Joint Sealants
Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
Showing changes from Rev 5
to Rev 6
in Joint Sealants.
---
title: Joint Sealants
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### A working joint shall not be sealed when it is at its widest (cold) or narrowest (hot) dimension.
−### The sealant cures to whatever width exists at the moment of application and must then accommodate movement in both directions from that cured state. {note}
+### The sealant cures to whatever width exists at the moment of application and must then accommodate movement in both directions from that cured state.
### A joint sealed wide open on a cold morning is placed almost entirely in compression as the structure warms; sealed tight on a hot afternoon, it is placed almost entirely in extension as it cools. {note}
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#### Type S is a single-component sealant supplied ready to use; Type M is a multi-component sealant mixed before application. {note}
+#### The sealant type shall be specified as single-component or multi-component, selected for the joint configuration and application method required.
+
```datasheet
label: Sealant Type (ASTM C920)
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#### Grade NS is a non-sag (gunnable) sealant that stays in place on vertical and overhead joints; Grade P is a pourable, self-leveling sealant for horizontal joints. {note}
+#### The sealant grade shall be specified as non-sag or pourable, matched to the orientation of the joint being sealed.
+
```datasheet
label: Sealant Grade (ASTM C920)
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#### A Class 25 sealant withstands ±25% of the joint width; Class 50 withstands ±50%; Class 100/50 withstands +100%/−50%; lower classes are 35, 25, and 12.5. {note}
+#### The sealant movement class shall be specified, sized to the anticipated movement the joint is designed to accommodate.
+
```datasheet
label: Sealant Movement Class (ASTM C920 / C719)
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#### The Use designations identify the substrates and service the sealant is qualified for, including Use T (traffic-bearing), Use NT (non-traffic), Use I (immersion), Use M (mortar/masonry), Use G (glass), Use A (aluminum), and Use O (other specified substrates). {note}
+#### The Use shall match the joint's substrates and service.
+
```datasheet
label: Sealant Use (ASTM C920)
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```
−#### The Use shall match the joint's substrates and service.
−
## Sealant Modulus {toc}
### Modulus is the stress a sealant develops as it is strained. {note}
+### A low-modulus sealant exerts low pulling force on the joint faces at a given movement, which is important where the substrate is weak or porous — a high-modulus sealant on soft stone or weak mortar can pull the substrate apart (cohesive substrate failure) before the sealant itself fails. {note}
+
+### Low-modulus sealants shall be used on stone, weak masonry, and joints with large movement.
+
```datasheet
label: Sealant Modulus
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```
−### A low-modulus sealant exerts low pulling force on the joint faces at a given movement, which is important where the substrate is weak or porous — a high-modulus sealant on soft stone or weak mortar can pull the substrate apart (cohesive substrate failure) before the sealant itself fails. {note}
−
−### Low-modulus sealants shall be used on stone, weak masonry, and joints with large movement.
−
### Medium- or high-modulus sealants suit strong substrates and traffic joints.
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#### Silicone offers the best ultraviolet and long-term weathering resistance and the widest service-temperature range, making it the default for exposed exterior weatherseal joints. {note}
+#### Neutral-cure silicone (oxime or alkoxy) is used on masonry, stone, metal, and coated substrates because its cure by-products do not corrode metal or attack masonry. {note}
+
+#### Acid-cure (acetoxy) silicone releases acetic acid as it cures, which can corrode some metals, etch masonry, and stain stone; its use is limited to glass and other non-reactive substrates, typically interior glazing.
+
```datasheet
label: Silicone Cure System
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```
−#### Neutral-cure silicone (oxime or alkoxy) is used on masonry, stone, metal, and coated substrates because its cure by-products do not corrode metal or attack masonry. {note}
−
−#### Acid-cure (acetoxy) silicone releases acetic acid as it cures, which can corrode some metals, etch masonry, and stain stone; its use is limited to glass and other non-reactive substrates, typically interior glazing.
−
#### Standard silicones are not paintable and shall not be used where paint must adhere over the joint.
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### The relationship between a sealant joint's width and the depth of the sealant bead — the shape factor — is the single most important geometric property of a working joint. {note}
+### For a properly proportioned joint, the sealant is installed in an hourglass profile that is wider at the bonded faces than at its thin mid-section, so that as the joint moves, the strain concentrates in the thinner middle and the bond lines at the faces are relatively protected. {note}
+
+### A bead that is as deep as it is wide (a near-square cross-section) develops high stress at the bond lines under the same movement and tears loose. {note}
+
+### As a general rule for working joints in the common width range, the sealant depth at mid-joint shall be approximately half the joint width, within the manufacturer's stated limits, with minimum and maximum bead depths the manufacturer specifies for the chosen sealant.
+
```datasheet
label: Sealant Width-to-Depth Ratio (working joints)
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```
+### A working joint shall be wide enough that the anticipated movement is within the sealant's movement class at that width, and not so wide that the bead sags or exceeds the sealant's maximum.
