Sealing crack between sill plate and foundation

A homeowner in Portland, Oregon posted a question about her 100-year-old house, where the wood sill plate (the beam resting on top of the foundation wall) doesn't sit tightly against the concrete foundation below it. The gaps range from tiny (1/16 to 1/8 inch) to as wide as a half-inch to full inch in spots, and in a few places she can see daylight through the crack. On one side, the foundation had leaned outward and pulled the sill plate with it; she had a support post installed to stop further movement, but the gap itself remains.

These gaps let outside air leak straight into the house, since there's no solid barrier — what builders call a capillary break — between the wood and concrete. She's planning to install metal reinforcing plates (Universal Retrofit Foundation Plates) along the sill, which will cover these areas and make them hard to reach afterward. That's why she wants to seal the gaps first, before the plates go on.

She's considering a flexible polyurethane caulk, backed in the wider spots with a foam backer rod (a cylinder of foam pushed into a gap before caulking, so the sealant has something to grip and doesn't sink in too deep), rather than options like liquid-applied flashing or spray foam.

For homeowners with older houses and stone or concrete foundations, this is a familiar problem: uneven, crumbling foundations often leave gaps under the sill plate that leak air and invite moisture. Sealing these gaps before covering them with structural hardware is a detail worth thinking through carefully, since access disappears once the work is done.