ᐅ Floor Insulation – Underfloor Heating

Hello,

Here is my next question.

The floor structure is as follows:

- No insulation under the slab
- 20 cm (8 inches) concrete
- Torch-on membrane G 200 S4
- 13 cm (5 inches) insulation
- 6.5 cm (2.5 inches) screed
- 1 cm (0.4 inches) floor covering

I have agreed with the plumbing company that we will handle the insulation design for the floor ourselves.

The contractor’s quote included 6 cm (2.4 inches) WLG 035 + 5 cm (2 inches) WLG 035 + 3 cm (1.2 inches) Rolljet 045.

The Rolljet 045 was already supplied by the contractor. I will source the insulation boards myself since it is significantly more cost-effective.

Here are the options I have checked using the U-value calculator (ubakus):

- General purpose EPS insulation board 035 DEO/WAB General purpose insulation board for €1,305.71 including VAT
Calculated U-value 0.265
- Austrotherm insulation board XPS-Top 30 SF WLS 032 for €1,590.78 including VAT
Calculated U-value 0.247 (additional €285.07)
- IKO Aluminum PIR insulation board Enertherm WLS 023 for €2,065.45 including VAT
Calculated U-value 0.19 (additional €474.67 or €759.74)

Is the following calculation for comparing U-values 0.265 versus 0.19 correct?

0.075 W/m²K (difference in U-value) x 84 kKH (annual heating degree hours) x 100 m² = 630 kWh annual savings x €0.065/kWh = €40.95 heating cost savings per year

This would mean the additional cost is amortized after 18.5 years. Less if gas prices rise.

How would you decide? Since we are doing this ourselves, the extra cost would be affordable, but if it is not necessary, naturally we'd spend the money on something more sensible.

PS: My buddy recommends laying a PE foil under the insulation and on top of the torch-on membrane. My plumbing company says this is not necessary. What do you think?

What exactly is your question now?

You can do it, but you don’t have to; it’s your own decision. €700 is not much for significantly better insulation, and energy prices will increase. I would go with WLS 023.

The 18 years seems plausible to me.

XPS appears to be less compressive, at least that was my impression. We have PIR on the ground floor and XPS on the upper floor.

WLS 032 doesn’t make sense.

ypg schrieb:
So, what exactly is your question now?

1. Should I choose the cheapest insulation with a worse U-value or the most expensive insulation with a better U-value?
2. Should I place a PE foil under the insulation despite having a bitumen membrane?
3. Is my calculation reasonable?

boxandroof schrieb:
You can do it, but you don’t have to; it’s up to you. €700 is not much for significantly better insulation, and energy prices will go up. I would choose WLS 023.

The 18 years seem plausible to me.

XPS is less compressive, at least that was my impression. We have PIR on the ground floor and XPS on the upper floor.

WLS 032 doesn’t make sense.

Yes, WLS 032 doesn’t provide enough benefit for the extra cost. My gut feeling also leans towards PIR. Is it as easy to install as EPS? Like cutting with a utility knife and then fitting the pieces together? I’ll call the company again later; I’m always interested in a second opinion...

I would avoid the so-called Rolljet system and instead use EPS WLG035 150 kPa (kilopascal) for the full thickness (varying heights depending on pipe installations on the screed), followed by a simple PE foil on top. The heating pipes would be placed on that. Regards

Unfortunately, the company insists on the Rolljet approach because that’s how they always do it. At the time, I accepted it without insisting on a change and took what they proposed. Never change a running system.

We will probably decide on the more expensive option since we won’t have another chance to change it. That eases our conscience, the insulation industry lobby has done a good job, the extra cost will be balanced out by the initially unplanned DIY work, and hopefully we will live there for at least 50 years so it will pay off.