How to Create a Truly Insulated Slab 

Very well spoken and presented.

Great explanation

Here in Northern Europe we make many slabs with even more insulation. First 12 inches of Macadam under the whole slab, that is compressed well. This will drain and be the capillary break layer. The macadam is carefully adjusted with laser level. Then you lay out the border/edge beams for the slab. These are made by aproximately 4 inches of eps insulation and only a thin glassfiber board on the outside. The beam is 16 inches high. On the inside of the beam 12 inches of eps insulation then fills out the whole slab except next to the edge. Most of the slab will then be 4 inches of concrete on top of 12 inches eps insulation. But next to the edge there will be 12 inches thick (and 12 inches wide) concrete beams. With 2-4 inches of eps insulation on the outside.

What prevents frosty from taking the edges? A frost protected shallow foundation, insulates the entirety of the slab so the frost doesn’t get under the edges.

I learned about insulated raft slabs after becoming interested in building science. Yet I don't see them being used very often by anyone, including the "building science types".

I am surprised that it is not a code requirement in Missouri for there to be insulation in a slab foundation. In PA we are required to have either 2 or 4 ft of continuous insulation from the top of the slab, depending on the climate zone you are in. The insulation can be either verticle or horizontal or a combination of both.

If you're adding a thermal break, then it's no longer a monolithic slab, but rather a perimeter foundation & inner floating slab? Edit: Nevermind. I watched again & see where you poured traditional stemwall below frost depth.

Great idea, I've always wondered what slab on grade didn't have this. What's the additional cost of adding the foam as a thermal break?

Makes a lot of sense. I don't understand why this isn't standard, especially for northern climates.

Thank you for the video. I think this is a great idea to do to all homes. I wish you took it one step further and installed radiant heat into that slab before it was poured. It would have been more efficient than any other type of heat you can put in that home. Even still, what you did is 100% better than what is usually done.

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We got more advanced syustem here in europe ... the whole slab is inside a foam jacket... wall included. Only a small part of the heat is exiting from the slab ... more exit from walls and even more from the roof. There is a need for continuous insulation around the house.

Isn't the standard the Frost Protected Shallow Foundation (FPSF) where the insulation goes on the outside of the foundation along with a horizontal insulation extending out X inches depending on your Air Freezing Index? Just a thought.

How does this differ from frost protected shallow foundations? I would like to have one of those on my next house.

Your insulation design is a good idea but it only provides thermal separation from the outside concrete. If you had hydronic tubes to also heat the slab and your thermostat was set on 76, the slab would be 78 or higher. The example you used of granite counter tops was a good analogy. Imagine the comfort of heated floors.

How do you finish the 1 1/2 rigid foam insulation on the stem wall/curb? I plan to pour slab on grade with hydronic heat and will have R14 under the slab spray foam or rigid. I will also have rigid foam on the stem wall interior and exterior. Do you bevel 45 degrees the rigid foam on the interior? I plan to use 2 inches on the exterior too and I’m wondering a good detail on how to finish? Thank you, Charlie

That blue foam against your pre-poured exterior "stem wall" will also serve as an expansion joint I would think as well? Am I to understand that this slab has no radiant heat pipes installed inside it as well?

Is there any risk of upheaval or subsidence with the interior slab being uncoupled from the perimeter footing?

So your stud wall (2x6 5.5” wide) sits partially on top of the 4” curb wall and partially on top of a piece of insulation board correct?

Hi Jake, How do you handle the slab insulation at the door openings?

they're using XPS for insulation. However, is it possible to use EPS of some density?

Jake, I just watched your video and had a few questions. I’m a homeowner in Ohio, adding a covered screened in patio to my home. The base will be concrete 14x18. The contractor wants to do 1” blue Dow insulation foam board in the concrete pad. 3-4” gravel base and 4” concrete. I’ve never heard of using the blue Dow. Is this a common acceptable practice that I should not be concerned about? Please help.

The parameter of the slab must have insulation. There needs to be a 4 ft slope from the slab with ridged foam for frost!

#1: so that foam is not under the entire slab? #2: How do I know if I can just trench and not build forms? Is it just a judgment call?

Modern building need to be built without any thermal bridges. Slab edge detail like helps to reduce thermal bridging and should be standard. In fact not only should be standard but the foam thickness should be increased to achieve higher u-values suitable for the climate. This is standard when building towards Passivhaus standards.

Heated floor

What holds the slab to the foundation? Couldn’t they separate?

Thermal Nosebleed

do it out of icf...

Jake, I think you're mixing up emissivity with thermal conductivity in the context you're using. Different surfaces feel like different temperatures to human skin (which is basically a calorimeter) because the thermal conductivity of different materials.

Ummmm how is the slab connected to the exterior concrete wall?? if it is not well I will not get into the problems that will happen LOL 🙂

If you are adding foam why not 6 or 10 inches