Question on cotton rugs!

Hey there,
A girl at that keeps her horse at the same barn as me was telling me the other day that putting cotton rugs underneath other rugs can keep your horse cooler.
I had never heard this before, it confuses me as to how having an extra rug on could keep your horse cooler.
For instance, the other night she stated how she thought it would be too hot put just a canvas rug on her horse without a cotton underneath overnight.
I'm always up for learning new things but this confuses me so if someone could please explain it to me, it would be much appreciated!

Thank you!

I'd think any extra layer would create more heat.

A cotton sheet, by itself would be much cooler than a waterproof sheet or blanket.

Welcome to the forum!
I agree with Lexiie...a double layer should be warmer.
It's late summer there. Why do you need to cover at night? Is it to keep the horse clean? Whatever you do use I would think it should be lightweight.

Something that wicks moisture away can be cooler, but cotton doesn't fit that bill. When I dress for the barn in cool (but not cold weather) I'd much rather wear a wicking long sleeve shirt than cotton under my jacket. The wicking layer keeps me from getting that sticky, humid feeling under the jacket, but a cotton shirt just holds it against my skin and feels much hotter & uncomfortable. I imagine horses would be subject to the same effect.

A cotton on its own would be just fine unless she means a mesh type cotton?