Cat 5 and 6 are not sheilded (well, some "six and better" is -but it is not part of the standard.. Don't know what kind of obstruction you are looking at - but cat 5 comes in different jacket thicknesses - as much as 25% difference in diameter. Non Plenum FT4) cable is GENERALLY thinner than FT6 (plenum). Here in Ontario FT6 is required if it is run in an air plenum (above suspended ceiling where the air return is open, for example) and it is almost twice as expensive as FT4. Some of the pricier FT6 has a thin teflon type covering which is quite tough, and very easy to pull.