Envelope and protection

Sealants & Membranes

These are the least visible materials in a building and among the most consequential. A vapour control layer, a damp-proof course, a breather membrane and a bead of sealant all do one job: they control where moisture and air are allowed to go. When they are placed on the wrong side of an assembly, or interrupted, the assembly can behave quite differently from the intent.

These entries describe what each product family is and what the decision depends on. They contain no waterproofing specifications, no condensation risk analysis and no instruction on where a given layer should sit in a given build-up — that is assembly design, and it belongs with a qualified professional.

27 reference entries · educational materials reference · suitability always depends on the complete assembly and qualified professional review

Scope

What this category covers

Where the boundary of this part of the library sits, including what it deliberately leaves to qualified professionals.

Joint sealants and the movement joints they occupy
Damp-proofing, tanking and below-ground moisture control products
Vapour control, air-tightness and breather layers and their tapes
Not covered: which layer belongs where in an assembly, waterproofing design, condensation analysis

Start here

Frequently referenced entries

A starting point, not a ranking — these are simply the entries most people arrive looking for.

Silicone sealant →A flexible cured-rubber jointing sealant associated with sanitaryware, glazing and wet junctions, water-tolerant but in most grades not paintable.Vapour control layer →A membrane intended to restrict water vapour passing into a construction, close to opposite in intent from a breather membrane and routinely swapped with one.Breather membrane →A membrane that resists liquid water arriving from outside while staying relatively open to vapour passing outwards, and frequently confused with a vapour control layer.

All entries

Every sealants & membranes entry

The complete set for this category, in reference order.

Entry describes

Role in an assembly

Part of a building

Silicone sealant →A flexible cured-rubber jointing sealant associated with sanitaryware, glazing and wet junctions, water-tolerant but in most grades not paintable.

Polyurethane sealant →A tougher and usually paintable elastic jointing sealant associated with construction joints where movement is combined with abrasion or contact wear.

Acrylic frame sealant →A paintable water-based filler for small static gaps at skirtings, architraves and frames, which is neither a movement-joint nor a wet-area material.

Damp-proof membrane →A continuous sheet or liquid-applied moisture-control layer associated with floors and slabs, and a different product from the course built into walling.

Damp-proof course →A moisture-resisting strip or course built into walling as it is constructed, distinct from the continuous area membrane associated with floors and slabs.

Vapour control layer →A membrane intended to restrict water vapour passing into a construction, close to opposite in intent from a breather membrane and routinely swapped with one.

Breather membrane →A membrane that resists liquid water arriving from outside while staying relatively open to vapour passing outwards, and frequently confused with a vapour control layer.

Cementitious tanking slurry →A rigid cement-based coating bonded to masonry or concrete, used only as one element of a scheme designed by a suitably qualified waterproofing specialist.

Air-tightness tape →Adhesive tape used to join membranes, boards and junctions so a control layer stays continuous, with performance decided by the substrate and by the sequencing.

Expanding foam →A canister-dispensed polyurethane that expands as it cures to fill irregular voids, where the expansion itself is the main thing to understand about the material.

MS Polymer Sealant →A moisture-curing hybrid jointing and bonding sealant that accepts paint and is free of isocyanates and acetic acid, with its own limits on movement and immersion.

Butyl Sealant →A non-curing butyl rubber compound, supplied as a gun grade and as preformed tape, that stays permanently soft instead of setting into an elastic rubber.

Polysulphide Sealant →An elastic joint sealant, usually mixed on site from a base and a curing agent, associated with joints meeting fuels, solvents or prolonged contact with water.

Fire Sealant →A gun-applied mastic formulated to expand and char when heated, which has meaning only as part of a specific tested detail rather than as a product chosen from a shelf.

Backer Rod →A compressible foam rod set into a joint to back the sealant, control how deep the bead is and stop it bonding to the base of the joint. It is not a sealant.

Flashing Tape →A self-adhesive membrane strip used to manage liquid water at openings and junctions, and a different product from the tape used to keep an air control layer continuous.

