Hello, we've just moved into a brand new build and we find one of the rooms in the upper level very cold. We find the cold air return on the floor is letting a lot of cold air into the room. Being the winter time this makes the room very cold when the door to the room is closed. This will be our newborns bedroom hence the door needing to be closed. CAN WE BLOCK THE COLD AIR RETURN TO KEEP THE ROOM WARMER? Thanks in advance for any help regarding this.

I see the idea for this BUT you better check with the heating contractor that did your new home. Depending upon the exact layout of all the duct work and equipment it could cause a problem by blocking it off. And BTW cold air returns are NOT supposed to put out cold air. They are intake air only so something else might be wrong with your system to start with.

Anything could be missing but I doupt if it would be a baffle since they are not usually installed to start with.

Perhaps the cold air return is passing through an unconditioned space, or pipe seam has somehow separated causing cold air to convect back to the room. Either way you should have someone look at the system. With new construction you should have some kind of warranty support.