Attic Floor Insulation: How Many Centimetres of Wool Do You Need
Short answer: a 16 cm reinforced concrete ceiling below an unheated attic needs 14 cm of mineral wool (λD 0.036) to bring U down to 0.24 W/(m²·K) and meet the maximum permitted U = 0.25 for continental Croatia. In theory 133 mm is enough, but wool is sold in fixed thicknesses, so you pick the next one up. On the coast (U ≤ 0.30) 120 mm is enough, and for the stricter target of energy renovation programmes (U ≤ 0.20) you need 180 mm. The insulation calculator works this out from the layers of your ceiling.
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Why insulate the ceiling rather than the roof
When the attic is not used or heated, you insulate the ceiling above the top heated floor. The area is smaller than the roof, there is no cutting between rafters and the job needs no scaffolding. Heat rises, so an uninsulated ceiling below the attic is one of the biggest heat losses in a house.
The attic above the insulation stays cold and needs ventilation. If you plan to convert the loft into living space later, you insulate the roof instead of the ceiling – in that case use the pitched roof option in the insulation calculator.
What U value the regulation requires
The Croatian Technical Regulation on the Rational Use of Energy and Thermal Protection in Buildings sets the maximum permitted thermal transmittance U in Annex B, Table 1, for new buildings and for work on existing ones. The limit depends on the mean monthly temperature of the coldest month at the location (θe,mj,min), for rooms heated to 18 °C or more:
- Energy renovation programmes set a stricter U ≤ 0.20 in their technical conditions for a ceiling below an unheated attic inland and U ≤ 0.25 on the coast.
- For your building the design and the call conditions decide; the calculator only checks the target you enter.
| Element | θe,mj,min ≤ 3 °C (inland) | θe,mj,min > 3 °C (coast) |
|---|---|---|
| Ceiling below an attic, flat and pitched roof | 0.25 W/(m²·K) | 0.30 W/(m²·K) |
| External wall, wall to a garage or attic | 0.30 W/(m²·K) | 0.45 W/(m²·K) |
How to calculate the thickness – step by step
- 1. In the insulation calculator choose the element “ceiling to an unheated space” with the unheated space above.
- 2. Enter the existing layers from the bottom up: 10 mm plaster and a 160 mm reinforced concrete slab.
- 3. Add the wool as a new layer and enter the declared λD from the product's declaration of performance.
- 4. Enter the target U (e.g. 0.25) and the thicknesses the manufacturer offers, e.g. 100, 120, 140, 160, 180, 200 and 240 mm.
- 5. The calculator returns the theoretical thickness, the next available thickness and U before and after the work.
Example: an 80 m² ceiling above a heated floor
Without insulation, the gypsum plaster and concrete slab give U ≈ 3.39 W/(m²·K). With 20 °C in the rooms and 0 °C in the attic, about 5432 W flow through 80 m² of ceiling. The table shows the same construction with wool λD 0.036:
- Target 0.25 (inland): 133 mm in theory, you buy 140 mm, U 0.24.
- Target 0.30 (coast): 109 mm in theory, you buy 120 mm, U 0.28.
- Target 0.20 (renovation programmes): 169 mm in theory, you buy 180 mm, U 0.19.
- Going from 200 to 240 mm cuts the heat flow by only another 43 W – each extra centimetre brings less.
| Wool (mm) | U (W/(m²·K)) | Heat flow through 80 m² (W) |
|---|---|---|
| 100 | 0.33 | 521 |
| 140 | 0.24 | 382 |
| 160 | 0.21 | 338 |
| 200 | 0.17 | 273 |
| 240 | 0.14 | 230 |
What to watch during installation
- Lay the wool without gaps and do not walk on it; compressed wool loses thickness and with it thermal resistance.
- Two layers with staggered joints reduce heat flow through the joints between slabs or rolls.
- If people will walk in the attic, plan a walkway or rigid boards as the insulation manufacturer instructs.
- Insulate and seal the attic hatch – otherwise warm, moist air escapes through it.
- Around chimneys and light fittings follow the instructions of the chimney and lighting manufacturers.
Common mistakes
- Thickness “by habit”: 100 mm of wool gives U 0.33 and does not meet 0.25.
- The λ of a different product entered instead of the value from the declaration of performance of the wool you buy.
- Area taken from the house footprint instead of only above heated rooms.
- Lower heat flow through the ceiling taken as an equal saving on the bill – total losses also depend on walls, windows and ventilation.
Sources and assumptions
λD 0.036 is a typical declared value for mineral wool from the calculator's database; replace it with the value of your product. This text is for information and does not replace a design.
Frequently asked questions
How many centimetres of wool does an attic floor need inland?
With a 16 cm concrete ceiling and wool λD 0.036, 133 mm in theory; you buy 140 mm, which brings U down to 0.24 W/(m²·K), below the permitted 0.25.
Is 12 cm of wool enough on the coast?
Yes, the same ceiling needs 109 mm in theory, and 120 mm gives U 0.28, below the permitted 0.30.
Is it worth installing more wool than required?
U 0.20 from renovation programmes needs 180 mm. Beyond that the gain shrinks: through 80 m² of ceiling the difference between 200 and 240 mm is only 43 W.
Does the type of wool change the required thickness?
Yes, the thickness grows with λ. Enter the λD from your wool's declaration of performance and the calculator recalculates the required thickness.
Calculate yours: U-value and insulation