Insulating the basement ceiling. Which formwork and Styrofoam to use for the ceiling?

Hello

I am currently building a basement. Above it will be an open shed.
I am currently working on the formwork for the ceiling. I want to lay a 5cm thick Styrofoam insulation on the formwork, then the reinforcement on top of that. Concrete and done.
Now the Styrofoam gets significant pressure while pouring. Later, when the formwork is removed, there is no more pressure.
Can someone tell me which Styrofoam I should use?

Steven

You probably have to calculate that yourself. EPS comes in different compressive strengths, if it gets more intense you need XPS. Entire houses are supported on that. Why don’t you pour the ceiling and then insulate from below with any panels afterward?

Hello Alex

it will be about 400KG per m².
I think the structure is consistent. If I pour first and then insulate with Styrofoam, there will be a lot of cavities between the Styrofoam and the concrete. If I lay the Styrofoam on the formwork and then pour, there are no cavities.
That's what I think.

Steven

The subsequent insulation corresponds to common practice. The boards are glued. This is how new buildings are constructed and cellar ceilings are insulated afterwards. No idea if they won't simply fall off when you remove the formwork.

Hello Alex

I also fear the falling off. That is why I will insert anchors for drywall into the Styrofoam before pouring (5 pieces per board) and screw a stainless steel screw into these anchors. The screw will protrude about 3 cm. That should be enough to hold the boards forever.

Steven

I think you are inventing a construction there that simply isn't necessary and is also more expensive and time-consuming. Pour the slab and stick EPS underneath, and that's it.