Is a comprehensive insulation concept reasonable for a KfW70 house?

An insulation concept should always consider the entire building. All components of the energy concept (insulation, heating, ventilation) must be coordinated with each other, both in terms of building physics and calculations. In your case, the manufacturer must demonstrate within the framework of a building energy consulting report (GEB) by means of calculation with software approved by BAFA that the desired KfW standard is achieved with their house concept. In this process, the individual components of the energy concept can be offset against each other if the required balance value according to KfW specifications is met.

It is therefore quite possible that this required value is achieved with the offered windows (Ug value 1.0). In that case, a higher quality triple glazing is not necessary. In addition, windows only make up about 10-15% of the total facade area (normal case, without large-area fixed glazing), so energy savings here are generally not cost-effective compared to the additional costs. Insulation of the opaque building envelope is more effective.

Demonstrating the KfW standard only with a fossil energy source (gas) and a gas condensing boiler can become a problem in 2016: too high primary energy demand.

For the roof insulation, the manufacturer should describe the component structure in detail, including all materials used. The use of vapor barriers/vapor retarders in the form of special, building authority-approved sheets should be minimized or avoided if possible and replaced by suitable component constructions using other building materials. The sheets cannot be practically processed 100% airtight on site—which would be a prerequisite—and also have a theoretical service life of max. 50 years, which, however, has not yet been proven in practice.

I would also minimize ventilation systems in terms of the ventilation concept according to DIN 1946-6, as far as possible. The entire building technology should be as simple and robust as possible. Ventilation systems are not. They cost money and require maintenance. If the ventilation concept allows it, I would, for example, try to prove the necessary air exchange rate only through trickle ventilators in the window rebate. In windowless rooms (toilets, interior bathrooms), through decentralized exhaust air, possibly also with heat recovery (WT).

You advise this to your customers? So far, I found your posts really great, but without ventilation, it’s hardly feasible anymore for a working person... plus the maintenance effort is very low and the installed technology is rather minimal.

I’m more interested in what you see as an alternative to the vapor barriers?

Hello Sebastian, I try to keep the proportion of complicated, expensive, and failure-prone technology as low as possible. House construction in Germany has become too expensive and is also overregulated, see [Energieeinsparverordnung 2016]. Of course, I share the basic idea of building energy-efficiently, but not the extreme excesses anymore. Building in Germany has become 36% more expensive since 2000, among other things due to the tightened requirements of the [Energieeinsparverordnung]. The balance of the whole effort seems to me to be at best break-even for private builders, not to mention a financial payback within a reasonable period of 10 years.

Personally, I am not a fan of ventilation and air conditioning systems and elaborate building services technology, at least not within the scope of a single-family house, when a healthy indoor climate can also be created with traditional methods, prudence, and knowledge of building physics and climatic relationships. But Germans as a whole tend to lean toward technical solutions...

In place of films with defined sd-values as a vapor barrier/vapor retarder, materials/wood-based materials with comparable sd-values can be used in the roof area, following the principle "tighter on the inside than on the outside." For example, OSB 3 inside, a wood fiber insulation board ([DWD-Platte]) outside in a cavity insulation. The material combination must be chosen according to the situation. Sometimes the films cannot be avoided; I try to manage without them as much as possible because I have found enough decomposed films from the 80s/90s in attic rafters during renovations and building surveys. On the other hand, I have inspected some attic rafters in which these films, due to incorrect combinations, misunderstood building physics by the executing company, and processing errors (leaky joints, damage during installation, non-sealed joints, etc.), have led to massive moisture problems within two heating seasons.

This is also a consequence of construction becoming increasingly complicated. It is no longer error-tolerant.