Tag Archives: insulation

Air sealing the roof

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.

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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.

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Do-over dilemma

Blower door test – after insulation

Double duty

Attic insulation – foam board component

Stuffing the attic – Part 2

Stuffing the attic – Part 1

Spatial challenge

Advancing on the attic

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Porch enclosure – stair insulation

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.

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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.

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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.

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Well, with that done, I can start to think about drywall and painting!

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Porch enclosure – wall insulation

I knew where I was going with the wall insulation – I had sketched it out when I was planning for the porch enclosure.

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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.

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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.

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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.

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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!

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Porch enclosure – ceiling insulation

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.

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Cut-and-cobble

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.

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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.

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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.

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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.

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Another rationale was that the rock wool also acts as a fire wall. Remember – rocks don’t burn.

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Adding more floor components

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:

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  1. Insulation
  2. Vapor barrier
  3. Welded wire mesh
  4. Pex tubing

Insulation

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.

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Vapor barrier and wire mesh

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.

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PEX tubing

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.

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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.

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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.

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