It's a nice product for a bathroom.

JC is correct, not a product for wet or damp areas.

Ditto J.C. above. Bad idea. I suspect he is telling you its fine because he doesn't want to replace it. Who wrote the specs for the work ? If there are no specs, while I would be concerned about his electing to use MDF trim in a bathroom (tells me is more concerned with price than quality), I do not think you can get him to eat ripping and replacing it. I'd chalk it up to a little bit of a lesson learned (you need detailed specs). Explain that you would like him to get matching poplar trim (I'm not a fan of finger jointed pine) and that you will be willing to absorb some of the cost. Its cheaper to resolve it now. I say you probably cannot get him to absorb all the costs because sometimes it is wiser to pick your battles and maintaining a good working relationship with the contractor is a good objective. I would keep in the back of my mind that if he is willing to install MDF in a bathroom he's not that particular. So I would keep an eye on his work and hold your last payment until you are happy.

MDF swells in the presence of moisture. Moisture is also known as "humidity". If this is a bathroom with a full shower/bath that is going to get daily showers/baths then the humidity in the bathroom will reach 100% at least once/day. This humidity is the real "MDF killer". If this is a powder room that has only throne and sink, you have a much better chance of the MDF working out over the long term. It will still have a slightly shorter life than a bookshelf...but it should hold up for a decade or so.

Thank you so much for all of your insightful comments and suggestions. I will take your comments to heart and will ask my contractor to get a poplar substitute and absorb the cost myself. In fact, I will email him right now. If the MDF does last a decade, I should be still alive and in this home (I hope) and will not want to deal with finding a match, replacing it and dealing with whatever costs. Heck, I probably would not even find a match. I did the specs and design for the bathroom. My contractor even asked if I would order materials based on my specs however I was not confident and felt it was his job as I am not a professional, just a huge design and houzz fan. I truly appreciate your helpful advice! Wish me luck!! :)

I wish I could hire Classic Connecticut Homes to do my reno, Cancork Floors to do my flooring and JC Sterling FIne Furniture for my couches!!!