ᐅ Remove and replace old insulation?

Hello everyone,
we have purchased a house from the early 1980s (solid construction, the roof is still in very good condition). We are currently deciding whether to re-insulate the sloping roof areas. We have spoken with an energy consultant, a drywall contractor specializing in insulation, as well as various tradespeople and friends.

To better assess the situation, here are the details:
The double-glazed wooden windows will remain, the masonry includes an air gap and 40mm (1.6 inches) of insulation, and two roof windows in the sloping roof will be replaced. The roof is currently insulated between the rafters with 140mm (5.5 inches) of glass wool (one side covered with aluminum foil). In front of the roof tiles, there is a kind of mesh membrane, and on the inside, drywall panels are attached to counter battens.
The existing insulation extends from the roof gable all the way down, and the attic floor ceiling is also insulated. The knee wall (dwarf wall) of the rooms is about 1.2m (47 inches) high and solidly built, with insulation behind it.

To receive funding, the effort would be too great. The plan is to increase the existing rafters to 200mm (7.9 inches) depth and insulate with glass wool having a thermal conductivity of 0.032 W/mK. The drywall contractor would remove the knee walls and rebuild them with drywall. He said it would be sufficient to do this only for the two children’s rooms and the bathroom. The bedroom and stairwell would remain as they currently are.

Does such partial insulation really make that much difference?
Is it at all sensible (considering thermal bridges, etc.) not to insulate all areas anew (bedroom, stairwell)?
If the roof needs to be replaced in about 15 years, would it then be better to insulate more effectively?

We are very torn because this was not originally planned, and we don’t know how much benefit the effort will bring. The costs for the three rooms will be just under 10,000 euros (plus electricity and labor, removal and disposal of the old insulation, etc.).

What should we do? We are really unsure...

It would be really great if someone could provide an expert opinion on this or if anyone has already done something similar. 🙂

Alibert87 schrieb:

We are currently deciding whether to add new insulation to the sloped roof areas. We have spoken with an energy consultant, a drywall specialist who focuses on insulation, as well as various tradespeople and friends.

So, there were professionals on site who inspected the house.
You have formed your opinion based on that.
Just proceed as planned; none of us know your house personally.
What has the energy demand of the house been so far?
Whether it will ever pay off in the long run is anyone’s guess.

Nida35a schrieb:

There were experts on site who inspected the house.
You formed your opinion based on that.
Do as planned, none of us know your house.
How high has the house’s energy demand been so far?
Whether it will ever pay off, only a crystal ball can tell.

Thank you for the reply!
Since we are not yet living in the house, I can’t say how it is in summer and winter or whether the current insulation is sufficient.
According to the energy certificate, the energy consumption is about 150 kWh.

Does that mean you save about 20 percent energy?

Energy performance certificate or consumption certificate? Who lived there before? Obtain old bills to see how much heating was used, in all rooms? Then it might be possible (!) to roughly estimate it.

Buschreiter schrieb:

Needs-based certificate or consumption certificate? Who lived there before? Get old utility bills showing how the heating was used, for all rooms? Then you might be able to roughly estimate it.

My energy certificate states that the building has an energy consumption of 150 kWh.
Older tenants lived there who heated more than adequately, according to the daughter’s statement.

Is it worth upgrading from glass wool insulation with a thermal conductivity of 0.035 to 0.032?