Curtain Wall and Insulating Glass Sealant Guide

Two different jobs in one facade

A curtain wall facade uses sealants for two different jobs. The weather seal keeps water and air out of the joint. The structural seal bonds the glass to the frame and carries load. They are different products with different requirements, and confusing them is a serious specification error.

Weather seals need flexibility, weathering resistance and durable adhesion to glass, metal and stone. Structural seals need documented strength, adhesion to the exact substrates and compatibility with the whole glazing system, including gaskets, spacers and coatings.

Structural glazing needs project design

Structural glazing is engineered for each project. The sealant, the joint size, the substrates and the loads are calculated together, and the system usually needs documented approval for the specific build-up. A product described for structural glazing in general is not automatically approved for your project.

Compatibility testing is part of the process: the sealant must be tested against the glass coatings, gaskets, spacers and setting blocks actually used. A change in any component can change the result, so substitutions after approval need re-checking.

Insulating glass secondary seals

An insulating glass unit has a primary seal that holds the spacer and a secondary seal that gives the unit its strength and gas retention. Silicone and polysulfide are the usual secondary seal materials. The choice depends on the unit design, the gas fill and the frame system.

Secondary seal products are applied in production, under controlled conditions, and the unit manufacturer — not the site team — normally selects them. Site teams should not substitute a site-applied sealant for a factory secondary seal.

Site application and quality

Site-applied weather seals need clean joints, correct backing, proper joint size and enough cure time before the facade is exposed to water or movement. Cold, wet or dusty conditions during application are a common cause of later leaks.

Keep records: product codes, batch numbers, colours, application dates and weather conditions. Facade leaks are investigated months later, and records are what separate a product problem from an application problem.

Weather seal design

A weather seal joint must be wide enough to absorb movement and shallow enough to cure and tool properly. The joint width is calculated from the expected movement and the movement capability of the sealant, with a safety margin. Too narrow a joint overstrains the sealant; too wide a joint wastes material and looks poor.

Backing rods set the sealant depth and prevent three-sided adhesion. The rod should be slightly larger than the joint width so it stays put, and it should be a type that does not react with the sealant. Open-cell rods suit joints where air must escape; closed-cell rods suit most other joints.

Substrate preparation on facades

Facade substrates — glass, anodised aluminium, coated metal, stone, concrete — each need their own preparation. Glass needs cleaning; metals may need a primer; stone may need sealing against staining. A single preparation method for a whole facade is rarely adequate.

Site conditions during application matter as much as preparation. Rain, dust, frost and high wind all harm adhesion. Facade sealing should be planned around weather, not forced through it.

Movement and thermal cycling

Facades move daily and seasonally. Aluminium expands roughly twice as much as glass per degree, so a glazed unit in a metal frame sees differential movement at every temperature change. The weather seal absorbs that movement thousands of times over its life.

Sealant fatigue — gradual loss of adhesion or cohesion after many cycles — is the long-term failure mode. It is why movement capability, joint size and adhesion all matter together, and why a product that passes a single pull test can still fail after years of cycling.

Inspection and maintenance

Facade sealants should be inspected on a schedule, not only when a leak appears. Look for adhesion loss at the edges, splits in the middle of the bead, hardening, discolouration and staining of neighbouring materials. Early replacement of a failed length is cheap; a leak that reaches interiors is not.

Replacement joints need the old sealant fully removed and the joint re-prepared. New sealant over old failed sealant is a short-term patch, not a repair.

Spandrel panels and opaque areas

Spandrel areas — the opaque panels between vision glass floors — use sealants under different conditions from vision glazing. They face higher temperatures from solar gain on dark panels, and the sealant must handle that heat without softening or staining.

Spandrel sealants also meet insulation, backpans and fire-stopping materials. Compatibility with those neighbours must be checked, since a sealant that suits glass and aluminium may react with an insulation facing or a fire-stop material.

Access for replacement is often worse in spandrel areas than in vision areas. A sealant chosen for a spandrel joint should be selected for long life, because replacing it later may need scaffolding or panel removal.

Leak investigation on facades

Facade leaks rarely come from where the water appears inside. Water travels along joints, flashings and framing before it shows, so the entry point may be metres from the stain. Investigation starts outside, with a systematic water test of one area at a time, not with resealing everything at once.

Common entry points are failed weather seals, blocked drainage paths, unsealed fixing penetrations and discontinuous flashings. A sealant failure is only one candidate; drainage and flashing failures are at least as common.

Document the investigation: test areas, results, photographs and the products found in each joint. A facade with mixed sealant types from past repairs needs identification before any new product is applied, since incompatible sealants do not bond to each other.

Glazing gaskets and sealant interaction

Glazing systems combine gaskets and sealants, and the two must be compatible. Some gasket materials — particularly certain rubbers and plasticised PVC — migrate plasticiser into sealants, softening them or causing adhesion loss. The system supplier should confirm gasket and sealant compatibility for the exact combination used.

Setting blocks and edge blocks also matter. They position the glass, carry its weight and keep the sealant joint size consistent. A missing or misplaced setting block changes the joint geometry and can overload the sealant on one side.

During installation, protect the sealant joint from damage by following trades. A bead cut by a later trade, or contaminated with paint or mortar, is a leak waiting for the first storm.

A note on product selection

Guides on this site describe product types in general terms. They do not promise that any particular product suits a particular job. Before ordering, match the product code, the manufacturer's published description and your application's requirements, and confirm anything not stated in writing with the supplier.