Breathable Membranes for Roofs & Walls

A breathable membrane is a synthetic sheet that blocks liquid water while allowing water vapour to diffuse through it. In roofing it sits between the rafters or sarking board and the tiles; in walls it sits between the structural frame or masonry and the external cladding. Getting the specification wrong causes two types of failure: trapped moisture leads to timber rot, mould and insulation degradation; using a non-breathable membrane where a breathable one is required creates the same condensation risk from the inside.

The term “breathable” is informal. The correct technical term is vapour-permeable, and the key measurement is the Sd value (equivalent air layer thickness, in metres), the lower the Sd value, the more vapour can pass through.

Roofing Membranes: Types and Specifications

There are two broad categories of roofing membrane, plus a third often confused with them:

High-resistance (Type HR), the old standard, bitumen felt and some early polypropylene underlays. An HR underlay has a vapour resistance above 0.25 MN·s/g.¹ The roof space above the insulation has to be ventilated, and where insulation follows the rafters a clear 50 mm air path is kept between it and the underlay.¹ Now rarely specified in new work.

Low-resistance (LR) / Breathable, the current standard for most new and re-roofing projects. An LR underlay has a vapour resistance of no more than 0.25 MN·s/g, equivalent to an Sd of about 0.05 m, and many products are well below that.² Because vapour from the roof space can pass through it, a cold loft needs far less ventilation than with an HR underlay.¹

Vapour control layers (VCLs), these are the opposite: high vapour resistance, typically an Sd of tens of metres or more (Proctor’s reinforced polythene Procheck 500, for example, is declared at 533 MN·s/g, just over Sd 100 m), placed on the warm side of insulation to prevent moisture-laden air from reaching the cold zone.³ Commonly confused with breathable membranes; they serve a completely different purpose.

Pitched Roof Membrane Specifications

NHBC Standards Chapter 7.2 and BS 5250:2021, Management of moisture in buildings, are what membrane specification is normally worked to for new build and for renovation subject to Building Regulations. The old title, code of practice for control of condensation in buildings, belongs to the withdrawn 2011 edition.⁴ Neither document is a regulation: BS 5250 is a code of practice and the NHBC chapter is a warranty requirement.

Neither Approved Document C nor Approved Document L puts a number on a membrane’s vapour resistance. What Part C does is set Requirement C2, that the roof adequately protect the building from precipitation and from interstitial and surface condensation, and Part L governs the thermal performance around it.⁵ The figure that gets quoted as though it were regulatory, a vapour resistance of no more than 0.25 MN·s/g for a low-resistance (LR) underlay, comes from BS 5250’s classification, not from the Approved Documents, and it is often misquoted as an Sd of 0.25 m when it is closer to Sd 0.05 m.¹ The product standards BS EN 13859-1 and -2 set out how underlays are tested and how their characteristics are declared.⁶ Which value a particular build-up needs is a condensation risk question, not a fixed threshold.

Products and What They Cost

Breathable roofing membranes are typically sold in rolls of 50 m × 1 m (50 m²). Indicative 2026 UK retail prices, including VAT:

In September 2026 Toolstation listed Rhinovent Ultra breathable membrane, 1 m × 50 m, at £79.49 including VAT.⁷ Onduline’s heavier Ondutiss Air 120, also 1 m × 50 m, was £113.03 a roll there.⁸

These are material costs only. Roofers outside London typically charge about £150–£300 a day per person; membrane installation is normally included within a full re-roofing quote rather than priced separately.

Wall Breather Membranes

Breather membranes behind external cladding, timber, metal, render on frame, serve the same principle: they act as a secondary weather barrier and allow any moisture that reaches the cavity or back of cladding to evaporate outward rather than saturate the insulation or timber frame.

The membrane goes on the outside of the structural frame (or external insulation), under the cladding. This is the opposite side of the construction to a vapour control layer.

