Trust me, sheets will allow too much light through. Why would you not use cardboard instead?

Did you try this? Any shareable results? I'm about to try using bedsheets as landscape weed barrier underneath mulch.

I saw another thread that recommended polyester/satin-ish sheets because said cotton sheets breakdown too quickly.

From my experience the majority of weeds you will get... will be from new seeds carried in from above growing down into your mulch.... Not from seeds germinating from below. And if you let any weed get its roots into the sheets..... It seems like it would be a bare to get out.

If you needed to use something under mulch to aid in keeping unwanted plant growth in check then you are not putting enough mulch down, however, cotton would be much better than a synthetic fiber because the cotton would do one of the four things mulches are supposed to do, provide organic matter to the soil. Synthetic fabrics do not do that and are one reason the commercial landscape fabrics are not good to use in the garden.

Put down enough mulch and the unwanted plants that grow from the soil most often won't. The mulch will not stop unwanted plants that get seeded in the mulch, though.

kimmq is kimmsr

I assume the OP is considering sheets as a first layer and then covering it with something, but perhaps not. It would solve the problem of light getting through and would certainly look less like...a bunch of sheets that way. :-]

As to water, I would think water will penetrate fine if it's below a cover layer. If there is no cover mulch and they are on a slope, they could shed some water if they get very dry. You'd have to try it, and hit it with a lot of water from a watering wand and see what happens.