Thursday, December 18, 2008

Insulating the pool

Since this is my first posting about the pool, let me show you around a bit.  South is to the left.  You can see all along the south side of the pool there is a two-foot-wide shelf.  The water over that shelf will be 12 inches deep in the shallow end and 18 inches deep in the deep end.  In the middle of the south side you can see where the hot tub will go.  At the far end (west), you can see the shelf that will eventually support the automatic cover vault.  To the left of the pool, at the far end, you can see the pump vault that will hold the pumps below ground, behind baffles, which should make them completely silent.  The top of the wooden form will be about three inches below the coping around the pool, so you can see that the coping is about 18 inches above grade on the near side.  That coping will be about 16 inches wide, and will form a sort of bench seat most of the way around the pool.  At the far end, the grade level will be raised to match the coping height, which because of the slope of the back yard will only be about six inches.

The blue stuff you are looking at is the 2 inch thick Dow Styrofoam Highload 40 insulation, most of which is glued (yes glued, with polyurethane foam) to the soil behind it.  On the bottom of the pool, the insulation sits on 4 inches of crushed drain rock, which sits on a geotextile fabric membrane.  The next thing to go into the pit is the plumbing and rebar grid, and after that we shoot gunite.

Heat losses for most pools are dominated by evaporation. We're going to be getting a safety cover, and one of the side effects of these covers is that they are close to vapor-tight. In the summer months, we'll have the cover off most days, but in the spring and late fall we'll keep it closed for most of the day and only open it to go swimming. As a result, I'm expecting evaporative losses to be quite small -- perhaps a half inch a month or so. On our 18x48 foot pool, that's 270 gallons/month. Multiply by 2270 kJ/kg (water's heat of vaporization), and that's 72,000 BTU/day.  [Update: this evaporation estimate was dead on.]

Direct heat loss through the cover will be the largest remaining heat loss.  The 24-hour average outside temperature in spring is around 60 degrees, and the safety cover will be something like R-1, so loss from a 85 degree pool would be 520,000 BTU/day.  During the early spring and late fall, we'll probably lower this loss by putting a bubble-type cover over the safety cover.  (I wasn't able to find an automatic safety cover which insulates.)  The combination of the two covers will be around R-3, so losses will be around 180,000 BTU/day.

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 of warm water headed towards the San Francisco Bay.  [Update: water table never gets to the bottom of the pool.]

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.

The blue Styrofoam has an R value of R-10. It is designed to deflect 5% at 40 psi, and is rated for continuous (dead) load of 13.3 psi. 10.5 feet of water plus 8 inches of concrete will be 5.25 psi, well within the dead load rating of the Styrofoam. The stuff should compress 0.33 mm when the pool is filled, which I don't think will cause cracking anywhere (thermal expansion is probably more than that).

The insulation changes the bottom of the pool from R-2 to R-12, so now I expect to lose 90,000 BTU/day, which is a savings of 447,000 BTU/day.  To put that in perspective, on an average day my solar panels will deliver around 40,000 BTU each.  I think I can squeeze 12 of them up onto the roof.  The insulation is saving about as much heat flow as the panels put in!

To attach the insulation to bare dirt on the walls and shelves, I glued them on with Tap Plastics X-30 two-part expanding polyurethane foam, which I sprayed on with a pair of Wagner power sprayers.  The trick here was to hold the panels firmly (with a few hundred pounds of force) against the dirt while the polyurethane expanded, which takes about 30 minutes.  Once we did that the panels really stuck on well.

Note: I wore complete face covering, goggles, and breathed through an activated charcoal filter to take out the volatile organics.  It was impossible to get the crew to wear the same protective gear.  In the end, everything that was exposed got coated in a fine mist of polyurethane.  The warning label says skin will develop an allergy to the stuff with repeated exposure.  If you try this yourself, be careful, and be careful with your crew!

The soil at the bottom of the pool is firm clay, covered with filter fabric, covered with drain rock.  Initially I tried gluing the insulation to the drain rock, but this doesn't work well.  The insulation does not sit flat against the drain rock, and it rocks around a bit.  Worse still, the polyurethane is quite springy, and was deflecting about a quarter inch under my body weight.  This made me realize that the combinations of shelves, drain rock, and insulation is very bad, because the insulation on drain rock might settle, which would cause the pool to hang from the shelves, which would cause the gunite shell to crack.  If I had it to do over again, I would eliminate the soil shelves, which are only there to reduce the cost of the gunite by $1500 or so.

