What is the best way to insulate a floating slab on grade foundation?

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Insulation on interior or exterior of frost wall - or both - for floating slab?

Marc-Andre Roy Oct. 4, 2021Last updated: Oct. 4, 2021

I am building a foundation for a floating slab. The ~4' foundation walls sit on footings.Is it better to insulate on the inside or outside of the foundation walls, or both inside and outside?

Responses (5)

Marc-Andre Roy
Marc-Andre Roy Oct. 4, 2021, 2:23 p.m.

Thank you. To be clear, there is no basement. This is a floating slab with a 4' frost wall (on footings) around the periphary. The contractor is suggesting that 2" insulation on interior or frost wall is all that is needed. Frankly, I have seen so many different approaches online hence my confusion. Any help clarifying would be appreciated.

Mike Reynolds Oct. 5, 2021, 9:37 a.m.

It will work fine, but it is a lot of excavation and a lot of concrete for no reason. Many builders and even homeowners have it in their head that you 'have' to go below the frost line, but that is just out of familiarity because it's what was always done. That contractor is factoring dirt as insulation, where if you use 'insulation' as insulation you can do it all pretty much on the surface. 1 foot of dirt insulates about the same as 1 inch of EPS foam insulation, so you only have to go down 4 inches instead of 4 feet. 

Where are you building Marc-Andre? 

Marc-Andre Roy
Marc-Andre Roy Oct. 6, 2021, 2:23 p.m.

Thanks. Building in area of Wakefield QC. I would have preferred monolithic slab but contractors indicated more complicated, less experience with this,  potentially more difficult for permitting, and no real cost savings. 

Mike Reynolds Oct. 6, 2021, 2:52 p.m.

Any type of contractor will steer their clients towards what they like and are familiar with, that's not new. These slabs are actually less complicated because they come with step by step instructions and training for installers, but it may look complicated to anyone who hasn't built one.  Permitting is never an issue either. As for cost, sometimes it is more expensive sometimes its cheaper, that's really on a case by case basis and what you are looking for.