Cutting Edge Insights
with Dr. Lee

This Is Not a Bridge to Nowhere

Q:  Hey, that picture isn’t very sexy!

A:  I know; it’s a picture of a project I have going now at my house.  I’m currently in the “it has to look worse before it looks better” phase of operations.

Take a close look at the thin blue strip above the window.  That’s a ½ inch thick strip of insulating foam; it’s about 10 inches high and 80 inches wide.  It covers the window header.

Take a look at the left and the right of the window.  The white is dense-packed cellulose insulation.

I ripped out all the existing fiberglass insulation in the 2×4 wall cavity so that I could caulk all 4 corners in each stud cavity and spray foam all electrical penetrations (improved air barrier).

Q:  Why wasn’t the existing insulation good enough?  It had been there since 1969, correct?

A: 1.  Imagine trying to fit a piece of fiberglass insulation around an electrical box.  There will always be some imperfections in the fit.  Imperfections mean air leakage.

2.  Yes.

Q:  Why did you dense pack the wall instead of using spray foam?  I hear that spray foam also fills wall cavities better than traditional fiberglass insulation.

A: 1.  Open cell spray foam isn’t as good of an air barrier as dense pack cellulose.

2.  Closed cell spray foam is a vapor barrier, and you do not want a vapor barrier inside your wall.

3.  Both open and closed cell spray foam are subject to off gassing (VOCs), which can be worse if the chemicals in the mix are not applied in the correct proportions.

4.  Dense pack is less expensive than spray foam as well. A better solution at a better price.

5.  Dense pack cellulose is orders of magnitude more environmentally friendly than spray foam (it has a lower GWP, Global Warming Potential) – more on this in the future.

Q:  What’s the significance of “bridge” in the title to this article?

A:  Oh, I almost forgot.  Back to the blue strip of foam.  A “thermal bridge” is an area of framing/structure through which heat and cold can freely flow via conduction.  In the picture, the window header, which may be in direct contact with the exterior brick, is a thermal bridge from the inside to the outside.  In the winter, heat from the (eventual) interior drywall surface is easily conducted through the window header to the brick, like a vicious cycle, making it harder and harder to keep such a room at a comfortable temperature.

You can’t stop the flow of heat via conduction; you can only slow it down.  You’ll notice that the wall area on each side of the window was furred out from the studs with ½” plywood – most likely, that was a 1969 effort to counteract thermal bridging by introducing a ½” air gap.

Leaving the ½” plywood in place accomplishes 2 things:

  1. I was able to dense pack an extra ½” of cellulose.  Not a big deal at R-4.2 per inch, but better than nothing.
  2. I was able to place the blue foam against the window header. It is the same thickness as the ½” plywood and it is a 2021 method to reduce thermal bridging. This is way better than leaving an air gap, which is a 1969-style anti-thermal bridge.

Q:  But the blue foam is R-5 per inch and the cellulose (at 4 lbs. per cubic foot) is R-4.2 per inch, that’s not a great deal better!

A:  You can’t dense pack a ½” deep cavity.  Solid foam was the only option.

Q:  What type of foam is that?

A:  XPS.  Short for extruded polystyrene.  Not to be confused with EPS – expanded polystyrene.  Still not to be confused with polyisocyanurate foam.

Can you believe that my local lumberyard employees did not know the difference?

More on the differences in a future article.

Q:  If I am going to remodel my (house, apartment complex, commercial building), and I see opportunities to decrease thermal bridging in a similar way, is there a limit to how much I can increase the energy efficiency of the wall assembly?

A:  Yes.  R-value, the measurement by which we compare insulating efficacy of different materials, pertains to resistance to heat flow.  You can’t stop heat flow; you can only slow it down.  You can’t create energy; you can only limit how fast it flows from one surface to another.  And you can’t fill your entire wall with insulation; you have to have some structure present.  And here is the dissonance: structure is strong (wood, steel, concrete); but it is very poorly resistant to the flow of heat.  Achieving balance between structure and energy exchange is always the challenge.

As you think about that, keep in mind that utility rates, building materials, and labor keep going nowhere but up.  As you tackle any remodel project, always ask yourself whether you can design a functional solution that also consumes less energy.

Until next time,

Dr. Lee Newton

What real estate investing, building science, or development questions do you have?  I’ll be happy to answer them here.  Send them to


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