Building a basement shop

The foundations of a subterranean workspace

By Steve Maxwell

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A growing number of contractors now offer site-applied spray-on foam, and this works very well. But if you’re interested in saving money with a DIY strategy, look for 24″ x 96″ rigid sheets of foam with rabbeted edges. These are made specifically for finishing basement walls. Position the sheets long, edge to long edge, against your wall, nestle wood strapping into the grooves formed by neighbouring sheets of foam, then fasten the wood to the masonry wall using masonry screws sunk into predrilled holes. The wood holds the foam to the wall and also provides an anchoring surface for wallboard.

One basement location that’s almost always insulated poorly is the place where the first-floor joists meet exterior walls in basement ceilings. Standard practice for insulating these areas involves stuffing spaces between floor joists with insulation batts, then attempting an ill-fated plan to seal this insulation behind poly vapour barrier. But there’s simply no way to seal effectively around all those joists. Air leakage, cold-weather condensation and mould growth inevitably develop behind the batts at the points that touch the outside members of the floor frame. It’s almost universal. If there is batt insulation around floor joists, pull it back and take a look. That grey or black stuff on the back face of each piece is mould.

Professionally applied spray foam also works exceptionally well where floor joists meet outside walls; however, you don’t necessarily need to call in a contractor to enjoy the same benefits. For a mould-proof, DIY-friendly finish, cut pieces of 2″-thick extruded poly foam so there’s approximately 3/4″ of space all around the perimeter of each piece. Glue the foam to the 2-by lumber forming the outside rim of the floor frame using construction adhesive or hot-melt glue. As you work, maintain the 3/4″ gap all the way around each piece of foam, then finish by filling this gap with expanding polyurethane foam from a can. This last step completes the insulation layer perfectly, while also preventing warm, moist indoor air from migrating to the back face of the foam where it would cool, condense and cause trouble during winter.

You can woodwork on a concrete floor, but some kind of subfloor will make your legs and feet feel much better at the end of the day. Interlocking oriented strand board (OSB) tiles (24″ x 24″) are made with a dimpled plastic membrane on the bottom face for basement subfloor applications. They protect against upward moisture migration, while also allowing the subfloor to be more tolerant of an actual leak. In tests I’ve conducted, OSB basement floor panels even withstood several days of total submersion without any significant side effects.

You should also consider boosting sound insulation while finishing your basement space. Resilient channel is one method to use. Lengths of sheet-metal strips screw into ceiling joists and offer a surface that accepts screws anchoring drywall. Resilient channel breaks most of the physical connection between drywall and the floor frame above, reducing sound transmission upstairs.

You can apply resilient channel to walls, but I prefer staggered 2×4 wall studs on a 2×6 wall plate. With a layer of fibreboard sheathing underneath 5/8″-thick drywall, you have a solid wall that’s easy to anchor things to and is quite quiet.

Finishing your basement is the easiest way to get workshop space. Do it right, and you’ll have a warm, dry and comfortable place to enjoy your favourite hobby.

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