Vapour Control Layers: Sd Values, Correct Positioning in Roof and Wall Build-ups, BS EN ISO 13788

Quick Answer: A vapour control layer (VCL) is installed on the warm (internal) side of the insulation to limit water vapour diffusing from heated rooms into the cold side of a construction, where it would condense (interstitial condensation). Its resistance is measured by its Sd value (vapour diffusion-equivalent air layer thickness, in metres): a true vapour barrier has an Sd ≥ ~100m, a vapour control layer typically Sd 2-100m, and a "breathable"/vapour-open membrane Sd ≤ ~0.3m. The golden rule is "warm side tight, cold side open" — high resistance inside, low resistance outside — verified by interstitial condensation analysis to BS 5250 and BS EN ISO 13788.

Summary

Interstitial condensation — condensation forming within a wall or roof build-up rather than on its surface — is one of the most damaging and least understood moisture problems in construction. It rots timber, corrodes fixings, degrades insulation performance, and grows hidden mould, all out of sight until the damage is structural. The vapour control layer is the primary defence, and getting its type and position right is the difference between a dry, durable build-up and a slowly rotting one.

The mistake that causes most VCL failures is conceptual: builders treat any plastic sheet as "the vapour barrier" and staple it wherever convenient, or worse, install a vapour-tight layer on both sides of the insulation, trapping any moisture that does get in with no way out. The physics is directional. Vapour moves from warm/humid (inside) to cold/dry (outside) in a UK heating climate. The build-up must resist vapour entering from the warm side and allow any that gets through to escape on the cold side — "warm side tight, cold side open".

This guide explains Sd values (the only meaningful way to compare membranes), where the VCL goes in roofs and walls, why the warm-side/cold-side ratio matters, the special risk of internal wall insulation and warm-deck roofs, and how interstitial condensation risk is assessed to BS EN ISO 13788 (the Glaser method) and managed under BS 5250.

Key Facts

• Vapour control layer (VCL) — a membrane/layer on the warm side of insulation that restricts vapour diffusion into the construction
• Sd value — vapour diffusion-equivalent air layer thickness, in metres (m); the standard measure of vapour resistance. Higher Sd = more resistant
• μ value (mu) — water vapour resistance factor of a material (dimensionless); Sd = μ × material thickness
• Vapour barrier — Sd ≥ ~100m (effectively vapour-tight): polyethylene sheet, foil-faced boards, AVCL membranes
• Vapour control layer — Sd ~2-100m (restricts but doesn't fully block)
• Vapour-open / breathable membrane — Sd ≤ ~0.3m (allows vapour to escape): used on the cold side
• Warm-side rule — VCL goes on the internal/warm side of insulation, behind plasterboard/finish
• Cold-side rule — the outer membrane (roofing underlay, breather membrane) must be MORE vapour-open than the VCL, so trapped vapour can escape
• BS 5250:2021 — Management of moisture in buildings. Code of practice (the UK condensation control standard)
• BS EN ISO 13788 — Hygrothermal performance of building components — interstitial condensation assessment (Glaser method)
• 5:1 guideline — a common rule of thumb is the warm-side layer should be at least ~5× more vapour-resistant than the cold-side layer
• Air-permeable (LR) vs vapour-permeable (HR) roofing underlay — BS 5250 terminology; affects whether roof void ventilation is needed
• Service void — running cables/pipes through a battened service void on the warm side avoids puncturing the VCL
• Sealed laps — VCL laps and penetrations must be taped/sealed; an unsealed VCL is largely worthless

Quick Reference Table

Detailed Guidance

Understanding Sd Values (Stop Saying "Vapour Barrier")

You cannot compare two membranes by feel or by the words on the packaging. The only meaningful figure is the Sd value — the thickness of still air (in metres) that would offer the same resistance to water vapour diffusion as the membrane. An Sd of 100m means the membrane resists vapour like 100 metres of still air.

• Sd ≥ 100m — vapour barrier / air-and-vapour control layer (AVCL): polythene, foil membranes, foil-faced boards
• Sd 2-100m — vapour control layer: OSB, some membranes
• Sd ≤ 0.3m — vapour-open / breathable: breather membranes, vapour-permeable roof underlays

Sd = μ × d, where μ is the material's vapour resistance factor and d is its thickness in metres. This is why a thin foil membrane (very high μ) and a thick masonry wall can have comparable Sd values.

