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Insulating the underside of cast in place concrete garage slab

rden | Posted in Energy Efficiency and Durability on

We have a 1960’s 2-story detached garage.  It has poured in place concrete walls and a on-way structural concrete garage floor slab supported ½ way the width of the garage with a steel beam and 2 steel posts.  The space below the garage is currently just used for storage and is a ‘walk out’. It only has 7’-2” clear between the floor and underside of concrete slab above.  The beam cuts that to 6’-5” clear.

We are about to have part of the structural slab repaired by demoing out a portion, shoring, splicing in new reinforcing steel and pour new concrete due to moisture issues and corrosion.  A new floor coating would then be installed to control the water & salt issues.  We are in zone 5 of Michigan. We are hoping to move our workshop to part of the space below the garage to get more space inside our house. I am considering installing a split system to help making the shop comfortable with the temperature swings & hopefully condensation?  The garage space above is unheated and not insulated.

The question is how to insulate the ceiling and walls.  I am not going to occupy it for more than a few hours on the weekend but am not thrilled by the idea of leaving my tools in an unconditioned space. Mainly concerned about condensation & rust with temperature swings. I am considering 2” of closed cell foam on the ceiling to keep 7’-0” building code clear.  I am also considering finishing out just half of the space.

Any comments are welcome! Right now we are trying to figure out if this makes any sense.  I don’t need perfection but I don’t want to make a big mistake either. Thanks!

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  1. Expert Member
    NICK KEENAN | | #1

    Around here spray foam needs to be covered for fire protection. I'd go with 2" of R-MAX sheet insulation all around, taped at the joints, gaps filled with can foam. It is fire-rated so it doesn't need to be covered, although your local authority may or may not agree. . There are nylon anchors made for attaching it to concrete

    On the walls you'd want to cover it with something you can hang tools from.

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