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Efflorescence or Mold Growth?

Efflorescence is often confused with mold growth.  Both of them occur in damp environments and exhibit similar growth characteristics, so the confusion is understandable.

What is efflorescence?

Efflorescence in Basement

  • Efflorescence occurs when water migrates through cement structures, carrying salts to the surface.  These salts are then deposited on the surface when the moisture evaporates, leaving behind a crystalline like structure.

Where is efflorescence typically found?

  • The phenomenon typically occurs in concrete or brick construction materials, often during the initial dryout.  Large amounts of water are present in the materials during their initial installation.  Just pouring a foundation and slab for a 2,000 sqft home introduces thousands of gallons of water.  Therefore, we most often find efflorescence in either new construction or existing buildings with water failures.
  • Basements are common locations for efflorescence in existing homes and buildings.  This is a sign that water is migrating through the concrete structure.

Where does the moisture come from that causes efflorescence?

Efflorescence on Concrete

Eurico Zimbres

Pinpointing the exact source of the moisture can be difficult, as numerous entry points are possible:

  • Ground water saturation.  Capillary action can easily draw water from wet soil into masonry.
  • Rain water.  Either through a leak or through vapor drive.  This occurs when the sun hits a saturated wall and drives the moisture inward.
  • Sprinklers / artificial water sources.  Occurs when sprinklers saturate an exterior wall, causing water to enter the wall and later retreat, carrying salts to the surface.
  • Water vapor.  Airborne moisture forced through masonry walls can provide sufficient water to cause efflorescence. For example, during the winter months, warm, humid indoor air will move towards the cool exterior.

Does efflorescence require an inspection?

  • Efflorescence itself is not a health problem.  However, it is an indication of building’s failure to properly manage water.  Therefore, if efflorescence is present, there is a significant chance mold growth or other moisture issues may be present.  For example, while efflorescence on a garage floor will not cause a problem, the accompanying moisture can cause mold growth on items stored on the floor.  If the problem is significant, contact a mold inspector or concrete waterproofing company.

Project Report > Inspection for Efflorescence in a Basement


Efflorescence, not mold.

  • RH: 69%  Temp: 69 deg F.  CO2 = 650 ppm   CO = 0 ppm
  • Basement was extremely humid due to a wet concrete skim coat being applied to the floor.
  • Visible mold growth was found behind the base trim at the landing of the basement stairs.
  • Client had applied tape over the visible mold growth to contain the area.
  • Once tape was removed the inspector was able to confirm moderate amounts of mold growth along the bottom 2 inches of sheetrock.
  • Sections of sheetrock were broken due to water damage.
  • Elevated moisture levels were noted in sheetrock to a height of approximately 10 above the floor.
  • Concrete block foundation was slightly damp with some minor efflorescence found on the mortar joints.
  • Stair stringers and supporting framing members did not register and elevated moisture levels or visible mold growth.
  • Backside of sheetrock wall found under stairs did not exhibit any signs of water damage or mold growth.
  • Ventilation in basement is minimal and could be increased to allow for the removal of relative humidity and CO² from the rooms.


  • Client may wish to treat concrete wall with a water-sealer membrane to help minimize water vapor diffusion.

Got a question? Ask it here and we'll post the answer below

I have a 2-3 foot long, 8in. wide covering of white cotton like substance on my block wall in basement. It also looks like ther might be some sort of bugs in it, maybe spiders as I saw 5or6 spiders very near the area. Can you help???

There are three possibilities.  Mold, efflorescence or spider web.  The easiest way to determine if it’s efflorescence is to spray water on the substance.  If it’s mold (or a spider web), it won’t change much.  If it’s efflorescence, it will dissolve.  Finding spiders amid mold growth is rare.  I would assume you’re looking at a insect related web/nest.  Feel free to upload a photo for further investigation.