All I can ask is you keep it in mind. The download folder in total may be around 5 GB, but I don't yet have one. The Windows.old folder, where previous data is stored, created during the install my be larger than 15 GB, depending on how much and type of data.
If the system needs 10% free for normal operations, I would think that might put you close enough for it to be a possibility, but not a certainty. I just bring this up because of the message you received. It could be something else entirely.
Edit: The only other thing which may be involved is if you had some type of link to another drive or location where the install could not access it.