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Setting-Block Sizing and Durometer Errors: The Quiet Failures That Stress IGU Edge Seals
A setting block is the smallest part in a glazed unit and one of the few that can quietly shorten the life of the most expensive one. Get the durometer or the length wrong and nothing fails on the line, at the water test, or in the first year; the insulating glass unit simply settles, its edge seal carries a load it was never meant to, and the unit fogs years later as a sealed-glass defect that was really a glazing-accessory decision.
This is how the two errors happen, what they do, and how ASTM C864 and the GANA rules prevent them.
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What does a setting block actually do?
A setting block carries the dead load of the glass and holds the lite off the frame so the edge of the insulating glass unit never bears on metal. It is a structural support, not a seal, which is why it is dense and hard rather than cellular. On the Window and Door Systems page the block is specified as glazing-grade solid silicone in the 80 to 90 Shore A band per ASTM C864 Option I, placed at the glass quarter points per the GANA Glazing Manual.
Two numbers govern the part: its durometer, which decides whether it holds the load without creeping, and its length, which decides whether the bearing stress stays low enough that it does not creep anyway.
Why does a soft setting block shorten IGU life?
The first error is durometer substitution: a 60 or 70 Shore A block goes in where the drawing said 80 to 90, usually to clear inventory or because the line grabbed the nearest silicone. The softer block creeps under the constant glass dead load, the lite settles, and the IGU edge seal, the perimeter that keeps the unit hermetic, takes a sustained stress it was never designed to carry.
The failure is invisible for years and then shows up as premature fogging, which reads as a sealed-unit defect rather than a block that was three durometer points too soft. ASTM C864 Option I exists precisely to define the dense-elastomer band; treat a durometer substitution as a spec change that requires engineering sign-off, not a supply convenience.
Why does block length matter as much as hardness?
The second error is the length shortcut: the area arithmetic gets skipped and a standard short block goes under an oversize lite. Bearing stress is load divided by area, so a block that is too short concentrates the glass weight, raises the stress at the bearing face, and creeps even at the correct durometer. The lite settles out of square, edge-seal stress rises, and the unit fogs early.
The GANA Glazing Manual sets the rule: two blocks per lite at the quarter points, with length scaled to glass area at roughly 0.1 inch per square foot of glass, and width matched to the IGU thickness with clearance. The arithmetic is not optional; it is the difference between a block that supports the glass and one that punches into it.
Where the load and the seal meet
Both errors converge on the same victim: the IGU edge seal. The block holds the glass; the edge seal keeps the unit sealed; and when the block fails to hold, the edge seal inherits a job it cannot do for long. That is why setting-block discipline is really IGU-durability discipline, and why insulating glass unit performance is qualified separately per ASTM E2190 while the block that protects it is governed by ASTM C864 and the GANA placement rules.
The curtain-wall scale of the same problem, with anti-walk side blocks and the dead-load arithmetic, lives on Curtain Wall Glazing and Perimeter.
The Two Setting-Block Errors and What They Cost
| Error | Mechanism | What it causes | The rule that prevents it |
|---|---|---|---|
| Durometer substituted too soft | Block creeps under dead load | Glass settles, edge-seal stress, early fog | ASTM C864 Option I, 80 to 90 Shore A band named |
| Block too short for the lite | Bearing stress concentrates | Creep at correct durometer, lite out of square | GANA length, about 0.1 inch per square foot |
| Blocks off the quarter points | Load path not where designed | Uneven bearing, edge stress | GANA placement, two blocks at quarter points |
| Block walks out of position | No positive retention | Glass bears on metal at a corner | Keyed or notched die-cut geometry |
Qualitative summary of the failure modes on the parent application page. The block supports the lite; the IGU edge-seal durability it protects is qualified separately per ASTM E2190.
How conversion prevents both errors
The fix is to make the block a controlled part rather than a line-side judgment call. H-O die-cuts setting blocks from dense glazing-grade solid silicone to the durometer band and the length the GANA arithmetic returns, with keyed or notched geometries that key into the pocket so the block cannot walk under vibration or thermal cycling.
We do not extrude or mold; we convert dense sheet and strip to the drawn dimension, kiss-cut on liner where the line wants peel-and-place, with the durometer certificate and lot-code TDS records traveling with the order under our ISO 9001:2015 certified quality management system. Send the glass build and the lite sizes and engineering returns the block length, the band, and the placement as drawn dimensions.
Frequently asked questions
What durometer should a window setting block be?
Dense elastomeric setting blocks per ASTM C864 Option I sit in the 80 to 90 Shore A hardness band, and dense solid silicone in that band is the most commonly specified material. A softer 60 or 70 Shore A block creeps under the constant glass dead load, lets the lite settle, and loads the IGU edge seal, shortening sealed-unit life. Name the 80 to 90 Shore A band on the drawing rather than just the word silicone, and treat any durometer substitution as a spec change requiring engineering sign-off.
How long should a setting block be?
The GANA Glazing Manual scales block length to glass area at roughly 0.1 inch per square foot of glass, with two blocks per lite at the quarter points and width matched to the IGU thickness with clearance. A block that is too short concentrates the bearing stress and creeps even at the correct durometer, so the length is as important as the hardness. Carry the calculated length on the drawing as a controlled dimension rather than defaulting to a standard short block.
Why does a setting-block error show up as IGU fogging?
Because the block protects the IGU edge seal. When a block is too soft or too short it creeps, the lite settles, and the edge seal, the perimeter that keeps the unit hermetic, carries a sustained stress it was not designed for. The unit then loses its seal and fogs, often years after handover, so the symptom reads as a sealed-glass defect when the root cause was a glazing-accessory decision. Insulating glass unit durability is qualified separately per ASTM E2190.
Can H-O die-cut setting blocks that will not walk out of position?
Yes. H-O die-cuts dense glazing-grade solid silicone setting blocks to the durometer band and the GANA length, and can add keyed or notched geometries that key into the pocket so the block resists migration under vibration and thermal cycling. We convert dense sheet and strip rather than molding or extruding, kiss-cut on liner where the line prefers peel-and-place, with durometer certificates and lot-code TDS records on the order. Send the glass build and the lite sizes and engineering returns the block dimensions as drawn.
Have engineering size your setting blocks
Send the glass build and the lite dimensions and we will return the block length from the GANA rule, the 80 to 90 Shore A band per ASTM C864, the quarter-point placement, and a die-cut geometry that keys into your pocket.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM C864 Option I (dense elastomeric gaskets, setting blocks, and spacers); GANA Glazing Manual (setting-block placement and sizing); ASTM D2240 (durometer).
- ASTM E2190 (insulating glass unit performance); NAFS-2022 / AAMA/WDMA/CSA 101/I.S.2/A440-22 (the unit performance classes the glazing supports).
- Manufacturer technical data sheets for the glazing-grade solid silicone family named on the parent application page.
This article summarizes setting-block failure modes and the ASTM C864 and GANA sizing rules cited on the parent application page; it is not glazing design guidance for any specific unit. Per-grade durometer values are confirmed on the manufacturer's current technical data sheet at quote time, and the project glazing spec governs. Insulating glass unit durability is qualified separately per ASTM E2190.
All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a die-cut converter and does not extrude or mold elastomers in-house.