+
```datasheet
label: Typical Exterior Working Joint Width
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```
−### For a properly proportioned joint, the sealant is installed in an hourglass profile that is wider at the bonded faces than at its thin mid-section, so that as the joint moves, the strain concentrates in the thinner middle and the bond lines at the faces are relatively protected. {note}
−
−### A bead that is as deep as it is wide (a near-square cross-section) develops high stress at the bond lines under the same movement and tears loose. {note}
−
−### As a general rule for working joints in the common width range, the sealant depth at mid-joint shall be approximately half the joint width, within the manufacturer's stated limits, with minimum and maximum bead depths the manufacturer specifies for the chosen sealant.
−
−### A working joint shall be wide enough that the anticipated movement is within the sealant's movement class at that width, and not so wide that the bead sags or exceeds the sealant's maximum.
−
### Joint widths are [[drawing: as detailed on the drawings]] and are sized from the calculated thermal and structural movement and the selected sealant's movement class per ASTM C1193.
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### Cylindrical sealant backing conforming to ASTM C1330 limits how deep the sealant is installed, establishes the hourglass shape, and acts as a bond breaker at the back of the joint. {note}
+### The backer rod shall be sized larger than the joint width — typically about 25% oversize — so it is compressed into the joint and stays in place without being stretched.
+
```datasheet
label: Backer Rod Oversize (compression fit)
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```
−### The backer rod shall be sized larger than the joint width — typically about 25% oversize — so it is compressed into the joint and stays in place without being stretched.
−
### The backer rod shall be installed at the depth that produces the specified sealant bead depth.
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#### The cellular structure of the rod is selected for the joint. {note}
+#### Closed-cell polyethylene backing (ASTM C1330 Type C) resists water absorption and suits most joints but can outgas through a puncture. {note}
+
+#### Open-cell backing absorbs water and is limited to dry, vertical joints. {note}
+
+#### Bi-cellular backing (ASTM C1330 Type B) has a closed skin over a partly open structure and tolerates surface puncture without outgassing, making it a robust general choice. {note}
+
+#### Open-cell rod shall not be used where water can reach it.
+
```datasheet
label: Sealant Backing Type (ASTM C1330)
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```
−#### Closed-cell polyethylene backing (ASTM C1330 Type C) resists water absorption and suits most joints but can outgas through a puncture. {note}
−
−#### Open-cell backing absorbs water and is limited to dry, vertical joints. {note}
−
−#### Bi-cellular backing (ASTM C1330 Type B) has a closed skin over a partly open structure and tolerates surface puncture without outgassing, making it a robust general choice. {note}
−
−#### Open-cell rod shall not be used where water can reach it.
−
# Sealant Schedule by Joint Location {toc}
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### Expansion and control joints accommodate the structure's planned movement and shrinkage. {note}
+### Vertical expansion and control joints shall take a non-sag low-modulus sealant of a class matched to the joint movement; horizontal non-traffic joints shall take a self-leveling sealant.
+
```datasheet
label: Concrete / Masonry Expansion-Control Joint Sealant
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```
−### Vertical expansion and control joints shall take a non-sag low-modulus sealant of a class matched to the joint movement; horizontal non-traffic joints shall take a self-leveling sealant.
−
### On masonry, a non-staining low-modulus sealant (Use M) shall be used to protect the masonry from edge staining and from being torn by a high-modulus bead.
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### Joints around windows, doors, louvers, and other openings seal the gap between the frame and the surrounding construction against air and water. {note}
+### Perimeter-of-opening joints are weatherseal joints and shall be sealed with a low-modulus silicone or hybrid compatible with both the frame finish (often coated aluminum, Use A) and the wall substrate.
+
```datasheet
label: Opening Perimeter Sealant
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```
−### Perimeter-of-opening joints are weatherseal joints and shall be sealed with a low-modulus silicone or hybrid compatible with both the frame finish (often coated aluminum, Use A) and the wall substrate.
−
### Coordinate perimeter-of-opening joints with [[sync/air-barriers]] so the perimeter seal is continuous with the air- and water-resistive barrier.
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### Acoustical sealant seals the perimeter and penetrations of sound-rated partitions to prevent flanking sound through gaps; it is a non-hardening, non-skinning sealant that stays flexible to maintain the seal. {note}
+### The perimeter and penetrations of sound-rated partitions shall be sealed with a non-hardening, non-skinning acoustical sealant that stays flexible to maintain the seal.
+
```datasheet
label: Interior Sealant
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```
−### The perimeter and penetrations of sound-rated partitions shall be sealed with a non-hardening, non-skinning acoustical sealant that stays flexible to maintain the seal.
−
### General interior non-working joints and gaps before paint shall be filled with paintable acrylic latex (ASTM C834).
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### The longer manufacturer warranty depends on the applicator approval, compatibility testing, and field adhesion testing required above being met during construction, so they are prerequisites to it. {note}