CoolSky FAQ
CoolSky FAQ
CoolSky is the name the Solar Fools community (solarfool.org) uses for the passive heating-and-cooling lineage that Steve Baer and Zomeworks developed, not a single current product. The community is led by people who worked with Baer directly, including Jose Baer, Bruce W. Davis, and Paul Hildebrandt, and the lineage runs from the DrumWall through the Cool Cell to Double Play. This FAQ answers only from the compiled knowledge base, states conditions with every measurement, and says "The KB does not establish this" wherever the archive is silent.
For the curious
What is CoolSky?
CoolSky is the Solar Fools name for Baer's Cool Cell and radiative sky cooling system, so it labels a lineage rather than one product (Solar Fools). The lineage starts with the DrumWall, Baer's passive solar heat-storage wall of water-filled drums, which the archive describes as predating the Cool Cell and Double Play by three decades. The Cool Cell is Zomeworks' passive climate-control system, with water reservoirs in the ceiling and a roof radiator and absorber that radiate heat to the night sky. The archive dates the Baer House DrumWall to 1971, while a 1981 manual dates it to 1972, so the sources disagree by a year (DrumWall).
Does it work without a compressor?
In summer, the Cool Cell brochure describes cooling with no electricity, no refrigerants and no compressor: ceiling water absorbs heat by day and the roof radiator and absorber cool it by night, with thermosiphon circulation (no pump) recharging the reservoir overnight (Cool Cell). Winter is different, because one of its modes uses a low-power pump to circulate water through the roof for solar gain, while two others use a south-wall absorber by natural thermosiphon with no pump (Cool Cell). The archive records the same asymmetry in the brochure's own words: solar gain is less reliable than radiant cooling, so winter needs pumps, louvers and more storage (Night-sky radiative cooling). Compressor-free also does not mean fan-free in every configuration, since the productized Double Play lists an optional fan (Double Play).
How comfortable is it, and where has it been measured?
The only built-and-measured cooling case in this set is the 2004 Bruce Davis Studio in Albuquerque, a 236 sq ft converted-garage space, where the progress report's temperature chart (July 22 to August 6, 2004) shows comfort through days with 95F highs, with a gas water heater adding load (Bruce Davis Studio). A 2025 summary reports the studio held 73.5-78F while ambient ranged 58-94.5F, a slightly different maximum than the report's 95F (Double Play). That is one studio over about 16 days, and Davis's "great satisfaction" is the owner's report rather than a measurement (Bruce Davis Studio). A separate 2006 New Mexico field study on unglazed pool collectors projected 25-89% power savings across 11 climate zones, which is a statewide projection and not a measured comfort result for any one building (Independent validation). Community-reported; no published test data in this KB: solarfool.org cites "100% of the cooling and 95% of the heating" and 65-75F indoors at 110-120F outside (Solar Fools). The KB does not establish this kind of multi-season monitoring or an occupant comfort survey for any Cool Cell or Double Play building.
Will it work in my climate?
The KB does not establish performance for any specific non-arid location, so this answer sets out conditions and does not predict your site. The vendor says Cool Cell systems work best in dramatic climates, where cool nights divide hot days and winter is sunny (Cool Cell). The archive notes that its measurements were taken under clear, dry New Mexico skies and describes humidity and cloud dependence as "severe, not marginal", so New Mexico results should not be extrapolated (Night-sky radiative cooling). The same article cites net outgoing radiation of 98.9 W/m2 under clear sky against 15.1 W/m2 overcast, and a 2019 study from a search snippet that it says should be independently re-verified, with a surface 6-7C above ambient in humid Hong Kong against 4.9C below in dry Stanford (Night-sky radiative cooling). Its own working inference is that night-sky cooling may be only supplemental, or even reversed, in humid subtropical peak cooling months (Night-sky radiative cooling).
Is it for retrofit or new construction?
The archive documents both kinds of precedent but no general procedure. Retrofits include the Davis Studio, a converted half-garage that a 2004 shareholders letter calls a "two-month-old retrofit" (Bruce Davis Studio), and a 20 ft x 8 ft cargo container fitted with roof water bladders, though sources disagree on bladders versus an interior reservoir (Zomeworks container prototype). Purpose-built test structures include the 1988 Andy Shack, a 10 ft x 10 ft test cabin on Zomeworks grounds (Double Play). The Cool Cell brochure calls warehouses and storage buildings the optimal application, and it describes roof-integrated designs without saying they are for new construction (Cool Cell). The KB does not establish a retrofit procedure, a structural-load check for overhead water, or a residential retrofit at scale.
How does cost compare with a heat pump?
