ᐅ Attic insulation upgrade from KfW 55 to KfW 40 standard applied to the attic floor

Hello everyone,

We are planning to build a single-family house in Baden-Württemberg with the following current key specifications:
- approximately 12x12 m (39x39 ft) external dimensions
- basement with about 16 cm (6 inches) perimeter insulation and a good layer (exact height unknown) of glass foam gravel underneath (within the thermal envelope)
- 49 cm (19 inches) Poroton exterior wall filled with perlite (U-value around 0.16)
- windows/doors adapted to the exterior wall (U-value currently unknown, but roughly at the same level as the facade)
- gable roof (attic outside the thermal envelope)
- domestic hot water heat pump or geothermal heat pump with an appropriate annual performance factor to qualify for BAFA incentives
- photovoltaic system on the roof
- controlled ventilation system with heat recovery

Now, regarding the gable roof, I have the following question. For a KfW-certified house, besides the exterior walls, basement, and openings in the facade (windows, doors), the roof also needs to be properly insulated. Is it sufficient to insulate the attic floor itself (about 30-40 cm (12-16 inches) of insulation material, then covered with wood to allow access to the attic—the technical term escapes me), or is it necessary to insulate directly under the roof (between or below the rafters) to meet KfW 55 or, ideally, KfW 40 standards?

I believe that insulating the attic floor would be much easier as a DIY project and thus more cost-effective than insulation between or below the rafters. Or are the costs for both methods comparable?

Which material would you recommend for roof insulation in a KfW 40 house?

Are we mistaken in thinking that with the materials mentioned above, achieving a KfW 40 standard is fairly straightforward? Or is the effort of upgrading from KfW 55 to KfW 40 economically unjustified?

So far, we have not yet had a conversation with an energy consultant from the list.

Thanks in advance,
Best regards
Ann.

One of the most common methods is overlay insulation. This type of insulation is cost-effective and can be done as a DIY project with some basic carpentry skills.

Exactly, I want to install overlay insulation / floor slab insulation instead of rafter insulation because I think it is easier to do myself and also more affordable.

Am I mistaken? And is this sufficient with the appropriate U-value to meet KfW 55 / 40 standards?

You will need to ask this question to your energy consultant, who will also certify everything for the KfW.
Achieving KfW40 will be challenging, not because of the roof, but due to the basement.
The step from KfW55 to KfW40 is usually not economically viable.

I’m not very familiar with this topic, just a couple of quick comments on the concept.

- Are you aware of how “thick” 49cm (19 inches) really is? You have to really want that.
- With 49cm (19 inches), I hope you don’t just get a 0.16 U-value... you can achieve that already with 36cm (14 inches). So with that wall thickness like a castle wall and using perlite, I hope you get below 0.1.
- Your overall concept in Baden-Württemberg, depending on the other equipment, will be between 2500 and rather 3000 per square meter—that’s clear to you?
- From what you’re aiming for, it sounds more like KfW 40(+) standard; KfW 55 is “easier” to achieve.
- About the roof, as I said, I’m not an expert… you’ll have a cold roof, meaning it’s outside the thermal envelope. Better have a carpenter prepare an offer for you just to be safe. I wonder if it’s reasonable for me to want to build a house like that with probably 250–300 sqm (2690–3230 sq ft) at that standard and then insulate the roof myself… but of course, everyone has to decide that for themselves.

Is this going to be a "1.5-story" single-family house? Then you will have sloped ceilings in some rooms on the upper floor.

This means there will be a transition from heated space to the roof in those areas. At a minimum, that part of the roof must be insulated. In my opinion, the rest of the roof can remain uninsulated if the ceiling between the upper floor and the attic is insulated.

face26 schrieb:

- With 49cm (19 inches) you hopefully don’t have just a 0.16 U-value... you can achieve that with 36cm (14 inches). So with that wall thickness and perlite insulation, you should hopefully get below 0.1.
- From what you’re planning, it sounds like you’re aiming for something close to KfW 40(+), KfW 55 is “easier.”
- Regarding the roof, as I said, I’m not very experienced... so you’d basically have a cold roof, meaning “outside” the thermal envelope. I’d recommend getting a quote from a carpenter just to be safe. I wonder if building a house of maybe 250-300 sqm (2700-3200 sq ft) to that standard and then insulating the roof yourself is really reasonable... but that’s up to each individual.

If you check, for example, the Schlagmann website and select the T8 49 brick -> 0.16 W/(m²K) U-value (approximate price 8 EUR per brick). The newer T7 49 brick has a U-value of 0.14 (price about 8.5 EUR per brick). Is the 50 cents per brick justified by the 0.02 improvement in U-value?

Well, I’m not aiming for any specific standard. I simply find the non-air source heat pump better than an air source heat pump and see no reason not to use well-insulating materials. Of course, keeping cost-effectiveness in mind.

I don’t necessarily have to reach KfW 40; I’m just wondering if it’s still economically worthwhile to go from KfW 55 to 40, or if it’s mainly driven by ecological considerations. @apokolok also mostly dismissed it. A basement inside the thermal envelope is probably challenging for a KfW 40 house? KfW 55 should be achievable though—right, or maybe not because of the basement?

Compared to perimeter insulation on the basement, I think that insulation on top of the floor slab or between floors from the upper floor to the basement would be much easier to do yourself. I understand it won’t save tens of thousands of euros though...

Grobmutant schrieb:

Is this going to be a “1.5”-story single-family house? Then you will have sloped ceilings in some rooms upstairs.
That means there will be a transition from heated space to roof. At least that part of the roof needs to be insulated. The rest of the roof can, in my opinion, remain uninsulated if the ceiling between the upper floor and attic is insulated.

Sorry, forgot to mention. No, it’s two full stories with about 13x11 meters (43x36 feet) external dimensions.