Double duty

The closed cell spray polyurethane foam (SPF) came in handy while solving a problem with the attic insulation.

We used salvaged four inch XPS boards under the roof joists in the attic. Because salvaged material is rarely perfect around the edges, I was left with gaps where the boards meet.

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


These gaps needed sealing to prevent warm air and moisture from migrating into the insulation assembly. The associated energy loss — and even more so the wetting of the roof assembly — would be counterproductive or even dangerous.

Taping over those gaps didn’t seem feasible. I thought about using spray foam cans to fill the gaps. That would would be a big and expensive job.

Sealing the seams with closed cell SPF, however, is relatively straight forward, particularly with our installer and his attention to detail.


During the SPF installation, the installer “over sprayed” the top of the wall to properly seal the interface between the XPS boards and the masonry wall. From there he moved on to the ceiling, and sealed the joints between the boards with foam.

I got the gaps between the boards closed up. But it doesn’t entirely solve the problem of air or moisture from migrating into the roof assembly. There will always be some imperfections, leaving small pathways. My plan to manage that moisture is a good subject for another blog post.

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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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