The heating bill is too high, the radiators run constantly, and still there are cold corners. Often the answer is not a new heating system — it is better insulation. A poorly insulated house loses heat faster than the system can produce it. Retrofitting insulation is the most effective energy improvement you can make — and often the cheapest.
Here is an overview of options, costs and the right sequence.
Where does the heat go?
In a typical unrenovated detached house, heat loss is distributed roughly as follows:
- Roof and loft: 25–30%
- Exterior walls: 25–35%
- Windows and doors: 20–25%
- Floor and foundation: 10–15%
- Ventilation: 10–15%
This is why sequence matters: start where the loss is greatest and the cost is lowest.
Loft insulation — cheapest and most effective
Cost: 10,000–35,000 DKK for a detached house Saving: 10–15% of the heating bill Payback period: 2–5 years
Loft insulation is the low-hanging fruit. Extra mineral wool or cellulose insulation is laid on top of the existing layer, up to 300–400 mm total thickness. The work takes a day for most houses.
Important: Check that the vapour barrier is intact. A leaky vapour barrier allows warm, moist air to penetrate the insulation, where it condenses and can cause mould. Houses from before 1970 often lack a vapour barrier — one must be installed during loft insulation.
Avoid blocking ventilation openings in the eaves. The roof structure must be able to ventilate freely to avoid condensation.
Cavity wall insulation — great effect, low cost
Cost: 20,000–50,000 DKK Saving: 15–25% of the heating bill Payback period: 3–7 years
Insulation material (glass wool granules, EPS beads or cellulose) is blown into the cavity between the outer and inner leaf of the wall. The work takes a day and only requires small holes in the mortar joints, which are made good afterwards.
Prerequisites: The cavity must be at least 50 mm wide and empty. The masonry must be sound — cracks and defective mortar joints must be repaired first, otherwise moisture penetrates the insulation.
Limitation: Not all houses have a cavity wall. Solid masonry (early 20th century villas, art deco houses) requires internal or external insulation.
External facade insulation
Cost: 150,000–400,000 DKK Saving: 20–30% of the heating bill Payback period: 10–20 years
The most effective but also the most expensive solution. Insulation batts (100–200 mm EPS or mineral wool) are fixed to the outside of the wall and clad with render, brick slip, timber or panels.
Advantages: Eliminates cold bridges, protects the masonry from weathering, and provides a new facade. No reduction of internal floor area.
Disadvantages: Changes the appearance of the house. May be problematic for listed or architecturally significant buildings and in local plans with facade requirements. Requires adjustment of window reveals and roof overhangs.
Internal insulation
Cost: 40,000–150,000 DKK (depending on scope) Saving: 10–20% Payback period: 5–15 years
Insulation batts are fixed to the inside of exterior walls with a new vapour barrier and plasterboard. Typically 50–100 mm insulation.
Advantages: Preserves the external appearance. Can be carried out room by room. Disadvantages: Reduces room size (50–100 mm per exterior wall). Requires relocation of electrical sockets, radiators and skirting boards. Risk of moisture in the structure if the vapour barrier is not correctly installed.
Floor insulation
Cost: 30,000–100,000 DKK Saving: 5–10% Payback period: 10–20 years
The most difficult and expensive retrofitting measure. A ground slab requires excavation or a new floor build-up. Crawl spaces can be insulated from below with fixed batts (easier and cheaper).
Floor insulation makes most sense when the floor is being replaced anyway — for example during underfloor heating installation or following moisture damage.
Which houses benefit most?
Detached houses from the 1960s–70s. Typically 50–100 mm insulation in walls and 100–150 mm in the loft. Large improvement potential.
Brick villas (1900–40). Solid masonry without insulation. Loft insulation and internal insulation give the greatest effect.
70s type houses. Cavity walls with 75–100 mm insulation that may have settled. Top-up blown insulation or external insulation.
Holiday homes. Minimal insulation — retrofitting significantly extends the usable season.
Grants and finances
Energy companies provide grants for insulation — typically 3,000–25,000 DKK depending on the measure and saving. The BoligJob scheme provides a tax deduction for labour costs. Always apply for grants before work begins. See more at Sparenergi.dk.
A comprehensive insulation upgrade can raise your energy rating by 1–3 levels — from F to D, or from D to B. This increases the property value and permanently reduces heating costs.
Mistakes that make things worse
Insulation can cause problems if done incorrectly:
- Vapour barrier on the wrong side. The vapour barrier must be on the warm side — otherwise moisture condenses in the insulation.
- Blocked ventilation. Insulation that closes off ventilation in the eaves or crawl space causes condensation and mould.
- Cavity fill in a leaky wall. If the mortar joints are poor, rainwater penetrates and saturates the insulation. Repair joints first.
- Missing sealing. Cold bridges at windows, base profiles and beam ends must be sealed — otherwise insulating the rest is pointless.
- Too thin insulation. 30 mm internal insulation is almost without effect. Minimum 50 mm, ideally 80–100 mm for a noticeable difference.
Material choices
The most commonly used insulation materials:
| Material | Thermal conductivity | Cost (DKK/m², 100 mm) | Properties |
|---|---|---|---|
| Mineral wool | 0.034–0.037 | 50–100 | Fire-safe, standard choice |
| Glass wool | 0.032–0.035 | 40–80 | Lightweight, fire-safe |
| Cellulose | 0.038–0.040 | 60–100 | Recycled material, good moisture balance |
| EPS | 0.032–0.038 | 40–80 | Lightweight, inexpensive, not fire-safe |
| XPS | 0.030–0.036 | 100–180 | Water-resistant, for foundations and ground slabs |
How to move forward
Start with the loft. It is the cheapest, easiest and most effective place to insulate. Then walls — cavity fill if possible. Floor last, and ideally as part of a larger floor renovation. And remember: insulation without the correct vapour barrier can cause more problems than it solves.
If you experience a cold house or a high heating bill, insulation is often the most effective single measure. Combined with new windows and possibly a heat pump, it can significantly improve the energy rating. There are also grants for energy renovation that make the investment more manageable. If you are renovating a holiday home, the same principles apply — but pay attention to vapour barrier requirements in buildings with intermittent heating.
Sources: Danish Energy Agency, BYG-ERFA — insulation, Sparenergi.dk