Silicone vs Polyurethane Sealant: Which Building Joint Fits

Silicone sealant suits glazing and metal joints needing long UV life; polyurethane suits paintable concrete and masonry movement joints, floors and abrasion exposure. This holds for building joint sealants classed under ASTM C920. The movement class, uses and paint limits on each product's data sheet decide the final choice, because grades within each chemistry differ.

• ASTM C920-18(2024), the active ASTM specification for elastomeric joint sealants, classes them by movement as class 100/50, 50, 35, 25 or 12.5 (ASTM International, 2024).

• In a Florida exposure study started in 1983, 13 sealants from six makers were evaluated at 22, 30 and 41 years (Construction Specifier, 2025).

• One single-component polyurethane data sheet calls for 7 days of cure before overcoating, with compatibility and adhesion testing of the paint (Sika, 2026).

• Caulk put in place between 1950 and 1979 may contain as much as 40 percent PCBs (US EPA, 2026).

This comparison covers cold-applied joint sealants on building facades, plazas and decks. It does not cover structural glazing, insulating glass edge seals, highway or airfield pavement joints, or hybrid sealants such as MS polymer, which have their own selection rules.

How do silicone and polyurethane sealants differ on building joints?

Silicone and polyurethane sealants differ on building joints mainly in UV resistance, paintability, traffic rating and the substrates they are rated for under ASTM C920. ASTM C920-18(2024) classes each sealant by type, grade, movement class and use, and it is similar, but not identical, to ISO 11600 (ASTM International, 2024). Its use codes relate to tests on standard specimen substrates of mortar, glass and aluminum (ASTM International, 2024), so the use codes on a data sheet show which test substrates a product was classed on.

The table below compares one neutral-cure building silicone with one single-component polyurethane, property by property, with the test or basis for each value.

ASTM C920 use is the row to check first: the polyurethane carries Use M for mortar and Use T for traffic (Sika, 2026), while this silicone carries Use G for glass but neither M nor T (Dow, 2025). Use codes rate products, not chemistries: one ultra-low-modulus silicone is rated class 100/50 for uses T, NT, M, G, A and O (Dow Corning, 2011).

Why do published service life figures for silicone and polyurethane sealant differ?

Published service life figures for silicone and polyurethane sealant differ because each source measures a different thing, under different exposure, at a different date, and some come from makers of one chemistry. The table below sets the figures side by side with the basis for each.

The polyurethane row spans 5 to 20 years because the 5 to 8 year figure applies to urethane under direct UV exposure (Hixson, 2020), while the 20 year figure is a general external life expectancy used for costing (GreenSpec, 2005).

Three more factors widen the spread. First, no classification predicts it: ISO 11600 does not predict sealant durability (GreenSpec, 2005). Second, the field decides: ASTM C1193-25 lists compatibility with adjacent materials, installation techniques or deficiencies, joint design, maintenance and environmental exposure among the factors that can affect service life (ASTM International, 2025). Third, two of the sources are tied to silicone: the 2007 manual is a silicone maker's, and the author of the 2025 Florida study works for a silicone maker (Construction Specifier, 2025). That study, on glass and painted aluminum panels with sealants sold in 1983, found silicone outperformed polyurethane and acrylic in nearly every performance category after 40 years (Construction Specifier, 2025).

When is silicone sealant the better joint choice?

Silicone sealant is the better joint choice for glass, glazing frames, metal panels and other nonporous joints exposed to sun, where the joint will not be painted. One neutral-cure building silicone is rated ASTM C920 class 50 for uses NT, G, A and O (Dow, 2025), which covers glass and aluminum but not traffic. One building consultant writes that silicone adheres to nonporous substrates better than any other sealant, including glass, metal, ceramic tile and porcelain (Hixson, 2020).

Silicone also suits cold-weather work. One maker states that silicone sealants can be applied in subfreezing temperatures provided the substrate is frost- and moisture-free, with slower cure (Dow Corning, 2007). The same maker claims a polyurethane sealant can get 2 to 3 times stiffer in cold temperatures, while silicone keeps its low modulus (Dow Corning, 2007).

