We just had a fireplace insert installed with stainless steel flue pipe all the way up. The flue pipe within the chimney is double wall insulated with some white padding around it which we think is called thermaguard. The 3 foot length of pipe that extends above the roof is single wall. The guy who installed it suggests we replace the 3 foot above the roof pipe with a four foot insulated pipe to improve draft and also reduce creosote buildup. What do you think?