Stucco on the Chimney (Fireplace Part XIV)

Whew, I'm way behind on updating the blog! In fact, I just realized that we stucco'ed the entire chimney without taking a single picture of the process. Oh well, here's a so-so picture of the finished product.

I used the Quickrete one-step Stucco product that comes in 80 lb bags - "just add water." I applied it directly to the cinder blocks (wetted down with water) with no mesh or lath and it seemed to stick just fine (time will tell). At first, I tried to achieve a very smooth finish, but found this was somewhat futile for a couple of reasons... First, our stucco skills (those of mine and the stone mason who is still sticking with me) were not really up to the task. Actually, we could keep the finished surface smooth, but it took an inordinate amount of time and cold-joints (between day one and day two) were especially hard to disguise. Secondly, the product that I chose has fiberglass embedded in the mix to reduce cracking in the finished product. It seemed that when I tried to wet-float the surface (to get it smooth) after the initial cure, the fibers would raise up out of the finished surface.

Which leads me to another point. Although the product says "one step" right on the bag, it was necessary to apply the stucco in two steps. In fact, the small-print instructions on the back of the bag actually recommend a second step using another Quickrete product called "stucco finish coat." After rubbing my arms against the first coat, and feeling the itching sensation that results from exposure to fiberglass, I decided that a finish coat is a necessity for indoor applications or anywhere you might anticipate a person touching the surface.

Whereas we applied the first coat (with fiberglass) at 1/4" to 3/8" thick, it was only necessary to apply the second coat (without fiberglass) at 1/8" thick. The fiberglass also helped to hide the cinder block mortar joints which were still barely visible after the first coat of stucco dried. With scaffolding already in place, we were able to put the finish coat on the entire surface (400 square feet) in one day so there were no cold joints in the final surface. The mason also suggested (wisely) that we apply the finish coat from the top down so that drips and dribbles would not mar our finish coat.

Where the chimney passed through the roof, I had to figure out a few details. Common sense (and codes in some regions) dictate that chimneys have 2" of clearance to combustable structural members where the chimney passes through the roof. How then does one fill this space? In the picture to the left, you can see the copper step flashing that keeps rain from entering the house through this gap. The copper step flashing provides no measure of insulation to the outside. In fact, one can see daylight between the pieces of flashing.

Into this 2" gap, I stuffed un-faced fiberglass insulation. It is important to use un-faced insulation, because the paper facing could present a fire hazard, but unfaced fiberglass is even nastier than faced insulation. Cutting and stuffing this material (now labeled as possible cancerous) into the gap was a nasty job. I'm glad that I've got no fiberglass insulation anywhere else in the house. Fiberglass insulation does not impede air flow (e.g. precious warm air leaving the house), nor is it pretty to look at, so I bought some thin aluminum flashing (in rolls) and bent it into a shape that would fill the gap just below the fiberglass insulation.

Painted flashing stock might have been more aesthetically acceptable, but I reasoned that paint was not good to have in contact with the chimney for the same reason that paper should not contact the chimney.

This aluminum flashing is also flexible enough to accommodate differential movement of the house structure and the chimney structure. Speaking of differential movement, we had winds that approached 30 mph while I was working on this detail on the inside of the house, and I could not detect any movement between he house and chimney whatsoever. Of course, the house or the chimney could settle over the years.