So we have the scene where Kylo Ren is talking to the helmet of Darth Vader. We see that it has been seriously deformed by the funeral pyre that Luke placed Vader on. Deformed to the point that it looks like the helm is actually melted to the mask. Now consider that Vader's suit was able to stand the rigors of space based on his ability to fly his TIE Advanced with no additional life support. That makes it really good at insulating Vader's body from the temperature extremes, both heat and cold.
Considering all this, it's highly possible that Vader's body didn't reach a great enough temperature to be fully cremated. Considering the deformity of the helmet, that would mean that Kylo Ren is keeping the decapitated head of his grandfather on his Star Destroyer.