The KB does not establish this. The 2006 New Mexico study showed 25-89% power savings but did not model installed system costs against modern heat pumps, and the archive says a 2026 cost comparison is missing (Why not scaled). The nearest figure is a vendor estimate from the c.2002 Cool Cell brochure for a storage building allowed to fluctuate between 40F and 95F: about 10 kWh/ft2 saved in the cooling season and 25 kWh/ft2 in the heating season, or $3.50/ft2 per year at $0.10/kWh (Cool Cell). That is an operating-savings estimate for a warehouse case, not an installed cost and not a heat-pump comparison, and the brochure also expects the radiator and absorber panels to last about 10 years in New Mexico (Cool Cell).
What maintenance, freezing, and leak risks are documented?
The Cool Cell brochure names water's high heat of fusion and expansion on freezing as the stated downside, managed by drain-back in winter rather than antifreeze, and a south-wall absorber that cannot drain back must be freeze-tolerant (Cool Cell). On leaks, the DrumWall's steel drums developed them, and a 2009 slideshow quote in the archive says plastic drums do not, with the Baer House drums later updated to white plastic (DrumWall). The same article calls drum lifetime the "primary engineering challenge" and says the long-term effect of anti-corrosion additives was not established by 1975 (DrumWall). Cool Cell's experimental "boogie" valves also leaked slowly (Cool Cell). The KB does not establish a maintenance schedule, inspection interval, water-treatment practice or leak-incidence rate, so none should be inferred.
What happens during cloudy weather, a heat wave, or a cold spell?
The documented dependency is on sky conditions: net outgoing radiation was 98.9 W/m2 under clear sky against 15.1 W/m2 overcast at the same temperature, a 6.5x difference (Night-sky radiative cooling). The brochure says close regulation, as for houses, requires radiator panels as large as the roof to be effective in marginal weather (Cool Cell). The only hot-spell data is the 2004 Davis Studio, which held 73.5-78F with ambient near 95F over about 16 days; that is a single small studio and not a heat-wave test (Bruce Davis Studio). For cold weather, the brochure warns the winter sun can disappear for days, so winter storage must be greater (Night-sky radiative cooling). The KB does not establish performance in an actual heat wave, a multi-day cloudy spell or a cold snap, so resilience claims remain unverified.
Where can I see one?
The KB does not establish a publicly confirmed site to visit. Zomeworks' research and development page, as the archive records it, describes two Double Play test outbuildings, one for sale and one used for meetings and tours, which is not an open invitation or a visit procedure (Zomeworks container prototype). The container is a prototype described as "to be tested in Albuquerque", and the archive says it does not yet settle which configuration was built (Zomeworks container prototype). A 2004 letter says Davis showed his studio to clients and prospects, which is a past private showing and not a public opening (Bruce Davis Studio). Photographs of the studio and the container are in the archive, at those two articles and in the archive photographs collection.
Can I build one, and are the patents expired?
The KB does not establish the expiration or maintenance-fee status of any granted US patent in the portfolio, so this FAQ makes no determination. The archive lists 21 Baer and Harrison patents and applications from 1969 to 2010, including US 5,513,696 (passive temperature regulating, granted April 1996) and US 6,357,512 B1 (passive heating and cooling, granted March 2002); it records that US 5,609,200 lapsed in 2001 and that one late application was never granted while another's grant status is unknown (Zomeworks patents). The Cool Cell article calls the 6,357,512 patent the 2001 patent, so the sources differ on that date (Cool Cell). The brochure describes connectors that push on by hand, but that is a vendor simplicity claim and not a build guide, and no article provides build instructions or a safety or code basis. This FAQ is not legal advice and does not determine current patent, code or permit status.
Why is this not everywhere?
The archive offers eight explanations and treats the question as open: it says it documents the problem thoroughly but does not resolve it (Why not scaled). They include industry inertia and split incentives between builders and occupants, no recurring revenue from passive systems, absence from canonical solar histories, a narrow institutional definition of passive cooling, and pilot projects with no route to adoption (Why not scaled). One 2009 Baer slideshow framing is that sellers of fuel regard periods of self-reliance as "epidemics", which is his analysis and not a demonstrated cause (Why not scaled). The archive's validation article calls non-adoption a market failure and not a technical failure, which is that article's own interpretation, and cost competitiveness remains an open question (Independent validation).
Technical questions
How did the system evolve?
The archive traces the lineage from the DrumWall, which began with water-filled barrels at Drop City around 1968 and reached the Baer House in 1971, to the Cool Cell, which was reborn in the late 1980s for remote telecom battery arrays (DrumWall; Double Play). The Cool Cell then moved to buildings with the 1995 corrugated-roof patent, switched to rooftop radiators in August 1999, and produced the c.2002 architectural brochure (Cool Cell). Double Play followed with the 2003-04 Davis Studio, and the archive attributes its origin differently: the Zomeworks page credits Norman Saunders, while a 2025 paper traces it to Baer's stock pond melter (Double Play). The 2026 container is a distinct later prototype, and the archive notes that sources disagree on whether it uses roof bladders or an inside reservoir (Zomeworks container prototype).
