not working to maintain the pool-temp.

    Things in the equation:
    - Fall: almost all days are sunny in Oct/Nov/Dec so the temperature inside of the dome would rise during the day. Not sure how much loss would happen at night in comparison to your insulating technique, and in comparison to only an auto-cover.
    - Spring: we’d like to swim in April (maybe even March). Nights start being less-cold in mid-March. Many days are windy though, some are Arctic-winds. It seems like wind-breaking might be important ... not only for helping with preserving some temp but also enjoyment of a swim. In general, spring has more rainy/cloudy days … sometimes in blocks of 2-4 days.
    - The low-profile enclosure has 2.5’ of air-space. Not sure how significant that is. I could see a door into this space only being open during entry/exit into the pool. Standard heights range up to 10’.
    Thanks in advance on thoughts,

    Enclosure links:

    1. Sorry about this, this post got caught by Blogger's spam detector because of the links.

      First, you need a cover year round, otherwise evaporation will cool the pool at an incredible rate. If evaporation is not fixed, nothing else matters. It looks like you have a good solution for this.

      That aquashield cover looks good! It looks like it would keep the rain off an automatic cover during the spring, which is the biggest problem that we have. When our automatic cover gets rain on it, the subsequent evaporation sucks huge amounts of heat out of the pool, even when I pump the rain off in a few hours. Evaporating 1/8 inch water off our 46x18 foot pool takes over 500,000 BTUs. That's as much as heating 2.7 inches of rainfall and then pumping it off.

      It doesn't look like the aquashield has a lot of R value. I'd guess something like R-1 (, and I'd guess R-0.75 for the aquamatic cover on the pool itself. If you are in the East Bay, your Dec-Jan probably averages 48 degrees like mine. So an 800 square-foot pool would lose 350,000 BTU/day through the two covers. That's not bad at all, especially since the aquashield is clear. The two covers together will probably transmit, even when dirty, at least 33% of the incoming solar radiation to the water... that's 280,000 BTU/day. Making up 70,000 BTU/day, plus bottom losses, plus whatever you lose when you go swimming for an hour or two a day, shouldn't be too hard.

  13. Ian, What cost benefits have you noticed from insulating your pool? I am an architect working with a client in Ohio on an energy efficient addition to their home which includes a lap pool.

  14. Anonymous (Ohio),

    The simple analysis, mostly at

    For four months out of the year, I would need to heat the pool if I didn't have the bottom insulated. During those 4 months, I lose 450k BTU/day more out the bottom without insulation. I'm being charged $1.36 per 100k BTUs, so if I ran the heater to replace that heat it would cost $560/year. If gas prices shoot up, which I think is likely, that savings will go up.

    The cost of insulating my pool was about $8000: $4300 materials and $3500 labor. That number was high due to mistakes I made elsewhere. If I'd put the pipes inside the main insulation envelope as is done in Australia, material and labor would have been lower. If I'd been less concerned about settling because I didn't have horizontal steps in the dirt, the labor costs would have been lower. If I'd nailed the insulation to the dirt rather than gluing it, I would have saved $1500 in polyurethane.

    So was $8000 worth saving $560/year? Maybe. I'm glad I did it, because I gained other benefits (experience) besides just saving money. If it was straight economics, given what I know now, I'd find a contractor who was happy to nail the insulation to the soil, and ensure that the dig had only one horizontal surface (the bottom), and be done.


  15. Hi,
    You talk about savings and cost to heat pool?
    what water temperatures in pool and outside air temperature are achieved..? Obviously ground temperature have a factor to contemplate installing insulation in the pool . Some companies I have spoken to say 90% of heat loss is thru surface so a cover is only needed. So now I am confused... I am thinking of insulating my new pool. And using solar panels to heat in warmer months and then switch to heat pump! Where we live in summer average daytime temp is say 32 c night 18c ... Winter day time 17c and night you think I would benefit from installing insulation.. I would like to have my pool Say 28c all year round? Any info would be great to this layman....

    1. If you don't have a pool cover, nearly all of your heat loss will be through the top, via evaporation. Way over 90%.

      Even if you have a cover, if it rains on your cover, you have to be good about pumping that rainwater off, or you will lose a ton of heat through evaporation.

      Once you have a dry cover, loss through the top might be less than loss through the bottom for most of the winter. You suggested your average outside temperature is 11 C = 52 F in winter. Your average ground temperature is probably 13 C = 55 F. You have a lot more area to the ground that to the surface, and the concrete bottom of the pool is about R-0.5 compared to R-0.5 to R-1 for the top. So you'll probably lose more out the bottom unless you insulate it.

      Should you insulate? Like I suggested in my last reply, find a contractor happy to nail the insulation to the soil, ensure the bottom is the only horizontal surface in the dig, and be done. You didn't mention which country you are in. If it's UK or Australia, you might be able to find such a contractor. Otherwise, maybe not.