What the official numbers say
Natural Resources Canada compares the two products in its guide Keeping the Heat In.
Silicone fills joints up to 25 mm with a backer rod. It handles movement of 12 to 50% depending on the product and lasts more than 20 years. It can be used indoors and outdoors.
Polyurethane also fills joints up to 25 mm with a backer rod. It handles movement of up to 50% and lasts more than 25 years. It is for exterior use and dries slowly.
On these numbers alone, it is almost a tie. The difference lies elsewhere: sun, paint and the surface.
Sun and weather
An exterior joint lives outside all year. It gets summer sun, rain, snow and frost.
In Quebec, the same joint goes from July heat to January frost, with thaws in between. The product must stay flexible through all these swings, or it hardens, cracks and lets water in. This is where the two families differ.
This Old House notes that polyurethane breaks down in the sun if it isn't painted. On an exposed façade where the joint will stay bare, that is a point to consider.
Architectural silicone is designed for these conditions. Dow's data sheet describes DOWSIL™ 791 as virtually unaffected by sun, rain, snow and ozone. It cures on contact with moisture in the air. However, it can't be applied to a frosty or wet surface: preparation matters as much as the product.
Paint: the real deciding factor
This is often the question that settles it. Will the joint be painted?
If yes, choose polyurethane. NRCan says it can be painted, and the paint also protects it from the sun.
If not, silicone has the edge. The DOWSIL™ 791 data sheet states that paint generally doesn't adhere to it. So you choose a sealant colour that matches the siding. We offer several colours, from whites to browns, including greys and bronze, and we compare them on site with your siding.
Joint movement and the ASTM C920 standard
Building materials move. Brick, aluminum and wood expand in the heat and contract in the cold. The joint has to follow without tearing or pulling away.
For professional sealants, the American ASTM C920 standard classifies products by the movement they can handle. The higher the class number, the more movement the joint accepts.
According to its data sheet, DOWSIL™ 791 meets ASTM C920, Class 50, with a movement capacity of ±50%. That is what allows it to be used on façades and curtain walls, where joints move a lot.
One tip: whatever the product, look at its data sheet. A cartridge that mentions no standard and no movement percentage doesn't tell you how the joint will perform.
The surface and the installation
The surface matters as much as the product.
On wood, This Old House notes that silicone bonds poorly. The wood must be sound, dry and well prepared. If it is rotten, it must be repaired before caulking, or protected with aluminum cladding. Wood you want to paint calls for polyurethane instead.
On brick, stone and concrete, Dow's data sheet presents 791 for connection and perimeter joints, and for façades.
On aluminum, the 791 data sheet states that it adheres without primer to many materials. It is also the sealant we apply at the joints of aluminum cladding.
A good sealant poorly installed won't hold. A few rules apply to both families.
The surface must be clean and dry. The Government of Quebec recommends removing peeling paint, dust and old caulking. The DOWSIL™ 791 data sheet rules out application on a frosty or wet surface.
From 6 mm wide, a backer rod goes in, according to NRCan. The sealant then bonds to both sides of the joint and keeps an even thickness.
Curing takes time. According to its data sheet, DOWSIL™ 791 cures fully in 7 to 14 days, on contact with moisture in the air. Polyurethane, for its part, dries slowly according to NRCan.
Our choice: the DOWSIL™ line
On our job sites, we apply professional silicones from the DOWSIL™ line, including 791 and CWS. We choose them based on the surface and exposure.
Joints done with DOWSIL™ 791 carry a lifetime warranty on materials and labour. The conditions are written in your contract. As for tools, we apply our beads with an Adfast gun.
That said, we don't impose anything. If your specs call for polyurethane or another product, we install it. If you supply your own sealant, we install that too, and we explain what that changes for the warranty.
We also work as subcontractors for general contractors, using the product specified in the specs. And we caulk year-round, summer and winter.
Frequently asked questions
Can you put silicone over polyurethane?
We don't do it. A new joint applied over an old one depends on how well the old one is bonded. The Government of Quebec recommends cleaning joints well, old caulking included, before applying a new one. We remove all of the old sealant before applying the new one.
Does silicone stick to wood?
Poorly, according to This Old House, especially on bare or damaged wood. The wood must be sound, dry and prepared. If the joint must be painted, polyurethane is a better fit. If the wood frame is in poor condition, aluminum cladding can protect it.
Which one lasts longer?
According to NRCan, silicone lasts more than 20 years and polyurethane more than 25 years. These are theoretical material lifespans. In the sun, unpainted polyurethane breaks down faster, according to This Old House. Installation and exposure make the real difference.