The scenario:

  1. Slab on grade construction.
  2. House built 1938, slab poured in 1950's we think. San Francisco area.
  3. New framing, new hardwood floor, new baseboards. Completion 6 months ago.
  4. Altriset and Termidor (both) pumped under slab to control prior termite issue.
  5. Adjacent room is unfinished basement space. There's concrete but no particular source of debris.

Every week or so we vacuum up what seems to be sand from the top of the hardwood floor. It seems to blow out from under the baseboard. What could cause this?

sand on hardwood floor doorway, sand on finished side unfinished side, showing new quake bolts and shear wall from rear

  • 5
    Are you confident that it is sand and not wood dust? – bib Aug 6 '13 at 12:09
  • "Adjacent room is unfinished basement space." - Can you post pictures of this area from the unfinished side of the wall? – Freiheit Aug 6 '13 at 13:57
  • How often do you go to the beach? – Jason Aug 6 '13 at 13:57
  • Definitely sand: all particles are sharp and rocky. Pictures posted. Beach travel rare. – Bryce Aug 6 '13 at 19:14
  • The floor is cleaned every other Monday, the picture represents a week and half of sand accumulation. – Bryce Aug 13 '13 at 5:30


The answer was: airflow. The sill plate does not sit firmly on the foundation, leaving a gap. Differential air pressure in the basement and the room results in a small wind, which brings dust and (yes) bits of sand from the concrete or mortar.

A little bit of spray foam (using the gun, not the instant-clogging single use cans) solved the problem.

  • Please tell me more about spray foam that does not come in an instant-clogging single-use can. – ArgentoSapiens Nov 20 '13 at 16:02
  • I have the Great Stuff Pro 14 Foam-Dispensing Gun. Way better than the insta-clog cans. – Bryce Mar 18 '14 at 18:17

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