Ahmet Alptekin Kasar
Founder, Managing Director
Updated: 2026-10-21
This is a spec-level explainer for developers, contractors and specifiers working in Hungary and the wider CEE region. It covers what the number means, how it is derived and how to read it on a Declaration of Performance (DoP) or a technical data sheet (TDS). It does not pick a membrane for you: product selection is covered in our guide to choosing a vapour control membrane, and the full BK45 and BD70 data (weight, tensile strength, roll size, documents) are in our BD70 and BK45 membrane article (in Hungarian).
What the Sd value means
Water vapour diffuses from the warmer, more humid side of a construction towards the colder, drier side. It passes through every building material, only at very different rates. The Sd value turns that resistance into a single length: the thickness of a still air layer that would hold vapour back to the same degree. Hence the name, equivalent air layer thickness, and the unit, metres.
Two declared values show the range:
- BD70 breathable membrane, Sd 0.02 m: it resists vapour like 2 cm of still air. Its Declaration of Performance (DOP-BBARIYER-L4S-BD70/082025-01) gives 0.02 m after artificial ageing as well.
- BK45 vapour control layer, Sd 20 m: it resists vapour like 20 m of still air (Declaration of Performance, DOP-BBARIYER-LW-BK45/082025-01).
So Sd is a resistance, not a percentage and not a flow rate. Lower Sd means more vapour-open; higher Sd means more vapour-tight.
Sd = μ × d
For materials, vapour resistance is expressed by the water vapour resistance factor μ (mu). It is a material property and says how many times more a material resists vapour than a still air layer of the same thickness. Multiply it by the layer thickness d in metres and you get the layer's Sd value:
Sd = μ × d
A worked example with an insulation board: ODE declares μ 100 for its Isıpan XPS (EN 12086). At 10 cm that gives 100 × 0.10 m = 10 m; at 5 cm, 100 × 0.05 m = 5 m. Same material, half the thickness, half the Sd. For membranes the manufacturer declares Sd directly; the BK45 DoP states 20 m.
Where layers sit in series, their Sd values add up. That is why the hygrothermal check looks at the warm-side layers together against the cold-side layers together.
- ODE Isıpan XPS, 10 cm: μ 100 × 0.10 m = Sd 10 m (μ: ode.com.tr/ode-isipan, EN 12086, checked 2026-10-21)
- ODE Isıpan XPS, 5 cm: μ 100 × 0.05 m = Sd 5 m (same μ, half the thickness)
- BK45 vapour control layer: Sd declared directly by the manufacturer, Sd 20 m (DoP DOP-BBARIYER-LW-BK45/082025-01)
- Rule: tighter layers on the inside (warm side), more open layers towards the outside; layer Sd values add up.
Why the warm side must be tighter
In the heating season the indoor air is warmer and carries more moisture than the air in a cold loft or outdoors, so vapour moves outwards. If the warm side lets more vapour into the construction than the cold side lets out, moisture accumulates in the cold zone and can condense. Wet insulation performs worse, timber stays damp, mould follows.
The fix is the order of Sd values: high Sd on the warm side, low Sd on the cold side. On a loft floor (attic floor) this reads as follows:
1. On the slab, under the wool: a vapour control layer such as BK45, Sd 20 m.
2. In the middle: the insulation, for example 2 × 12.5 cm glass wool with the second layer cross-laid.
3. On top of the wool: a breathable cover such as BD70, Sd 0.02 m.
20 / 0.02 = 1,000: the bottom layer resists vapour a thousand times more than the top one, so whatever little moisture gets in can leave upwards. The reverse, a vapour-tight foil on top of the wool, would trap moisture in the insulation; it is good practice never to place one there. Whether a given ceiling needs a vapour control layer at all is the subject of our loft-floor decision guide. For pitched roofs the same principle applies with a different build-up: insulation between rafters.
A declared Sd only holds in the building if the layer is continuous. An untaped lap or an unsealed penetration lets vapour bypass the membrane, and no Sd value makes up for that. We recommend taping all laps and turning the membrane up the walls and taping it there.
The Hungarian EKR limits: at least 20 m below, at most 0.03 m above
Hungary's energy efficiency obligation scheme (EKR) sets two numerical conditions for attic-floor insulation in its measure catalogue: a vapour control layer of at least Sd 20 m under the insulation, and a breathable dust-protection membrane of at most Sd 0.03 m over fibrous or blown insulation (Decree 18/2025 (VII. 31.) EM, Annex, points 1.3.1.1 g) and 1.5.1.1 g), in Hungarian).