Instead, I ripped up all the bottom insulation that the crews had installed and reinstalled it myself.  I had already compacted the soil, first with the Bobcat, then with my feet (the crew were actually laughing at me), then with a heavy roller.  Then I compacted the drain rock with the heavy roller.  Then I cut the insulation into 15" x 24" squares and hand placed each one, tweaking the rock placement and vibrating the panel so that the rocks settled into a configuration which was flat for each panel.  That took about four partial days, and only got finished because Martha got into the pit with me.  I read later that a skilled laborer with one assistant is expected to place 100 square feet of under-slab insulation per hour, so I was about two times slower than that.

I'm still paranoid about settling, so I'm going to have the south and west walls of the pool done with #5 40 ksi vertical rebar on 6 inch centers.  #5 is strong enough to actually support the entire pool weight (450,000 pounds filled) on the soil shelves, should the shelves not creep under that load.  I also put a 1 foot chamfer on the edge of the shelf under the cover vault.  The fillet of gunite that fills that chamfer, along with the rebar, will spread the torque from the pool wall hanging on that shelf.

[Update: I ended up with a mix of #3 40 ksi and #4 60 ksi rebar, with about half the strength necessary to hold up the whole pool. I figured I only had to hold up one end, and the rebar guy objected strongly to #5 -- he bent most of the rebar with his right knee!]

Another issue is the bond strength between the XPS insulation and the gunite.  I've since built the pump vault, which is insulated as well, and the XPS/concrete bond there is perhaps only 1 to 3 pounds per square inch shear strength.  If I assume 1 psi for the pool shell, then it will take about 140,000 pounds of shear, which is substantially less than the 450,000 pound filled weight of the pool.  This shear strength is comparable to the shear strength of the polyurethane bond to the soil.  This pool shell will be sitting on its bottom.  The polyurethane is substantially more springy than the polystyrene, and I think it will deflect the 0.3mm in shear that will happen when we fill the pool.

I did not predict the cost well.  I used 130 2 foot by 8 foot panels, with a small amount left over that I am using to insulate the pool piping.  These cost $2800.  I used 30 gallons of the X-30, which cost me about $1500.  It took me and a crew of three guys four days to glue on the side panels, and I think that labor cost me $2500.  The bottom I did myself, but the labor for that would probably have been another $1000 or so.

You can try to compare that cost to the cost of the panel array.  The problem with the comparison is that the panels deliver the most heat when its warm, when you don't need it.  The second problem is that the panel array is limited by the size of my roof, so I can't just spend an arbitrarily large sum on panels.

That said, standard FAFCO-type panels are something like $300 each, and produce 25,000 BTU/day.  To generate the same amount of heat saved, you'd need 18 panels, which would cost $5400.  However, the panels don't really work at all in November and April, when you need the heat most.  The panels I'm using are SunEarth EP40s, which are glazed and insulated copper panels, and cost around $1100 each.  These will deliver around 35,000 BTU/day, and will work in November and April.  But I'd need 13 to match the insulation's savings.  And that would cost $14,000!  So I'm pretty confident the insulation will be a win.

I'm not quite done with the insulation.  You can see a pile of pink insulation at the southeast corner of the pool.  This is standard XPS, and it will be going around all the pool piping.  I tried to find XPS pipe insulation, but the stuff is hideously expensive because it is all custom cut from large billets, usually for municipalities who are insulating their sewer pipes against frost.  So instead I'm gluing together a little box of 2" XPS around all my pipe runs.  Because the pool pump will be running 24 hours a day, the heat losses from the pipes can actually be comparable to the pool shell itself if not insulated.  If you are building a pool, you might want to consider insulating your pipe runs even if you don't insulate the shell, because insulated pipe runs don't suffer any of the structural and installation issues that come with the shell.

Side note: I had a hell of a time finding a Highload 40 distributor. I ended up calling Dow directly, who referred me to White Cap Insulation in San Francisco, who sold me the stuff for $21.58 per 2" x 2' x 8' sheet. Note that standard 20 psi extruded polystyrene has a dead load rating of over 6 psi, so it would have been technically okay for my 5.25 psi of dead load. If I wanted to save around $600, I would probably have gone for the thinner stuff. It would also be easier to buy. I'm hoping that the extra-sturdy foam is buying me a little margin, which is nice to have because I don't have examples of other pools that have been built this way.  [Update: somehow I miscalculated the amount of gunite in the pool, and it's a bit larger than my estimate here.  So, I'm glad I have lots of extra margin on the load carrying capacity of the insulation.]

Side note two: the space down at the bottom of the deep end experiences really wild temperature swings right now.  Because it is down low, it is radiatively coupled to the sky and not much else.  When the sun is shining down there, it can get well over 100 degrees.  As soon as the sun goes down, the temperature plummets, and because the pool is a depression, chilled air tends to stay down there.

Side note three: If you read this post and insulate your pool, or know of a pool that's been insulated, you might leave a comment so that there is some repository of success stories on the internet for this idea. And please please post if there has been a problem with an insulated pool.