EPC and insulation: which works are a priority?
Which insulation works are a priority to improve the EPC score for rental?
Understanding heat loss
Before starting works, you need to understand where your property loses heat. An energy audit identifies the sources of heat loss:
| Component | Share of heat loss | Priority |
|---|---|---|
| Roof | 25 - 30 % | Very high |
| Walls | 20 - 25 % | High |
| Glazing (windows, doors) | 15 - 20 % | High |
| Floor | 7 - 10 % | Medium |
| Ventilation | 15 - 20 % | Variable |
| Thermal bridges | 5 - 10 % | Variable |
Every property is different. A top-floor apartment loses more through the roof than a mid-floor one. A ground-floor apartment loses more through the floor. The energy audit allows you to prioritise based on your specific situation.
The EPC certificate already contains recommendations. Certified assessors identify weak points and suggest the most impactful works.
Roof insulation: first step
The roof is the most significant source of heat loss and often the simplest to address. Hot air rises: a poorly insulated roof lets 25 to 30 % of heating energy escape.
| Technique | Cost per sqm | Thickness | Ideal situation |
|---|---|---|---|
| Glass/rock wool (loft) | EUR 20 - 40 | 20 - 30 cm | Unconverted loft |
| Rigid panels (pitched roof) | EUR 40 - 80 | 12 - 20 cm | Converted loft |
| Sprayed polyurethane foam | EUR 35 - 60 | 10 - 15 cm | Irregular spaces |
| Sarking (external) | EUR 100 - 180 | 12 - 20 cm | Roof renovation |
For a top-floor 80 sqm apartment, loft insulation with mineral wool costs EUR 3,000 to 6,000 and gains 1 to 2 EPC levels. It is the most cost-effective investment for a landlord.
Wall insulation: the most visible gain
Walls account for 20 to 25 % of heat loss. Insulation can be done externally (more effective) or internally (less expensive).
| Method | Cost per sqm | Advantages | Disadvantages |
|---|---|---|---|
| External (EWI) | EUR 80 - 180 | Eliminates thermal bridges, no loss of floor area | High cost, co-ownership approval |
| Internal (IWI) | EUR 40 - 90 | Cheaper, no co-ownership approval | Loss of floor area, residual thermal bridges |
| Cavity injection | EUR 15 - 30 | Very affordable, quick | Only if existing cavity wall |
Cavity injection is the most economical solution if your property allows it. In Belgium, many 1950s-1980s buildings have uninsulated cavity walls. The intervention takes one day and costs EUR 1,500 to 3,000 for an apartment.
In co-ownership, external insulation requires a general assembly decision. Internal insulation can be carried out individually.
Glazing replacement
Single glazing is still present in many Belgian dwellings, especially those built before 1980. Replacement with double or triple glazing offers a significant EPC gain.
| Glazing type | U-coefficient (W/sqm K) | EPC impact |
|---|---|---|
| Single glazing | 5.8 | Very unfavourable |
| Standard double glazing | 2.8 - 3.0 | Medium |
| HR double glazing | 1.0 - 1.3 | Good |
| Triple glazing | 0.5 - 0.8 | Excellent |
Switching from single to HR double glazing divides window heat losses by 4 to 5. The cost (EUR 150 - 350/sqm of glazing) is recouped in 8 to 12 years through heating savings.
For let properties, HR double glazing offers the best value for money. Triple glazing is mainly relevant for new builds or major renovations.
Optimal works order for a rental property
The ideal sequence to maximise EPC improvement on a controlled budget:
- Roof — best return on investment, essential prerequisite
- Walls — cavity injection if possible, EWI or IWI otherwise
- Glazing — replace single glazing, upgrade to HR
- Heating — replace if the boiler is over 20 years old
- Ventilation — double-flow HRV to complete the envelope
This sequence is not universal. If your 25-year-old oil boiler is the issue and the roof is already insulated, heating replacement will be the priority.
Plan works outside the letting period where possible. A rental management software helps coordinate lease deadlines, quotes and grant applications.