Is it hard to keep the space station warm?

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Warm is pretty easy. Cool is harder.

The outside of the ISS can reach temperatures as high as 250 degrees F (121 C) on the sunny side and as low as -250 degrees F (-157 C) on the shady side. Inside the ISS are plenty of things that generate heat - such as human bodies, laptop computers, pumps, and other electrical devices. It takes a lot of work and complicated thermal control systems to remove that heat from its sources and transport it outside where it can be radiated to space.

For the parts of the ISS that do need active effort to keep warm, that is accomplished using simple electrical resistance heater pads, like the one shown in the below picture. They work on a simple premise - the thin pad has a wire running back and forth and back and forth many times within it. That wire is attached to an electrical source and electricity flows through the wire. The circuit has resistance and resistance results in heat. The wire gets warm, so the pad gets warm, and so whatever surface it is adhered to will also get warm. A thermostat will measure the temperature in that vicinity and the value of that temperature will be used to turn the electrical circuit for the heater pad on and off. There are hundreds of these pads throughout the vehicle.

It is important to use these heater pads to keep the shell of the vehicle warm, because if the temperature drops below the dew point, condensation will form on that surface. Accumulations of water can cause problems with electrical equipment and can promote the growth of microorganisms.