ᐅ Insulation of the Ground Slab: Yes or No?

Hello,

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

If you want to heat your house, you should insulate all the exterior components, including the concrete slab. These slabs are usually made of reinforced concrete, and you can imagine how much heat is lost through them.

Besides, due to energy efficiency regulations (building codes), you won’t be able to avoid insulating your concrete slab.

I have never heard of a concrete slab being left uninsulated...
Where does that idea come from? How else are you supposed to meet the thermal insulation requirements?

Regards

This is about an additional insulation layer of approximately 8 cm (3 inches), not a vapor barrier or anything similar. Perhaps you did not fully understand that.

Also, I cannot recall the energy efficiency regulations stating that the ground slab must be insulated...

I believe he means an additional insulation layer beyond the standard insulation.

As my predecessor already mentioned, I think the standard insulation is 8cm (3 inches).

If you insulate more, for example, you can achieve a slightly improved KfW rating. For instance, you might currently have a KfW 70 house with an energy-saving regulation calculation of 60.58, which is what we are planning at the moment.

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

Hello E.Curb,

E.Curb schrieb:
I have never heard of not insulating a concrete slab... How do you even get that idea? How else are the thermal insulation requirements supposed to be met?

The question is not about "not insulating," but rather about testing the relevance of additional insulation underneath the slab.

As a rule, strip footings are installed around the perimeter below the slab down to the frost line, followed by the common capillary-breaking layer along with a vapor barrier. The most reasonable approach—including a cost-benefit analysis, in my opinion—is to insulate above the slab; aside from the fact that the perimeter is insulated anyway. All those tedious calculation examples from planners in Brussels (by 2025 only passive houses, etc.) primarily serve their "green conscience"; no one can yet say what happens to insulation under the slab over the years.

A good example is the currently valid Energy Saving Ordinance 2009. Every builder must insulate their house so well that without a ventilation system mold would otherwise develop; even with monolithic construction systems, this can sometimes be recommended. Great progress—insulate here and demonize old-style window ventilation—yet on the other hand, the consequence is mandatory mechanical ventilation of houses to prevent mold. Or another telling example: Exterior insulation systems (ETICS) are installed, which, in the event of future tightening of energy regulations, must be disposed of as hazardous waste before new insulation materials can be applied.

@krausf
Adding insulation solely to the slab will not achieve KfW 70 or KfW 55 certification.

Best regards

katinka schrieb:
This is about an additional insulation of approximately 8 cm (3 inches).

What was the thread title again? Insulating the slab – yes or no?
Or: insulating the slab in addition to some kind of standard insulation (whatever a STANDARD INSULATION might look like)?

You have been talking all the time about whether to insulate a slab or not. And the correct answer is: yes.

Whether you need additional insulation depends on your energy performance certificate. What standard do you want to achieve? If you want to know when additional insulation pays off, have an energy consultant calculate it for you. No one can say that generally.
But basically: insulate your house as well as possible so that your energy demand is lower and you can size your heating system accordingly. When it becomes uneconomical is hard to say in a forum.

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

I didn’t mention vapor barriers at all.

katinka schrieb:
Besides, I can’t recall the energy saving regulations stating that the slab must be insulated...

That’s correct, the energy saving regulations do not explicitly require additional insulation, how could they?

According to the energy saving regulations, you have to meet two requirements.
1. Primary energy demand
2. Heat transmission loss, which is the heat loss through the building envelope. This also includes the slab. In calculating heat transmission loss, the U-value plays a very important role.
Heat transmission loss, simply put, is an indication of all losses. Losses are small when the U-value is low and large when the U-value is high. If you do not insulate your slab, the U-value is about, I estimate, 3.0 W/m²K (0.53 Btu/ft²·h·°F). With an insulated slab, say 8 cm (3 inches) to 10 cm (4 inches), you get a U-value around 0.25 – 0.30 W/m²K (0.04 – 0.05 Btu/ft²·h·°F). Your other building components usually have U-values in the range of 0.15, 0.2, 0.25, 0.3 W/m²K.
If suddenly a large area appears with a U-value of 3.0, the required overall value cannot be met.

Regards

Bauexperte schrieb:
@ krausf
Simply adding extra insulation to the floor slab alone will not achieve KfW 70 or KfW 55 standards.

Kind regards

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 extra insulation of the floor slab, it is possible to reach KfW 55. However, the cost of this additional insulation is not proportional to the benefit gained from the repayment grant.