Applied to declared values:
- BK45, Sd 20 m: meets the lower limit, which reads "at least 20 m".
- BD70, Sd 0.02 m: below the 0.03 m upper limit.
- A membrane with Sd 0.2 m (the value on the BBarrier ELIT 90 and GOLD 90 data sheets) is still vapour-open, but above 0.03 m: 0.2 / 0.02 = 10, ten times tighter than BD70.
- A membrane with Sd 15 m (BBarrier PLATINUM PRO 160) is a tighter layer, but on its own it does not reach the 20 m minimum.
On an EKR job, the label "breathable" or "vapour barrier" does not decide compliance; the declared number does.
Reading Sd on a DoP and a data sheet
You will find Sd in two documents:
- Declaration of Performance (DoP): the document behind the CE marking in which the manufacturer declares the product's performance. It carries a reference number (for example DOP-BBARIYER-LW-BK45/082025-01) that you can cite in a specification.
- Technical data sheet (TDS): the manufacturer's product description. On Bilnur (B-Bariyer) sheets, Sd appears in the row "Water Vapour Transmission Properties (Sd)", test method TS EN ISO 12572, unit m, with the result in the last column.
What to check:
1. The unit. Sd is in metres. Between 0.02 m and 20 m there is a factor of 1,000, so a misplaced decimal is not a detail.
2. The aged value. The BD70 DoP also declares Sd after artificial ageing (0.02 m). If a document only gives the as-new value, note it.
3. The grade. A data sheet covers one grade. The ELIT 90 sheet does not give Sd for ELIT 100–190, GOLD 90 does not cover GOLD 120, PLATINUM PRO 160 does not cover PLATINUM PRO 180/200, MET 90 does not cover MET 130 or PLATINUM MET 170, and ALU PRO TEK 150 does not cover ALU PRO TEK 110. Check the sheet of the exact grade.
4. NPD. "No Performance Determined" means no value is declared for that characteristic. It is neither zero nor a pass.
5. The delivered batch. It is good practice to request the DoP for the delivered product and to match product name and DoP number against the specification.
More on CE marking and DoPs: our CE marking guide for construction products.
Common misreadings
Sd versus g/m². Grammage is mass per area, not vapour resistance. BK45 weighs 45 g/m² at Sd 20 m; BD70 weighs 70 g/m² at Sd 0.02 m. The lighter membrane is a thousand times tighter. BBarrier ELIT 90 and MET 90 share the 90 g/m² grade, yet their Sd values are 0.2 m and 100 m: 100 / 0.2 = a factor of 500. A heavier grade is not automatically more vapour-tight.
Sd versus water column and W1. BD70's 259 mm water column and BK45's W1 water-tightness class describe how the membrane holds back liquid water. Sd is about water vapour. The two are independent: per its data sheet, BBarrier ELIT 90 is W1 and Sd 0.2 m at the same time, so it is watertight and vapour-open. "Watertight" does not mean "vapour-tight".
"Breathable" versus the EKR limit. A membrane can be breathable in everyday terms (Sd 0.2 m, say) and still miss the ≤ 0.03 m condition for the top cover of a Hungarian attic floor. Read the number, not the product category.
Sd versus thermal performance. Sd says nothing about insulation value. That is the job of λ and the U-value; see our U-value calculation guide.
Sd table: BBarrier membranes on one scale
| Membrane | Sd value | Against the EKR limit | Source |
|---|---|---|---|
| BD70 (70 g/m²) | 0.02 m, also 0.02 m after ageing | below the top-cover limit (≤ 0.03 m) | DoP DOP-BBARIYER-L4S-BD70/082025-01 |
| BBarrier ELIT 90 | 0.2 m | above 0.03 m | ELIT 90 data sheet (TS EN ISO 12572) |
| BBarrier GOLD 90 | 0.2 m | above 0.03 m | GOLD 90 data sheet (TS EN ISO 12572) |
| BBarrier PLATINUM PRO 160 | 15 m | below the vapour-control limit (≥ 20 m) | PLATINUM PRO 160 data sheet (TS EN ISO 12572) |
| BK45 (45 g/m²) | 20 m | meets ≥ 20 m | DoP DOP-BBARIYER-LW-BK45/082025-01 |
| BBarrier ALU PRO TEK 150 | 50 m | above 20 m | ALU PRO TEK 150 data sheet (TS EN ISO 12572) |
| BBarrier MET 90 | 100 m | above 20 m | MET 90 data sheet (TS EN ISO 12572) |
| ELIT 100–190, GOLD 120, PLATINUM PRO 180/200, MET 130, PLATINUM MET 170, ALU PRO TEK 110 | per the data sheet of that grade | to be checked | grade-specific data sheet |
| Ratio BK45 / BD70 | 20 / 0.02 = 1,000 | calculated |
The data sheets were downloaded from bbariyer.com.tr on 21 October 2026. The table shows where each membrane sits on the scale, not where it belongs in a build-up. That depends on the construction, the Sd of the other layers and the designer's specification; our membrane selection guide and the BD70 and BK45 article (in Hungarian) are the places for that decision.
