Gave a quote on a changeout wet the plenum and trunk is duct board. Customer wants price to relplace trunk line.. It's in great shape, so when I gave him a price to replace trunk line, he ask what is the "life expectancy" of the duct board. Does anybody know life span of duct board? It's in attic, never gotten wet and no air leaks noticeble.. Some good and bad stories welcome... Thanks in advance...