ᐅ Basement insulation: beneath the slab or beneath the screed?

Hello,

we are about to have the final discussions with our builder regarding the house specifications and have a question.

The detached house will be built with a waterproof concrete basement according to the 2009 Energy Saving Ordinance (no further energy efficiency standards like 70 or 55 planned). The basement area is 70m² (750 sq ft). To comply with the Energy Saving Ordinance, the basement walls will be insulated with 100mm (4 inches) perimeter insulation with a thermal conductivity of 0.040 W/(m·K).

The basement is primarily planned as a storage cellar, but I want to keep the option open to use it later for residential-type purposes (guest room, office, or hobby room).

For energy reasons and to avoid cold floors, I want to request special floor insulation as a custom option. I asked the builder about insulation under the basement slab.

The estimated price is 3,809 EUR. The type of insulation was not specified in detail; I expect a basic standard.

Since I find the price quite high, I am now considering insulation under the screed. What do you think about that?

My view:

Insulation under the slab:
+ No thermal bridges, probably the best energy solution
- High cost

Insulation under the screed:
+ Floor heats up faster (I don’t have to warm the entire slab when heating the basement only for a few hours)
+ Cheaper (though I don’t know by how much)
- Thermal bridges (though reduced by perimeter blocks as a first row)
- Reduced ceiling height (or higher floor construction)

What is your opinion on this? Thanks for your advice.

Aila

For that amount of money, you can easily get the screed installed in the basement, including 10cm (4 inches) of insulation. Don't forget to account for the increased floor height, as well as adjusting the stair risers and door openings.