ᐅ Is Insulation Under the Concrete Slab Beneficial? Experiences and Insights
Created on: 7 May 2019 17:18
L
lesmue79
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…)
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…)
Here are the reasons why we decided on this approach. It’s important to note that we did NOT consider whether it would ever pay off financially; that was not our main concern.
- By including the foundation slab within the thermal envelope, thermal bridges are very unlikely to occur. We use the basement as living space and expect to gain comfort from a pleasant, even floor temperature. Especially at the edges near the exterior components, cold drafts are eliminated. For the insulation of our foundation slab, the insulation layer was designed to be slightly larger than the slab itself, and we hope this prevents a thermal bridge at the junction between the foundation slab and the exterior walls.
- In case of a water damage in the basement (which is not unlikely, considering the water softener system, but that’s another topic), drying will be easier if the insulation is placed on the outside.
My layman’s understanding tells me that if I insulate only from the inside and not underneath the foundation slab, some components simply remain und insulated from below.
The only thing that had us really worried was whether the improved polystyrene below can withstand such high loads without getting crushed over time.
We’ll see….
- By including the foundation slab within the thermal envelope, thermal bridges are very unlikely to occur. We use the basement as living space and expect to gain comfort from a pleasant, even floor temperature. Especially at the edges near the exterior components, cold drafts are eliminated. For the insulation of our foundation slab, the insulation layer was designed to be slightly larger than the slab itself, and we hope this prevents a thermal bridge at the junction between the foundation slab and the exterior walls.
- In case of a water damage in the basement (which is not unlikely, considering the water softener system, but that’s another topic), drying will be easier if the insulation is placed on the outside.
My layman’s understanding tells me that if I insulate only from the inside and not underneath the foundation slab, some components simply remain und insulated from below.
The only thing that had us really worried was whether the improved polystyrene below can withstand such high loads without getting crushed over time.
We’ll see….
From a thermal bridge perspective, the answer is definitely yes; insulating under the concrete slab has clear advantages. Once installed, you won't have access to this area again, which is why I am a strong advocate for insulation beneath the slab—better more than less.
There is no need to worry about the XPS compressing. It is absolutely standard practice to insulate under the concrete slab, and I am not aware of a single case where this caused any problems.
There is no need to worry about the XPS compressing. It is absolutely standard practice to insulate under the concrete slab, and I am not aware of a single case where this caused any problems.
N
nordanney8 May 2019 11:09Snowy36 schrieb:
Mind you, we did NOT consider whether this would ever pay off; that was not the deciding factor for us.That is probably the crux of the matter. It will never really pay for itself, and you won’t feel a difference in the house.Let’s put it this way: Better is always the enemy of good. But it always needs to be balanced financially as well.
On the topic of payback: Insulation not only reduces energy costs but also increases the long-term value of a house. This is especially true for insulation under the slab, since it cannot be accessed again later and insulation requirements are steadily increasing.
It is always easy to say that you can skip this or that part because it does not affect living comfort... but overall, living in a well-insulated house is significantly more comfortable than in a poorly insulated one (assuming proper building services engineering).
It is always easy to say that you can skip this or that part because it does not affect living comfort... but overall, living in a well-insulated house is significantly more comfortable than in a poorly insulated one (assuming proper building services engineering).
Currently, the slab-on-grade has been offered with 10cm (4 inches) of insulation underneath. I was just informed that omitting the insulation would result in a minimum price of €2150 including tax. From what I have heard, 80 mm (3 inches) of insulation is sufficient for KfW 55 standard. This afternoon, I just need to find out how much the U-value of the slab worsens if the insulation is removed. Then I can theoretically calculate the impact on heating costs. As I said, my main concern is the financial benefit. The 100 mm (4 inches) or 80 mm (3 inches) of Styrodur that would be omitted still need to be replaced with gravel or some other material and compacted. That also incurs costs but definitely not €2150.
N
nordanney8 May 2019 12:21Tego12 schrieb:
but overall, living in a well-insulated house is noticeably more comfortable than in a poorly insulated one... whereas insulation on the floor slab does not mean the house is poorly insulated. And the second buyer doesn't care how the insulation was done or if there was any "creative accounting" for KfW (a German development bank). What matters to them is the actual energy consumption.Similar topics