Insulation – how much is needed?

We figured out that insulating the building from the inside with spray polyurethane foam (or, in short, spray foam) is the most suitable approach. It avoids potential conflicts with our masonry shell and will help with the moisture management in the brick walls.

The next question is: how much insulation do we need? We can look at it in terms of R-value (thermal resistance) or the depth of the spray foam layer, although both are somewhat proportionate to each other.

Here is what the building code says: R-49 for ceilings, R-19 for exterior walls and R-10 for basement foundations (Chicago Building Code, Chapter 18-13-102.1.1; Building thermal envelope insulation, Table 18-13-402.1.1). The Chicago Green Homes program requires R-52 for ceiling, R-21 for exterior walls and R-15 for basement foundations.

Having our eye on the zero-energy goal, it appears that more insulation or the highest possible thermal resistance is better. But there are limitations we have to wrestle with.

To keep the moisture management of the masonry shell intact, the whole interior wall assembly must have a perm rate of greater than 1. Closed cell spray foam has a better thermal resistance than open cell foam, but also lower perm rates. Limiting the closed cell foam to a 1 inch layer followed by open cell foam should yield the right perm rate and allow for the needed diffusion of water vapor through the wall assembly.

And then there is the space limitation. The building originally had no insulation. There was the outside masonry shell, a ¾ inch furring strip, followed by a ¾ inch wood lath and plaster assembly, which we removed.

Replacing the old 1 ½” interior wall assembly with 1 inch of closed cell foam plus dry wall, would only give us an R-value of around 6.5. Adding more insulation, beyond the 1 inch, would take away from the room size. Here are some scenarios:


My friend David Lemair knew about our effort to balance room size with R-value and pointed me to an article in Fine Homebuilding. I learned that spray polyurethane foam has a point of diminishing returns:

“… you would think that an R-40 wall full of spray foam would perform twice as well as a wall sprayed to R-20 with the same foam, but that is not the case.”

Source: Yagid, Rob; Spray Foam – What Do You Really Know?; Fine Homebuilding, June/July 2009

The article goes on to explain that the increased effectiveness from the R-20 to the R-40 wall is only about 2%. Open cell foam apparently reaches its point of diminishing return at 5 inches, closed cell foam already at 3 or 4 inches. No technical explanation is given to what causes that diminishing return, but I would really like to know!

The puzzle is coming together. We have determined that the closed cell foam must be limited to 1 inch to keep the perm rate greater than 1. It looks like open cell spray foam has its point of diminishing returns at 5 inches. That would give us a 6 inch insulation assembly with an R-value of about 24 that takes 4 ½ to 5 inches away from the room size. This is a good balance between R-value and room size.


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.

6 thoughts on “Insulation – how much is needed?

  1. Marcus, maybe the people at Home Energy magazine ( could help explain the diminishing returns?

  2. Thank you so much for posting the images and information from your building retrofit journey… I’ve learned a ton about green rehabbing strategies from reading your entries that we can apply to our building search and eventual similar project… when sherry daun told us about your blog a couple months ago, I read the whole thing from start to finish and have been following your progress ever since (and had a good commiserative laugh about your struggles to get any information out of the city offices prior to purchase re:ANLAP vacant lot acquisition, as we’d encountered the same run-around before)… great work guys! Hopefully Steve and I can come check out your progress sometime…

  3. On a sidenote I would like to know why you want to keep a perm rate over 1.0 thru the entire interior wall assembly? Are you encouraging Water Vapor movement through your wall? I am an architect and have always seen the recommendation that a vapor retarder or barrier be placed directly behind interior finish, in the form of plastic sheeting or kraft faced insulation. The idea being that as warm heated air passes through assembly towards cold exterior it releases moisture when it reaches the dewpoint. This condensation if allowed to release inside the framing or insualtion can cause rot, mold, decreased insul performance etc., The vapor retarder or barrier blocks the water vapor from reaching the cooler region.

  4. Interesting info I came across in a seminar… The below pdf is a report that illustrates a scenario that uses Fourier’s steady-state heat flow equation to evaluate the performance of a typical wall area with a prescribed R performance value. The results are quite profound… it indicates, basically, that the first inch of insulation represents 80% of the heat flow reduction in the system, the next inch 9%, the next inch 3%, the next inch 2%, the next two inches 1% each and the next four inches only 1%! So in the first 2″ you are getting 89% of the total performance realized in your insulation assembly. The key for success in insulation is to do a through job, sealing all the opening in your building envelope. The punch line simple… the first “R” is the most important. There is a rapid decline in performance thereafter.

  5. Tom, thank you so much for this information. Looks like this was the missing piece in the puzzle. I now understand the rationales behind the diminishing return – or – the dramatic decrease in heat flow reduction with increased insulation depth. Thank you again and I will make sure I pass this information on!

  6. Alex, this is a rather complex issue and I barely got to understand the dynamics with regards to my specific scenario (with the existing masonry shell in the Chicago climate). It is difficult to keep water vapor out of a wall assembly, so it would be wise to allow it to get out (allow for drying) – thus the minimum recommended perm rate of 1. In other words, a vapor barrier can prevent an assembly from getting wet, but it also can prevent drying. Placing a vapor barrier directly behind an interior finish may help during the cold season, but can cause condensation and moisture problems during the warm season.

    The following building science paper does an excellent job in deciphering the vapor barrier issue. It is good reading:

    With regards to my specific case (masonry shell in Chicago), I took additional clues from the following building science paper:

    Hope that this (or the references) will provide the answer to your question.

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