ᐅ Is Insulation Under the Concrete Slab Beneficial? Experiences and Insights

Hello everyone,

I am currently considering leaving out the insulation under the concrete slab. The house itself will be a bungalow with a floor area of 102 m² (1,098 sq ft), featuring an air source heat pump, underfloor heating, controlled mechanical ventilation, and in terms of plumbing, a circulation line in a timber frame house.

In principle, we ordered a KfW 55 house, but since we have not applied for any KfW 55 subsidies from the banks or government, it would theoretically not matter to me whether the house meets the 2016 Energy Saving Ordinance (EnEV 2016) standards or KfW 55 standards. Because we are not receiving any subsidies, we also do not require a blower door test.

From initial discussions, I have learned that for KfW 55 certification, about 80 mm (3 inches) of Styrodur/Jakodur insulation would need to be installed beneath the slab.

Honestly, I don’t mind if the concrete slab has a U-value of 0.20 W/m²·K (values are estimated) due to insulation, or 0.22 W/m²·K without insulation (also estimated values).

What is more important to me right now is whether I should spend the estimated 2,000 € on the insulation and installation in the ground, which would basically pay off after 20 years by saving around 20 € per year in heating costs.

Or if I should rather invest the 2,000 € in something else for the house that I can use more effectively or benefit from, such as upgrading fixtures or investing in a photovoltaic system (and yes, I know that €2,000 is not enough to cover a full PV system).

Or is every millimeter of insulation really crucial when using an air source heat pump and underfloor heating? (I understand electricity costs won’t get cheaper, so that’s why I’m also considering the photovoltaic system…)

We recently discussed this topic here... I would definitely do the insulation again anytime.

But even more important, I would strongly recommend the blower door test... it helps identify many areas where improvements can be made and where heat might be escaping...

I would skip the insulation. Better to spend a few more euros on improved insulation in the foundation slab and use the rest as a down payment for, for example, a photovoltaic system (smaller systems are not that expensive these days).

nordanney schrieb:

... use as a down payment for, for example, a photovoltaic system (smaller systems are not that expensive anymore).

What does that mean in euros or kWp, I would be interested to know?

nordanney schrieb:

I would skip the insulation. Better to spend a little extra on improved insulation for the concrete slab and use the remaining budget as a down payment, for example, on a photovoltaic system (smaller systems are not that expensive these days).

That’s exactly what we did. We omitted the insulation under the slab and upgraded the insulation on the slab from WLG035 to WLG023.

Andre77 schrieb:

What does that mean in euros or kWp? I’m curious.

You should be able to get prices from around €1,000 (always net, as a business you can reclaim the VAT) per kWp, depending on the manufacturer and components. I highly recommend the photovoltaics forum for more information.