# Why does the ISS have so much pressurized but uninhabitable space?

From this answer I learned that the International Space Station has over 900m3 of pressurized volume. However, the same answer points out that the inhabitable volume is under 400m3. It seems odd to me that so much space is kept pressurized when there's no chance of crew being there, if I understand "inhabitable" correctly.

If these areas are pressurized, what makes them uninhabitable? If these areas are uninhabitable, why are they pressurized?

• Please do not use the comment space for answers. Feb 21, 2018 at 16:16

At least some of it is because of equipment against the walls that eats up space.

The modules (US side at least) are cylinders, round on the outside, but when you look at images from the station you will notice it looks very square on the inside.

From the inside of the module's walls there are racks on the various sides, that are round against the outside, flat on the front, for equipment, storage, experiments, etc.

Need to track down some good pictures to explain that. Commenter provided one for Columbus, the ESA module.

A great document from a commenter about the design of the nodes and modules is in this PDF.

• While this does explain a lot, I don't know if it's the whole story. Inscribing a square in a circle would result in uninhabitable volume being around 60% as much as inhabitable volume. However, the actual volume numbers show 140% instead. Feb 20, 2018 at 15:07
• I agree it is not the whole story. Probably short answer is 'equipment inside habitable space' in total. I just offered the probably largest component of equipment. Feb 20, 2018 at 15:12
• your estimate would be true if the corners of the square touched the circle, however that would make the depth of the racks ridiculously shallow at the edges. If instead you make the sides of the square about half the diameter, you get the racks and space to run ducts. esa.int/spaceinimages/Images/2001/07/…
– user20636
Feb 20, 2018 at 15:55
• The purpose of ISS is to maximize the volume of (/for) science - the living space (along with storage, life support and infrastructure) is just an overhead, a necessary sacrifice of volume better spent on scientific equipment, preferably minimized. Skylab had an excessive amount of useless living space - a huge empty middle that served no purpose,
– SF.
Feb 20, 2018 at 17:03
• That cross-sectional diagram definitely helped me understand how the ratio happened, and @SF. made a good point that "large habitable area" is an unfortunate constraint rather than the inherent goal. Feb 20, 2018 at 19:40