The same arithmetic applies to insulation boards: a 10 cm XPS board has Sd 10 m, which matters at the plinth and on basement walls. See XPS vs EPS for plinths and basement walls.
Frequently asked questions
What does the Sd value mean in plain terms?
The thickness of still air, in metres, that would resist vapour as much as the layer. BD70 at Sd 0.02 m behaves like 2 cm of air; BK45 at Sd 20 m like 20 m.
How is Sd calculated?
Sd = μ × d, with μ the water vapour resistance factor and d the thickness in metres. ODE Isıpan XPS has μ 100, so 10 cm gives Sd 10 m and 5 cm gives 5 m. For membranes the manufacturer declares Sd directly.
Which Sd values apply to Hungarian attic floors under the EKR scheme?
At least Sd 20 m under the insulation and at most Sd 0.03 m over fibrous insulation (Decree 18/2025 (VII. 31.) EM, Annex, points 1.3.1.1 g) and 1.5.1.1 g)). Per their DoPs, BK45 (20 m) and BD70 (0.02 m) meet these values.
Is a heavier membrane more vapour-tight?
Not necessarily. BK45 is 45 g/m² at Sd 20 m; BD70 is 70 g/m² at Sd 0.02 m. Grammage measures mass; only Sd measures vapour resistance.
If a membrane is W1 watertight, is it also a vapour barrier?
No. W1 and water column refer to liquid water, Sd to water vapour. BBarrier ELIT 90 is W1 and Sd 0.2 m: watertight and vapour-open.
Where do I find Sd on a Bilnur data sheet?
In the row "Water Vapour Transmission Properties (Sd)", test method TS EN ISO 12572, in metres. Each grade has its own sheet, so check the exact grade you are specifying.
Membrane supply, DoPs and project enquiries: contact form · +36 30 485 0258 · info@euconx.com. We reply on the same working day.
Sources
1. BK45 Declaration of Performance, DOP-BBARIYER-LW-BK45/082025-01: https://euconx.com/downloads/dop/euconx-bk45-dop.pdf
2. BD70 Declaration of Performance, DOP-BBARIYER-L4S-BD70/082025-01: https://euconx.com/downloads/dop/euconx-bd70-dop.pdf
3. Bilnur / BBarrier: ELIT 90 technical data sheet (downloaded 2026-10-21): https://www.bbariyer.com.tr/dosya/teknik-tablolar/elit90-tds-168b19b1440f27.pdf
4. Bilnur / BBarrier: GOLD 90 technical data sheet (downloaded 2026-10-21): https://www.bbariyer.com.tr/dosya/teknik-tablolar/gold90-tds-168b19bbdba4e6.pdf
5. Bilnur / BBarrier: PLATINUM PRO 160 technical data sheet (downloaded 2026-10-21): https://www.bbariyer.com.tr/dosya/teknik-tablolar/platinum-pro-160-tds-168b19c4246ece.pdf
6. Bilnur / BBarrier: MET 90 technical data sheet (downloaded 2026-10-21): https://www.bbariyer.com.tr/dosya/teknik-tablolar/met90-tds-168b19bd3534b6.pdf
7. Bilnur / BBarrier: ALU PRO TEK 150 technical data sheet (downloaded 2026-10-21): https://www.bbariyer.com.tr/dosya/teknik-tablolar/aluprotek150-tds-168b19ac0993be.pdf
8. Decree 18/2025 (VII. 31.) EM, Annex, points 1.3.1.1 and 1.5.1.1 (Magyar Közlöny, in Hungarian): https://magyarkozlony.hu/dokumentumok/665fad784d41c3417b9f5eeeb2bab1fe61474464/letoltes
9. ODE: Isıpan XPS product page (μ 100, checked 2026-10-21): https://ode.com.tr/ode-isipan
10. EuConX: BD70 and BK45 membranes (in Hungarian): https://euconx.com/resources/bd70-es-bk45-parazaro-es-paraatereszto-foliak-az-ekr-szigetelesben
11. EuConX: How to choose a vapour control membrane
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