ᐅ Is a comprehensive insulation concept advisable for a KfW70-level house?

Hello everyone,

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.

Is that the Ug value (glass) of the windows or the overall Uw value? Otherwise, there is still perimeter insulation under the slab that could be considered. In the end, I think the decision for both comes down to the price. It sounds reasonable to me.

BeHaElJa schrieb:
Is that the Ug (glass) value of the windows or the overall Uw?

When I look at the building specification, I assume it refers to the Ug value (unfortunately..). That’s why I’m also a bit skeptical about this point.
The specification literally states: Windows are made of quality-tested 6-chamber PVC profiles [...]. The windows come with double-pane insulated glazing (U-value = 1.0 W/m2K) [...].
Therefore, I tend to think that we will be offered triple-glazed windows and will decide depending on the costs.

Exactly, that’s how I would do it – have them give you the additional cost (before signing) and have the contract reviewed... Phrases like “quality tested” and “high-quality plastic profiles” would make me a bit cautious (empty buzzwords).

How about the ventilation? A ventilation system?

In my opinion, windows and masonry need to be matched. I would definitely follow up with the company on that. It doesn’t help if the windows are “tighter” or have better performance values than the masonry, because then moisture will condense on the walls instead of the windows. It’s better to have good double glazing than poor triple glazing. I would also inquire again about the exact details of how this works.

BeHaElJa schrieb:

And have the contract reviewed... Phrases like "quality tested" and "high-quality plastic profiles" make me a bit suspicious (empty buzzwords).

That was the plan. As I said, that was just an initial building specification. When it gets more detailed, I want to see the exact specifications. A Fiat Panda is definitely a quality tested car (nothing ever fails on a Fiat), but I think everyone understands what I mean.

EveundGerd schrieb:
What about the ventilation? Ventilation system?

A central controlled residential ventilation system with heat recovery is planned!