Choosing the Right Membrane
The correct membrane depends on its position within the building envelope and the function it needs to perform.
A membrane may provide:
- airtightness;
- vapour control;
- windtightness;
- weather protection;
- or a combination of these functions.
Internal and external membranes should therefore not be treated as interchangeable.
Airtightness vs Vapour Control
Airtightness controls uncontrolled air movement through gaps in the building envelope.
Vapour control influences the movement of water vapour through construction materials by diffusion.
An internal membrane can perform both functions when it is installed continuously and all overlaps, edges and penetrations are properly sealed.
This distinction is important because even a vapour-control membrane cannot provide effective airtightness if junctions and service penetrations remain unsealed.
What Is an Sd-Value?
The Sd-value describes a material's resistance to water-vapour diffusion as an equivalent thickness of still air.
A lower Sd-value generally means greater vapour permeability.
A higher Sd-value means greater resistance to vapour diffusion.
However, the appropriate Sd-value depends on the complete wall or roof construction rather than simply choosing the most vapour-open membrane available.
Intelligent Vapour-Control Membranes
Humidity-variable or intelligent vapour-control membranes change their diffusion resistance according to surrounding humidity conditions.
They generally provide greater resistance to inward moisture movement during colder winter conditions while becoming more vapour open under appropriate drying conditions.
This can increase the drying potential of suitably designed roof and wall constructions compared with a membrane having one fixed vapour resistance.
Pro Clima INTELLO PLUS
Pro Clima INTELLO PLUS is a reinforced humidity-variable airtightness and vapour-control membrane suitable for use with fibrous insulation materials.
Its vapour diffusion resistance changes substantially according to humidity conditions.
This makes INTELLO PLUS particularly useful in wall and roof constructions where both winter moisture protection and seasonal drying potential are important.
It should be installed continuously on the appropriate side of the insulation and sealed at all overlaps, junctions and penetrations.
Pro Clima DB+
Pro Clima DB+ is a reinforced paper-based vapour-control and airtightness membrane.
It can be used in walls, roofs, ceilings and floors with suitable fibrous insulation systems.
Its paper-based construction can make it particularly attractive for projects where a more natural material palette is preferred.
As with all vapour-control membranes, its suitability depends on the complete construction and moisture design.
STEICOmulti membra 5
STEICOmulti membra 5 is an internal vapour-control membrane with a fixed Sd-value of approximately 5m.
It is intended for interior wall, roof and ceiling applications where a stable vapour-control layer is required.
Because its vapour resistance is fixed rather than humidity-variable, it performs differently from products such as INTELLO PLUS or STEICOmulti renova.
STEICOmulti renova
STEICOmulti renova is a humidity-variable vapour-control membrane designed particularly for renovation applications.
Its variable Sd-value allows its vapour resistance to adapt to changing moisture conditions within the construction.
This can make it particularly useful in roof and wall renovation where access or existing construction details make conventional membrane positioning difficult.
Internal Membranes & Airtightness
Internal vapour-control membranes normally also form part of the building's airtightness layer.
This means that installation quality is critical.
Overlaps, wall connections, timber junctions, service penetrations, windows and doors all need to be sealed continuously using compatible tapes and adhesive connection products.
A high-performance membrane with poorly sealed junctions will not create an airtight building envelope.
External Breather Membranes
External breather membranes serve a different function.
They are generally designed to provide windtightness and weather protection on the outside of insulation while maintaining relatively low resistance to outward vapour diffusion.
This helps protect insulation from wind washing and water ingress while allowing moisture within the construction to dry outward.
STEICOmulti UDB
STEICOmulti UDB is a highly vapour-permeable external windtight and roofing membrane.
It has an Sd-value of approximately 0.02m, making it very open to water-vapour diffusion.
It can be used in suitable roof and renovation applications and incorporates integrated adhesive zones to assist with creating sealed overlaps.
SOLITEX Roof Membranes
Pro Clima's SOLITEX membranes are external roof and wall membranes designed to provide weather protection while maintaining high outward drying potential.
Products such as SOLITEX PLUS / PLUS CONNECT can be used as roofing underlays in suitable constructions.
The correct SOLITEX product should be chosen according to roof construction, exposure and required mechanical performance.
Façade Membranes
Ventilated façades can require dedicated windtight and weather-resistant membranes behind the external cladding.
Products such as SOLITEX FRONTA QUATTRO are designed for use behind open-jointed façade systems where portions of the membrane may remain visible between cladding joints.
The membrane needs to withstand greater UV and weather exposure than a membrane concealed behind a fully closed façade.
SOLITEX FRONTA HUMIDA
SOLITEX FRONTA HUMIDA is a specialist breather membrane for use behind ventilated masonry or other pre-wall constructions.
It is designed to protect the wall and insulation from moisture associated with the ventilated cavity while still allowing outward drying.
It should therefore not simply be described as a generic temporary weather-protection membrane.
SOLITEX ADHERO 3000
SOLITEX ADHERO 3000 is a fully adhered external weather-protection membrane.
Full-surface adhesion can provide additional security during construction and can be particularly useful on suitable sheathing and timber construction where temporary weather protection is required.
