Tag Archives: insulation

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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Framing, insulation, and the real world

Now that I have picked up my last load of rock wool, it’s time throw it into the wall.

We have the perimeter walls in the front (or north two-thirds) of the building already framed and insulated. The back (or south third) is a slightly different beast, because I have no ceiling joists to which I can attach the perimeter wall framing. I had removed the ceiling joists to have enough room to fit the attic insulation.


I worked around this problem by anchoring the top plate of the perimeter wall framing into the masonry wall, and in that process carefully minimized any thermal bridging.


Other than that, the process was similar to what we did in the front of the building: We offset the framing by 3 ½ inches to fit the first layer of rock wool between the back of the studs and closed cell foam. Once the rock wool was in place behind the studs, we set the framing plumb and anchored the top plate to the wall. Last but not least, we installed the second layer of rock wool in between the studs.

This gives us an uninterrupted layer of closed cell foam and rock wool insulation, which greatly improves the thermal envelope because we practically eliminated all thermal bridges.

A real world issue

This worked out really well, because I did the framing and rock wool installation myself. In the real world, however, you probably have contractors doing this work. And this is where it gets tricky.

A carpenter doesn’t necessarily want to deal with insulation, and an insulation installer may not know much about carpentry. Yet both trades are needed at the same time to put this kind of insulation assembly together.

This scenario, where an installation tasks spreads across trades, is not an exception in an energy retrofit. Nor is the fact that contractors find themselves in the situation where they have to think outside the box, such as with the pipe insulation.

There are plenty of contractors out there. But finding the one who brings the right level of attention to detail, who can think on the spot, who is willing to schedule with you and other trades, and who exhibits some level of coordination skills, is like finding a needle in a haystack.

If you are a contractor looking for a way to future-proof your business, turn renaissance and turn on your critical thinking skills. I am pretty sure you won’t run short on projects.

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Last rock wool pick up

We had started to frame out the perimeter walls on the second floor, and at the same time insulate them with rock wool.

Well, the time had come to make one last trip to pick up the last batch of rock wool. If I measured and calculated correctly, this last batch should allow us to complete the 2nd floor insulation. I may need another bag for an odd job here or there. But the big task – the insulation of the building envelope – was about to be completed!


This felt like another milestone. The numbers are certainly impressive:

To insulate our building envelope I purchased 194 bundles (or bags) of rock wool.

That took care of the basement and 1st floor2nd floorand attic.






We unpacked, handled, fitted, and installed a total of 2,328 rock wool batts, each measuring 15 ¼ inches wide, 47 inches long and 3 ½ inches in depth (stud depth). At 4.975 square feet per batt, we installed a total of 11,581.80 square feet.

The total material cost added up to $6,348.37, including taxes. That translates into $0.55 per square foot of 3 ½ inch batts, or $0.16 per board foot (one board foot is one inch over one square foot).

That leaves us with a nice, comfortable, and quiet building interior. That’s right! The rock wool does not just provide thermal insulation, but also sound insulation.

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