I am obsessed with insulation. And in case you haven’t noticed, let me tell you about the stair insulation in the back porch. The perfect hybrid between ceiling and wall insulation: a combination of cut-and-cobble and some fluffy rock wool.
To address the air sealing, I again had to rely on cut-and-cobble pieces of XPS insulation underneath the stairs. And like with the ceiling, I carefully foamed around and between the pieces.
I had the idea of filling the space between the installed XPS and the bottom of the stair stringer with rock wool. Our rock wool batts that typically are installed in a framed wall were not really suitable here. But I found several bags of loose rock wool at my favorite gold mine, the Rebuilding Exchange.
To find a way around gravity, and to add an extra layer of insulation, I attached another sheet of XPS insulation to the bottom of the stair stringers. That allowed me to stuff the space with the loose rock wool without it falling down.
Well, with that done, I can start to think about drywall and painting!
I had room for a double stud wall using standard two by fours. The 1st half (outer part) of the double stud was as already in place. I installed it when I put up the exterior sheathing. This wall was ready to receive the rock wool insulation.
With the 1st half (outer part) of the wall completed, I could start framing out the 2nd half (inner part). To minimize thermal bridging, the studs from the 1st and 2nd half are offset from each other.
The two layers of rock wool alone (one layer for each half of the double stud wall) add up to a R-value of 30. With an additional one inch layer of XPS insulation on the outside, the R-value climbs to R-35.
I am often asked why I opted for rock wool and not the cheaper fiberglass insulation. Well, rock wool insulation is easy to cut, shape, and install. It allows one to fill all nooks and crevices, like spaces behind electrical boxes.
But more importantly, I consider rock wool a low cost fire insurance. Again, rock wool is made out of rocks. And rocks don’t burn!
It doesn’t need to be cozy (at least not all the time), but it shouldn’t be freezing. That would be the expectation for the workshop and storage in the enclosed basement portion of the back porch.
To get there with the minimal amount of space conditioning, we need a fairly decent amount of insulation with an airtight enclosure. I got the walls airtight through careful caulking of the XPS insulation and exterior sheathing. I now had to turn my attention to the ceiling.
If you follow the online musings of green building and energy geeks, you will have heard of cut-and-cobble. It is declared counterproductive by some, ridiculed by others, yet beloved by tinkerers. And sometimes it is simply an option that makes sense, like in our case.
Cut-and-cobble is the process of taking XPS or ISO insulation and cut it into strips to fit it (or cobble it) in between the floor joists. But cut-and-cobble alone does not provide an airtight assembly. It takes some canned spray foam (or caulk) to fill all the gaps between the insulation and framing and abutting insulation pieces. And yes – it can be tedious. Thus the different opinions on this process.
To start with the ceiling insulation I used salvaged two inch thick XPS boards. The key to facilitate the foaming around the edges, is to leave a ? inch gap all the way around. That gap makes it easy to insert the straw from the foam can and get it filled to the full depth of two inches.
Cut-and-cobble is also an excellent way to use up scraps. And I had plenty of four inch XPS scraps from our 2nd floor ceiling-gone-wrong. These scrap pieces were turned into the the 2nd layer of insulation, and this time I fit them tightly between the joists.
I had filled six inches of the nine and a half inches between the joists with insulation. The remaining three and a half inches were lending themselves perfectly to rock wool insulation.
Another rationale was that the rock wool also acts as a fire wall. Remember – rocks don’t burn.
I get to play a game that I know! And the game is called “installing a radiant floor slab”.
I outlined in the last post the installation of the aggregate base for the concrete floor. The gravel had to be carefully screened to assure that I have the right slopes towards the two floor drains.
And now I get to play with the next four components of the radiant floor slab assembly:
Welded wire mesh
Installing the insulation was a bittersweet process. Bitter, because the four inch XPS boards I used came from the very carefully installed attic insulation assembly, which I had to take down again. Sweet, because I got to reuse the insulation and it didn’t to go waste.
I mentioned that the aggregate base was finished with the correct slopes towards the floor drains. That means that I had to line up the seams of the insulation boards with the slope ridges and valleys. If not, I would end up with suspended and wobbly boards that would crack or break.
I again paid attention to the bond breaks around the future radiant floor slab. A bond break is a piece of vertical insulation that will thermally separate the concrete floor from the adjacent foundation wall and footings. This assures that the heat in the radiant floor slab is effectively transferred into the room and not syphoned off into the foundation wall or other thermal mass structures.
Even though I had an effective capillary break with the open graded aggregate base, I still needed an effective vapor barrier under the concrete floor slab. A large 6 mil polyethylene sheet would do that job. I carefully cut it to size and fit it around the sump, floor drains and footing. To prevent it from shifting around while installing the welded wire mesh, I taped it along the edges.
The radiant floor slab will be heated with hot water. To get the hot water into the slab, I used ½ inch PEX tubing, which I attached to the welded wire mesh with zip ties.
I opted for two heating zones. Zone number one is heating the future workshop to the west. Because this section needs to be kept reasonably warm, I spaced the PEX tubing six inches on center along the edges and 12 inches on center towards the center.
Zone number two is the eastern half of the space and just needs to be kept above freezing. For that reason I spaced the PEX farther apart. I also made sure avoid PEX tubing in areas where I need to anchor into the future concrete floor, such as under the future steps and bottom plate that separates the workshop from the rest of the space.
I feel I’m getting seasoned – like a routine is setting in. It is a nice feeling, supported by the confidence that you know what you are doing.
The underground plumbing for the back porch is done. I now can focus on the concrete floor installation. And I’m dealing with literally the same principles, design and process as for the basement floor.
The basement section of the back porch will be enclosed. And we would like to heat the enclosed portion during the cold season as needed. That means we will have insulation under the floor slab, bond breaks around the edges, and PEX tubing in the concrete for the radiant floor slab.
We used a 100% recycled ¾ inch stone (ASTM C33 #57, or IL DOT CA7). The beauty of this material is that it is “open graded.” In other words, it has no fines and a lot of pore space. As such it is an effective capillary barrier and prevents soil moisture from rising up towards the concrete slab – which in turn helps with the issue of moisture management.
Shoveling the gravel into the hole was the easy part. But I also had to screen the gravel so that I end up with a consistent surface on which I can lay the insulation. The screening was complicated by the two floor drains, because I had to make sure that all the various floor sections slope into the right direction.