Wood has been used for hundreds of years as ornate mouldings including crown mouldings much larger than 4 1/4". If installed properly there should be absolutely no issues with "warpimg".

MDF changes dimension also.

It is a lower percentage than solid pine, but changes in every dimension.

Solid wood has no appreciable change in length.

Thank you very much for all the responses above, especially for mongoct's very detailed information.

Yes, the contractor said with prime wood crown, there is a 50% chance of opening at the joints when humidity or temperature changes. He might not know the tricks of how to do it properly.

The rooms have great lighting during the day because of the patio doors. I will have LED recessed lights installed there. So I prefer the sharper profile of pine moulding.

If the wall is not strictly straight, would MDF be better than pine?

Really appreciate your help.

The joints that open/close with humidity/etc. are the corner joints. Scarf joints(joining pieces to pieces) can be glued/nailed.

Mitered joints can open because of the joint being essentially free.

Inside corner joints should be coped, not mitered. That negates much of the movement and minimizes the spaces if movement does occur.

Outside miters can be glued/nailed, but given enough movement, the wood will over come the nails.

Knowing the area of the country and closeness to the ocean would help with determining the weather(humidity/etc.)

There is another option, one that works well for crown molding. Plastic materials. No movement to speak of, so that problem disappears. Painting it makes it look no different from painted MDF and only slightly different than painted wood.

Handymac, thank you very much for the detailed information. I only saw MDF or wood crown materials. I always thought plastic materials look cheap and avoided that. But if they look no different from MDF, and would not move, I'll take a look at the real materials.

I don't know if the general contractor would be able to do coped corners and scarfed running joints properly.

I am living in the south SF bay area, so the climate is mostly dry.

If a contractor(or his workers) cannot do scarf joints and coped corners, they cannot be a decent trim carpenters.

Period.

Those are some of the most basic knowledge required.

Handymac, thank you very much for your help.

Even with coped corners versus mitered joints and scarfed and glued running joints, the corner joints of pine crown moulding might still open due to humidity changes, while MDF materials would not. But these proper installation techniques can minimize the chance of possible opening. Is my understanding right?

Thank you very much.

Correct, but wood has been used in this manor for hundreds of years with little or no issue. The real reason people like to use MDF is that it looks better than the FJ moulding that became all the rage and is much cheaper that solid wood. Anybody who tells you otherwise is blowing smoke as that was the entire reasoning behind MDF moulding.