If it is aluminium, the screen should weigh less than the foil, so i guess your micrometer is not that precise.
Anyway, the foil could tear and is only one foot wide and it may cause condensation and mould.
I prefer stainless steel which seems more durable, but a thicker aluminium screen may withstand pressure and oxidatian well enough.
Some people have claimed that aluminium foil somehow looses its shielding power over time, which raises another question.
I am not in favour of screening except for blocking emf from directly around devices like computers and smps-s, using several millimeters of solid metal here as the distance is short and the signals are strong.
Shielding all over the place appears to me as an unnatural solution, you might block unmeasurable but vital natural fields too, and it could also give a false sense of safety as lots of shit won't be blocked so easily.
In addition it is very difficult to avoid cracks and slight openings, or inproper earthing ( even the ground-wire could be polluted ).
E.g. as a test i just covered a smps with a 1 meter long strip of shielding material, ungrounded : at the far end of it still high levels of electrical fields are measurable, thus instead of shielding it transports and spreads pollution, and it may even pick up a variety of weaker signals from far away.
If the ground is clean emf-wise, grounded shielding may help to reduce static electrical fields, but you may as well ground yourselves then for better results.
I don't think that it's a problem with the micrometer itself, but tip force and abrasion are bigger problems when you're measuring something so small and soft. Reducing the diameter of a cylinder by half reduces its volume by about four fold, so if I'm squashing the wire by 0.0005", that would explain a little more than half of the discrepancy. (Squashing it by 0.001" would explain all of it, but I used a pretty light touch - lighter than the built-in slip ring.)