The joy of insulating.

I've decided to take a break from posting on topics in [quasi] chronological order. To that end, I think I'll get into how I've been insulating the exterior walls of this house. Overall, I've been quite pleased with how the process has been going - it's always nice when things work out according to plan.

Knowing that I would be insulating with Roxul batts, the walls were built so that their dimensions would suit "off-the shelf" insulation - in this case, "ComfortBatt" for 2x4 walls at R14 per layer (three layers).

Joy of Cooking - Old Man Luedecke

Let me begin by first saying that my first choice for wall insulation would have been cellulose - if I weren't building this house myself. I plan on blowing plenty of loose-fill cellulose into the attic myself, but the problem with the walls is that I can really only insulate relatively small areas of the wall at one time and I was unable to think of a practical way to use cellulose insulation while working in this manner. So, the choice to use mineral wool in the walls was a compromise towards making things more "buildable". All this being said, I have to say that working with the Roxul batts has been great (if not an actual joy). The batts themselves have qualities that make them much more "user friendly" than fibreglass batts and have been working quite well for this application.

The insulation is in three layers:

1. The innermost layer fits between the studs in a conventional way. There are more studs and headers to work around in this part of the wall since it is all structural framing.
2. In the middle layer, the batts are "woven" horizontally into the 3 1/2" void between the inner and outer studs.
3. The third layer is also installed in a conventional manner between the outer studs. Since this part of the wall is not structural there are fewer studs and headers to work around.

A closeup of the lead photo showing the three layers more clearly. Notice that the "layer 2" batts have been installed horizontally into the framing - sometimes where the framing is a little tighter, it is better to cut these batts in half before "weaving" them in.

Looking at the "ComfortBatt"s a little more closely, it is easy to see why working with these mineral wool batts is so different from working with fibreglass. First of all, the batts are stiffer - some batts are stiffer than others but they all generally have about the same consistency as a loaf of bread, with the difference in consistency similar to the difference between fresh and stale bread. Secondly, the batts have a "soft side" and a "crispy side" - sort of like bread that has been toasted only on one side. Third, the consistency of the mineral wool batts makes them more resilient to pressure than fibreglass - when bending, pulling, pushing, cutting or compressing, these batts tend to hold up better to more vigorous handling and have a certain amount of "memory" for their original shape.

The "crispy side" is showing on the left. Notice the difference in texture and that a label is printed on the "crispy side".

I find that it works best to install the batts so that they face a certain way, depending on which layer they are in. Layer 1 batts are installed with the "crispy side" out. Layer 2 batts are installed with the "crispy side" in (facing the "crispy side" of the layer 1 batts) - the reason being is that the "crispy side" of the batts hold up much better to the friction created between the two layers as layer 2 is pulled into the wall. Layer 3 batts can be installed either way, but I like to have the "crispy side" facing out.

Looking again at the earlier closeup, you can see how I installed the batts so that the "crispy side" of layer 1 and layer 2 face each other - this makes sliding layer 2 in there a lot easier on the batts (they otherwise tend to pull apart by catching on each other).

Working around the gussets isn't as big a deal as some might guess. For layers 1 and 3, I have been cutting a thin wafer out of the side of the batts where they butt up against a gusset. In layer 2, the batts are pulled into the framing horizontally, so cutting is generally not required until the row comes up underneath a row of gussets. At this point I have found it easiest to "rip" a few batts into pieces 6" wide (the height of the gussets), then cut the 6" stock into chunks that fit between the gussets - once the voids between gussets have been filled, normal pulling of full sized batts resumes.

Within the circles you can see how the batts have had a thin wafer sliced from them to fit around the gusset. When cutting out the wafer, it is enough to gauge the depth of the cut by eye (I generally aim for about 1/2" for the 3/4" thick gussets). Cutting the batts this way is typical for layers 1 and 3. Additionally, this photo shows EPS insulation filling a small space adjacent to the window opening. I have been using rigid foam to fill any space 1 1/2" or narrower - I keep a few sheets of 1 1/2" thick EPS on hand for this purpose.

Here (forgive the bad lighting in the photo) it is possible to see how the layer 2 batts come up underneath the gussets, and how the space between the gussets is filled with a small block cut from some pre-cut 6" stock. Also note the 1/2" XPS filling the narrow space between the two studs on the right.

Update - Here's a better photo of what I was trying to show in the photo above. The 6" piece in the photo is cut to fit between the gussets before carrying on. Notice the single plate between the upper and lower studs - this is balloon frame, so the framing is light and non-structural.