What insulation is suitable for a 50-year-old house?

My 85-year-old father is determined to insulate his 50-year-old detached house (double-layer masonry) using blown-in insulation. It is a double-layer masonry. I believe that the 50-year-old plastic windows with double glazing as well as the front door should also be replaced. He thinks that is unnecessary and says they are still good and that only double glazing would be used again anyway. He doesn’t see any energetic improvement in that. It is a detached single-family house with just under 100 sqm of living space. The attic is not developed and in the ceiling of the upper floor (apart from the commonly used glass wool) nothing is insulated. Under the roof tiles there is still a roofing membrane that was typically installed in the mid-70s.

What do you think? And what happens if you now "fully blow" the walls and later replace the windows or install a patio door instead of a window? Will everything then fall out again?

Honestly, I wouldn't advise on what to implement here but rather to bring in a real expert (I believe an energy consultant in this case) and have them inspect the property on-site. They can then best determine which measure achieves which improvement and how that relates to the blown-in insulation if there is insistence on using it.

My 85-year-old father has made up his mind

to have it done, he lives more contentedly afterwards and has made his environmental contribution and it does no harm to the house.

My 85-year-old father has made up his mind to insulate his 50-year-old single-family house (double-shell masonry) using blown-in insulation.

Energetically, measures like this offer one of the best cost-benefit ratios. Congratulations on having such a father.

The attic is not developed, and there is no insulation in the ceiling of the top floor (except for the previously common glass wool).

And now the great son comes up with a brilliant idea and makes his dad happy. Go to the hardware store and buy insulation rolls by the meter. Then just lay 2x12 cm of insulation material (tension felt or similar) on the top floor ceiling. Very cheap, yet highly effective. I assume that given the age of the house, there is sufficient ventilation in the roof.

There are different materials for blown-in insulation. It is possible to blow in flakes, mostly made of stone wool. These interlock relatively well and there is only minimal crumbling when replacing the windows. However, minimum air layer thicknesses of 4 cm are required there. Alternatively, granulates, mostly EPS beads, can be blown in. These have a slightly better insulation value and the air layer thickness can be smaller. However, they crumble significantly more during window replacement. Overall, the mentioned suggestions for blown-in insulation and ceiling insulation are reasonable. Replacing windows also leads to energy savings. New double-glazed windows are significantly better than the 50-year-old ones – but in any case, especially in combination with blown-in insulation, triple-glazed windows would be installed. If consultation is needed, I would commission an ISFP (Individual Renovation Roadmap) from an energy consultant. The cost minus subsidies should be 300-500€. Blown-in insulation, window replacement, and ceiling insulation could afterwards be subsidized with 20% (up to 60,000€ per calendar year with ISFP). Without ISFP, still 15% / 30k. If only blown-in insulation and ceiling (in own labor) are to be done, skip ISFP and subsidies, it is not worthwhile.

Energetically, this is one of the measures with the best cost-performance ratio. Congratulations on such a father.

And now comes the great son with a brilliant idea and makes dad happy. Go to the hardware store and buy insulation by the roll. Then just lay 2x12 cm of insulation (compressed fleece or similar) on the top floor ceiling. Very cheap, also very effective. I assume that, given the age of the house, there is sufficient ventilation in the roof.

The wonderful daughter, who is already 59 and definitely doesn’t crawl around there insulating anything herself. New windows would be useful first, I’ve been told. That probably won’t happen, as he is bothered by the restlessness.