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Q Can I install ceramic tile over rigid foam insulation on a floor? I’m planning to put 2” of rigid foam over an existing subfloor on a raised building with no basement, then another layer of plywood on top, with tiles over electric infloor heating. I’m concerned about the floor flexing downwards because of the foam, allowing tiles to crack. Cubic Zirconia Veneers
A This is the exact approach we took at my son’s house when we built it, and it has worked perfectly for more than 5 years. That said, you’ll want to make sure a few key details are covered.
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First, you’ll definitely want to use an uncoupling membrane over the top layer of 5/8” plywood subfloor. This greatly reduces the chance of tile cracking and grout failure. Uncoupling membrane is a dimpled plastic sheet that allows tiny amounts of side-to-side movement of the tiles in relation to the subfloor. I’d never install any kind of ceramic tiles without an uncoupling membrane no matter what kind of floor was underneath. A company called Schluter invented modern uncoupling membranes and they still do the best job. Their DITRA-HEAT product is an uncoupling membrane that’s also made to accept infloor heating cables, so it’s perfect for your situation.
You’ll also need to make sure your floor assembly is as rigid as possible. Use large-head structural screws (not deck screws) to secure the top layer of plywood through the foam and at least 2” into the underlying floor joists. CAMO and Headlok are two brands of structural screws I’ve used and like. Drive these screws every 10” to 12” along each joist.
Like other brands of electric infloor heating, you shouldn’t rely on DITRA-HEAT exclusively for space heating. It’s really meant to make the floor warm while the main heating system keeps air warm.
Q Does spray foam still need a poly vapour barrier applied over it when used between wall studs and rafters?
A In cold climates like ours here in Canada, vapour barriers prevent warm indoor air from infiltrating wall and roof structures, then cooling enough as it moves towards the outdoors to cause water to condense out of the air within the building frame. Preventing this trouble is why sheets of clear plastic are applied to the warm side of walls and ceilings when using fibre-based insulation, but the situation is often different with spray foam.
Broadly speaking there are two types of spray foam in the world, closed cell and open cell. Closed cell spray foam acts as its own vapour barrier when it’s thick enough, and this is one reason it’s such an amazing product. Minimum thicknesses required for built-in vapour barrier action vary from brand to brand, but 3” will be enough with any type of closed cell foam I’ve researched.
One more thing . . . if you’re applying spray foam between rafters to create an open, cathedral ceiling, this means the roof surface to get very hot in summer. Regular asphalt shingles will fail prematurely under this heat, but fibreglass shingles stand up just fine. So will a metal roof.
Making a basement sump hole
Q How can I make a hole in my concrete basement floor to install a sump pump? We’ve been dry for years, but last spring we had a flood and I want to make sure it never happens again.
A Your concrete basement floor will be about 4” thick, and the task involves removing a circle of concrete to accept a plastic sump liner that sits below the floor surface. I’ve done this by drawing a circle on the concrete that’s a little larger than needed for the sump, then drilled a series of 3/4” to 1” diameter holes on that circular outline using a rotary hammer. It then becomes easy to break out the concrete – something like breaking out a stamp from a perforated sheet.
Steve Maxwell has been helping Canadians make the most of their homes since 1988. Visit him online at baileylineroad.com and join 30,000 people who get his Saturday morning newsletter each week.
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