Posted by: SherlockHomesSY | 24 Dec 2010

Common Complaints: Dust!

Attic Air Leakage Locations

(c) Carson-Dunlap, Illustrated Home

DUST! Part 1 of 2: Holes

Typical causes
– Shell Duct Leakage
– Duct Leakage
– Tight Walls & Unsealed Ceiling
– Ventilation systems
– Hot Attics
– Clogged Dryer Ducts

Dust is almost always the result of air leakage, either through the shell (walls, floors, and ceilings) or ductwork.

Two blog posts are needed to discuss this common complaint.  First, we’ll look at it from how it gets in from outside.  The second post talks about dust coming in from strange places.

What is Dust? It’s a bunch of stuff: pollen, dirt, mold, lint, and insulation – just to name a few.  Let not forget drywall, flour, sawdust and other things.  Let’s consider the sources.

The bulk of the pollen load comes from outside.  It’s especially bad in the spring when the plants are blooming and wind is raging.  Thank God for rain!  When the pine trees do their thing, everything in north Georgia is covered in yellow for a week.

Dirt, too, comes from outside.  The fine grains of soil (rock, and decayed organic matter (e.g., leaves, grass, flowers, any anything else from animals and vegetation) easily blow in the wind and get tracked in on our feet.

I’ve done enough testing to know mold spore counts are seasonally and climatically high too.  Outdoors, good.  Indoors, not so good.  Its function is to make dirt out of anything organic: trees, grass, weeds, paper, cotton, leather, etc..

Lint is usually fibers falling off our clothes.  Some of it is organic, such as wool or cotton.  It can also be polyester, nylon, or any other synthetic, plant, and animal fibers.

Until I started diagnosing houses, I never gave insulation as a source of dust a thought.  This is in the attic, walls, and crawlspace. However, the more houses I help diagnose and correct, the more I’m convinced that dirt, pollen, and insulation are the major sources of dust in our homes.  Why? Mold spores are too small to see and these things originate outside our living areas.

My House. When I started home inspecting in 2002, some company was letting us know the air in our houses was up to 90X more polluted than the air outside—quoting the United States Environmental Protection Agency (EPA).  They told us that if we put their $2000 gadget in our heating and cooling systems, our indoor air quality would be better.  It sounded appealing, but I was too broke to buy it.  Today, they’re either out of business or they’ve changed their business model, but I don’t hear or seal their advertising anymore.

In the meantime, my air filters clogged with dust every two weeks, especially after I started remodeling.  At the time I did something clever, but I didn’t even consider small holes to be any big deal.   When I did the air leakage test (aka Blower Door test) on my own house,  I knew right away why my house was so full of dust nowadays. I didn’t tape the drywall I hid behind paneling.

After that I made may house a laboratory to teach myself how to air seal houses and ductwork.  I was pleasantly surprised when my air filter really didn’t need to be cleaned, even after 6 months.  I don’t think we accumulate as much dust in year than we did in a weak.  My lungs, family, and electronics continue to thank me!  A major source of dust really is from outside.

Air Leakage and Duct Leakage. I’m willing to bet that company advertising their whizzy gizmo is out of business because it didn’t really work.  First, if the source of dust is holes in the shell (walls, ceilings, and floors) a filter or any kind in the heating and cooling system isn’t going to work well—especially if there are holes in the ductwork.  It would work great in a tight house, but why would you need it (unless you happen to more sensitive to dust than most people)?

Unsealed Ceilings. I have to applaud efforts builders are taking, somewhat accidentally, to make tighter houses.  The trend away from crawlspaces has been going on for a long time.  Concrete slabs leak far less air into and out of the house.

Windows are a lot better. Building paper is in use. Sill gaskets are between the wall and slab.   Many houses in California are now covered with stucco.  A big advantage of stucco is that it makes a more air tight shell.

But a strange thing is happening in these newer houses!  They are incredibly dusty, but it isn’t dirt.  Its insulation.  Where? The attic.  How? Holes around light outlets, canned lights, smoke alarms, duct registers, any anything else poking through the ceiling.  Why? It’s pulled in by the heating and cooling system.

My first clue the house I’m looking out is tight is a filthy return filter.  Most of the time, there are streaks on the wall or ceiling around the return air duct.  (The tendency is to have one large, very short return, so all the sucking is happening in one place.)  If this condition is really bad, it tells me the ductwork or something else with the heating and cooling system is hosed too.  Usually, the ductwork is too small for the equipment or there is unbalanced supply and return ductwork.

I know from experience that when the ceiling is sealed, the dust goes away!


The key to sealing out dust from outside is air sealing and duct sealing.  Finding and fixing air leakage, including duct leakage, is found on our home performance website.

In Conclusion

This time, we covered where dust comes from outside.  Next, we’ll look into Dust from Strange Places!


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