The external wall of a room measuring $$2\ \text{m}\times3\ \text{m}$$ consists of a layer of white pine of thickness $$d_{pine}=2.0\ \text{cm}$$ and a layer of rock wool in succession. The external temperature is $$36\ \text{K}$$ below the indoor temperature. The thermal conductivity coefficients are $$K_P=0.10\ \text{W m}^{-1}\text{K}^{-1}$$ for white pine and $$K_W=0.04\ \text{W m}^{-1}\text{K}^{-1}$$ for rock wool. The thickness of the layer of the rock wool, so that the thermal conduction rate $$P_{cond} =\frac{dQ}{dt}$$ across the wall does not exceed 120 watt (assuming no loss of heat during conduction and no other way of heat transfer other than conduction), is