A new experiment is described on heat transfer in a Rayleigh-Bénard chamber modulated by an external flow. The results show a linear response of thermal gradients to small Reynolds numbers, here resolved with micro-Kelvin resolution at room temperature. It approaches a hyperbolic behaviour at large Reynolds numbers. This response is nowhere reminiscent of singular behaviour in boundary layer theory. It may, instead, represent a cellular model for describing heat transfer in porous media. An application to correlating residential gas-energy usage and local weather data is included.