ᐅ 20 cm of 035 insulation plus additional polystyrene underfloor heating?

Good morning,

I am currently renovating a house. We are going to install underfloor heating on the first floor. Between the first floor and the ground floor, we have installed 20cm 035 glass wool insulation (between the joists) (approximately 8 inches). The first floor is not heated—it is essentially a garage or storage space. On top of that we laid OSB boards. I just found out that an additional 14cm (approximately 5.5 inches) will be installed above the OSB boards, consisting of polystyrene insulation, impact sound insulation, underfloor heating, and of course screed. I believe the finished surface is included in that layer thickness, but I am not completely sure.

My question is—do you really need the polystyrene insulation if you already have 20cm insulation under the OSB? I don’t want to save on insulation, but I also don’t want to overdo it if it doesn’t help. Or is it simply impossible to install underfloor heating directly on OSB (or directly on the mats on top of OSB) without some kind of insulation underneath?

CF

Hello,

frankentux schrieb:
.... We will install underfloor heating on the first floor. Between the first floor and the ground floor, we have installed 20cm 035 glass wool insulation (between the joists) (there is no heating planned on the first floor – it is basically a garage or storage room).

frankentux schrieb:
....- but I am not sure about it.

With such investments, you should always be sure!

frankentux schrieb:
My question – do you really need the polystyrene insulation if you already have 20cm insulation under the OSB?

You should also consider the energy saving regulation §9, annex 3.
However, an additional 14 cm seems quite extensive. The person who proposed this measure should be able to provide a justification!
For dry screeds and insulating floor finishes, compared to wet systems, more insulation is needed below the heating layer.

Underfloor heating must be carefully designed to ensure comfort and energy efficiency.

Best regards.

Thank you for the quick response. What I meant by 14cm (5.5 inches) was 14cm (5.5 inches) including the screed. I’m not sure how thick the polystyrene panels will be that are installed directly on the OSB boards – I estimate around 6cm (2.4 inches). So the finished floor height should be at least 14cm (5.5 inches) above the OSB boards. That would be 20cm (7.9 inches) of 035 insulation underneath, 22mm (0.9 inches) OSB, probably 6cm (2.4 inches) of polystyrene, and then pipes and screed (possibly other layers, e.g., impact sound insulation).

From your answer, I understand that additional insulation above the OSB is not only sensible but necessary – that is reassuring :-)