Passive Shading and Low-Energy Ventilation Against Summer Overheating in Lightweight Military Shelters

Lightweight fabric shelters are the default habitation in humanitarian relief, disaster response and military deployment, and they overheat severely in summer, yet field evidence on low-energy countermeasures is almost absent for the temperate–continental climate of south-eastern Europe. Two identical heavy-canvas tents (12.96 m2, 25.3 m3) of a legacy military type were monitored simultaneously in Bucharest during August 2026 in seven paired 48 h configurations combining a standard camouflage net (≈40% transmittance) laid on or spaced 6 cm above the roof, a reflective roof foil, and mechanical extraction at nominal rates of 2–8 h−1. The unmodified tent reached 49.5 °C at an outdoor maximum of 32.6 °C and doubled the outdoor diurnal amplitude. Shading dominated; in paired comparison the net laid on the membrane lowered the daytime overheating by 4.1 K, the spaced, unventilated net was 5.6–7.4 K cooler than the ventilated bare tent, and the sensitivity of the overheating to irradiance fell from 1.1–1.9 K to 0.5–0.7 K per 100 W m−2, whereas extraction alone had no effect on the unshaded tent that could be resolved from week-to-week weather, and the differences between a roof foil and the net at equal ventilation (0.9–1.6 K) were within the resolution of the two uncalibrated loggers. Irradiance-adjusted estimates across weeks, reported as indicative, place the spaced net 6–8 K below the bare tent and the benefit of extraction under shade at about 1 K. All covers kept the tent 0.4–1.8 K warmer at night. The whole ventilation campaign consumed 3.9 kWh (0.22 kWh per ventilated tent-day), an amount a single photovoltaic module can supply. The most effective cooling device is therefore the cover that field units already carry, provided it is kept off the canvas.