It's exactly like being trapped in a thermos. But! There are space suit concepts that use an airtight helmet, but the rest of the body effectively is exposed to vacuum, supported and protected using a skin tight water-permeable fabric that prevents you from swelling up or burning in the UV. One of the benefits of this kind of space suit is that SWEATING WORKS IN SPACE!!! The sweat carries away heat (and further consumes it in the phase transition) when it instantly flashes to vapor, cooling one's body. Sweating is a human super power and we can leverage it to perform our own temperature regulation in a vacuum! I've always thought that was super cool.