Floor slab insulation "Yes!" or "No!"

Hi,

to be honest, I don't quite understand the whole discussion.

If you want to heat your house, you should insulate all exterior components. Including the [Bodenplatte]. This usually consists of reinforced concrete, and guess how much heat is lost there.

Besides, you won't be able to avoid insulating your [Bodenplatte] just because of the energy saving ordinance.

I have never heard of not insulating a [Bodenplatte]...
How did you even come up with the idea? How else is one supposed to meet the thermal protection requirements?

Regards

This is about an additional insulation of approximately 8 cm. Not about a vapor barrier or anything similar. Maybe you didn't quite understand that.

Also, I cannot recall that the [Energieeinsparverordnung] states that the floor slab must be insulated...

I think he means an additional insulation apart from the standard insulation.

As my predecessor already wrote, I believe 8cm insulation is standard.

If you now insulate more, for example, you achieve a slightly changed KfW value. For example, it might be that you currently have a KfW 70 house with an energy saving ordinance calculation of 60.58, as we are currently planning.

With additional insulation, it might even be that you achieve KfW 55. This would again have the advantage that firstly you save heating costs and secondly you even receive a repayment bonus from the KfW.

Hello E.Curb,

Not insulating a slab on grade, I've never heard of that... How do you even come up with that idea? How else are you supposed to meet the thermal protection requirements?

The question is not "not insulating," but rather the experimental clarification regarding the meaningfulness of additional insulation below the slab on grade.

As a rule, strip foundations are laid all around below the slab on grade down to the frost depth, and then the well-known capillary-breaking layer along with foil covering is installed. The then most reasonable way – also from a cost-benefit perspective – is, in my opinion, further insulation above the slab on grade; aside from the fact that it is insulated all around anyway. All these dreadful calculation examples from planners in Brussels (as of 2025 only passive houses, etc.) primarily serve their "green conscience"; no one can yet say what will happen to insulation below the slab on grade over the years.

A good example of this is the still valid Energy Saving Ordinance 2009. Every builder has to insulate their house so heavily that, without a ventilation system, mold would otherwise form; even with monolithic systems it may be advisable. Great progress – insulating here and damn the forced ventilation of windows from earlier years – on the other hand, the consequence of mandatory automated ventilation of houses because otherwise mold formation threatens. Or, also a telling example: an ETICS is installed, which upon future tightening of the Energy Saving Ordinance must be disposed of as hazardous waste before new insulation materials can be applied.

@ krausf
With additional insulation of the slab on grade alone, you will achieve neither KfW 70 nor KfW 55 status.

Kind regards

This is about an additional insulation of approx. 8 cm.

What is the thread called again? Insulating the floor slab yes or no?
Or: additionally insulating the floor slab on top of some standard insulation (whatever a STANDARD INSULATION might look like)?

You have been talking about insulating a floor slab yes or no the whole time. And the correct answer is: yes.

Whether you need additional insulation is told to you by your energy performance certificate. What standard do you want to achieve? If you want to know when additional insulation pays off, then have an energy consultant calculate it for you. Nobody can say that in general terms.
But basically: Wrap your house as warm as possible so that you consequently have less energy demand and can dimension your heating system accordingly. From when it becomes uneconomical is difficult to say in a forum.

Not about a vapor barrier or anything like that. Maybe you didn’t quite understand that.

But I didn’t even talk about a vapor barrier.

Besides, I can’t remember that the Energy Saving Ordinance says that you have to insulate the floor slab...

True, the Energy Saving Ordinance says nothing about additional insulation, how could it.

According to the Energy Saving Ordinance you have to meet two requirement values.
1. Primary energy demand
2. Transmission heat loss, which is the heat loss through the exterior components. And that also includes the floor slab. In the calculation of the transmission heat loss, the U-value plays a very important role.
The transmission heat loss is, simply put, a total of all losses. And the losses are small when the U-value is low and large when the U-value is high. So if you don’t insulate your floor slab now, then you have a U-value of, well, I estimate around 3.0 W/m²K. With an insulated floor slab, say 8cm - 10cm, you have a U-value of about 0.25 - 0.30 W/m²K. Your remaining components are also in the range of 0.15, 0.2, 0.25, 0.3.
If then suddenly a huge surface with a U-value of 3.0 appears, the required value can no longer be met.

Regards

@ krausf
Adding insulation to the floor slab alone will not achieve either the KfW 70 or the KfW 55 status.

Kind regards

So we are building a KFW 70 house, which, thanks to the additional heat pump, has an energy value of just under 60.

The architect said that with additional insulation of the floor slab, one can reach KFW 55. But the price for this additional insulation is disproportionate to the benefit from the repayment grant.