Convert an Imperial thermal resistance R-value of $R\text{-}21$ (units: $\text{h}\cdot\text{ft}^2\cdot^\circ\text{F/Btu}$) to the metric equivalent RSI value (units: $\text{m}^2\cdot\text{K/W}$). (Take $1 \text{ ft} \approx 0.3048 \text{ m}$, $1 \text{ Btu} \approx 1055.056 \text{ J}$, and a temperature difference ratio of $1.8 \text{ } ^\circ\text{F} = 1 \text{ K}$)