For modelling the rate of air flow through the opening, empirical relationships in the ASHRAE Fundamentals Handbook seem relevant here.
Volumetric flowrate
Equation 37 on page 25.13 of the 1997 version, section "Flow Caused by Thermal Forces" may be useful for calculating flow rate:
$$Q=C_D \cdot A \cdot \sqrt{2 \cdot g \cdot \Delta H_{NPL} \cdot (T_i - T_o)/T_i} $$
where:
$Q$ : airflow rate, [$\frac{m^3}{s}$]
$C_D$ : discharge coefficient for opening [$-$]
$A$ : Area of opening, [$m^2$]
$g$ : gravitational constant, $9.81 \space \frac{m}{s^2}$
$\Delta H_{NPL}$ : height from midpoint of lower opening to NPL (Neutral Pressure Level, "the height at which the interior and exterior pressures are equal"), [$m$]
$T_i$ : indoor temperature, [$K$] (assuming $T_i>T_o$)
$T_o$ : outdoor temperature, [$K$] (assuming $T_i>T_o$)
The value for $C_D$ that takes into account interfacial mixing of the bidirectional flow of air through the opening is Equation 38:
$$C_D={0.40}+{0.0045}|T_i - T_o|$$
Density
Density can be calculated from equations 11, 22, and 27.
Specific Enthalpy
The volumetric flow rate $Q$ and density, combined with the enthalpy difference ($65^{\circ}C$ @ 100% RH vs. $25^{\circ}C$ @ 50% RH) of two points on this psychrometric chart on page 6.11, should permit you to calculate heat transfer out of the building via this natural convection air flow, assuming air pressure is near $101.325 \space {kPa}$.
If air pressure is not close to $101.325 \space {kPa}$, then the set of equations referenced by the Situation 3 table of the "NUMERICAL CALCULATION OF MOIST AIR PROPERTIES" section on page page 6.10 can be used instead to calculate specific enthalpies as a function of dry-bulb temperature $t$, Relative humidity $\psi$, and absolute pressure $p$.
