ᐅ Insulation Under Reinforced Concrete Slab for KfW 55 Standard
Created on: 20 Feb 2018 22:07
T
turbotopo
Hello everyone,
We are going to build a prefabricated house on a concrete slab foundation. Our builder told us that with KFW55 standard, insulation beneath the slab is not required.
Now we are wondering whether we should add it anyway and if it is a worthwhile investment.
The concrete slab:
Reinforced concrete slab as a load-bearing element made of in-situ concrete (c20/25, approx. 15-20cm (6-8 inches) thick, edge reinforced) with reinforcement according to structural requirements.
Costs for the insulation:
120 mm (5 inches) insulation with thermal conductivity class 040 under the slab of the residential building: 3115€
160 mm (6.3 inches) insulation with thermal conductivity class 040 under the slab of the residential building: 4930€
Will these costs ever be recovered through energy savings?
Best regards
We are going to build a prefabricated house on a concrete slab foundation. Our builder told us that with KFW55 standard, insulation beneath the slab is not required.
Now we are wondering whether we should add it anyway and if it is a worthwhile investment.
The concrete slab:
Reinforced concrete slab as a load-bearing element made of in-situ concrete (c20/25, approx. 15-20cm (6-8 inches) thick, edge reinforced) with reinforcement according to structural requirements.
Costs for the insulation:
120 mm (5 inches) insulation with thermal conductivity class 040 under the slab of the residential building: 3115€
160 mm (6.3 inches) insulation with thermal conductivity class 040 under the slab of the residential building: 4930€
Will these costs ever be recovered through energy savings?
Best regards
T
toxicmolotof22 Feb 2018 12:55So, 6cm (2.4 inches) of insulation on the slab, and you want to achieve KFW55?
Do you have access to the calculation documents? As a layperson, I find it hard to believe that this would be compatible.
Do you have access to the calculation documents? As a layperson, I find it hard to believe that this would be compatible.
T
turbotopo22 Feb 2018 13:12What exactly should I have changed? More insulation?
The supplier is always open to discussion.
The supplier is always open to discussion.
T
toxicmolotof22 Feb 2018 13:57How are we supposed to know? What does the energy calculation say? What does the expert who is supervising the confirmation for the KFW say?
You insulate only as much as specified by the calculation from the energy consultant (or whoever is responsible for the thermal protection verification) to meet KfW 55 standards. If they say 6cm (2.4 inches) below the screed is sufficient, then that should be the case—although I personally find it unlikely.
For a building constructed to KfW 55 standards, the energy consultant recommends a total of 20cm (8 inches) of insulation with a thermal conductivity of 0.035 W/(m·K). This results in a building component with a U-value of 0.164 W/m²K. However, this is only an example; conceptually, other solutions are certainly possible.
For a building constructed to KfW 55 standards, the energy consultant recommends a total of 20cm (8 inches) of insulation with a thermal conductivity of 0.035 W/(m·K). This results in a building component with a U-value of 0.164 W/m²K. However, this is only an example; conceptually, other solutions are certainly possible.
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turbotopo23 Feb 2018 23:17And how exactly is your insulation divided?
So far, 6 below and 14 above. We will either flip it exactly or lower everything. Building completely with ETICS (External Thermal Insulation Composite System) and wrapping it all around seems logical. In addition, a slab foundation; with strip footings, it would be different again.
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