Way more goes into keeping you dry then just a number with a k after it.
Is it 100% seem sealed? If not it can be 50k and you will still get wet? Is it a 1layer fabric, two or three layer? What membrain is it? Some stop breathing if they get saturated, so you get soaked by sweat. Some can take abuse better then others too.
If you wear a tall tee it doesn't matter since the cotton will pull water up so your fucked any way.