ᐅ Is a comprehensive insulation concept advisable for a KfW70-level house?
Created on: 14 Nov 2015 10:29
S
sn4tch
Hello everyone,
Yesterday, we received the first plans for our single-family home. Here are some key details:
Our approach regarding insulation and KfW standard is that we want to build a KfW 70 house with a gas condensing boiler. We prefer to invest the extra costs that a heat pump would require into better insulation instead. Our principle is that it makes more sense to reduce energy demand rather than just supplying energy cheaply.
Now, I’m wondering whether the overall insulation concept of the house makes sense as is or if there is a need for changes:
Now my questions:
If you need any additional information, just ask!
Thank you in advance for your support.
Yesterday, we received the first plans for our single-family home. Here are some key details:
- Frisian-style house with 162 sqm (1740 sq ft) of living space across ground floor and attic
- on a south-southwest facing plot
- built with solid construction and facing brickwork
Our approach regarding insulation and KfW standard is that we want to build a KfW 70 house with a gas condensing boiler. We prefer to invest the extra costs that a heat pump would require into better insulation instead. Our principle is that it makes more sense to reduce energy demand rather than just supplying energy cheaply.
Now, I’m wondering whether the overall insulation concept of the house makes sense as is or if there is a need for changes:
- The structure of the external walls is described as follows: 17.5 cm (7 inches) aerated concrete, 16 cm (6 inches) cavity insulation with thermal conductivity 0.035 W/mK, 4 cm (1.5 inches) air cavity, 11.5 cm (4.5 inches) facing brickwork. This results in an overall wall thickness of nearly 50 cm (20 inches) and a U-value of 0.16 W/m²K.
- The insulation of the roof slopes is planned with 24 cm (9.5 inches) cellulose insulation with thermal conductivity 0.040 W/mK, resulting in a U-value of 0.20 W/m²K.
- The elements above the ground slab are specified as 14 cm (5.5 inches) insulation with thermal conductivity 0.035 W/mK plus 6 cm (2.5 inches) screed. U-value not yet known.
- The windows (this surprises me somewhat) are specified with double-glazed insulated glass units with a U-value of 1.0 W/m²K. I would have expected triple glazing here, which could be easily installed on request.
Now my questions:
- Does the overall insulation concept make sense, or would you recommend any changes? Please keep in mind that we do not want to switch from the gas condensing boiler to a heat pump.
- What are your thoughts on the windows? Would it be sensible to use triple glazing with a U-value around 0.7 W/m²K (exact specifications would need to be reviewed)? The current specification seems like a significant weak point in the building envelope.
If you need any additional information, just ask!
Thank you in advance for your support.
An insulation concept should always consider the entire building. All components of the energy concept (insulation, heating, ventilation) must be coordinated both in terms of building physics and calculations. In your case, the manufacturer must demonstrate, as part of a building energy consulting report (GEB), using BAFA-approved software, that the desired KfW standard is met with their house design. Individual components of the energy concept can be balanced against each other, as long as the required overall energy balance according to KfW guidelines is achieved.
It is therefore possible that this required value can be reached with the offered windows (Ug-value 1.0), making a higher-quality triple glazing unnecessary. Furthermore, windows typically account for only about 10-15% of the total facade area (in standard cases without large glazed areas), so energy savings here may not justify the additional costs. Insulating the opaque building envelope is more effective in that regard.
Demonstrating the KfW standard with only a fossil fuel energy source (gas) and a gas condensing boiler could become problematic by 2016 due to a high primary energy demand.
For roof insulation, the manufacturer should provide a detailed description of the component structure, including all materials used. The use of vapor barriers or vapor retarders in the form of special, officially approved foils should be minimized or, if possible, avoided altogether and replaced by suitable component structures using other building materials. These foils cannot be made completely airtight on site—which is a necessary condition—and although they have a theoretical maximum service life of 50 years, this has not yet been demonstrated in practice.
Ventilation systems as part of the ventilation concept according to DIN 1946-6 should also be minimized as far as possible. The entire building services should be as simple and robust as possible. Ventilation systems are not; they cost money and require maintenance. If the ventilation concept allows, I would try, for example, to meet the necessary air exchange rates only through trickle vents in the window frame. In windowless rooms (toilets, interior bathrooms), decentralized exhaust ventilation, possibly also with heat recovery, can be used.
It is therefore possible that this required value can be reached with the offered windows (Ug-value 1.0), making a higher-quality triple glazing unnecessary. Furthermore, windows typically account for only about 10-15% of the total facade area (in standard cases without large glazed areas), so energy savings here may not justify the additional costs. Insulating the opaque building envelope is more effective in that regard.
