Insulation preps – plugging the 3” gap

This thing is persistently following me: I think I completed a task, only to learn that I am merely halfway through.

I cut the three inch thermal break around the first floor edge and thought I was done. But something in the back of my mind kept nagging me:

How does this three inch gap work with the spray foam insulation?

The spray polyurethane foam (SPF), in particular the open cell foam has a very high expansion rate (1 in 100). When the foam is applied to the basement wall and ceiling, it would bubble up through the three inch thermal break and make a big mess.

While thinking about how to resolve this problem, I had a ‘that pile of trash is now my treasure’ moment.


I had a pile of two inch XPS insulation scraps lying around. I had moved them out of the way a number of times and was ready to throw them out. Thank God I didn’t!

The scraps were easily cut into three inch wide strips that I then jammed into the three inch wide thermal break gap. Not only did I successfully plug that gap, but I did so maintaining the effectiveness of the thermal break.

I only hope that experiences like this won’t turn me into a hoarder.

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.

2 thoughts on “Insulation preps – plugging the 3” gap

  1. I like the logic of what you’re doing with the 3″ thermal break on the floor, my question is how do you intend to cover it? Are you planning to cover all the brick with foam all the way up the building?

    On another note, I put 6″ of spray foam in my basement 2 years ago, and the results are phenomenal (2″ gap from the wall, then a 2×4 stud wall for a total value of R-36 give or take). My basement is now significantly warmer than the main floor in the canadian winters that i’m facing. The closed cell foam was roughly double the cost of fiberglass, but the benefits of air sealing, and the elimination of any chance of mould growth in the future are well worth the price. Really enjoying the blog so far

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