Hello I'm redoing a bedroom 10&10 house built in 1889. Should I used reflectix insulation over the slats of wood or take the slats out and install r15 insulation . Thanks

I wouldn't use that at all. I don't even think it's meant for such use.
By slats do you mean lath? Have you removed the plaster? If not, and the plaster is OK, I wouldn't remove any of it. What is the house sided with? Are the windows original?
Most of the heat loss is out windows and doors. If you haven't dealt with them first, you are throwing money away. Either put in wood/vinyl windows complete or retrofit sashes with jamb liners. Take off the apron under the inside sill and insulate in that slanted space, which tends to be drafty in old houses.
Plus you have to be aware of the need for a house to breathe, to let moisture out without it condensing when it hits cold air somewhere between the inside wall and the outside sheathing. I suppose if you really want to do all that demo and hauling, insulate with fiberglas with vapor barrier, and drywall and tape/compound/sand or skimcoat, and deal with trim and baseboards, and paint, go ahead. Ton of work for a bedroom, which to my mind can be colder than the living parts of the house.