I bought a used gas fireplace/stove that came with more chimney vent pipe than I will need to install in my manufactured home. The double wall vent pipe chimney is 7" diameter with a 4" pipe inside. The chimney will go through the ceiling and then through the roof. I believe it will be exactly vertical from the stove through the roof with no bends. Can you tell me what to look for in your store for a piece to keep the vent pipe away from the ceiling and going through the 4" of insulation above?