  We report the temperature dependence of the effective spin-mixing conductance
between a normal metal (aluminium, Al) and a magnetic insulator
($\text{Y}_3\text{Fe}_5\text{O}_{12}$, YIG). Non-local spin valve devices,
using Al as the spin transport channel, were fabricated on top of YIG and
SiO$_2$ substrates. By comparing the spin relaxation lengths in the Al channel
on the two different substrates, we calculate the effective spin-mixing
conductance ($G_\text{s}$) to be $3.3\times10^{12}$~$\Omega^{-1}\text{m}^{-2}$
at 293~K for the Al/YIG interface. A decrease of up to 84\% in $G_\text{s}$ is
observed when the temperature ($T$) is decreased from 293~K to 4.2~K, with
$G_\text{s}$ scaling with $(T/T_\text{c})^{3/2}$. The real part of the
spin-mixing conductance ($G_\text{r}\approx 5.7\times10^{13}~
\Omega^{-1}\text{m}^{-2}$), calculated from the experimentally obtained
$G_\text{s}$, is found to be approximately independent of the temperature. We
evidence a hitherto unrecognized underestimation of $G_\text{r}$ extracted from
the modulation of the spin signal by rotating the magnetization direction of
YIG with respect to the spin accumulation direction in the Al channel, which is
found to be 50 times smaller than the calculated value.
