Thermally speaking, our staircase from the front door to the 2nd floor unit is “un enfant terrible.”
Because of dimensional deficits due to the existing staircase, the envelope insulation along the 1st floor level solely consists of two inches of closed cell spray foam.
This may sound pretty potent to some, but with a wall insulation goal of around R24 to R30, two inches of closed cell foam is merely mediocre. The staircase will always be cooler in winter and hotter in summer than the adjacent interior spaces. Because it’s a transit space it doesn’t matter that much, with the exception that it has a negative impact on the thermal load of the adjacent rooms.
To solve this shortcoming, we separated the entire staircase into its own thermal unit that gets insulated on all sides. This in combination with air sealing should compensate for the inept exterior insulation along the 1st floor level.
To get there, we installed two inches of the closed cell foam along the 1st floor and 2nd floor exterior walls.
Because the exterior masonry wall on the 2nd floor narrows from three to two wythes, we ended up with enough room to switch from the 1st floor furring strips to regular framing along the 2nd floor level. We filled all those framing cavities with rock wool, adding another R15 insulation value.
To stay with the seamless thermal separation we filled the ceiling joist cavities with rock wool…
… and all framing cavities of the flanking interior walls.
Thermally isolating the staircase by surrounding with with an added insulation value of R15 (less at the framing studs) wasn’t the ideal solution, but given the restrictions, it was the most practical process towards minimizing thermal inefficiency.
Although I usually enjoy writing blog posts, this one doesn’t necessarily fall in the “fun” category. I am talking about my well intended roof insulation that required a partial do-over.
I did a very thorough job, starting with rock wool insulation between the roof joists, followed by four inch thick XPS foam board that we mounted under the roof joists and then airsealed with close cell spray foam. I subsequently discovered that my insulation assembly was upside-down and that I had created a cold roof deck. So I started the process of removing the carefully installed XPS insulation, which ultimately should be installed on top of the roof deck.
With the XPS insulation removed, I needed a new vapor permeable air seal. It needs to be vapor permeable to allow for seasonal drying of the roof assembly. Out of the handful of methods available, using half inch drywall in place of the XPS boards seemed to be the simplest and most reliable solution.
Once we had the drywall mounted under the roof joists, I made sure we mudded and taped it carefully to create an effective air barrier.
To seal the edges, I installed two by twos with a small gap that I filled and sealed with foam.
With my new air barrier in place, I started rebuilding the ceilings where needed and then I moved on to installing the ventilation duct work.
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.