Silicone sealant is the weaker choice where the joint must be painted, since one neutral silicone data sheet says it should not be used if the sealant is intended to be painted, because paints do not typically adhere to most silicone sealants (Dow, 2025). On porous stone it needs a test first: excess crosslinker in silicone sealants can migrate into porous material and leave a dry-looking area beside the joint in rain (Dow Corning, 2007). Check the cure type as well: acetoxy silicone releases acetic acid, which limits its use on some substrates due to potential corrosion, although acetoxy samples showed the best long-term performance in the Florida study (Construction Specifier, 2025).

When is polyurethane sealant the better joint choice?

Polyurethane sealant is the better joint choice for any joint that will be painted, including concrete and masonry movement joints, and for floor and plaza joints under traffic. One single-component polyurethane is rated ASTM C920 class 35 for uses T, NT, O, M, A and I (Sika, 2026), which includes mortar and traffic. A building consultant notes that urethane sealants resist abrasion well, making them suitable for use on floors (Hixson, 2020).

Polyurethane also suits new and damp concrete. One data sheet allows application to green concrete 24 hours after pour and to damp concrete 1 hour after getting wet, once standing water is removed, and warns that low-strength concrete may impair adhesion (Sika, 2026). The same consultant reports that urethane sealants are more compatible with barriers, wraps and flashings than neutral-cure silicone sealants (Hixson, 2020).

Polyurethane has three limits on exposed joints. Painting over it is recommended, because exposure to the sun's UV rays causes chalking if it is not painted (Hixson, 2020). One data sheet limits sealant depth to between 6 mm and 13 mm and allows movement of no more than 35% of average joint width (Sika, 2026). The same data sheet rules out contact with bituminous or asphaltic materials (Sika, 2026), so check what sits behind the joint before choosing polyurethane against a membrane.

How do you switch from polyurethane to silicone sealant?

You switch from polyurethane to silicone sealant by removing the old polyurethane completely, cleaning the joint walls to sound substrate and testing adhesion before resealing the whole elevation. One silicone maker states that when a failed organic sealant is cut out and replaced, the old sealant must be completely removed (Dow Corning, 2007). A contractor's study found that silicone and polyurethane materials are not generally good adhesive partners (Walsh Construction, 2014), so a thin skin of old polyurethane left on the joint wall is a likely bond line failure.

Before cutting, test old caulk in buildings constructed or renovated between about 1950 and 1979, since it may contain PCBs (US EPA, 2026). In the US, PCB removal and disposal follow 40 CFR Part 761 (US EPA, 2026); elsewhere, check the national rules on PCB waste. The silicone maker cited above also notes that its removal procedure may not suit joints containing PCBs (Dow Corning, 2007).

The maker's sequence runs: cut the old sealant away, clean the joint by wire brush, grinding, saw cutting or solvent, blow out dust with oil-free air, let the joint dry, prime if recommended, install backer rod, seal, and check adhesion on a test joint after 7 to 21 days of cure (Dow Corning, 2007). If a solvent is used, read its SDS first and follow the flammable-liquid rules for your jurisdiction, such as US OSHA 29 CFR 1926.152 on construction sites (OSHA, 2026). Grinding or saw cutting concrete or masonry joint walls can expose workers to respirable crystalline silica; in the US this falls under OSHA 29 CFR 1926.1153 (OSHA, 2026), and elsewhere under the national rule.

Lubinpla can organize a switch like this from your own data sheets, SDS and site history into one cited document for your review.

What drives the cost of silicone and polyurethane sealant over service life?

The cost of silicone and polyurethane sealant over service life is driven by how often the joints must be replaced and what each replacement needs, more than by the sealant itself. In its example of a building with a 50 year design life, ASTM C1193-25 notes that proper maintenance could include complete sealant replacement, perhaps more than once, depending on the sealant's polymer base and formulation (ASTM International, 2025).

Four factors set that cost. Replacement interval comes first, and the published figures differ widely, as the service life table above shows. Access comes second: on a high facade, every replacement needs access equipment such as scaffolds or lifts, a cost that recurs with each replacement interval. Repainting comes third for polyurethane, since painting over urethane is recommended to avoid UV chalking (Hixson, 2020). Primers come fourth: a primer adds another material and step to the work (Walsh Construction, 2014).

Warranty terms also shift cost. One consultant reports that after compatibility testing and documented field peel adhesion, sealant makers offer 20 year weatherseal and adhesion warranties for neutral-cure silicone (Hixson, 2020).

