I agree that in cases where there is a good, uncompromised tile liner in place that the trapped air can work as an insulator. But there are some concerns about relying on air for the insulation. First, it's hard to keep a continuous airspace around the perimeter of the liner. If the chimney has flaws and the uninsulated liner is in direct contact with the flawed area, there will be greater heat transference to this flawed area. Second, when an exterior chimney is at zero degrees, the heat loss through the masonry may counteract the insulating value of the air column. Another solution to installing an insulated liner in a tight space is to use an insulated liner like M&G Duraliner. It has a thinner profile and is available in round or oval, flex or rigid pipe.