Annoying dust-up in my basement
November 14, 2007 5:18 AM   Subscribe

Help me unravel the enigma of the "Mysterious Basement-floor Chalk Dust."

Ever since work was completed on my new, finished basement six months ago, parts of the floor have been playing host to a fine, chalk-like dust. I'd snap a picture of the culprit, but the color of the dust is very similar to the gray paint on the floor, and my photoshop skills aren't up to increasing the contrast.

Here are the relevant details:
-A fine, white dust. Reminiscent of talcum powder, but with smaller granularity. So fine it cannot be swept away, but must be removed with a moist cloth.
-Seems to appear next to the walls and entrance to my basement bathroom, ie, in areas with slightly elevated humidity. FYI, I keep the interior at a constant 55% humidity with a dehumidifier. (This is a comfortable level that doesn't overstrain my electric bill, so if possible I'd prefer not to lower the humidity setting.)
-Limited to the floor only. The walls, which are sheetrocked, are unaffected. Although I'm guessing that the concrete behind the sheetrock might have a buildup as well.
-Slow to develop. Takes two weeks to reappear after removal.

At present, my basement is hardly chalk city central, but I want to nip the problem in the bud. I'm about to lay wall-to-wall carpeting in the room, and I'm concerned that a buildup of chalk over time will trigger my allergies.

Marathon googling has come up with zilch, apart from a few technical articles. I'm at a loss. Is this a common problem in finished basements, and is there a way to counteract it?
posted by Gordion Knott to Home & Garden (7 answers total) 1 user marked this as a favorite
posted by lyam at 5:27 AM on November 14, 2007

Beat me to it...
posted by jon1270 at 5:28 AM on November 14, 2007

Talk to your contractor -- this sort of thing shouldn't be happening. A solution probably lies in applying a sealant, such as this one.
posted by beagle at 5:41 AM on November 14, 2007

Was a continuous vapor barrier installed under the slab? (something like 10 mil poly sheeting). The fact that you cite it as being near the exterior (?) walls makes me think it could be missing/have gaps near the perimeter.

FYI, efflorescence is a powder or stain resulting from deposition of water-soluble salts. In your case, it could be due to interior moisture (shower, cooking, etc) which can cause condensation at lower temperatures and can provide the moisture necessary to trigger the mechanism for efflorescence.

Was the concrete slab sealed after work was complete and before interior partition walls installed? It could be that the sealer was not fully installed and gaps are evident near walls. Consider installing a new coat of sealer and see if this helps.

Clear water repellents, silicone and acrylic coatings are often used for preventing moisture. This coating prevents efflorescence from recurring by reducing the amount of water absorbed. However, applying a coating without stopping the mechanisms causing that efflorescence, may lead to degradation of the concrete. (if it's ground-source moisture and not interior-source moisture).
posted by mightshould at 5:58 AM on November 14, 2007

If it isn't effloresence, could it be drywall dust from your newly refinished basement walls?
posted by kuujjuarapik at 6:06 AM on November 14, 2007

Response by poster: mightshould, I'm not certain if a vapor barrier was placed under the slab, but various water-preventative plastics were placed in the foundation area. The building was designed, built and inspected to be up to current code.

The current sealant on the floor is a gray, industrial-style concrete paint. Maybe a second coat of this, or a transparent sealant on top, would do the trick?

Also, I wonder if raising the temperature of the floor or room might improve things (this could easily be accomplished via my radiant heating system, though I hesitate to burn more oil than necessary . . .)
posted by Gordion Knott at 6:10 AM on November 14, 2007

Best answer: Gordion Knott,
Is the gray paint all the way to the walls? If not, that's a good thing to try, as painting/sealing the perimeter is not too difficult.

You are not going to be able to make a considerable difference in the slab temperature without considerable expense (and waste of energy.) Reducing the dew point/condensation could be tackled, but that would be a next step if the sealant failed to solve it.

If you believe the vapor barrier was installed correctly then the most likely source of chalking is the interior condensation. However, I've seen many, many code-approved buildings that were not 100% properly done....

In the future: If you are planning on installing a synthetic hardwood-type flooring, consider placing a vapor barrier down prior to the flooring. If you are planning on a glue-down vinyl tile or carpet, take a moisture test prior to work to ensure the correct type adhesive for proper bond.
posted by mightshould at 6:36 AM on November 14, 2007

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