ᐅ 25% energy savings achieved by only replacing the roof covering.

Hello, our house, built in 1972, has a top floor ceiling area of 175 m² (1885 sq ft). In 2004, I insulated it with two layers of 4 cm (1.5 inches) Styrofoam. Since 2002, I have been keeping a detailed record of the average winter temperature, renovation measures, and gas consumption. When we bought the house in 2002, the consumption was 39,000 kWh. In 2004, after insulating the top floor ceiling with 8 cm (3 inches) Styrofoam, the consumption dropped to 33,000 kWh. Then we replaced more windows, and over the last 10 years, the consumption has stabilized at around 28,000–29,000 kWh.

Last year, we installed a new roof covering. The old one had a 30-degree (30°) pitch, wooden sheathing with asbestos cement slates, two single-glazed windows, a very drafty attic, and high humidity. The new roof includes Braas Protegon roof tiles, photovoltaic panels, new windows, a draft-tight attic, and completely dry air.

Gas consumption last heating season, without any change in heating habits, was 21,500 kWh. That’s a 25% saving. The mild average winter temperature should have accounted for only about a 3% saving.

My guess to explain this significant energy reduction: The attic floor, concrete with 2 cm (0.75 inches) insulation and screed, is covered only about 30% with a wooden floor. The rest is just the two layers of 4 cm (1.5 inches) Styrofoam, installed offset. Because the previously drafty attic was not airtight, the Styrofoam boards were lifted by wind suction forces. As a result, most of the insulation was almost ineffective.

Does this explanation for the large energy savings seem reasonable?

I am now planning to insulate the entire attic with at least 20 cm (8 inches) thickness: a 20 cm (8 inches) Styrofoam layer with wooden flooring for the walking area, and 20–24 cm (8–9.5 inches) of glass wool on the outside.

You have converted a cold roof into a warm roof, meaning you included the attic within the thermal envelope.
I interpret "windproof" as including a vapor barrier (foil), which is correct.
You won’t achieve the same savings again by insulating the ceiling; you might reduce heating demand by a few percent more (5-10%).
Please report back afterwards; there are also discussions about renovating older buildings here in the forum.

I wouldn’t do any more work in the attic. If the roof is watertight, there’s no point. Above all, you shouldn’t let it get too cold up there.

Maybe you could focus your efforts on the basement ceiling? Has it been affected at all?

If the building is now completely airtight, you actually need to heat, dehumidify, or ventilate it in a controlled way. Otherwise, it won’t work well in the long run. Unless you don’t use the space at all during the entire winter.

I was simply asking whether my theory is correct that the existing 8 cm (3-inch) polystyrene insulation was mostly ineffective. The unoccupied attic alone, with just the new roof covering, saved about 6,000 to 7,000 kWh of energy! That seems almost impossible.
I just completed a detailed energy calculation with Ubakus.
8 cm (3 inches) polystyrene insulation everywhere and functioning properly = 3,200 kWh. This corresponds to the expected heat loss through the upper floor ceiling with the current insulation.
Middle area with 8 cm (3 inches) polystyrene covered by wood, so the insulation works! = 1,050 kWh
Outer area: insulation not covered by wood, so the insulation is completely ineffective due to air currents! = 8,500 kWh
Adding the outer and middle areas results in 9,500 kWh energy loss. Minus the expected 3,200 kWh equals 5,300 kWh difference, meaning excessive heating.
Assumption: last winter was somewhat milder, which could have reduced energy consumption by up to about 1,000 kWh.
1,000 kWh + 5,300 kWh = 6,300 kWh, which corresponds to my observation that I have likely been overheating by about 5,000 kWh each year due to faulty insulation.
Therefore, I will now re-insulate the upper floor ceiling to achieve an even better U-value of 0.15.