I have an upland 207 I'm looking to install. It's going through an exterior wall on the first floor. There is a large section of wall that is brick for this purpose. the thing is, from the edge of the thimble there is only about 12" of brick before you hit wood studs. I thought that was too close to just use a clay thimble, so I thought I would use a section of 8" double wall pipe. Is this a good idea? Can I just butt the double wall up against the clay liner in the chimney and seal the seams with heat stop?