The Capillary Break – Your First Line of Moisture Defense

by Erik North on December 4, 2011

What is a Capillary Break


Because what is a capillary break but something that stops water?

Sounds a little like a caterpillar taking a rest, doesn’t it? Capillary breaks are one of those things, like caffeine, that people have known about forever, but in the modern world have become way more important. You obviously know about the caffeine thing, assuming you’ve had your morning coffee. But what is a capillary break? And why is Erik so insistent on time off for caterpillars again?

Because controlling moisture driven capillary movement is one of the most important defenses for your house against moisture and humidity.

What is a Capillary Break?

Have you ever wondered how water, as gravity bound as the rest of us, travels up a tree? We’re pretty sure the Keebler’s elves are not hauling water up the tree by the bucketful, so how? The water moves by ‘capillary action,’ molecular interaction between liquids and solids which can move liquids up against gravity. Take a paper towel and dip it in water…the water soaks right up the fibers against gravity.

How strong is this molecular action? Scientists estimate that water could move up 6 miles of concrete, against gravity. That’s ten times higher than the tallest building currently being even thought of. That’s a pretty powerful force moving water through your house.

Capillary Break Materials

A capillary break is a hydrophobic material which stops capillary movement. Traditionally, this could have been courses of copper, tin or lead, or a layer of solid slate. The sheet of metal or rock would be built right into a horizontal course of the construction. In modern construction, this can be any material of sufficient thickness and water impermeability to stop capillary movement. This can include various metals, glass, foam or plastics.

Why are capillary breaks important? It should be pretty evident (unless that 6 miles of moisture rising up concrete didn’t impress you). But I’ll put it plainly: capillary breaks are your house’s main defense against water below your foundation entering your house.

The Capillary Break In Action

Beneath the foundation slab a capillary break can be created with a layer of gravel and a polyethylene plastic or a sand layer with geotextile drainage mats.

A capillary break can be created between the slab and foundation walls by applying Drylok or a similar foundation water proof treatment.

When the slab is sufficiently cured and prior to pouring the foundation walls, 2 coats of Drylok will stop capillary water movement up the foundation wall.

A sill seal (a strip of closed cell foam) is installed between the top of the foundation wall and the sill plate (the pressure treated wood member sitting atop the foundation wall). Any wood construction in the basement should use pressure treated lumber for bottom plates and include a sill seal foam strip between the concrete slab and the wood.

I’ve said many time, foundations and basements are giant holes we dig in mud and clay then act surprised when things get wet. Capillary action moves water aggressively through porous materials like concrete. Strategically used capillary breaks help keep your basement and house dry.

{ 2 comments… read them below or add one }

gale August 20, 2013 at 9:51 am

man i wish you’d been writing the c.e. course i’m doing instead of just being the site i went to for clarification. ever think of doing that?


Erik North August 20, 2013 at 7:28 pm


Thanks for the comment. Funny you mention, I just contacted a local adult ed organization about having an energy audit class.



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