Radon Barrier →A polymer sheet manufactured and tested for resistance to soil gases, whose usefulness rests on a verified continuous installation rather than on the sheet itself.

Cavity Drain Membrane →A studded polyethylene sheet used in drained below-ground protection, where water is collected and removed by a designed drainage and pumping arrangement rather than resisted.

Wet Area Tanking →A thin bonded waterproofing layer, liquid applied or supplied as flexible sheet, formulated to be tiled over and detailed with its own corner, pipe and drain components.

Waterstop →A profiled, hydrophilic or injectable strip cast into concrete joints to interrupt a leakage path inside the structure that no surface-applied membrane can reach.

Pre-Applied Bonded Membrane →A composite sheet made for concrete to be cast directly against it, developing an adhesive bond so that water cannot travel along the interface between sheet and structure.

Bentonite Panel →Natural swelling clay held between geotextiles or bonded to a carrier, which forms a dense low permeability gel when it hydrates under restraint and closes minor defects as it does so.

Chemical injection damp-proofing →A silane or siloxane cream or fluid introduced into drilled holes in an existing mortar bed, lining the pore structure from inside the wall rather than coating its face.

Door perimeter seals →Manufactured seal sections fitted dry into a frame kerf, a rebate or the edge of a leaf, compressed and released every time the door moves rather than cured into a joint.

HDPE geomembrane liner →A thick polyethylene sheet laid on a prepared formation and joined by fusing panel to panel, where the seam itself is pressure tested rather than inspected by eye.

Liquid-applied air and water barrier →A control layer rolled or sprayed onto sheathing and cured in place, monolithic with no laps at all, where wet film thickness replaces lap integrity as where the risk sits.

Pre-compressed sealing tape →An impregnated open-cell foam strip supplied compressed on a roll, which recovers slowly to fill a joint by pressure alone and seals from the weather side outwards.

Browse by role

By the role the material plays

The same entries, clustered by what they typically do in an assembly. A material can appear in more than one cluster because it genuinely plays more than one role.

Substrate and base

Backgrounds and bases that other materials are fixed or applied to.

Weather barrier

Layers that shed or resist rain, wind and weather.

Moisture and air control

Thin layers that control where moisture and air are allowed to move.

Internal surface

The visible inner face people see, touch and walk on.

Acrylic frame sealant →

Jointing and fixing

Materials that bond, seal, support or connect other materials.

Where they appear

Parts of a building this category touches

An orientation aid only — it does not imply that any entry is suitable for that part of a particular building.

Foundations and groundworks · 10
Structural frame · 1
External walls · 16
Roofs · 7
Internal walls and ceilings · 14
Floors · 12
Wet areas · 3
External works and landscape · 3

Elsewhere on Build Design Hub

Planning guidance for this subject

This library explains what materials are. These sections help with choosing between them and preparing for the conversation.

Materials planning guides →Planning and comparison guidance for material choices.Building envelope checklists →Preparation before envelope conversations.Material maintenance guides →Planning upkeep for sealed and jointed surfaces.

Building Systems Encyclopedia

The assemblies these materials sit in

A material page explains what a substance is. These pages explain how the assembly around it is organised, what roles its components perform and which interfaces matter.

Building envelopeEnvelope Control Layers and Continuity →The continuous planes running around an enclosure — air, vapour, water and heat — and the interruptions that decide whether that continuity actually exists.Building envelopeEnvelope Junctions, Interfaces and Terminations →The places where continuous layers stop, turn, change plane or are pierced, and several duties collide inside a very small amount of construction.Structure and supportBelow-Ground Structures, Earth Retention and Water Control →Assemblies built against retained ground, where lateral earth pressure and the water held in that ground decide the design together rather than one at a time.

Materials & Systems Encyclopedia

Reference entries for building materials, product formats and system components — what they are, what their behaviour depends on and what to ask about them.

Browse the full Materials Library →

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