Key Specifications for Wall Membranes

• Water vapour resistance: BS 5250 treats a wall breather membrane as one with an Sd below 0.12 m (0.6 MN·s/g); moisture must escape easily, so lower is better on timber frame walls²
• Water penetration resistance (W1): must resist liquid water; tested to EN 13859-2
• Air permeability: many modern breather membranes also act as air barriers; check whether air-tightness is needed in your spec
• UV exposure resistance: if cladding will be delayed after membrane installation, check the product’s rated UV exposure period (this varies widely between products)

Wall Membrane Cost Comparison

Flat Roofs: Different Rules Apply

Breathable membranes in flat roofs are rarely the primary waterproofing layer, that role belongs to single-ply (TPO, PVC, EPDM) or built-up felt systems. However, on warm flat roofs, a VCL is critical on the warm side of the insulation to prevent interstitial condensation, and some inverted roof specifications use a vapour-permeable layer in combination.

Do not install a standard pitched-roof breathable membrane as the waterproofing on a flat or near-flat (less than 12.5° pitch) roof. The fall allowances and joint details are entirely different.

How to Choose the Right Membrane

1. Identify the construction type: pitched warm roof, pitched cold roof (rare in new build), timber frame wall, masonry cavity wall, flat roof.
2. Check the Sd value requirement against BS 5250 or your structural engineer’s specification. An LR roof underlay is no more than 0.25 MN·s/g (about Sd 0.05 m) and a wall breather membrane is below 0.6 MN·s/g (about Sd 0.12 m).
3. Check the pitch: most breathable roofing membranes have a minimum pitch rating (typically 12.5°–15°). Below that you need a more water-resistant product.
4. Check certification: BBA (British Board of Agrément) certificate or equivalent European Technical Assessment confirms the membrane meets the declared performance values. UKAS suspended the BBA’s accreditation on 26 February 2026, so while that stands the BBA cannot issue new accredited certificates; a certificate issued before that date remains valid, and a European Technical Assessment or a certificate from another UKAS-accredited certification body carries the same weight.⁹
5. Detail the laps correctly: BS 5534’s minimum horizontal laps for an unsupported underlay are 225 mm at 12.5–15°, 150 mm at 15–35° and 100 mm at 35° and above, with side laps of at least 100 mm; manufacturer instructions take precedence and must be followed to maintain BBA certificate validity.¹

Laps and Taping

Breathable membranes are only as good as their laps and seals. Untaped or underlapped joints allow wind-driven rain to track through. Use the tape recommended by the membrane manufacturer, not a generic butyl tape, to maintain the BBA certification.

Common Mistakes

• Fitting the membrane the wrong way up. Most breathable membranes are directional, the logo and text face outward (to the rain). Installed reversed, it may not shed water or perform as its certificate assumes.
• Using a roofing underlay on a wall or vice versa. Product ratings for roofing and wall use differ; always check the application category on the BBA certificate.
• Confusing VCL and breather membrane. A VCL on the outside of your frame is a serious construction defect, it traps moisture inside the structure.
• Omitting ventilation with high-resistance felt. If you install a Type HR underlay without ventilating the roof space, condensation is likely.
• Excessive UV exposure. Most membranes degrade under prolonged sunlight. Cover with tiles or cladding within the manufacturer’s stated period.

Building Regulations Summary

In England, breathable membranes in new construction fall under:

• Approved Document C: resistance to weather and ground moisture⁵
• Approved Document L, Volume 1: thermal performance, a correctly installed breathable membrane allows the insulation beneath to perform to its rated value; a poorly installed or wrong-type membrane can cause enough moisture loading to degrade insulation performance materially¹⁰

A like-for-like re-roof is not automatically outside Building Control: stripping a roof back to the rafters and rebuilding it counts as renovating a thermal element under Approved Document L.¹⁰ Where more than 50% of the roof’s surface is renovated, the whole roof should be upgraded to the Table 4.3 U-value of 0.16 W/m²K, so far as that is technically, functionally and economically feasible.¹⁰ That makes it building work, so it needs a building control application unless the roofer is registered with a competent person scheme for replacing roof coverings.¹¹

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.