Probably... but a cost/benefit analysis suggests that it might not be feasible.

Yesterday, in the process of getting 220V power up to where the compressors* will sit, I learned a bit more about the construction of this house. Carlos (the engineer/contractor who built the house, and still owns the two bottom floors) went somewhat overboard in its construction, by local standards at least. The roof is about fourteen inches thick, made of bricks set edgewise on the inside, and outside covered with concrete painted a painfully bright white** to reflect heat. On a sunny day you really need sunglasses to be up there. The ceiling inside is 14 feet above the floor at the highest point of the dome. I would guess that not a lot of heat is getting through that roof, but I have no real data to back up that assertion.

I do have data to estimate what my air conditioning costs will run me, however. Making the hopefully not unwarranted assumption that my A/C will run for four hours per day in April and May only, 36,000 BTU with a SEER of 10.3, at a cost of about 21 cents/kWh***, my annual operating cost should run about $180.

Insulating the roof would not obviate the need for the A/C, because we have discovered to our dismay that SWMBO simply cannot tolerate high heat. If the temperature in the house is in excess of 85 degrees F (29C) she is literally incapacitated. Not just uncomfortable, but completely unable to function. If insulating the roof saved (arbitrarily) 40% on my A/C operating costs, it would take decades to recover the cost, and I don't know how well the foam would stand up under the intense sun. Probably they have that part figured out, though...

So, for better or worse, the A/C contractor will be here later today to do the installation. A pair of 18,000 BTU mini-splits, heat-pump systems for cooling and heating, operating independently in each room. We will keep the inside temperature between 75-80 degrees to minimize usage, we're both comfortable at that temperature. About $1800 including installation.

Let the adventure begin!

tanstaafl.

*Two 18,000 BTU (1.5 Ton) units, one for the bedroom, one for the kitchen/dining room.

**When flying over Texas desert on the way down here one time, I was absolutely astonished to see whole subdivisions, hundreds (thousands?) of houses, all lined up in rows with black roofs.

***Electricity pricing here is amazingly complicated. Our electrical usage is low enough that most of it is at the "Basico" rate of about 6 cents per kWh, some of it at the "Intermedio" rate of about 10 cents. But on the months that the A/C runs, most of that additional usage will fall into the "Exceso" rate, of about 21 cents per kWh. If I ran the A/C all year long, I would fall into the dreaded "DAC" rate ("Domestico Alto Consunción") in which every single kWh I used would be billed at the Exceso rate of 21 cents per kWh. My neighbors across the street with a big house and swimming pool are paying eight times as much as I am per year for electricity, but are probably using only 50% more kWh. They're on the DAC rate, I'm not. I have made a fantastically complex spreadsheet that accommodates all the variables involved in projecting an electric bill: the three tiered rate structure, the DAC penalty, the cost effectiveness of solar panels with amortization times, sales tax, projected rate increases based on past performance, pesos vs dollars, number of meters, months per billing cycle, wattage of appliances, etc. Interestingly (to me, at least, probably not to you guys) our computers (my PC, SWMBO's iMac) are using exactly as much electricity as the rest of the house combined. After 10 months of operation, the computers have consumed 1167 kWh, the rest of the house is at 1166. The A/C will be split across the two meters so the "meter parity" will continue.

db