A proper chimney - part 1

I'm choosing the chimney as my starting point for getting back into more detailed construction-related posts since it seems to follow nicely from where I left off with a post about the air-tight ceiling, which I ended by showing a photo of two foam boards acting as a place-holder for my future chimney.
Installing the chimney was pretty straight-forward with the biggest consideration aforehand being careful measurement. A "proper chimney" should go straight up (without any bends), while all clearances to combustible materials (from bottom to top) are maintained, and while also having the chimney "land" where the stove is supposed to go.
Prepare for many a photo "below the fold".

The finished install: In this case the chimney had to land in just the right spot so that the stove could sit correctly on it's tiled pad next to the air-supply duct I roughed in oh-so-many years ago. There wasn't much wiggle-room to play with but fortunately luck was on my side here, and there happened to be just room enough to find a path through the roof framing. Measure thrice, cut once, I say.

Burning Down The House - Talking Heads

Let me begin where I left off then, after I had removed my stylish, but temporary XPS ceiling panels.

1. I climbed up into the attic and dropped a plumb-bob down from the underside of the roof deck - all the way to the basement floor - a distance of about 25' or so. On the basement floor, I had placed a piece of cardboard which accurately represented the stove's lateral dimensions and the centre of the stovepipe's connection point.
2. I spent some time fiddling around with the plumb-line and cardboard, measuring to various combustible components of the house, until the "sweet spot" was revealed (note: this process goes a LOT faster if you have a helper down below). I then marked that spot on the underside of the roof deck and drove in a screw from which I cold hang the plumb-line.
3. With the plumb line hanging from that centre screw, I was able to measure from it to mark the trusses for the additional framing required to support the weight of the chimney assembly.

After marking from the plumb line, new structure was added according to the chimney manufacturers instructions. Notice that this extra framing is not attached to the trusses, but rather to the ceiling joists which hang from the bottom chord of the trusses. As the weight of the chimney assembly is not-so-insignificant I nailed in extra hangers - one for each corner of the support. With the framing in place, the missing portions of the plywood air-barrier where completed and sealed with tape.

Rock-wool insulation fills the spaces between the new structure.

From below, you can see that I framed the new chimney support using 2x6 which allowed it to extend downwards just far enough to penetrate the air barrier and come flush with the lower plane of the strapping (2x2) for the ceiling service cavity. I used silicone to seal around the perimeter of this opening rather than tape.

At this point I was still quite a way away from being ready for drywall, but a small square was needed in order to follow through with the chimney installation. A bead of silicone was applied around the bottom edge of the support structure before lifting the drywall into place and securing it.

With the drywall in place, I was now able to fit the steel chimney support to the completed framing. This steel support is a decorative metal flange welded to a heavy gauge steel ring. Air-sealing the flange to the drywall was not-so-easy. After considering a few different options, I decided that the best way was to make a cured silicone gasket on the back side of the flange by applying, then gently smearing, several layers of silicone around it's square perimeter (messy work!). I built the gasket up until it stood just a little proud of the flange, so that it was then compressed against the drywall by tightening the finish screws at the corners. With flange-ring secured in place with the finish screws, I then nailed through the steel ring into the wooden framing to make a load-bearing assembly.

Back up in the attic now, the next step was to drop the steel "bucket" into the support ring, which you can see resting in there already in the picture above. The bucket has a lip around the top edge which transfers the weight of the chimney to the steel ring. With the bucket in place, I then fit the stovepipe adapter (left) to the bottom of the first section of insulated chimney pipe (top) and lowered both into the bucket so that the adapter's crimped end extends down into the conditioned space below.

With the chimney pipe resting on the assembly, it's weight keeps everything together snugly so that no further fasteners are needed. After speaking with the inspector, I was allowed to add some rock-wool insulation to the small triangular voids in the corners but not inside the ring itself. In terms of air-sealing, I had to be content with the tight fit of all the components, as using sealant between them (silicone or otherwise) is not permitted.

From the inside, the completed support assembly, with stovepipe adapter ready to adapt a double-wall stovepipe to the insulated chimney pipe.