I use sheet aluminum. Similar to rolls of thin gauge aluminum flashing.

Attach with small sheet metal screws. At the front edge on the bottom of the countertop overhang I'll run a thin bead of clear silicone on the edge of the metal to seal and adhere the Al to the wood, and to soften the edge of the aluminum.

Can't see any of it unless you lay down on the floor and look up.

Thanks mongoct, that's what I was looking for in HD the other day- couldn't find anything big enough (I want to cover the whole 24"x 24" area) That was my exact plan, just can't find it...and the tops are being installed today!

laminate will work too, we sandwich our countertops between what we call backer(on the bottom) and the laminate on top. Contact cement to hold it on

Your wood should be no problem. Dishwashers come with sufficient moisture protection built in. Think, they are designed to go under a particle board and laminate counter. The only time moisture escapes from the DW is that rush of steam comes out when you open it.

Ikea sells a Fixa diffusion barrier for their whirlpool DW. It supposedly protects the exposed counter edge outside the DW, against steam escaping when you open the DW. Some people apply metal tape to the front underside edge of the countertop for the same effect.

But, I would be reluctant to put plastic sheeting or something like that under the counter unless the DW manufacturer recommends it. You could create more problems by trying to solve a non-problem. Sometimes, we can over-think these things. Dishwashers are made to be leakproof right from the manufacturer.

would a coating of silicone work as moisture/vapor barrier?

>> sheet aluminum. Similar to rolls of thin gauge aluminum flashing.

You might have to go to a building supply if you can't find it wide enough at the big box store. There are places that cater more to the trades that have nearly everything.

8 year old post.

Treated with poly or boric acid?