Category Archives: building shell

Staircase insulation

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.

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How was that possible?

A couple of years back, after all major air sealing, we had a blower door test result of 2.1 ACH @ 50 pascal. This February the results dropped to 0.62 ACH @ 50 pascal in a follow up test! How was that possible?

From what I have read and heard, once the major air sealing is completed, it becomes increasingly difficult to tease out additional improvements in the blower door test results. And a drop from 2.1 to 0.62 ACH @ 50 pascal is – well – a massive improvement.

Let’s start by looking at what I mean by…

Major air sealing

In one sentence: We took good care of the building envelope.

On the 1st floor we had an application of closed cell foam followed by a layer of open cell foam. The 2nd floor just got one layer of closed cell foam. The closed cell foam on both floors was acting as our air barrier across the masonry wall plane.

The closed cell foam only provides a functioning air barrier if it is diligently installed. On a sloppy spray job, you may have to contend with leaks in your air barrier. I point to some examples here: on the open cell job on our 1st floor.

We air sealed the top of the attic with drywall, which was taped and mudded. The gaps around the attic edges were sealed with another bead of foam.

All penetrations through the building envelope were diligently sealed, typically with foam. Examples are:

  • The ERV fresh air intake and exhaust
  • The range hood exhaust
  • Any electrical conduits leading to the exterior
  • Any low voltage conduits leading to the exterior
  • The supply and return lines to our minisplits
  • Etc.

You can seal around electrical conduits, but that still leaves with a big hole – the conduit itself. To plug the conduit, we diligently used duct putty.

I made sure we had decent weather stripping on our exterior doors. And then there were the windows. We had good quality replacement windows on the 1st floor, while the 2nd floor still had the old double hung vinyl windows. However, I made sure the perimeter of each window was properly sealed.

This was our baseline that gave us the blower door test result of 2.1 ACH @ 50 pascal. Now let’s take a look at what may have caused the drop to 0.62 ACH @ 50 pascal.

The 1st floor

I took a mental walk through all improvements on the 1st floor, major or minor, since we completed the major air sealing.

There was some additional air sealing on the kitchen back door. This is a fancy way of saying, “I installed some additional weather stripping.” The back door was pretty air tight to begin with, though, so I am not sure that this effort contributed that much.

There was the transom window over the kitchen back door. It had a temporary window that most likely was not properly air sealed. I didn’t replace it with a properly sized window until I ordered the replacement windows for the 2nd floor last fall. This should have contributed to the reduction in air leakage.


I suspect that the biggest impact lies with the air sealing work I did on the 1st floor casement windows. Regular readers may recall my air leakage problems in the corners of the casements. The manufacturer was not able to resolve the issue, but was gracious enough to refund me the money for the leaky windows.

As described in a past post, the problem came down to 1/16 inch and I was determined to plug that gap. I invested a little research time online and found a single coated, low density, PVC foam tape, 1/16 inch thick. I installed it on the operable part of the casement, right across from the gaskets on the casement frame. The foam tape pushed against the gaskets, closed the gaps, and eliminated the air leaks. I could tell it did, because the cold drafts on windy winter days disappeared.



2nd floor

Like on the 1st floor, I did some additional air sealing around the 2nd floor back door. The most significant improvement may have come from my work on the door threshold, which was not properly sealed.

But the biggest reduction in air leaks must have come from the 2nd floor replacement windows, which I installed last fall. I thought I had done a good job air sealing the old double hung windows after we finished with the spray foam installation, but the blower door test numbers tell me otherwise.


I clearly underestimated the amount of air leakage from the 1st floor casement windows and the effect of good quality replacement windows on the 2nd floor. Or, I overestimated the effect of my air sealing efforts on those old double hung windows.

The more I think about it, the more I am convinced that the windows were the major contributors to the drop from 2.1 to 0.62 ACH @ 50 pascal.

So, maybe it is not that difficult to improve your blower door results after all major air sealing after all! Or maybe I have a unique definition of what “major air sealing” means.

