Silicone vs Polyurethane Sealant: Which One Does Your Scope Actually Need?

The two sealants get lumped together as "caulking" on a lot of specs, but they’re built for different jobs. Using the right one in the wrong spot is one of the more common reasons a joint fails early — not the workmanship, the product selection.

Silicone: internal, cosmetic, low-movement joints

Neutral-cure silicone is the standard for internal wet areas — bathrooms, kitchens, benchtops, shower recesses. It colour-matches well, cures fast, and gives a clean, tight line against tile, stone and laminate. It’s not built to carry structural movement, and it’s not the right choice for anything exposed to UV or heavy weathering long-term — it will yellow and degrade faster than polyurethane outdoors.

Polyurethane: external, structural, high-movement joints

Polyurethane is the workhorse for anything outside the building envelope — expansion joints, precast panel joints, facade and cladding seams, window and door perimeters. It has far better UV resistance and a higher movement capability, which matters on joints that expand and contract with temperature or carry structural load. It’s also the better choice for trafficked joints — car parks, warehouse slabs — where a sealant needs to take abrasion as well as movement.

The detail that actually matters: width-to-depth ratio

Neither sealant performs to spec if the joint geometry is wrong. Backing rod sized to the correct width-to-depth ratio — generally close to a 2:1 width-to-depth for movement joints — controls how the sealant flexes without tearing at the bond line. Get this wrong and even the right product will fail prematurely.

Working to a specification

On any commercial or multi-residential job, the sealant schedule should already specify the system. Where it doesn’t, or where a detail looks unbuildable on site, the right move is to raise an RFI rather than substitute and hope — a fire-rated or structural joint sealed with the wrong product doesn’t carry its rating regardless of how neat the bead looks.