June 5, 2020

Both materials are porous and therefore wick or suck water. Small pores can suck water to a greater height than large pores. Materials with the tiniest pores suck the most. Did you know that concrete has very tiny pores?

Same with the capillary size of wood fibers. Turns out that tree height is limited by the height that the capillaries can suck water. We owe Lord Kelvin and his equation for figuring out that if you fill in the pores or make the pores bigger you can control how high the capillaries can suck.

Under our slabs, we provide this capillary break with large holes — between the stones underneath. For the perimeter walls, we have used similar strategies, stones and sand near the walls and then maybe some bitumen to fill in the pores of the concrete walls. In the 1800's and early 1900's, mass walls were built on top of stones that provided a capillary break. Now concrete footings are simply placed and once the concrete has cured, then the foundation walls are set on top of the footing. Recent tradition is to use nothing at the footing to deal with capillary rise. This is a big deal now that we use our basements for living. We insulate them, cover with gypsum board and now we have to deal with the footings differently.

So, coat the top of the footing and keyway if applicable with a fluid-applied capillary break or a membrane without pores that adheres to the footing before you set the concrete perimeter walls and you are good to go.

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