And here is another thought: I made it my job throughout our deep energy retrofit to stay on top of all air sealing efforts and moisture management issues. That was my responsibility. There was no hole in the building envelope I didn’t know about or I didn’t make sure was sealed properly.

Most other remodels, gut-rehabs, energy retrofits, or even new construction that follow conventional methods probably won’t have that one person who is in charge of tracking the air sealing. There are a number of contractors and tradespeople coming and going, all doing their own thing, but no one person has a real incentive to pay attention to details – sometimes not even the details of their own work. Air sealing is probably not on their mind, or may not even be a concept to some.

Usually there is the general contractor, who, one could assume, would be in charge of managing all contractors and tradespeople and making sure air sealing is tracked and accounted for. But the success stories seem to be more an exception than the rule.

Building green has an uphill battle ahead with a lack of skilled labor, with contractors that often don’t think an inch beyond their own trade, and with no one on the job site who is responsible for tracking air sealing from the planning stages to the final execution.

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A magic 0.6 – as airtight as it gets

Reaching a major milestone is a reason to celebrate. But what do you do when you exceed all expectations and it takes you by surprise?

Old buildings like ours are notoriously leaky. Even a lot of newer buildings are. That’s something you will notice when you try to escape the cold drafts in your own home on a windy winter day. We know how leaky our building was before we started with the deep energy retrofit, because we conducted a blower door test on the 2nd floor unit. It leaked a whopping 4,763 cfm @ 50 pascal.

The blower door basically depressurizes the building or unit by 50 pascal. At that point you take a reading of the airflow. In our case, the 4,763 cfm @ 50 pascal is the air volume that is leaking through the building or unit. To get to a metric that has a common denominator, the airflow is converted into air exchanges per hour or ACH @ 50 pascal. We ended up with 13.9 ACH @ 50 pascal.

Our results were not that unusual. Blower door test results often range from 7 – 15 ACH @ 50 pascal, depending on the building.

What are the air leakage standards?

Air leakage standards in the U.S. are a moving target. There is no national standard; instead, a builder or homeowner can pick from a list of requirements. Here are some examples:

The PHIUS standard discontinued to use the metric of air exchanges per hour (ACH), and instead replaced it with cubic feet per minute (cfm) of air infiltration per square foot of gross building envelope area, measured at 50 and 75 pascal. The rationale behind this metric is that it can be scaled, unlike ACH. Because ACH is based on the building volume, it allows larger buildings to have a greater rate of air leakage compared to smaller buildings. And where do air leaks occur? At the surface of the building enclosure. Thus it appears logical to measure air leakage based on the gross building envelope area.

How did we do?

We ran a blower door test on the 2nd floor unit after we finished with the spray foam insulation and major air sealing. The results were 720 cfm @ 50 pascal or 2.1 ACH @ 50 pascal. That was a remarkable improvement over the 13.9 ACH @ 50 pascal and caused some excitement.

About a year later we conducted a blower door test on the finished 1st floor unit during a project tour organized by the Chicago Community Loan Fund.That test gave us a reading of 630 cfm @ 50 pascal which also equated 2.1 ACH @ 50 pascal.


I have learned over the past years through reading and talking with green builders that once the major air sealing is done, shaving the last few 1/10th off an ACH @ 50 pascal is painstaking work. It involves a lot of chasing, tracking and sealing small air leaks.

All our major air sealing work was completed. My hope was that I would be able to get a final result of just under 2 ACH @ 50 pascal by plugging the last few leaks I could find. That would have made me very happy!

In for a major surprise…

This past week, I hosted another tour of our deep energy retrofit. It was organized by Eco Achievers, our LEED rater and also a PHIUS+ Rater. Part of the tour was another blower door test demonstration. This time we tested the 1st and 2nd floor unit together.


The reading of 385 cfm @ 50 pascal seemed unreal. So much so that I asked to start the test over again including another recalibration of the blower door equipment. Yet we got the same reading for a second time.

385 cfm @ 50 pascal for the 1st and 2nd floor unit equates 0.62 ACH @ 50 pascal! If I use the PHIUS metric, I come up with 0.05 cfm50 per square foot of gross envelope area.

