Frost protected slab with insulation 'blanket' on ground - use drain tile?

[Tom Sestito]

Hi all, as the thread title suggests I'm building a heated slab, with insulation under and 6' out from the edge.

The engineer doesn't specify a drain tile....

Questions:
- Any experts think I need a drain tile on the outside (my gut says yes)
- would it go on above the perimeter insulation or below? OR would it go 6'out at the edge of the insulation?

I've searched all over and can't find an answer to my second question. Note that I'm in an unorganized territory and there are no permits required so I don't have to satisfy an inspection - but I still want to do it right.

 

[jack stand]

What's your soil's and groundwater levels?
Well draining soil, high and dry (groundwater) and you may not need anything and that slightly pitched outward sub surface insulation is moving whatever surface water percolating through the soil away from where you don't want it. If you do, just at/below your 6' insulation.

 

[WisJim]

When we built my garage and shop, my perimeter insulation was 2 feet down and extends out 2 feet (I think) and I installed drain tile above the insulation close to the building. I have good drainage but thought this was cheap insurance against water problems.

 

[WisJim]

With our sandy soil, my wife was able to dig the trench on one side of our 40 foot building and put in the tubing in an afternoon. She's a good worker but mostly her back does shoveling better than mine does.

 

[ConCretin]

I don't see any need whatsoever for a drain tile. Deal with water at the surface with proper grading.

 

[Tom Sestito]

What's your soil's and groundwater levels?
Well draining soil, high and dry (groundwater) and you may not need anything and that slightly pitched outward sub surface insulation is moving whatever surface water percolating through the soil away from where you don't want it. If you do, just at/below your 6' insulation.

The soils are mostly sand above clay. The clay layer varies between 6' and 1' below grade. We are lakeside (this is for a cottage) with a slight slope above us draining down to the lake.

I think the best solution is as you say, out to the edge of the 6' insulation. The insulation itself, and properly graded ground will drive any surface/subsurface water to the outside.

There's no point putting drain tile at the foundation and under the insulation. And I don't think I have enough room to put it above the insulation. So that leaves it out on the edge.