Curtain Wall Mullion & Thermal Break: Plugs, Dams, Air Baffles & Spandrel Insulation

A mullion plug is a closed-cell foam block compressed into a hollow extrusion cavity, at sills, heads, splices, and anchor penetrations, so the cavity cannot bypass the wall's air and water line. The working material is a firm closed-cell foam with designed interference against the actual extrusion section: the firmest SCE neoprene grade is the standard for irregular cavities (CFD per ASTM D1056 on the TDS, UL 94 HF-1 lines present), EPDM foam serves regular sections, and crosslinked PE takes positions where water stands against the plug.

Specify by extrusion section, not by nominal block size. [1]

An end dam closes the end of a horizontal member's drainage gutter so collected water exits through the weeps instead of pouring into the vertical mullion cavity. Without it, the gutter becomes a duct feeding water into the frame, and the building reports a leak far from the actual cause. Dams are to the gutter section, in EPDM foam for general duty or crosslinked PE where water stands (near-zero absorption per ASTM D3575 on the TDS), and their positions belong on the shop drawing matched to the weep positions.

The AAMA 501-series water mockup is where missing dams surface. [10]

They change the details they isolate, and the honest answer lives at assembly level. A closed-cell strip on a metal-to-metal contact interrupts a conduction path and raises the local interior surface temperature, which is what stops condensation at that detail. Whether the whole frame's U-factor moves is a question for NFRC 100-framework rating and ASTM C1363 hot-box or modeling work on the section, the strip contributes its thermal-conductivity data line (ASTM C518 on the TDS) to that model.

Specify the strip for the detail's surface-temperature outcome, and let the section modeling claim the U-factor. [5]

NFPA 285 qualifies exterior wall assemblies with combustible components against fire propagation, at assembly level, held by the wall system supplier for a specific construction. If your wall is on an NFPA 285 path, the spandrel insulation is part of the qualified stack: it must be the material, grade, and thickness in the test file, and substitutions are engineering events with the system supplier in the loop.

Material data lines still inform the case, surface burning per ASTM E84 (on the ArmaGel TDS) and smoke density per ASTM E662 (on the SOLIMIDE TDS), but no material data line substitutes for the assembly result. State the qualification path on the RFQ. [6]

Choose by the binding constraint. If the cavity is thin and the thermal target is fixed, aerogel blanket wins: high thermal resistance per unit thickness (conductivity per ASTM C177/C518 on the TDS), hydrophobic behavior, E84 data lines.

If weight, conformability, and fire-smoke behavior lead, and the cavity is protected and drained, polyimide foam wins: extremely low density, E662 smoke-density lines, and useful acoustic absorption (C423) that can double-duty in shadow boxes. Both arrive to the cavity; both belong inside whatever assembly-level fire qualification the wall carries. [9]

Bare metal crossing the thermal line. An anchor clip or splice sleeve conducts heat outward, the interior face at that detail drops below the room's dew point in cold weather, and the result is a streak or stain that gets reported as a leak. The fix is an isolator in the conduction path, an XLPE break strip on the contact line, a dense EPDM pad where the part is clamped under bolt torque, confirmed by the section's condensation-resistance modeling at the design interior humidity and exterior condition.

Bring the detail and those design conditions to the RFQ; the pad geometry follows. [3]

The classic cause on this page is a stuffed-not-designed plug or baffle. A foam block cut by hand and pressed into a cavity holds the day it goes in; thermal cycling walks it, the interference relaxes, and the bypass opens slowly, so the ASTM E283-style performance drifts with the seasons rather than failing at once. Baffle butt-splices behave the same way as compression relaxes.

The fix is converting discipline: plugs to the extrusion section with designed interference, the D1056 class chosen for retention through movement, and splice geometry (shiplap, interlock) instead of butt joints. [8]

Yes. Kitting is a converting service: the plugs, dams, baffles, break pads, and insulation panels for one unit or one elevation arrive bagged together, labeled to the detail numbers on your shop drawings, so the line and the field crew stop sorting loose foam. Kiss-cut-on-liner formats suit factory unitized lines; compression-fit parts with no adhesive suit field stick installation. Kitting also closes the omission gap this page keeps warning about, an empty pocket in the kit is visible in a way a missing BOM line never is.

Ask for kitting on the RFQ with the detail list.

No, and the distinction matters. The structural thermal barrier inside a thermally broken extrusion, the poured-and-debridged polyurethane or the rolled-in polyamide strut, is part of the extrusion itself, engineered and supplied through the extruder. What H-O converts is the layer around it: isolator strips and pads at assembly details, anti-condensation parts, plugs, dams, baffles, and spandrel insulation.

If your detail needs both, the extrusion barrier comes from the extrusion supplier and the converted parts come from H-O, and the two are coordinated on the shop drawing.

Usually, yes. Send the system manual's accessory pages, the incumbent TDS, or physical parts, and engineering cross-references to families H-O converts, matching chemistry, closed-versus-open cell, ASTM D1056/D3575 class, water-absorption behavior, the relevant data lines (C518 thermal, E84/E662/UL 94 where called), and the geometry.

The result is an engineering cross-reference verified against the vendor TDS before quoting; where a part sits inside an assembly-level qualification (NFPA 285 path, tested mockup), the re-qualification consequence is called out honestly rather than papered over.

Every mullion and thermal component is made-to-order to your drawing, including samples. We keep common plug, dam, and baffle materials on hand for faster turnaround.

Samples typically ship in 3–5 business days for common configurations on materials we keep on hand. Standard production runs ship about 2 weeks after drawing approval, including kitted and kiss-cut-on-liner configurations. Expedited service is available when a mockup or glazing date is closing in. MOQ varies by material and part. Send the drawing and quantity through the form below for a specific lead-time commitment with your quote.

Three siblings complete the system. Curtain Wall Glazing & Perimeter covers the glazing pocket: setting blocks per ASTM C864, structural glazing spacer tape, pressure-plate gaskets, and weep baffles. Fenestration Tapes & Gaskets carries the cross-system tape library and the tape-versus-gasket decision tree. Fire-Rated Glazing & Opening Protectives takes the fire-code programs: rated glazing gaskets and intumescent seals. Impact, Blast & Acoustic Glazing takes the blast-resistant stacks and acoustic curtain-wall insulation.

The Fenestration industry hub indexes the full family.