Its diffusion resistance is designed to retain outward drying potential while protecting the underlying construction.
Roof Retrofit Membranes
Roof renovation often presents different airtightness challenges from new construction.
Where the internal ceiling finish is being retained, the airtightness layer may need to be installed from above during roof renovation.
Specialist membranes such as Pro Clima DASATOP and selected STEICO renovation membranes are designed for these situations.
Pro Clima DASATOP
DASATOP is a humidity-variable renovation vapour-control membrane designed for a sub-and-top installation over and between existing rafters during roof renovation from the outside.
This makes it particularly useful when the internal ceiling remains in place and an airtightness layer cannot easily be installed conventionally from below.
The complete roof construction still needs to meet the manufacturer's moisture-design requirements.
Pro Clima DA
Pro Clima DA is an airtight vapour-control membrane designed for external insulation and roof-retrofit applications.
It can be installed over suitable roof sheathing beneath external insulation and can also be used in selected retrofit arrangements between insulation layers.
It should not be described simply as a “high-humidity vapour barrier”.
Vapour Barrier vs Vapour-Control Membrane
The terms are often used interchangeably, but technically they are not always the same.
A vapour barrier generally describes a material with very high resistance to vapour diffusion.
A vapour-control membrane moderates vapour diffusion but may still provide meaningful drying potential.
Humidity-variable membranes go further by adapting their diffusion resistance according to environmental conditions.
Membranes & Wood Fibre Insulation
Wood fibre insulation systems are often designed as vapour-open wall and roof constructions.
This does not mean membranes are unnecessary.
Internal airtightness and vapour control can reduce moisture entering the insulation through uncontrolled air leakage, while an external windtight breather membrane protects the insulation from weather and wind washing.
Both layers need to be considered as part of the complete system.
Membranes for Traditional Buildings
Membranes can be used appropriately in traditional and historic buildings, particularly in roof and framed construction.
However, they should not automatically be introduced into every solid masonry wall simply because airtightness is desirable.
The existing construction, moisture behaviour and drying paths should first be understood.
In traditional buildings, the objective should be to improve airtightness without unnecessarily restricting the building's ability to dry.
Airtightness & Moisture Risk
Uncontrolled air leakage can transport significant quantities of moisture into insulated roof and wall constructions.
Creating a continuous airtightness layer can therefore improve both energy performance and moisture safety.
However, airtightness must be combined with appropriate ventilation and vapour-control design.
Making a building more airtight without considering indoor humidity and ventilation can create a different set of problems.
Sealing Membrane Junctions
A membrane is only as effective as its connections.
Critical areas include:
- membrane overlaps;
- wall-to-roof junctions;
- floor junctions;
- windows and doors;
- electrical cables;
- pipes and ducts;
- timber beams;
- and changes in substrate.
Compatible airtight tapes, primers and adhesive sealants should be used according to the membrane and surface being connected.
Frequently Asked Questions
What is an airtight membrane?
An airtight membrane forms part of a continuous layer designed to prevent uncontrolled air leakage through the building envelope.
What is a vapour-control membrane?
A vapour-control membrane moderates the diffusion of water vapour through a wall or roof construction.
Many internal vapour-control membranes also perform as airtightness layers.
What is an intelligent membrane?
An intelligent or humidity-variable membrane changes its vapour diffusion resistance according to surrounding humidity conditions, increasing the drying potential of suitable constructions.
What is the Sd-value of INTELLO PLUS?
INTELLO PLUS has a humidity-variable Sd-value of approximately 0.25m to greater than 25m, depending on moisture conditions.
What is the Sd-value of STEICOmulti membra 5?
STEICOmulti membra 5 has a fixed Sd-value of approximately 5m.
What is the Sd-value of STEICOmulti UDB?
STEICOmulti UDB has an Sd-value of approximately 0.02m, making it highly vapour permeable.
What membrane should I use for a roof retrofit?
The correct membrane depends on how the roof is being renovated.
Products such as DASATOP, STEICOmulti renova and Pro Clima DA are designed for different roof-retrofit arrangements.
What is the difference between an internal membrane and a breather membrane?
An internal membrane generally provides airtightness and vapour control.
An external breather membrane generally provides windtightness and weather protection while allowing outward vapour diffusion.
Do I need to tape membrane overlaps?
Yes.
Where the membrane forms part of the airtight or windtight layer, overlaps and junctions need to be sealed using appropriate compatible products.
Do breathable membranes eliminate condensation risk?
No.
Membranes form only one part of the complete moisture-management strategy.
Insulation, vapour control, airtightness, external exposure, ventilation and indoor humidity all influence condensation risk.
Can Roundtower help select the correct membrane?
Yes.
If you provide the wall or roof build-up, Roundtower Lime can help identify an appropriate airtightness, vapour-control or breather membrane and associated sealing products.
Need Help Choosing a Membrane?
If you are unsure whether your project requires an intelligent vapour-control membrane, fixed vapour-control layer, roof underlay, façade membrane or roof-renovation membrane, contact Roundtower Lime with details of the construction.
Our technical team can help identify the appropriate membrane and compatible tapes, primers and connection products.