How do water-on-top, water-on-side, and louvered systems differ?
The archive documents Hay's Skytherm, with water bags on a flat roof under movable insulation, where the roof water is itself the radiator (Night-sky radiative cooling). The Cool Cell instead keeps water in the ceiling and connects it by thermosiphon to a separate roof radiator and absorber, so it works in a conventionally roofed building (Cool Cell). On the side, the DrumWall is a south-facing wall of drums behind glazing whose heat flow is uncontrolled, while the Davis Studio used a vertical wall array to avoid leaks over his drawings and Baer preferred horizontal sub-ceiling arrays (DrumWall; Double Play). Aluminum louvers below the ceiling mass make heat flow controllable, and the brochure says opening them doubles or triples the radiant flux (Cool Cell). The KB does not establish a measured comparison between vertical and horizontal arrays, so the Baer and Davis preferences are documented design choices and not performance findings.
What sizing and nightly heat-rejection numbers exist?
These figures come from different systems and conditions and are not sizing recommendations. Heated-versus-unheated plate experiments on clear, dry New Mexico nights showed night radiation can exceed 9.5 W/ft2 (Cool Cell), and Daniels (1964) gave 10 to 35 BTU/ft2/hr in hot clear climates, which the archive converts to 2.9-10.3 W/ft2 (Night-sky radiative cooling). Bliss's 1961 Arizona house used a 280 ft2 black cloth radiator and stored 120,000 BTU of "cold" in a rock pile, about 2 tons of refrigeration equivalent (Night-sky radiative cooling). The 2006 New Mexico field study used ten radiator panels across 11 climate zones and projected 25-89% power savings (Independent validation). The only built Double Play case is the Davis Studio, with 127 sq ft of radiator for a 236 sq ft studio and 200 gal of storage, a ratio that describes that installation alone (Bruce Davis Studio). The KB does not establish a radiator-area or storage-per-floor-area rule for a house, nor a measured nightly heat rejection for a Cool Cell or Double Play building.
What structural and plumbing details are documented?
The Sol-Mod drawings are ingested raw material that has not yet been compiled into a FAQ-eligible article, so this FAQ does not reproduce their specifications; the nearest compiled treatment is the Double Play article, which notes that the vertical array was developed further in the Sol-Mod's vertical water Trombe wall. At the level the archive compiles, the Davis Studio used vertical 4-in PVC storage pipes and 12 polypropylene radiator mats on the garage roof, and the sources disagree on the pipe count, 42 in the progress report against 38 in a 2025 paper (Bruce Davis Studio; Double Play). The Andy Shack used a sub-ceiling array of 8-in water-filled PVC pipes, a black rooftop radiator and aluminum louvers (Double Play). The KB does not establish flow rates, pump specifications, valve schedules or structural support and loading details for overhead water.
What has failed or changed over time?
Documented failures include the DrumWall's steel drums, which developed leaks, and the Cool Cell's experimental boogie valves, which leaked slowly and once lost a whole day's heat in a night (DrumWall; Cool Cell). The long-term effect of anti-corrosion additives in the drums was also not established by 1975 (DrumWall). The design changes that followed were separate decisions: the Baer House moved from steel to white plastic drums around 1991, and the Cool Cell moved from wall radiators to rooftop radiators in August 1999 and to ceiling tanks (DrumWall; Cool Cell). Date differences such as 1971 against 1972 for the Baer House are source disagreements and not failures. The KB does not establish a failure log or post-occupancy reliability record for any Cool Cell or Double Play building.
What are the operating modes and controls?
The c.2002 Cool Cell brochure describes five modes: summer day, with the ceiling water as a heat sink; summer night, when the roof radiators cool and thermosiphon recharges the reservoir; winter day, when a low-power pump circulates water through the roof; and two south-wall absorber modes that use thermosiphon with no pump (Cool Cell). Aluminum louvers control heat flow, and the brochure describes the controls as simple (Cool Cell). The boogie valves were floating plastic check valves used in earlier di-thermal-wall experiments and not in the final ceiling-tank design (Cool Cell). These are brochure descriptions that depend on climate, so they are not a universal control sequence, and the KB does not establish a louver schedule, set points or automation logic.
What is the code and permitting status?
The KB does not establish this: nothing in the compiled articles records jurisdiction-specific approval, a permit pathway, plumbing or structural code compliance, or an inspection history for any Cool Cell, Double Play or DrumWall installation. The only legal material is patent history, which is not code or permit approval (Zomeworks patents). Overhead water load, freeze protection and jurisdictional differences are therefore open unknowns and not settled points. This FAQ is not legal advice and does not determine current patent, code or permit status.