Advancing on the attic

It’s time to move up onto the 2nd floor, where the attic is calling.


A lot of things have to happen here that are insulation and green roof driven. But let’s start with the insulation piece.

Heat rises through the building and would escape at a staggering rate through the attic, unless it is properly insulated. While we are content with an R-value of 26 to 28 in the walls, we will need to shoot for R 60 in the attic to effectively reduce our overall energy demand.

I can’t recall how many different insulation strategies I devised, all because of a scheduling issue. The original idea was to use polyurethane spray foam (SPF) in the attic. Because the SPF would stick to the roof deck, any repairs to it would need to happen first. We don’t want to touch the roof for the next few years, which takes the spray foam option off the table.

Instead I needed an insulation material with a good R-value that does not stick to the bottom of the roof deck, is easy to install, and has some recycled content.

It turned out that rock wool (or mineral wool) could be a perfect candidate. An added benefit is that rock wool is significantly less expensive than SPF but offers the same or better insulation value.


If I fill the space between the 2 by 10 old growth roof joists with three layers of R-15 rated rock wool, I would still end up 15 units short of our R-60 goal. Adding a four inch layer of insulation foam board (XPS) under the roof joists would yield another R-20. It would get us to an R-value of 65 (R-45 for the three layers of rock wool, plus the R-20 for the foam board). Mission accomplished!

Edit: The described insulation assembly does not comply with important  building science principles. For more information read the following post: Do-over dilemma


I considered a variety of products that I could add under the roof joists. The foam board made the cut because it would be easy to install and it is likely that I find the material in the salvage and reuse market.

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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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