topics · updated 2026-07-19
Designing a miniature Double Play experiment
confidence: medium volatility: warm verified: 2026-07-19fresh
Can a Double Play-style passive water thermosiphon be shrunk to a 2.5 cubic-foot outdoor cat shelter in South Texas? Scaling physics says use a small pump, not passive circulation. Climate physics says the region's humidity may weaken or even reverse the archive's core night-sky cooling mechanism in summer -- exactly when a cat needs it most. The honest answer is: build it as an instrumented, monitored experiment, not as a cat's sole line of defense against extreme heat.
Prompted directly by a request: design an experiment to build a small (2.5 ft³) Double Play-style structure in OSB with PEX plumbing to heat/cool an outdoor cat shelter in South Texas, instrumented with a Raspberry Pi Zero. This page is the synthesis across five research angles; the actual step-by-step build+test protocol is the companion output artifact.
Site correction (2026-07-19): the user identified the actual build site via a hyperlocal weather station — San Antonio, TX (29.41°N, 98.48°W), not the Rio Grande Valley the initial climate research was based on. The two are ~220 miles apart with meaningfully different climates (San Antonio: average August high ~97°F, RH swinging ~83% at 6am to ~48% at 3pm, occasional winter freezes; RGV: even hotter/more humid on average, freezes almost never happen). See Night-sky radiative cooling § Climate dependency for the full comparison. The San Antonio station’s own overnight humidity readings (~78–91% on its most recent sample day) suggest the humidity-driven radiative-cooling concern still applies at this specific site, just somewhat less severely than the RGV data alone implied.
The honest answer, up front
This is a legitimate, interesting experiment — but it should be built and tested as an experiment, not deployed as a cat’s only shelter from the start. Two findings from this research round should change the design from a naive scale-down of the archive’s New Mexico-proven system:
- Passive thermosiphon circulation is marginal to non-viable at this scale — use a small 12V pump instead, which also matches how Zomeworks’ own production Double Play already works.
- Night-sky radiative cooling is significantly weakened by humidity, and one study found it can run several degrees above ambient in humid subtropical conditions comparable to a South Texas summer — precisely the season a cat most needs cooling. This is not independently re-verified from primary literature and should be treated as a strong caution rather than settled fact, but it means the archive’s core mechanism cannot be assumed to work here the way it does in New Mexico.
What this means for the design
Lead with proven cat-shelter fundamentals, and treat the Double Play mechanism as an added, monitored, and initially unproven layer:
- Follow established shelter dimensions and layout: instrumented but sized per Bideawee/Alley Cat Allies precedent — roughly 24”×24”×18” interior for a single cat (larger than the strict minimum, since 2.5 ft³ ≈ 4,320 in³ comfortably allows it), a single off-center 5–6” entrance, elevated off the ground, shaded, never on concrete or asphalt.
- Because PetMD’s thresholds put heatstroke risk at 104°F+ body temperature and Texas-specific guidance flags ambient ≥90°F with no airflow as dangerous, summer ventilation and shade are non-negotiable design elements, independent of whether the thermosiphon experiment works. A sealed, insulated box that traps heat would be actively dangerous in a South Texas summer if the cooling mechanism underperforms.
- Build the OSB shell from Exterior-rated panels, not Exposure 1 (West Fraser) — sealant cannot compensate for the wrong OSB grade under continuous outdoor exposure.
- Any paint, sealant, or adhesive the cat could contact must be a named animal-safe product, fully cured before the cat has access — cure times range from 24 hours to 30 days depending on product (My Chemical-Free House). Note the specific caution that oil-based sealants raise off-gassing concerns on surfaces that reach elevated temperature — directly relevant to a box that is, by design, a solar heat sink.
- Balance insulation with a deliberate vent path — a sealed, insulated box without airflow risks condensation and mold in a humid climate even if it solves the thermal problem (High Country Spray Foam).
- Circulate the water loop with a small 12V DC pump (~16W, sub-1-GPM range) rather than relying on passive thermosiphon, per the scaling analysis.
- Instrument it per the Pi Zero monitoring reference — interior temp/humidity, water temp, and a solar-irradiance proxy, logged unattended on solar/battery power — and collect at least one full summer of data before trusting the mechanism, exactly as Stickney and Baer validated night-sky cooling with instrumented plates before building it into Cool Cell.
What would make this a genuine contribution back to the archive
The archive has never tested its core mechanism outside arid New Mexico, and has no small-scale (sub-building) precedent at all. A rigorously logged South Texas prototype — even a modest one — would be a real, citable data point: does night-sky radiative cooling of a small water mass do anything useful in Gulf Coast humidity, or does it wash out entirely? That is presently an open, evidence-thin question this research round could only bound with proxy data (Hong Kong vs. Stanford humidity comparison, RGV climate normals), not answer directly.
Safety note
This page and its companion experiment protocol are written for a hobbyist building a genuine research prototype — not as installation instructions for an animal’s sole source of shelter. Given the real uncertainty about whether radiative cooling helps or actively hurts in this climate, any prototype should be validated with data (and ideally veterinary or experienced rescue-org input) before a cat’s wellbeing depends on it, and should always be paired with a conventional, already-proven shaded/insulated/ventilated shelter as the fallback.