  We present the thermal model of the Balloon-borne Large-Aperture
Submillimeter Telescope for Polarimetry (BLASTPol). This instrument was
successfully flown in two circumpolar flights from McMurdo, Antarctica in 2010
and 2012. During these two flights, BLASTPol obtained unprecedented information
about the magnetic field in molecular clouds through the measurement of the
polarized thermal emission of interstellar dust grains. The thermal design of
the experiment addresses the stability and control of the payload necessary for
this kind of measurement. We describe the thermal modeling of the payload
including the sun-shielding strategy. We present the in-flight thermal
performance of the instrument and compare the predictions of the model with the
temperatures registered during the flight. We describe the difficulties of
modeling the thermal behavior of the balloon-borne platform and establish
landmarks that can be used in the design of future balloon-borne instruments.