The design question is never "is there a vapour barrier?" — it's "what is the Sd of the warm-side layer, what is the Sd of the cold-side layer, and is the ratio enough to keep the dew point dry?"

The Golden Rule — Warm Side Tight, Cold Side Open

In the UK heating-dominated climate, vapour pressure is higher inside (warm, humid) than outside (cold, drier) for most of the year, so vapour diffuses outward through the construction. To prevent it condensing inside the build-up:

1. Resist vapour entering on the warm side — high-Sd VCL behind the internal finish.
2. Let any vapour that gets through escape on the cold side — low-Sd, vapour-open outer membrane.

If the cold-side layer is more vapour-resistant than the warm-side layer, vapour gets in faster than it can leave and accumulates at the cold face — interstitial condensation. A widely used rule of thumb is that the warm-side resistance should be at least ~5× the cold-side resistance. The fatal error is a double vapour barrier — vapour-tight layers on both sides — which traps any moisture that enters (through gaps, construction moisture, or a leak) with no escape route.

VCL Position in a Pitched Roof

For a cold roof (insulation at ceiling level, ventilated loft above): the VCL sits above the ceiling plasterboard on the warm side of the insulation, and the loft is cross-ventilated to BS 5250 (eaves and ridge ventilation) so any vapour reaching the loft is carried away. See warm roof cold roof.

For a warm roof (insulation at rafter/deck level, no cold void): the VCL goes on the warm side of the rafter-line insulation, directly behind the internal lining. The roofing underlay or covering on the cold side must be appropriately vapour-open, OR a ventilated air gap is provided between insulation and underlay. A warm deck flat roof with a vapour-tight covering (felt/EPDM) must have an effective VCL on the warm side, because the covering is the opposite of breathable — get the warm-side VCL wrong and moisture has nowhere to go. See flat roofing.

The choice between an air-permeable (LR) and vapour-permeable (HR) roofing underlay determines whether the roof void needs ventilation — a key BS 5250 decision.

VCL Position in a Wall

In a timber-frame wall: VCL on the warm side of the insulation (typically behind the internal plasterboard/service void), with a vapour-open breather membrane on the cold (external) face of the frame so the structure can dry outwards. The OSB sheathing's position and Sd matter — internal-side OSB can act as the VCL if sealed.

In internal wall insulation (IWI) on a solid masonry wall: this is the highest-risk scenario. Adding insulation to the inside face makes the existing masonry colder and wetter (the wall no longer benefits from internal heat), pushing the dew point into the original wall. A VCL is usually required on the warm face of the IWI, but the detailing must be precise — penetrations, junctions, and reveals are condensation traps, and the wall must be able to dry. Many IWI failures are condensation behind the insulation. See internal wall insulation and internal wall insulation damp risk.

In external wall insulation (EWI): the masonry stays warm and dry on the inside of the insulation, so interstitial risk is much lower and a warm-side VCL is often unnecessary — one of EWI's structural advantages over IWI. See external wall insulation.

Intelligent (Variable) Vapour Control Layers

Standard polythene VCLs are a fixed high resistance. Intelligent (humidity-variable) VCLs change their Sd value with ambient humidity: high resistance in winter (vapour-tight, keeps moisture out of the structure) and low resistance in summer (vapour-open, lets the structure dry back inward). They're particularly valuable in refurbishment and retrofit, where the cold-side construction is fixed and not as vapour-open as you'd design from scratch (e.g. IWI behind impermeable render, or roof refurb under an old impermeable underlay). They allow drying in both directions seasonally, reducing the risk of trapped moisture.

Detailing — Why VCLs Fail in Practice

A VCL is only as good as its continuity. The common installation failures:

• Unsealed laps — laps must overlap and be taped with compatible tape; loose-laid sheet leaks vapour at every joint
• Penetrations — every cable, pipe, downlight, and fixing through the VCL is a leak unless taped/grommeted; recessed downlights are notorious
• Junctions — wall-to-ceiling, wall-to-floor, and around windows must be sealed continuously
• Service void — best practice is a battened service void on the warm side of the VCL so first-fix electrics and plumbing don't puncture it
• Damage during follow-on trades — plasterers, electricians, and decorators tear VCLs; protect and inspect before closing up

A poorly detailed Sd 100m membrane with 30 unsealed downlight holes performs like no VCL at all. Continuity and air-tightness are inseparable from vapour control — which is why the modern term is AVCL (air and vapour control layer). See airtightness.

