Penetrating the Air Barrier.
Mounting the light/receptacle blocks to the exterior of the building could have been a mess if there was no foresight. It is hard to attach a trim block to a foam wall. The only solution was blocking attached to the studs directly behind the foam in the wall before the cavities were insulated. I used 2x6 stud lumber for that that purpose. It had to sit flush against the foam and had to be thick enough so that the siding would be just flush with the front of the block. I drilled 4 counter sunken holes: one in each corner and attached the block through the foam with 6" screws into the wall blocking. Once the siding is finished and the electrical elements are installed I will install a small plug in each hole. Without some thinking ahead, this would have caused a lot of headaches.
The plans called for a piece of conduit to be routed through the exterior 2x8 wall and into the electrical boxes. The exterior perimeter of the conduit would be sealed to the OSB at the interior. using tape and acoustical sealant. Once the wire is routed through the conduit it will be filled with spray foam. The detail looks fine on paper. However, drilling through a wall with fibreglass insulation is an uncertain task. Fibreglass has a bad habit of wrapping around anything spinning so we came up with the following procedure.
1. Drill a 1 1/8" hole through the exterior block on the wall
2. Drill through the 3" foam slowly until the bit breaches through the back of the foam and the fibreglass is visible.
3. Insert the 3/4" conduit (O.D. 1 1/8) from the outside and compress the insulation against the exterior side of the interior OSB. (my experience is that you can drill a hole through compressed insulation)
4. Insert a long 3/4" spade it into the conduit and push it through until you feel the insulation at the end of the conduit
5. Quickly drill until the tip spade bit just pierces the interior OSB.
6. From the interior, drill the piloted hole with a 1 1/8" spade bit until it just passes through the OSB.
After removing the conduit, the hole was inspected with a small flash light. The insulation was still filling the cavity and the hole left behind is large enough to pass the 3/4" conduit through the hole drilled in the wall and the conduit. Although the hole was drilled with a 3/4"bit, there is enough give in the insulation that a 1 1/8 conduit easily passes through with careful direction. After drilling is complete, the opening for the receptacle is cut with a jig saw (receptacle) or a hole saw (octagon box for a light). The conduit is glued to a 3/4" threaded section, passed through the electrical box and secured with a locking screw. The box/conduit assembly is the inserted into the opening in the trim block and secured with some screws. At the interior conduit was caulked to the OSB using acoustical sealant. A tape gasket was made with 3M All Weather Flashing tape and pushed back into the acoustical sealant. The gasket was taped around the conduit for an air tight installation. Once the wiring is installed, the conduit will be filled with spray foam. Photos 1-1 through 1-8 illustrate the process.
The original design for the house called for an interior stove pipe. The main reason for choosing an interior installation is performance. Warm chimneys perform better. After lighting a stove, bouyant gases rise through the flue pipe and deposit energy inside the chimney raising its temperature. Expanding gases move up the chimney and eventually spill out the top of the pipe. As the chimney gets warmer this effect is more pronounced. A pressure differential is created between the stove air intake and the chimney that causes draft. Once draft is established it can be maintained by supplying heat. When compared to an interior installation, a chimney installed outside needs more energy to establish and maintain an effective draft. My internet searches really haven't provided any hard numbers regarding the amount of extra energy required. Someday I may revisit this problem to see if there is some way to estimate the difference between the two scenarios.
Because of our floor plan, there was really one place to install the stove. There were some uncertainties surrounding the interior installation that I was not happy about.These uncertainties played a large part in the decisions to move our stove. The floor joist o.c. spacing, the proximity to the bedroom upstairs, having to create a chase in the middle of one of the bedroom walls, running the air intake under the slab, lack of an air tight installation, all added up to a complex problem that had a simple solution: move the stove to an exterior wall. When passing through a combustable wall, a stove pipe requires a radiation shield (thimble). The options for an air tight stove thimble in our marketplace are few and far between. After some research, ICC (http://icc-chimney.com/) produces a thimble that can be used for an air tight installation. Their EXCEL brand of chimney has a thimble (http://icc-chimney.com/en/excel, labeled on the web page as EWRSI). The first time I saw a reference to this thimble was in a Green Building Advisor article. The thimble required a framed opening of 10" x 10" and can fit into a wall with a thickness of 12" or less. A decorative telescoping section can be used to ensure that the stove pipe exits the assembly 4" beyond the wall. Our exterior wall is about 10 3/4" when the foam and OSB are included so rough framing had to be extended through the wall. Due to the wall needing a 2x4 (on face) wiring chase, the stove thimble had to be offset from the wall 1" bandsaw pine frame in order for our cladding material (ship lap board) to work out. Photos 2-1 through 2-5 illustrate the installation.
Photo 3-1 illustrates the current state of the home....starting to look red!
Photo 1-2. Drilling through with a 1 1/8" bit.