If you vent directly into the chimney and you are burning wood, the chimney does not get very hot. As a result you get a build up of tar inside the chimney.That can create 2 problems. The tar can run down into the fireplace and stain it, or more importantly, it can catch fire. A single walled liner is designed for traditional chimney's , not outside a chimney. The single wall keeps the inside not and safe. Inside a chimney a single wall liner is your best option. Take a look at  http://rockfordchimneysupply.com/  – they sell stainless steel liners and are fairly reasonable in cost! Also you might want to drop a weighted line with a sample of the liner down the chimney before you invest the cash. Just to be sure it fits OK. Chimneys can sometimes take some odd twists and turns. Lastly, you will want to swept your chimney annually to avoid soot build up.