Types of sealant: silicone, polyurethane, latex

The NRCan chart, in plain words

In its guide Keeping the Heat In, Natural Resources Canada compares caulks on three criteria.

Maximum width: the joint width the product can fill. Beyond 6 mm, NRCan recommends first installing a backer rod, a compressible foam rod that supports the sealant.

Movement: how much the joint can stretch or compress without breaking. The materials in a house move with heat, cold and humidity. The higher the percentage, the better the joint follows these movements.

Durability: the lifespan of the material. Keep in mind these are theoretical lifespans. They don't account for the quality of the installation, sun exposure or freeze-thaw cycles. On a wall in full sun, a poorly installed joint will last less long than the chart says.

Backer rod comes in several diameters, from 6 to 50 mm according to NRCan. You choose it based on the width of the gap, so it stays in place once compressed.

Silicone

According to NRCan, silicone fills joints up to 25 mm with a backer rod, handles movement of 12 to 50% depending on the product and lasts more than 20 years. It can be used indoors and outdoors. Ventilate well during application.

Its limit: it can't be painted. The DOWSIL™ 791 data sheet says so clearly: paint generally doesn't adhere to it. This Old House adds that it bonds poorly to bare wood. So surface preparation matters a lot.

There is also a difference between hardware-store silicone and architectural silicone. DOWSIL™ 791, for example, is designed for façades and curtain walls. It resists sun, rain, snow and ozone.

Polyurethane

Polyurethane, also called urethane at the hardware store, is the other major family for exterior joints.

According to NRCan, polyurethane also fills joints up to 25 mm with a backer rod, handles movement of up to 50% and lasts more than 25 years. It is for exterior use, dries slowly and can be painted.

This Old House adds a nuance: polyurethane breaks down in the sun if it isn't painted. It is a good choice when the joint will be painted over. Left bare in full sun, it ages faster.

Acrylic latex and siliconized latex

Acrylic latex is sold in every hardware store. According to NRCan, it fills joints up to 10 mm, handles movement of 7 to 10% and lasts 5 to 25 years depending on the brand. It cleans up with water and can be painted. NRCan advises against using it on steel.

This low movement explains why a latex joint often cracks on an exterior wall that moves. This Old House goes further and advises against it for leaky windows. This partly contradicts NRCan, which gives a wide lifespan range depending on the brand.

Siliconized acrylic latex does better. NRCan gives it a width of 13 mm, movement of 12.5 to 25% and more than 30 years of durability, indoors and outdoors. This Old House, for its part, presents it mainly as an indoor product. The sources don't all agree: read the manufacturer's data sheet.

Butyl, polysulphide, elastomeric and acoustical sealant

The NRCan chart also includes more specialized products:

• Butyl rubber: joints up to 13 mm, movement of 12.5 to 25%, 5 to 10 years, mainly outdoors.
• Polysulphide: joints up to 25 mm with backer rod, movement of 12 to 25%, more than 25 years. Mainly outdoors, it can also be used below grade and can be painted.
• Elastomeric: joints up to 25 mm with backer rod, movement of 50%, more than 30 years. It can be painted.
• Acoustical sealant: joints up to 16 mm, movement of 10%, 20 years. It doesn't harden and is for indoor use only, where it isn't exposed.
• Urethane foam: 10 to 20 years. Low-expansion foam is used around frames, before caulking.

What about hybrids?

You see more and more "hybrid" sealants, often called MS polymer. They are often presented as a middle ground between silicone and polyurethane.

The NRCan chart doesn't mention them by that name, and we found no official Canadian source that compares them. The numbers going around come mostly from blogs. If someone offers you a hybrid, ask for the manufacturer's data sheet and check which standard it meets.

We apply sealants from the DOWSIL™ line, including 791 and CWS, chosen based on the surface and exposure. If your specs call for another product, we install it. We can also apply a sealant you supply: we will then tell you what that changes for the warranty.

Joints done with DOWSIL™ 791 carry a lifetime warranty on materials and labour. The conditions are written in your contract.