Which way up do you lay underlay?

[rednaxela]

Hello all,

My dad has purchased some foam rubber underlay. One side has rubber ribs and the other is a single surface.
He asked me which way up it should go, and I didn't know.

I would be really grateful for any advice.

Many thanks in advance, Alex.

 

[Softus]

Ribbed side down. (For pleasure).

 

[rednaxela]

Many thanks Softus