Tag Archives: insulation

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:


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


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.


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.


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.

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Porch floor aggregate base

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.

But first things first: I have to install a proper aggregate base on which I can install the next layer, the XPS insulation.

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.

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Back porch foundation wall insulation

One big ticket item that is still sitting on my to-do list is the exterior insulation around the basement. Well, I did get the chance to practice a little, because our brand-spanking-new porch foundation wall was asking for some insulation before I started with the backfilling.

I had researched the insulation strategy a long time ago and knew which direction to go and that I would need XPS boards totalling four inches in depth. The question about where to get the XPS boards was also solved, thanks to my snafu in the attic insulation strategy.

I had started to take down some of the salvaged four inch XPS boards we had mounted to the bottom of the roof joists. I was now ready to repurpose them as foundation wall insulation. That was a tiny bit of silver lining in my attic insulation dilemma.


We cut the XPS boards to size so that they fit from the top of the footing (or bottom of the foundation wall) to the top of the foundation wall. To help with the moisture management, we placed a drainage membrane over the insulation.


The drainage membrane is a HDPE sheet with dimples on one side, creating a ¼ inch air gap between the membrane and insulation. That air gap prevents hydrostatic pressure from building up against the foundation wall. Any bulk water that enters the air gap immediately drains down, where the membrane connects to the footing drain which is embedded in open graded aggregate.

This assembly should keep the foundation wall dry and reasonably warm (or cold for that matter). With four inch XPS boards in place, I can expect an R-value of about 20, not counting the thermal mass of the soil behind it. That exceeds the Chicago Building Code requirement of R-10 (Chicago Building Code, Chapter 18-13-102.1.1; Building thermal envelope insulation, Table 18-13-402.1.1) and the Chicago Green Homes requirement of R-19 (Chicago Green Homes Program Guide, Version 2.0 – 210 Energy Use Reduction [210.3]).

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Do-over dilemma

Project specific factors usually determine what insulation method would be suitable. For our roof those factors were a target R-value of 60 or more, the existing low slope roof with its ten inch roof joists, the spatial constraints of the south attic, and the need for some roof deck repair down the road.

We reconciled these sometimes conflicting factors with an insulation assembly that started with three layers (total of 10 1/2 inches) of rock wool batts (R-45), followed by four inches of XPS foam boards (R-20). That should give us an R-value of 65 anywhere between the roof joists.


It looked like we had the perfect roof insulation for our project. Yet, it was also risky, which suddenly made it a lot less perfect.

What I neglected to consider in my schedule and material driven decision making, was the building science – the issue of moisture management.

From cold to wet to rotten

The current insulation assembly is so effective that I created a cold roof deck. During winter time, its temperature will be close to the outside air temperature. In other words, it will be around or below freezing.

Any water vapor in the assembly is likely to condense at the cold roof deck – or, to be more precise, the water vapor will be absorbed by the boards and the upper (cooler) sections of the roof joists.

That could drive up the material moisture content. Once the moisture content rises above 28%, rot or biodegredation may set in. This could compromise the structural integrity of the roof deck and roof joists.

See also: BA-1308: Moisture Control for Dense-Packed Roof Assemblies in Cold Climates: Final Measure Guideline

We did fairly successfully airseal the 2nd floor, including the roof plane. That is, however no guarantee that the roof assembly is vapor tight. Plus the stack effect will constantly pound the insulation assembly with warm and moist air. Some of it will get into assembly. But I am not sure if it has an effective way to get out again.

The accumulation of moisture in the assembly would probably be a slow and gradual process. To get to and sustain a risky moisture content would probably take years. So should I lose sleep over it?

Yes. And I already lost a lot of sleep over it! We plan to be in this building for many years to come. The aspect of durability is very important to us. It makes economic sense and is sustainable. Plus we plan to put us a green roof. I don’t want to ignore the problem now only to risk costly structural roof issues 5, 10 or 15 years down the road.

The fix

Our current insulation assembly is upside down. If I move the XPS insulation on top of the roof deck, I should be able to keep it warm enough to reduce the sorption risk (often referred to as condensation).


The rock wool insulation can stay in place as is. It is vapor open and allows for seasonal drying of the now much warmer roof structure. With that, the moisture content of the roof deck and roof joists should remain in the safe zone, which would be below 20%. (Mold growth is likely to occur if the moisture content is in excess of 20%)

I don’t like doing things twice. But in this case it is the lesser evil. I will now spend a few days taking down our carefully installed XPS insulation. Reinstalling it atop the roof deck will happen sometimes down the road.


Trying to determine how to insulate or ventilate a roof isn’t easy. Here is a list of articles that guided me in the process:

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Multi tasking

I’ve concluded the last post contemplating the real world issue of two different trades (carpenter and insulation installer) working on an insulation assembly at the same time.

How about bringing a third trade into the picture: The plumber.

The outdoor cooking concept

The last room to insulate is the 2nd floor kitchen. Back in the day, I had roughed-in the gas line for the stove, knowing that I need to fine tune this connection once we start framing and insulating.

To add resilience to our design, we plan on extending the gas line to the back porch, which would allow for outdoor cooking during the dog days of summer. Shifting the cooking onto the back porch keeps the unwanted cooking heat out of the conditioned and hopefully cooler building interior.

Task layering

Installing the the interior perimeter wall framing with the rock wool insulation has become almost routine. Integrating the gas line into the assembly would typically be the last step, similar to what we did on the 1st floor.

However, aligning and drilling the holes through the already installed studs filled with rock wool batts and then fitting in the gas line is like a puzzle you don’t really want to put together. I always wondered if there is an easier way. My friend Drew and I decided to give it a try and represent three trades at once: carpenter, insulation installer and plumber.

Rather than installing the gas line last, we drilled the holes into the studs and fitted sections of the gas line while we were assembling the framing with the insulation.

I am not sure if this was a faster method. But it was easier and more precise with less puzzling. Pre-drilling the studs while we put the framing together made a big difference because it allowed us to perfectly align the holes.

I’ve learned that you have to be on your toes and constantly think and rethink the task sequencing, because layering three trades into one task is, let’s say, unconventional.

The bump-out


The rethinking of sequencing was further complicated by the chimney bump out on the west facing wall. I did not want the framing to follow the bump out. That would make for complicated drywall installation and even more complicated kitchen cabinet fitting.

Instead, we opted to hide the bump out behind the framing. Yet we still had to fit the two layers of rock wool insulation.

Our solution was to frame the wall left and right of the bump-out with two by six lumber. That gave us the three and a half inches to fit the first rook wool layer between the framing and closed cell foam. We framed the chimney bump-out with regular two by four studs with one layer of rock wool. This gave us a continuous wall plane.

I would like to thank our friends Drew and Rubani for their help with the multi-tasking and for putting their minds into this job and keeping me out of trouble!

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