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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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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