U-value calculation: R = d/λ, surface resistances, Hungarian limits and four worked examples

Updated: 2026-10-19

This guide is the arithmetic behind our calculator. The calculator gives an indicative material and cost estimate per square metre, for example ODE Starflex glass wool with a BK45 vapour control layer and a BD70 breathable membrane on a loft floor. It is not a quote, and it does not calculate U-values. That part is done here, by hand, in four worked examples aimed at specifiers and contractors working on Hungarian projects. For the basics of λ and thickness, see how to choose insulation.

What the U-value measures

The U-value (thermal transmittance) is the heat flow in watts through one square metre of an element for each kelvin of temperature difference across it, in W/m²K. Lower is better, and the relationship is linear: an element at 0.15 W/m²K loses half the heat per square metre of one at 0.30 W/m²K under the same conditions.

Thermal conductivity (λ) belongs to a material; the U-value belongs to the whole build-up. That is why regulations set U-values, not λ. For design, use the declared value λD from the manufacturer's Declaration of Performance (DoP) under the relevant EN product standard, for the thickness you are specifying.

The method, step by step

1. Thermal resistance of each layer: R = d / λ, with d in metres and λ in W/mK, giving m²K/W.

2. Surface resistances. The official Hungarian calculation method published with the decree (Table 4.1, based on EN ISO 6946) gives these values (kormany.hu, calculation method, in Hungarian):

Horizontal values apply up to ±30° from the horizontal. One rule that is easy to miss: for an element separating a heated from an unheated space, such as a loft floor under a cold attic, Rsi is used on both sides, not Rse.

3. Add up and invert: Rtot = Rsi + R1 + R2 + … + Rse, and U = 1 / Rtot.

Thin membranes (vapour control layers, breathable membranes) add negligible thermal resistance and are left out below.

Hungarian requirements (as of October 2026)

Annex 1 of Decree 9/2023 (V. 25.) ÉKM lists the maximum U-values. We took them from the text in force since 2 November 2023 on the official legal database, checked on 19 October 2026. We found no amendment, but could not retrieve the version history, so check the current text before you design.

The values apply to the average U-value of each element; ground-contact values are equivalent values that include the ground. Under section 3 of the decree they apply to new buildings and to extensions and elements affected by an energy-saving renovation.

Example 1: loft floor with 2 × 12.5 cm glass wool

Heat flows upwards from the heated room into the cold loft, so Rsi = 0.10 on both sides. The slab itself (concrete or timber) is ignored, which errs on the safe side.

That meets the 0.17 requirement. The 6.41 m²K/W of the wool layer also exceeds the 6.25 m²K/W used for loft floors in Hungary's EKR energy-efficiency catalogue (Decree 18/2025 (VII. 31.) EM, annex, in Hungarian). The two layers are laid crosswise with staggered joints. Build-up details: loft floor insulation guide (in Hungarian).

Example 2: flat roof with XPS

ODE Isıpan XPS is published as "λ from 0.035 W/mK". That is the best published figure; the λD per thickness is in the DoP (EN 13164), and the final calculation should use it. Here we use 0.035 and count the insulation only.

Working backwards for 0.17 W/m²K: Rtot must be 1/0.17 = 5.88 m²K/W. Subtract Rsi 0.10 and Rse 0.04, and the insulation has to provide 5.74 m²K/W, so d = 5.74 × 0.035 ≈ 0.201 m, about 20.1 cm.

Round up, never down. Board thicknesses and the number of layers are agreed in the offer. On an inverted roof, with XPS above the waterproofing, EN ISO 6946 also requires a rainwater correction (ΔU) for cold water running under the boards. We compare the two build-ups in inverted vs warm roof.

Example 3: facade with white and graphite EPS

Heat flow is horizontal: Rsi = 0.13, Rse = 0.04. For 0.24 W/m²K the total is 1/0.24 = 4.17 m²K/W, leaving 4.00 m²K/W for the insulation if the masonry is ignored.

The wall's own resistance reduces these figures; the designer adds it from the masonry's actual λ. Graphite EPS reaches the same U-value with roughly 1–3 cm less thickness, depending on the white EPS grade. Note that the producer's list shows 20–100 mm boards, so thicker build-ups need to be agreed separately. Teknopanel is a potential partner: we source these boards on request and hold no stock.

