Low-modulus sealant
A low-modulus sealant deforms under low stress to take large cyclic movement (often +/-50%) without tearing the substrate, the right pick for masonry articulation joints.
Ask Chalkline about this →A low-modulus sealant is a sealant formulated to deform under low stress, so it can accommodate large cyclic joint movement (commonly rated to ±25% or ±50% under ISO 11600) without pulling the bond off the substrate. “Modulus” here is the stress the sealant exerts as it stretches: low modulus means it stays soft and gives way easily, which is exactly what a moving joint on a weak or brittle substrate needs.
It is the right pick for masonry expansion and articulation joints, render-to-window perimeters, and other movement joints, because a stiffer high-modulus sealant puts more force on the joint edges and can tear a masonry or render substrate as the joint cycles. High-modulus grades are for adhesive-grade bonding (cladding panels, trim), not for joints with significant movement. The same chemistry (PU, silicone, MS polymer) is sold in both low- and high-modulus grades, so the grade, not just the brand, has to match the job.
Specify the modulus and the ISO 11600 movement class on the scope of works, not just “PU sealant”. The classic failure is a high-modulus adhesive-grade product used in a movement joint: it cracks at the first thermal cycle. NCC 2022 Housing Provisions Part 5.6.8 wants masonry articulation joints at least 10 mm wide with a backer rod and a flexible sealant, and a low-modulus grade is what “flexible” means here. See polyurethane sealant and elastomeric.
Also known as: LM sealant, low-modulus elastomeric sealant, expansion-grade sealant.
Category: Materials / Sealants.
Related
See also
References
- ISO 11600 Building construction, Sealants classification and requirements (verified 2026-05-11)
- ABCB NCC 2022 Housing Provisions Part 5.6.8 (articulation joints in masonry) (verified 2026-05-11)
Last updated: 2026-05-30. Verified: 2026-05-11. Quarterly review for currency.