Interstitial Condensation Assessment — BS EN ISO 13788

For any non-standard build-up, the risk is checked by calculation. BS EN ISO 13788 sets out the Glaser method — a steady-state assessment comparing the vapour pressure profile through the build-up against the saturation (dew point) profile, layer by layer, to predict whether and where condensation forms and whether it can evaporate in summer. BS 5250 references this method and provides the UK climate data and acceptable-risk criteria.

The Glaser method has limitations (it ignores moisture storage, liquid transport, and rain) so for complex or high-risk build-ups, transient hygrothermal simulation (e.g. WUFI, to BS EN 15026) is used instead. For most domestic work, following BS 5250's pre-assessed standard build-ups and the warm-side-tight/cold-side-open principle avoids the need for bespoke calculation.

Frequently Asked Questions

Which side of the insulation does the vapour control layer go?

The warm (internal) side — behind the plasterboard or internal finish, on the room side of the insulation. Vapour travels from warm/humid interior to cold exterior in the UK climate, so you resist it entering on the warm side and let it escape on the cold side. Putting the VCL on the cold side, or on both sides, traps moisture in the construction and causes interstitial condensation.

What's the difference between a vapour barrier and a breather membrane?

They're opposites. A vapour barrier/VCL has a high Sd value (≥100m for a true barrier) and goes on the warm side to stop vapour entering the build-up. A breather membrane has a very low Sd value (≤0.3m) and goes on the cold side to let any vapour that got in escape outward while keeping liquid water out. Mixing them up — breather inside, barrier outside — causes condensation.

Can I have a vapour barrier on both sides of the insulation?

No — this is a classic and damaging mistake. A vapour-tight layer on both faces traps any moisture that enters the build-up (through gaps, construction moisture, or leaks) with no route to dry out, leading to rot and mould. The rule is "warm side tight, cold side open": resistant inside, vapour-open outside, so the structure can dry.

Do I need a VCL with external wall insulation?

Usually not on the warm side, because EWI keeps the masonry warm and dry on its inner face, moving the dew point into the insulation/outer render where condensation risk is low. This is a key advantage of EWI over internal wall insulation, which makes the original wall colder and wetter and almost always needs a carefully detailed VCL. Always check the specific build-up against BS 5250.

What Sd value do I need for a vapour barrier?

A true vapour barrier (air-and-vapour control layer) is generally Sd ≥ 100m. A vapour control layer that restricts but doesn't fully block is roughly Sd 2-100m. For refurbishment where the cold side isn't very vapour-open, an intelligent variable VCL (Sd ranging from ~0.25m in summer to >10m in winter) is often the safest choice because it allows seasonal drying in both directions.

Regulations & Standards

• BS 5250:2021 — Management of moisture in buildings. Code of practice (UK condensation control)

• BS EN ISO 13788 — Hygrothermal performance of building components and building elements — Internal surface temperature to avoid critical surface humidity and interstitial condensation (Glaser method)

• BS EN 15026 — Hygrothermal performance of building components — assessment of moisture transfer by numerical simulation (transient/WUFI)

• BS EN ISO 12572 — Determination of water vapour transmission properties (μ and Sd test method)

• Building Regulations Part C — resistance to moisture, including condensation

• Building Regulations Part L — conservation of fuel and power (insulation, must not increase moisture risk)

• Building Regulations Part F — ventilation (works with VCL/airtightness strategy)

• PAS 2035 / PAS 2030 — retrofit standards requiring moisture risk assessment

• BSI — BS 5250 — management of moisture in buildings

• GOV.UK — Approved Document C — resistance to moisture

• GOV.UK — Approved Document F — ventilation

• NHBC — Technical guidance on condensation — interstitial and surface condensation

• BBA — Membrane Agrément certificates — verified Sd values for specific products

• airtightness — air and vapour control as a combined layer

• warm roof cold roof — VCL position in roof build-ups

• thermal bridging — cold spots that drive surface and interstitial condensation

• interstitial condensation — diagnosing condensation within the structure

• internal wall insulation — the highest-risk VCL scenario