In short: Home energy efficiency means the same comfort with less energy — through insulation, windows, heating and habits — which lowers monthly bills.
Simply put: A sustainable choice is often a cost choice too: an upfront cost against a smaller bill every month. Work out the cost per year and how many years it takes to pay back: payback period = upfront cost ÷ annual saving. The saving is an assumption — it depends on energy prices, habits and the building — so get an estimate from an independent specialist. Do not put money you may need at short notice into an improvement.
Example: Insulation costing €6,000 that saves €600 a year pays for itself in 10 years.
Watch out: A "green" loan is no guarantee of a lower price — check the APR, term, fees and collateral, as with any loan. Grants and cheaper loans for renovation exist at times, but their conditions change: check them only with the body that runs the programme.
Example: Bulgaria — see the Ministry of Regional Development and Public Works (https://www.mrrb.bg), https://www.eufunds.bg and the National Decarbonisation Fund (https://www.bgeef.com).
Check in your country — look on the websites of your ministry for housing or energy and your national energy agency.
Source: European Commission, energy-efficient buildings — https://energy.ec.europa.eu/topics/energy-efficiency/energy-efficient-buildings_en