I just did a back of envelope calculation. I looked up atmospheric pressure in kilograms per square meter (10332) and multiplied by 20/293 (as T and P are proportional). 705 Kg tallies with billnotgatez and I'm happy with this as the force against the door.

The force holding the door shut is not the same as the force needed to open it. Doors have hinges. Making the not unrealistic assumption that the handle is on the opposite edge to the hinge then the force needed is half the force keeping the door shut. My answer is then 353Kg. Astonishing for what seems to be mild and reasonable conditions.