interior

Black Stains on Carpets and Ghosting of Framing 

We have investigated numerous units over the years that have been experiencing discoloration of carpeting due to particulate accumulation. In many units we also observe particulate deposits on window blinds, inside of kitchen cabinets and on appliance surfaces (microwave ovens, CD players and refrigerators). Additionally, we typically notice "ghosting" of wood stud members on the interior gypsum board surfaces of exterior walls. 


The problems are related to four deposition mechanisms: filtration, electrostatic attraction, thermal deposition due to Brownian motion and impaction. These deposition mechanisms cause tiny microscopic airborne particles to: 


The particle sources for the types of really tiny particles that can cause the problems observed are many: 


Aside from tiny dirt, concrete cutting and gypsum sanding most of these sources have something in common. Something has to get burned, cooked, fried or pyrolyized in order to create the size of particles required. There are a few other potential sources of particles that could provide them in the size range necessary. However, we typically dismiss them as not likely for obvious reasons: particles from industrial grinding or cutting operations, fumes from welding and carbon black from copiers or laser printers. Of course, it is possible that someone has unusual habits we don't know about or an unusual lifestyle that we don't notice. Just the same we don't typically notice any welding or industrial grinding going on inside the typical unit. We also don't often notice a Kinko's like business in most units. Ongoing renovations to interior gypsum surfaces do happen on occasion. 


Brownian Motion  

The oldest known deposition mechanism will be described first - Brownian motion. A long dead English botanist (yes, botanist not physicist), R. Brown, noticed tiny particles behave much like molecules with respect to vibration. Under the kinetic theory of gases, molecular vibrations are directly related to temperature. The higher the temperature the greater the molecular vibration. Brown observed that tiny particles in fluids get jostled or bounced around depending on temperature. The higher the temperature, the greater the jostling or movement of the tiny particles. He believed that the vibration of molecules in the fluid bounced the particles around. He was correct and became famous - hence the term Brownian motion. Okay, so far this is an interesting story, but how does this relate to our problem. 

Well, air is a fluid and particles in air get jostled around based on the temperature of the air. When air with all of these bouncing particles brushes against the interior of an exterior wall it gets cooled. Not all of the air gets cooled, only the air right against the interior surface of the exterior wall. We call this layer of air the boundary layer. Now it gets a little complicated. Not all of the air in this boundary layer cools at the same rate. Some spots are colder than others. Where there is higher heat loss such as at a wood framing member, say a 2x4, the air in the boundary layer right at this spot is colder than the air in the boundary layer just inches away. Well, the air doesn't bounce around as much in these cold spots which means the tiny particles in the air at these cold spots also don't bounce around very much. If the particles are not bouncing around very energetically, it is easier for them to get stuck to the surface of the wall. The particles in the air next to the wall tend to "plate out" on colder surfaces. Since every stud is a "thermal bridge" the outlines of the studs can be sometimes seen due to the accumulation of tiny particles at the cold spots. This is often called ghosting and was very common in the old days, before MTV, even before Ozzie and Harriet. It was common back then because we typically had poorly insulated walls (real cold spots) and lots of people that smoked. The smokers supplied the particulates. The tobacco particulates plated out on the cold spots, usually at the studs due to Brownian motion. 

Why doesn't this phenomenon happen as much today? Two reasons: our houses are better insulated and there are fewer smokers. Fewer people smoke inside and those that do typically accept the fact that smoking not only causes their house to smell but smoking also makes their house dirtier due to the tobacco particles plating out on cold surfaces. We point this out since many units we inspect contain heavy smokers and the problems in those units are usually due to tobacco particulates. Don't smoke inside and your white walls won't go brown, nor will your light-colored carpets discolor - same for your blinds. You should accept the consequences when you live in an ashtray. 


Electrostatic Attraction  


Impaction 

Impaction occurs when moving air carrying tiny particulates flings or throws the particulates against a surface. This typically happens when air moves under a door due to an air pressure difference. As the air rushes under a closed door it has to "bend" under the door. When air in the middle of the room (heightwise) has to go under a door, it moves downward. The particulates in the moving air have their own momentum (as do the air molecules). The air makes the curve and bends under the door, but the particulates don't - they travel in more or less a straight line and end up hitting the carpet directly under the door leaving a dark line. 


Filtration  


The Big Picture  

 • we have more of the right sized particulates  

• we have lighter colored carpets  We are not about to argue against light colored carpets However, let’s talk about the particulates.