FAQ

Can silicone and polyurethane sealant be used in the same joint?

Silicone and polyurethane sealant should not share one joint without testing, because they are not generally good adhesive partners (Walsh Construction, 2014). One polyurethane data sheet also states that it should not cure in the presence of curing silicone sealants (Sika, 2026).

Which sealant bonds better to concrete and masonry?

Both chemistries include products rated for concrete and masonry; the ASTM C920 Use M rating on the data sheet shows which were tested on mortar. The polyurethane compared here is rated Use M (Sika, 2026), the silicone compared here is not (Dow, 2025), and one ultra-low-modulus silicone is rated Use M and sold for precast concrete and masonry joints (Dow Corning, 2011). Choose polyurethane where the joint will be painted, and confirm either with a field adhesion test on your substrate.

Does polyurethane sealant need a primer where silicone does not?

Neither chemistry needs a primer by default. One polyurethane data sheet calls for priming on most substrates only where testing indicates a need or the joint will be immersed after cure (Sika, 2026). One silicone data sheet says primer may be necessary on certain porous surfaces (Dow, 2025).

How does movement capability compare between silicone and polyurethane sealant?

Movement capability depends on the grade, not only the chemistry. ASTM C920 defines classes from 12.5 to 100/50 (ASTM International, 2024), and the two products compared here are class 50 silicone and class 35 polyurethane (Dow, 2025; Sika, 2026).

Working a case like this? Lubinpla works it from your own documents and site history, step by step with your approval. Free to start, no card.

Lubinpla does not sell chemical products and names no brands in recommendations. Our reviewer also leads a specialty chemicals distribution company.

Sources checked 2026-10-13.

Sources

1. ASTM International. ASTM C920-18(2024) Standard Specification for Elastomeric Joint Sealants. 2024. https://www.astm.org/c0920-18r24.html

2. ASTM International. ASTM C1193-25 Standard Guide for Use of Joint Sealants. 2025. https://www.astm.org/c1193-25.html

3. Bull, E. (Momentive Performance Materials). 40-year study of silicone sealants: Silicone versus alternative chemistries. The Construction Specifier. December 2025. https://www.constructionspecifier.com/40-year-study-of-silicone-sealants-silicone-versus-alternative-chemistries/

4. Hixson, B. T. (Hixson Consultants). Sealant Performance and Compatibility: The Great Disconnect. Chamberlin. 2020. https://www.chamberlinltd.com/articles/sealant-performance-and-compatibility-great-disconnect/

5. Dow Corning. Dow Corning Americas Technical Manual, Form No. 62-1112F-01. Revised June 2007. https://www.fleetwoodusa.net/Documents_Guide/Products/_General/Sealant-Technical-Manual.pdf

6. Dow. Technical Data Sheet: DOWSIL 795 Silicone Building Sealant, Form No. 61-885-01. 2025. https://www.buildsite.com/pdf/dowcorning/DOWSIL-795-Silicone-Building-Sealant-Product-Data-2890459.pdf

7. Sika. Product Data Sheet: Sikaflex-1A, Version 01.03. August 2026. https://can.sika.com/content/dam/dms/ca01/l/sikaflex-1a.pdf

8. Mayer, P. (Building LifePlans). Whole life costing: Sealants. GreenSpec, first published in Building 2005. https://greenspec.co.uk/building-design/sealants-costs/

9. Walsh Construction. Sealant Adhesion and Compatibility Study Report. October 2013, revised February 2014. https://www.walshconstruction.com/wp-content/uploads/Walsh_Sealant-Study-Report_2013rev2014.pdf

10. US EPA. Polychlorinated Biphenyls (PCBs) in Building Materials. Last updated February 25, 2026. https://www.epa.gov/pcbs/polychlorinated-biphenyls-pcbs-building-materials

11. Dow Corning. Dow Corning 790 Silicone Building Sealant product information, Form No. 61-884N-01. 2011. https://berridge.com/resources/dow-790-sealant

12. US Occupational Safety and Health Administration. 29 CFR 1926.152 Flammable liquids, and 29 CFR 1926.1153 Respirable crystalline silica. Accessed 2026-10-13. https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.152 and https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.1153

Written by the Lubinpla team from the sources listed and checked against them before publication.