The chart runs the same sum for five catalogue materials. For a 0.17 flat roof, λ 0.031 needs about 17.8 cm and λ 0.043 about 24.7 cm.

Example 4: internal lining with mineral boards

Where a facade cannot be insulated from outside, internal insulation may be the option, for example Multipor mineral insulation boards: λ 0.043 (M3) and 0.045 (M2) W/mK, 6–15 cm, μ = 3, Euroclass A1. Türk Ytong is a potential partner, so these too are sourced on request.

We do not know the existing wall, so we use an assumed starting value of U₀ = 1.0 W/m²K, purely as an example. Its total resistance, surface resistances included, is then 1.00 m²K/W.

Internal insulation moves the dew point into the wall, so it needs a hygrothermal assessment, and junctions with floors and partitions remain thermal bridges. The figure shows an order of magnitude, not a design.

Where the simple method stops

R = d/λ assumes continuous, uniform layers. Three things make real elements worse, and designers handle them with corrections or detailed modelling:

• Thermal bridges where rafters, joists, slab edges or balconies interrupt the insulation. Rafter bays are covered in our pitched roof guide.
• Fixings, such as facade anchors or mechanical fasteners on flat roofs.
• Gaps and air movement between badly fitted boards or behind the insulation. This is why the loft floor wool goes down in two staggered layers.

Whole-building compliance, for example for nearly zero-energy buildings, also looks at the building's overall energy performance: see our nZEB requirements article (in Hungarian).

Frequently asked questions

How do I calculate the U-value of a wall?

Add up R = d/λ for each layer, add Rsi 0.13 and Rse 0.04 m²K/W, and take the reciprocal: U = 1/Rtot.

What U-value does Hungary require for external walls?

0.24 W/m²K under Annex 1 of Decree 9/2023 (V. 25.) ÉKM, and 0.17 W/m²K for flat roofs, heated roof spaces and floors under a loft (as of October 2026).

How much glass wool does a loft floor need for 0.17?

With λ 0.039 and the slab ignored, about 22.2 cm. The 2 × 12.5 cm build-up we supply gives about 0.15 W/m²K.

Why is Rsi used on both sides of a loft floor?

Hungary's official calculation method uses the internal surface resistance on both sides of an element between heated and unheated spaces; for upward heat flow that is 0.10 m²K/W.

Which λ should I use?

The declared λD from the manufacturer's DoP for the thickness specified. Catalogue "from" values are indicative only.

Does your calculator work out U-values?

No. It gives an indicative material and cost estimate per square metre. Use the method above, or your designer, for the U-value.

For quantities and an indicative cost, try the calculator. For a specific build-up, DoPs and a quotation, use the contact form or call +36 30 485 0258. We reply on the same working day.

Sources

1. Decree 9/2023 (V. 25.) ÉKM, Annex 1, U-value requirements (Hungarian legal database): https://njt.jog.gov.hu/jogszabaly/2023-9-20-8X

2. Calculation method for the energy performance of buildings, Table 4.1 (kormany.hu): https://kormany.hu/dokumentumtar/epuletek-energetikai-jellemzoinek-meghatarozasa-szamitasi-modszer-2

3. Decree 18/2025 (VII. 31.) EM, annex, EKR catalogue (Magyar Közlöny): https://magyarkozlony.hu/dokumentumok/665fad784d41c3417b9f5eeeb2bab1fe61474464/letoltes

4. ODE Starflex glass wool (producer page): https://ode.com.tr/ode-starflex-silte · EuConX: glass wool supply

5. ODE Isıpan XPS, producer page and DoP (EN 13164): https://ode.com.tr/ode-isipan · https://ode.com.tr/_upload/pdf/ODE_ISIPAN_PERFORMANS_BEYANI_-_EN_13164.pdf

6. Teknopanel Teknopor EPS: https://www.teknopanel.com.tr/tr-tr/urun-detay/teknopor-eps-urunleri-teknopor-isi-yalitim-levhasi

7. Teknopanel Grafit Teknopor: https://www.teknopanel.com.tr/tr-tr/urun-detay/teknopor-eps-urunleri-grafit-teknopor-isi-yalitim-levhasi

8. Türk Ytong, Multipor: https://www.ytong.com.tr/multipor-isi-yalitim-levhasi