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…)
B
boxandroof7 Oct 2019 10:47Grantlhaua schrieb:
According to our energy consultant, thermal bridges in the basement are minimal when underfloor heating is installed, so they can basically be ignored. Thermal bridges are not related to underfloor heating; on the contrary, heat losses can actually be higher there due to the greater temperature difference.
Insulation under the slab is usually not cost-effective. We installed it anyway because it’s no longer accessible afterward and because that was the original plan.
Passive house is a completely different concept with its own merits and doesn’t necessarily have to be more expensive in the end. However, in economic terms, it is probably difficult for it to outperform an efficient heating system with exhaust air heat recovery. With direct electric heating, heating costs can quickly become higher than with conventional construction, but you save a lot on technical equipment. That has its appeal.
There hasn’t been any sun here in the past few days.
Never use a wood stove in a Passive House – except in winter during very cold temperatures below zero.
The house overheats quickly.
Underfloor heating isn’t necessary; it’s really just a luxury.
Relying solely on electricity, for example with infrared heating, can still be quite expensive in a Passive House.
@Tom_Säuer how do you plan to ventilate?
Costs for a Passive House: they are manageable if you work with a Passive House builder for whom KFW 55 is considered unusual. If you go with a builder following the Energy Saving Ordinance, and maybe KFW 55, it becomes expensive.
Never use a wood stove in a Passive House – except in winter during very cold temperatures below zero.
The house overheats quickly.
Underfloor heating isn’t necessary; it’s really just a luxury.
Relying solely on electricity, for example with infrared heating, can still be quite expensive in a Passive House.
@Tom_Säuer how do you plan to ventilate?
Costs for a Passive House: they are manageable if you work with a Passive House builder for whom KFW 55 is considered unusual. If you go with a builder following the Energy Saving Ordinance, and maybe KFW 55, it becomes expensive.
Tom_Säuer schrieb:
I myself plan to insulate below the base slab with 30 cm XPS, You should discuss this carefully with the planner involved.
Using 30 cm (12 inches) against the soil seems like complete overkill to me, even for a passive house. Especially since XPS is relatively expensive. You can probably achieve better cost-efficiency by optimizing other building components.
G
Grantlhaua8 Oct 2019 06:29boxandroof schrieb:
Thermal bridges have nothing to do with underfloor heating; on the contrary, the losses are greater there due to the higher temperature difference.According to our energy consultant and the calculations he showed me, these losses are so minimal that they can basically be neglected.
B
Bauherr am L11 Oct 2019 20:53We are currently facing the same issue: our site is on a slope, meaning our "garden level" is partially underground by about one-third and opens out directly to the garden at ground level. A frost protection strip is planned at the ground-level sections. Aside from that, there is no insulation under the concrete slab. On top of the slab, we use insulation board with a thermal conductivity of 0.035 or 0.045 W/(m·K) and underfloor heating.
This obviously saves us a significant amount of money (perimeter insulation under the entire slab would cost around 5,000 to 6,000 euros).
There doesn’t seem to be a clear consensus regarding perimeter insulation under the slab, right? We are building "only" according to the energy-saving regulations.
We have not planned for any special insulating blocks either, so no "first course" with insulation blocks...
This obviously saves us a significant amount of money (perimeter insulation under the entire slab would cost around 5,000 to 6,000 euros).
There doesn’t seem to be a clear consensus regarding perimeter insulation under the slab, right? We are building "only" according to the energy-saving regulations.
We have not planned for any special insulating blocks either, so no "first course" with insulation blocks...
P
pffreestyler14 Oct 2019 11:59I don’t have professional expertise, but personally I think 045 insulation would be too little; I would go with the 035 instead.
In our initial offer, 10 cm (4 inches) of insulation was planned under the concrete slab, but calculations showed that this wasn’t necessary to comply with the energy efficiency regulations. So, we omitted it and saved 2,500 euros. Instead, I invested an extra 1,000 euros in better insulation on top of the slab – changing from 14 cm (5.5 inches) of 035 to 10 cm (4 inches) of 022 plus the 3 cm (1 inch) 045 Rolljet that our heating engineer insisted on. According to ubakus, this still improved the U-value from 0.24 to 0.187, and we saved 1,500 euros compared to the original plan.
In our initial offer, 10 cm (4 inches) of insulation was planned under the concrete slab, but calculations showed that this wasn’t necessary to comply with the energy efficiency regulations. So, we omitted it and saved 2,500 euros. Instead, I invested an extra 1,000 euros in better insulation on top of the slab – changing from 14 cm (5.5 inches) of 035 to 10 cm (4 inches) of 022 plus the 3 cm (1 inch) 045 Rolljet that our heating engineer insisted on. According to ubakus, this still improved the U-value from 0.24 to 0.187, and we saved 1,500 euros compared to the original plan.
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