Do I need to say that it took a while for this to sink in? Never in my wildest dreams would I have thought a less than or equal to 1 ACH @ 50 pascals would be possible on our deep energy retrofit. I guess it’s time to celebrate! And I have to find out how I managed to drop from 2.1 to 0.62 ACH @ 50 pascal.

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Window aluminum cladding

The 2nd floor windows were installed and air sealed, but still needed some protection from the elements. In short, I needed to add some aluminum cladding to prevent bulk water from seeping in around the windows.

I had an interesting experience when I had the aluminum cladding added to our 1st floor replacement windows. I learned that this job requires attention to detail and a specific skill set: Taking accurate measurements.

The contractor I hired for the 1st floor windows was somewhat deficient on both, so I ended up finishing the job myself. With that I felt fairly confident to take on all our 2nd floor windows … except that they are just a little higher off the ground. But that wasn’t a problem. I put up scaffolding so I had a safe working platform 20 feet off the ground.

window-47 window-48 window-49

I bought two coils of sheet aluminum, a bunch of caulk, and rented an aluminum brake (also called siding brake). Because of the elevation I moved slowly but deliberately. I have to admit, it was rather exhausting because of the concentration and focus it took. But after four days of measuring, cutting, bending, trimming, fitting, and caulking, I had all windows cladded. Again, just in time for the winter season. Hurray!

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2nd floor replacement windows

1st floor window cladding – or cluster

1st floor replacement windows

2nd floor replacement windows

My entrance into the world of energy efficient yet affordable windows started with a lot of research, and I mean a LOT of research. I had to acquaint myself with new vocabulary like “triple pane insulated glazing units,” or “solar heat gain coefficient.”

But it was worth it and it paid off. The replacement windows for the basement and 1st floor have served us well. They kept us comfortable on even the coldest days and keep our heating bills low.

I was very happy to tap into my acquired expertise again, this time to get our 2nd floor replacement windows installed. This time around it was a breeze, because our friend Drew and I had already done all the prep on the second floor window bucks a while back.

And on the purchasing side, I went around to check the specifications, performance values, and prices on triple pane insulated glazing units from local manufacturers. And I found what I was looking for – energy efficient windows that did not break the bank.

How did I know that I was looking at high performance windows? The National Fenestration Rating Council (NFRC) developed a national rating system for windows so that a consumer can compare apples to apples.

window-45 window-46

  • U-factor: 0.17
  • Solar heat gain coefficient (SHGC): 0.20
  • Visible transmittance (VT): 0.35
  • Air leakage (AL): ? 0.3 cfm/sf
Fixed casement
  • U-factor: 0.16
  • Solar heat gain coefficient (SHGC): 0.24
  • Visible transmittance (VT): 0.42
  • Air leakage (AL): ? 0.3 cfm/sf

I have written a lot about what these performance values mean, and rather than repeating myself let me link you to one blog post that summarizes the tech talk, or you can click on any of the other links in this blog post.

One thing that I will repeat – because it can’t be said often enough – is the importance of the air leakage metric. Why? Because any insulation is only as good as it is airtight. Take, for example, a ski jacket on a downhill slope. You left the front zipper partially open. Will the jacket keep you warm? No, because the cold air gets in. Same with windows.

The Efficient Window Collaborative recommends windows with an AL of 0.3 cfm/sf or less. A value of 0.3 cfm/sf is like a ski jacket with the zipper partially open. You really want to zip it up. Our target was a value of 0.05 cfm/sf or less.

Our NFRC label says less than or equal to 0.3 cfm/sf. However, to be certain about the actual performance, I asked the manufacturer for the test reports and found that they listed the actual air infiltration at 0.02 cfm/sf. Bingo!

Please note that most labels do not list the actual air leakage value. You will have to ask the vendor for the test data. It is probably the best question you will ask.

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I am just happy that we got the old double hung windows out and the replacement windows in before the winter hit. It will cut down on the heat loss, and changes the look from an abandoned apartment to something resembling a home.

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