25% energy savings through only new roof covering.

Hello, our house, built in 1972, top floor ceiling 175 m2, I insulated in 2004 with 2 times 4 cm Styrofoam. Since 2002, I have kept an exact record of the average winter temperature, modernization measures, and gas consumption. House purchase 2002: 39,000 kWh, 2004, after insulating the top floor ceiling with 8 cm Styrofoam: 33,000 kWh, afterwards replaced more windows, the last 10 years we had a constant consumption of about 28,000-29,000 kWh. Last year got a new roof covering, old: 30 degrees, sheathing with asbestos cement slate, 2 single-pane windows, attic very drafty and high humidity! New: Braas Protegon roof tiles, photovoltaic, new windows, attic windproof, absolutely dry air. Gas consumption last heating period, without change in heating habits, now new: 21,500 kWh. That is a 25% saving. ?? The last winter should have brought a maximum of 3% saving due to the average milder winter temperature. My assumption about the immense saving: The attic, concrete, 2 cm insulation with screed, is only about 30% additionally covered with a wooden floor. The rest is only the 2 times 4 cm Styrofoam, installed offset. Because the previously not windproof attic, the Styrofoam boards were lifted by wind adhesion forces. Therefore, the entire insulation was almost useless. Is this explanation for the very high energy saving reasonable? I will now insulate the entire attic with at least 20 cm new insulation, walking area 20 cm Styrofoam with wooden covering, outside 20-24 cm glass wool.

You have turned a cold roof into a warm roof, i.e. incorporated the attic into the thermal envelope. I interpret windproof as having installed a vapor barrier (foil), everything is correct. You will not achieve the same savings by insulating the ceiling again, a few percent (5-10) should still be possible to reduce. Please report back afterwards, there are also old building renovations here in the forum.

I wouldn't do anything more in the attic. If the roof is watertight, it doesn't make any difference anymore. Above all, you shouldn't let it get too cold up there. Maybe you could focus your motivation on the basement ceiling? Has it already suffered any damage?

If it is now completely sealed, you actually have to heat/dehumidify or ventilate in a controlled manner. Otherwise, it won't work well in the long run. Unless you don't go up there at all during the entire winter.

My question was only whether I am correct with my theory that the previous 8 cm Styrofoam insulation was almost completely useless. Uninhabited attic, energy savings of about 6-7,000 kWh just from the new roof covering! That is actually impossible. I have just conducted a precise energy calculation with Ubakus. 8 cm Styrofoam insulation everywhere and functioning = 3,200 kWh. Corresponds to the target heat loss of the upper floor ceiling with the existing insulation. Middle area: 8 cm Styrofoam covered with wood, so insulation is working! = 1,050 kWh Outer area: Insulation not covered with wood, so insulation is completely non-functional due to airflows! = 8,500 kWh Outer and middle areas combined = 9,500 kWh energy loss. Minus 3,200 kWh target = 5,300 kWh difference, so overheated. Assumption: Last winter was somewhat milder, which would have led to a maximum of about 1,000 kWh less energy consumption. 1,000 kWh + 5,300 kWh = 6,300 kWh and that corresponds to my observation that I probably heated about 5,000 kWh too much every year for all these years due to dysfunctional insulation. Therefore, I will now insulate the upper floor ceiling anew to achieve an even better U-value of 0.15.