Experiment design: miniature Double Play cat shelter for South Texas

The original brief: design an experiment to build a small (~2.5 ft³) Double Play-style structure in OSB with PEX plumbing, providing cooled/warmed interior space for an outdoor cat in South Texas, instrumented with a Raspberry Pi Zero to record temperature, humidity, solar radiation, and performance.

Site: San Antonio, TX (29.41°N, 98.48°W), per a hyperlocal weather station near the build site the user identified — not the Rio Grande Valley the initial climate research assumed. See Designing a miniature Double Play experiment for the correction; the humidity-driven radiative-cooling caution below still applies, just somewhat less severely than the RGV data alone implied.

Safety notice — read this first

This is a genuine open research question: does night-sky radiative cooling do anything useful at doghouse scale, in a humid climate, at all? The archive's own research round found real reason to doubt it — one study suggests a radiative-cooling surface can run warmer than ambient in humid subtropical air, which would make a sealed experimental box actively dangerous in South Texas summer heat if trusted uncritically. Do not make this prototype a cat's sole shelter. Build it as an instrumented add-on next to (or as an upgrade of) an already-proven shaded, insulated, ventilated shelter, collect at least one full summer of data, and only trust the mechanism once the data says to. If in doubt, consult a local vet or rescue organization (e.g. Alley Cat Allies) before deploying anything a real animal depends on.

1. Design decisions from research

2. Materials list

• Exterior-rated OSB (enough for a ~24"×24"×18" interior box with double-wall or single-wall + rigid foam insulation)
• 1–2" rigid foam board insulation (foil-faced for best performance)
• Named animal-safe exterior sealant/paint (e.g. ECOS Pet Dwelling Paint, or an equivalent 0-VOC exterior-rated product) — budget real cure time before assembly is "done"
• PEX tubing (largest diameter practical for the loop — favor low friction over compact routing) + push-fit fittings (no crimp tool needed for a one-off build)
• A small dark-painted coil or panel as the roof absorber/radiator (a flat-black-painted copper or PEX coil, sized to whatever roof area the box allows — expect only ~10-30W of radiative cooling power from a 1-3 ft² panel under ideal clear/dry conditions)
• A small 12V DC submersible pump (~12-16W, 60-130 GPH range) + a small solar panel/battery to run it, or share power with the monitoring rig below
• 1-2 gallon water reservoir (food-grade, opaque to limit algae growth)
• Elevation feet/blocks to raise the box off the ground

3. Construction sequence

1. Cut and dry-fit the OSB box first, unsealed, to confirm the 24"×24"×18" interior fits the plumbing and insulation layers without leaving the interior cramped below the target volume.
2. Seal all OSB edges and joints with exterior caulk before painting — edges are where OSB swells first.
3. Apply the animal-safe sealant/paint. Set the calendar for full cure before assembly continues — do not rush this step.
4. Install rigid foam insulation on interior walls, leaving the vent path (below) unobstructed.
5. Mount the roof absorber/radiator panel, plumb it via PEX (push-fit fittings) to the reservoir and pump, bleeding air from the loop carefully during fill — air locks are the most common failure mode in any thermosiphon-adjacent loop.
6. Cut and frame the entrance (5-6", off-center, midway height) and a separate small vent opening (screened, positioned to avoid direct rain ingress) elsewhere in the enclosure.
7. Add straw bedding (never fabric) once all sealants are fully cured and any residual odor is gone.
8. Elevate the finished box on blocks or rocks.

4. Instrumentation plan

Follow the full sensor/logging/power spec in Small-scale passive-thermal experiment monitoring. Summary:

• Interior temp + humidity: BME280 (I2C)
• Water temperature: DS18B20, waterproof-housing variant (1-Wire)
• Solar irradiance: BH1750 lux sensor with a correction factor (cheap/easy) or the open-hardware photodiode SolarSensor design (more accurate)
• Logging: Raspberry Pi Zero W, Raspberry Pi OS Lite, cron-scheduled Python script to CSV on the SD card — not a naive infinite loop
• Power: 8-10W solar panel + 6,600-10,000 mAh battery, TP4056 or MPPT charge controller, boost converter to 5V
• Enclosure: IP65/67 box for the electronics, separate from the cat's compartment, with cable glands and a vent plug for condensation control

5. Data collection protocol

1. Baseline period (2-4 weeks, pump and radiator installed but water loop optionally isolated): log interior temp/humidity, exterior ambient temp/humidity, and solar irradiance with the water system off, to characterize how the box behaves as passive insulation alone.
2. Active period (full summer, at minimum June-September): run the pump loop continuously, logging the same variables plus water temperature. Compare interior temperature against the baseline period and against unshielded ambient.
3. Compare against the climate literature's expectation: does interior temperature during clear nights show measurable cooling below ambient (as New Mexico data would predict), no measurable effect, or — the concerning case — periods where the water loop appears to add heat?
4. Use the KTXSANAN228 PWS dashboard as a live external reference — it's a real, continuously-logging station near the build site. Two uses: (a) sanity-check the Pi Zero's own ambient sensor readings against it after deployment, and (b) pull its historical daily/weekly/monthly data to characterize actual site conditions (overnight humidity, dew point, cloud-adjacent proxies) before committing to a full summer of testing, rather than relying only on regional normals.
5. Log qualitative cat behavior if a cat is actually present and using the shelter (approach/avoidance, time spent inside during peak heat) as a secondary, non-invasive signal — never as the primary safety mechanism.

6. Success / failure criteria

• Success: interior temperature during hot afternoons stays measurably lower than an unshaded control location, and the system shows no dangerous heat-trapping behavior (interior never exceeds safe ambient + a small margin during pump operation).
• Inconclusive: no measurable difference from passive shade/insulation alone — informative on its own (answers the open question), not a failure of the experiment.
• Stop condition: if logged interior temperatures during pump operation run above unshaded ambient on clear nights (the humid-radiative-reversal scenario), halt reliance on the mechanism immediately, ventilate the box fully, and fall back to a conventional shaded/insulated shelter while investigating.

7. Safety checklist before a cat has access

• All sealants/paints fully cured per manufacturer cure time (not just "dry to the touch")
• Entrance and vent both clear and screened
• Box elevated, shaded, not on concrete/asphalt
• Straw bedding in place, no fabric
• A conventional, already-proven shelter available as a fallback nearby
• At least one baseline data cycle completed before relying on the active system in extreme heat
• A plan to check on the shelter and the cat's condition regularly during the first deployed summer

Sources

See Designing a miniature Double Play experiment for the full research synthesis and citations.