Q. The off-white carpet in a spare bedroom looks just like it did when we had it installed seven years ago, except all around the edge near the baseboard looks dirty. It won't vacuum up, either. Is something about the heating system in the house? Do yo u have any advice at all on how to remedy this?
A. It's called "filtration soiling" and it's a product of how modern houses are built and function in their own often unpredictable environments. It has to do with differential air pressures from inside the house to outside the house.
Nature likes balance so the volume of air that occupies the inside of your house is always trying to come into equilibrium with the air pressures outside of the house. Things that effect and change the inside air pressure include everything from just th e fact that inside air is warmer or cooler than outside air to the operation of dryers, furnaces, bath or kitchen fans, fireplaces or whether or not the wind is blowing.
Air is nearly constantly passing from inside the wall spaces down where the floor meets the wall, or it could be moving in the other direction, and as the air escapes or enters under the baseboards it deposits microscopic bits of dirt and debris on the c arpet fibers much like an air filter collects dirt from your heating and cooling system.
The reason that we have had light colored carpets for years and we have had houses for centuries and not seen this sort of thing until just recently is simply that we build houses very differently now than we used to in times past. Both intentionally an d unintentionally we are building houses with ever more airtight skins.
Windows are rated higher and more preferable the more they resist the movement of air when closed and we sheath both the insides and outsides of our houses with sheets of sheathing or wall covering material that come in unbroken areas of from 32 to 48 sq uare feet per sheet which combined with house-wraps, siding-- even paint-- all work to create large areas of hermetically sealed building spaces. But we know that the natural balancing tendency of inside to outside air pressures is very persistent and n ow we have minimized through building techniques the infiltration points to wherever air can find its way in or out. It used to be that air came in-- or out-- at old loose windows, around and under doors and uninsulated walls. I've lived in houses wher e the curtains moved when the wind blew-- not so anymore.
Couple that air movement with its deposited dirt with light colored carpets and you've got a band of discoloration along walls that are themselves turned into unanticipated ventilation shafts.
What can you do about it? Well, not much easily. Researchers at the National Association of Homebuilders Research Labs in Upper Marlboro are working on studying ways of future prevention of filtration soiling but if you've got it now then the only way that I can think of involves pulling the carpet up and sealing the base of the wall.
Trim carpenters hold the baseboards up about a half an inch from the subfloor when they know wall-to-wall carpeting will be installed and the drywall installers use a foot tool to lift the lower sheet of drywall up about a half an inch to meet the upper sheet to form the mid-wall seam. This leaves a half-inch gap at the bottom of the wall exposing the rough framing. That's the culprit.
The gap needs to be exposed by removing the carpet and its tack strip. Then I would use a very elastomeric urethane or vinyl-acrylic caulk to fill and seal the gap, then re-install the tack-strip and carpet. I don't know if the fine dirt can be cleaned out of the existing carpet well enough for re-use. You may be into some new carpeting.
Unfortunately, building codes were behind the curve on this one. When housewrap is used under siding it needs to be carefully taped at the seams and windows to reduce to near elimination any air movement through the wrap especially at seams. Folks need to be reminded that building codes are minimum standards of conduct and are specifically for the safety of a building's occupants. The key for future prevention of this and other technology driven building problems ranging from sick building syndrome t o infiltration soiling is smart, heads-up builders who learn from the mistakes of the past.