You may already have tried this, but I was advised to limit the area in the ark available to young pigs - obviously as they get bigger you increase the floor area available to them. The idea being that the pigs wouldn't want to toilet where they actually sleep, if you give them too big an area then they have an inbuilt sleeping area and toileting area without having to go outside in the cold. We managed this by stacking up bales of straw around the edges of the ark which also served as extra insulation, but others have used temporary wooden sides which can be taken down as they grow. Good luck.