Demonstrating the KfW standard with only a fossil fuel energy source (gas) and a gas condensing boiler could become problematic by 2016 due to a high primary energy demand.
For roof insulation, the manufacturer should provide a detailed description of the component structure, including all materials used. The use of vapor barriers or vapor retarders in the form of special, officially approved foils should be minimized or, if possible, avoided altogether and replaced by suitable component structures using other building materials. These foils cannot be made completely airtight on site—which is a necessary condition—and although they have a theoretical maximum service life of 50 years, this has not yet been demonstrated in practice.
Ventilation systems as part of the ventilation concept according to DIN 1946-6 should also be minimized as far as possible. The entire building services should be as simple and robust as possible. Ventilation systems are not; they cost money and require maintenance. If the ventilation concept allows, I would try, for example, to meet the necessary air exchange rates only through trickle vents in the window frame. In windowless rooms (toilets, interior bathrooms), decentralized exhaust ventilation, possibly also with heat recovery, can be used.
S
Sebastian7915 Nov 2015 08:00Is that what you recommend to your clients? So far, I have found your posts really great, but for someone who works, it’s hardly feasible without ventilation... besides, the maintenance effort is very low, and the installed technology is rather simple.
What I’m more curious about is what alternatives you see to vapor barriers?
What I’m more curious about is what alternatives you see to vapor barriers?
Hello Sebastian,
I try to keep the amount of complicated, expensive, and failure-prone technology as low as possible. Building a house in Germany has become too expensive and overly regulated, for example, due to the Energy Saving Ordinance 2016. I fully support the basic idea of energy-efficient construction but not its extreme implementations. Since 2000, the cost of building in Germany has increased by 36%, partly because of the stricter requirements of the Energy Saving Ordinance. The overall result of these efforts seems to me, at best, a break-even for private builders, without considering an economic payback within a reasonable period of 10 years.
Personally, I am not a fan of ventilation and air conditioning systems or complex building technology, at least not for detached single-family homes, especially when a healthy indoor climate can also be achieved using traditional methods, careful planning, and knowledge of building physics and climatic conditions. However, Germans tend to favor technical solutions...
Instead of using membranes with defined sd-values as vapor barriers or retarders in the roof area, building materials or wood-based materials with comparable sd-values can be used, following the principle “more airtight on the inside than the outside.” For example, OSB 3 on the inside and a wood fiber insulation board (DWD board) on the outside, combined with cavity insulation. The choice of material combinations depends on the specific situation. Sometimes membranes cannot be avoided; I try to avoid them whenever possible because I have found many deteriorated membranes from the 1980s and 1990s in roof structures during renovations and inspections. On the other hand, I have also inspected roof structures where these membranes caused severe moisture problems within two heating seasons due to incorrect combinations, a lack of understanding of building physics by the contractors, and installation errors (leaky connections, damage during installation, unsealed joints, etc.).
This is also a consequence of increasingly complex construction methods. It is no longer fault-tolerant.
I try to keep the amount of complicated, expensive, and failure-prone technology as low as possible. Building a house in Germany has become too expensive and overly regulated, for example, due to the Energy Saving Ordinance 2016. I fully support the basic idea of energy-efficient construction but not its extreme implementations. Since 2000, the cost of building in Germany has increased by 36%, partly because of the stricter requirements of the Energy Saving Ordinance. The overall result of these efforts seems to me, at best, a break-even for private builders, without considering an economic payback within a reasonable period of 10 years.
Personally, I am not a fan of ventilation and air conditioning systems or complex building technology, at least not for detached single-family homes, especially when a healthy indoor climate can also be achieved using traditional methods, careful planning, and knowledge of building physics and climatic conditions. However, Germans tend to favor technical solutions...
Instead of using membranes with defined sd-values as vapor barriers or retarders in the roof area, building materials or wood-based materials with comparable sd-values can be used, following the principle “more airtight on the inside than the outside.” For example, OSB 3 on the inside and a wood fiber insulation board (DWD board) on the outside, combined with cavity insulation. The choice of material combinations depends on the specific situation. Sometimes membranes cannot be avoided; I try to avoid them whenever possible because I have found many deteriorated membranes from the 1980s and 1990s in roof structures during renovations and inspections. On the other hand, I have also inspected roof structures where these membranes caused severe moisture problems within two heating seasons due to incorrect combinations, a lack of understanding of building physics by the contractors, and installation errors (leaky connections, damage during installation, unsealed joints, etc.).
This is also a consequence of increasingly complex construction methods. It is no longer fault-tolerant.
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