The addendum nobody re-read
Addendum 3 drops at 4:17 PM on a Thursday, bid due Monday 2:00 PM. Buried on page 11, Section 084413.2.3 changes the IGU makeup from "1-inch insulating glass unit with low-e coating on surface #2" to "1-5/16 inch triple-silver low-e insulating glass unit with argon fill and stainless steel warm-edge spacer, SHGC not to exceed 0.23, VLT minimum 0.58." The elevations didn't change. The glazing estimator sees the addendum come in, scans it for "curtain wall framing" changes, finds none, and moves on. Four months later a submittal comes back rejected and the job just bought 40,000 SF of $38/SF IGU at a bid that assumed $21/SF.
That $680,000 delta is not hypothetical. It is the single most common six-figure mistake in commercial glazing, and it happens on roughly one in five bids with late-cycle addenda.
Unitized vs stick-built — the labor basis is completely different
Estimators who came up on storefront often price curtain wall as if it were a scaled-up storefront. It is not. GANA's Glazing Manual and the AAMA CW-10 guide are explicit that unitized and stick-built systems have fundamentally different labor bases:
- Stick-built: mullions, horizontals, pressure plates, and glass all set from swing stage or mast climber in sequence. Field labor is 0.28–0.42 hours per SF on a typical 10-story commercial elevation.
- Unitized: shop-glazed units delivered on racks, hoisted to floor, anchored to embeds. Field labor drops to 0.08–0.14 hours per SF but shop labor climbs, and the premium on the IGU and extrusions runs 18–25%.
- Crane and hoisting: a unitized job needs a crane or material hoist for the duration of enclosure — that line item alone is $45,000–$180,000 depending on building height and schedule.
- Anchor type: embed plates cast into slab edge (preferred) vs post-installed anchors into PT slab — the latter needs a PT scan and a structural engineer signoff per ACI and usually burns 0.8–1.4 hours per anchor.
Bidding a unitized system with stick-built labor rates will hand you the job at a 15–20% loss. Bidding a stick-built system with unitized rates loses you the job to somebody who can read a detail.
IGU makeup: where the glass dollars actually live
The IGU is 45–60% of the curtain wall material cost. The architect's performance spec drives the number, not the elevation drawings. Key variables that every glazing estimator should extract from 088000 and 084413 before sizing the buy:
- Low-e coating position: surface #2 vs surface #3 (and whether triple-silver is called out). A triple-silver coating is 2.5–3.5x the cost of standard low-e.
- Spacer type: aluminum box vs warm-edge (stainless, silicone foam). Warm-edge adds $1.80–$3.20/SF.
- Gas fill: air vs argon vs krypton. Krypton fills (needed for thin triples) can double the IGU cost.
- Tempered vs heat-strengthened vs laminated: hurricane zones, guardrail glazing, and ballistic specs each trigger different lites.
- Structural silicone glazing (SSG): two-sided or four-sided SSG requires Dow 983 or equivalent, adhesion testing, and an engineering signoff. SSG labor runs 30–45% higher than captured glazing.
The line item that always gets forgotten: perimeter sealant LF
Every curtain wall has two sealant joints — a weatherseal at the exterior and an air-seal at the interior — for every linear foot of perimeter against the adjacent construction. On a 40,000 SF curtain wall with average punched dimensions, that is roughly 9,500–11,500 LF of two-part silicone sealant per side. GANA and SWR Institute both recommend ASTM C1193 for sealant joint design.
"I audit bids for my former employer as a consultant. The number one miss I see, still, in 2026, is the perimeter sealant quantity. Estimators count window perimeter and forget the building perimeter where the curtain wall meets the slab edge, the spandrel, and the adjacent punched openings. It is a $40,000–$90,000 swing on a tower."
Rafael Beaumont, Glazing Consultant, formerly Chief Estimator at two ENR-ranked glaziers
Miscellaneous glass: the carve-outs that eat margin
084413 rarely contains only curtain wall. Tucked in the same section or cross-referenced in 088000 are: interior glass partitions, shower enclosures, mirrors, glass handrails, skylights, and decorative laminated feature glass. These items have completely different labor bases and often get priced at curtain wall rates. A laminated glass guardrail per IBC 2407 with a PVB interlayer and a top cap pulls 3–5x the labor per SF of a field-set curtain wall lite.
What changes when you get it right
A disciplined glazing takeoff extracts the IGU performance spec from 088000, maps it to the elevation square footage by type, verifies unitized vs stick-built against 084413 and the wall sections, counts perimeter sealant as a separate LF quantity on two sides, carves out the misc glass at its own labor unit, and — the easy part to skip — re-reads every addendum issued after the takeoff started. Glazing shops that follow that sequence run 9–11% net on tower work. The ones that skip the addendum re-read run 2% or bankrupt and nobody can ever quite explain why.