Fibre Reinforced Facade Encyclopedia: Installation
A facade sealant is an elastomeric compound gunned into a joint between panels to exclude water and air while accommodating the movement between them. It is a designed element of the joint, not a filler, and its selection is governed by one calculation and one uncomfortable fact: the calculation is the movement accommodation factor against the joint width, and the fact is that a sealant lasts a fraction of the life of the building it is sealing.
A sealant with a 25 percent movement accommodation factor in a 12 mm joint can absorb 3 mm of movement. If the panels move 5 mm the joint fails, and no amount of care in the gunning changes that.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Silicone, movement | Typically plus or minus 25 to 50 percent | The highest movement capability available |
| Polyurethane, movement | Typically plus or minus 20 to 25 percent | Lower, but tougher and paintable |
| Hybrid MS polymer, movement | Typically plus or minus 20 to 35 percent | Low stain risk, paintable, good adhesion |
| Minimum joint width | Typically 10 mm, never less than 6 mm | Width is set by movement, not appearance |
| Depth to width ratio | 1:2, with 6 mm minimum depth | A deep bead is a weak bead |
| Service life, silicone | Typically 20 to 25 years | Manufacturer and exposure dependent |
| Service life, polyurethane | Typically 10 to 15 years | Ultraviolet degradation is the usual limit |
| Building design life | 50 years or more | Two or more sealant replacements over that life |
| Staining risk | Highest with unmodified silicone on porous panels | Plasticiser migration |
Silicone, polyurethane and hybrids
Silicone is the highest performing chemistry for facade movement joints: the greatest movement accommodation, the best ultraviolet stability and the longest service life, adhering well to glass, metal and, with the right primer, to cementitious substrates. Its weaknesses are that most silicones cannot be painted, that the surface holds dirt, and that some formulations stain porous substrates.
Polyurethane is tougher, more abrasion resistant and paintable, and it is the traditional choice for concrete to concrete joints and for ground level joints where physical damage is likely. It accommodates less movement and degrades faster under ultraviolet, so its service life on a fully exposed facade is shorter, but it is more forgiving of a less than perfectly prepared substrate. Hybrids, usually silyl-terminated polyether or modified silane polymers, sit between the two: good movement capability, paintable, generally low staining risk on porous substrates, and often isocyanate and solvent free. They have become the default recommendation for cementitious panel joints on many projects.
Movement accommodation and joint width
Joint width is derived from the movement to be accommodated divided by the sealant’s movement accommodation factor, and is not an aesthetic decision. That movement is the sum of several components: thermal movement of the panel, which for glass fibre reinforced concrete comes from a coefficient of thermal expansion around 10 to 20 micrometres per metre per degree Celsius across the service temperature range; moisture movement, significant in cementitious panels and negligible in fibre reinforced polymer; structural movement including slab deflection, frame shortening, creep and any seismic drift; and construction tolerance, because a joint set out at 12 mm arrives somewhere between 8 and 16 mm. A plus or minus 25 percent sealant in a 10 mm joint gives only 2.5 mm of capability.
Staining, primers and bond breakers
Staining of porous panels by sealants is well documented and entirely avoidable. Plasticisers and uncured silicone fluids migrate out of the cured bead into the adjacent porous substrate, darkening the surface and attracting dirt to produce a permanent dark band either side of every joint. It is most associated with unmodified silicones on absorbent cementitious substrates and is worst where the panel is pale and matte. Use a sealant the manufacturer has tested against the actual substrate to ASTM C1248, prefer low stain or hybrid chemistries, and where doubt remains test on an offcut of the real panel left outdoors for several weeks.
The third control is the bond breaker. A bead must adhere to two faces only and be free at the back; if it also bonds to the back of the joint it is in three-sided adhesion, cannot deform properly, and tears. A closed cell or bi-cellular backing rod sets the depth and acts as the bond breaker, and where the joint is too shallow for a rod a bond breaker tape is used. Three-sided adhesion is the single most common cause of sealant failure on site.
Service life, and the argument for open joints
A silicone facade sealant has a typical service life around 20 to 25 years and a polyurethane rather less. A building has a design life of 50 years or more. A sealed facade therefore commits the owner to at least one, and realistically two or more, complete replacements of every joint on the building, each requiring full facade access, cutting out the old bead, preparing and priming, and regunning. On a tall building that is a major capital expense with a scaffold or swing stage attached to it.
That arithmetic is the strongest argument for an open jointed, drained and ventilated facade. In an open joint system there is no sealant in the weatherline at all: joints are open by design, pressure equalisation limits water entry, a baffle or labyrinth geometry deals with the rest, and the weather barrier behind the cavity is the actual line of defence. The consumables become baffles and gaskets, which are cheaper and simpler to replace than a gunned bead.
Sealants remain necessary at specific locations even on an open jointed facade: around windows and doors, at penetrations, at structural movement joints, at interfaces with other systems and at the perimeter. The objective is to minimise the length of sealed joint and to put every remaining bead somewhere that can be reached and replaced without dismantling the facade, with a replacement date in the maintenance schedule.
Standards and references
- ISO 11600 Building construction, jointing products, classification and requirements for sealants
- ASTM C920 Standard Specification for Elastomeric Joint Sealants
- ASTM C1248 Standard Test Method for Staining of Porous Substrate by Joint Sealants
- ASTM C1193 Standard Guide for Use of Joint Sealants, including joint design and depth to width ratio
- ASTM C1521 Standard Practice for Evaluating Adhesion of Installed Weatherproofing Sealant Joints
- AS/NZS 4284 Testing of building facades, water penetration and air infiltration
- GRCA guidance on joint design for GRC panels, and the panel manufacturer’s stated movement values
Related terms
Questions specifiers ask
Silicone or polyurethane for a facade joint?
Silicone has the greatest movement capability and the longest service life and is the usual choice for exposed movement joints. Polyurethane is tougher, paintable and better at ground level, but accommodates less movement and degrades faster under ultraviolet. Hybrid MS polymers sit between the two with a low staining risk on porous panels.
Why do dark bands appear either side of sealed joints?
Plasticisers and low molecular weight silicone fluids migrating out of the bead into the porous panel, darkening the surface and attracting dirt. Use a sealant tested against the actual substrate to ASTM C1248, prefer low stain or hybrid chemistries, and trial it on an offcut outdoors first.
What is three-sided adhesion and why does it matter?
A bead that bonds to the back of the joint as well as both sides cannot deform properly and tears as the joint moves. A closed cell backing rod sets the depth and acts as the bond breaker so the bead adheres to two faces only. It is the most common cause of sealant failure on site.
How long does a facade sealant last?
Typically 20 to 25 years for silicone and 10 to 15 for polyurethane, against a building design life of 50 years or more. That means at least one and usually two or more full replacements of every joint, each needing complete facade access.
Sizing joints for a moulded panel facade? We calculate panel movement and joint width as part of the shop drawings rather than leaving it to the sealant supplier. Talk to us about joint design
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.