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Community and Q&A

House is under negative pressure. Why?

Jeff Watson | Posted in General Questions on

I think my house is under negative pressure. There’s always a lot of dirt that builds up on the bottom flat horizontal part of my double-hung windows (this would be the surface that the bottom of the lower sash meets when you close the window). Technically it’s still the exterior.

All windows experience this dirt buildup. I cleaned them all thinking that it’s never been done in 50 years, but 1 month later they’re all dirty again. If I wiped the surface with a wet rag, the rag pretty much has mud on it. It’s not dust, it’s more like soil. Windows are closed year-round.

Is this a testament of negative pressure? The house has been air sealed to death, or are we just using our bathroom fan / microwave fan too much?

We have respiratory issues that started when we moved in & we know that the previous occupants died from lung cancer. We know something is wrong with the air in the house but can’t figure it out.
House: single story, double-wythe brick, basement, 1960s

– Walls: brick, 1″ paper-faced fiberglass, drywall
– Attic: loose fill fiberglass + cellulose, R60
– Basement walls: concrete, R11 unfaced fiberglass, plastic, drywall

Gas appliances:
– clothes dryer, vented to outside
– furnace, high efficiency, PVC for intake & exhaust
– water heater, PVC for intake & exhaust

– bathroom fan, 70cfm, exhausts through roof
– microwave fan, 150?cfm, exhausts through roof

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  1. Steve Knapp CZ 3A Georgia | | #1

    Many older homes with forced-air heating and cooling system experience exfiltration and infiltration, which may be what is going on here (although I suppose wind could force dust and dirt into your home or change the pressure balance as well).

    In my previous home, the carpeting near the baseboards quickly developed a permanent "stain" pattern because the HVAC fan would pull in outside air whenever it ran. The carpet acted as a filter and trapped particles as they entered the conditioned space.

    My new home is much tighter, so I haven't noticed any infiltration or exfiltration signs (plus, the floors are all tile and hardwood less likely to show obvious patterns). I also have an ventilation system that keeps the house over-pressurized.

    Long-story-short, air sealing your windows and other leaky areas will cut down on how much air is entering or exiting your home. You probably don't want to introduce active ventilation unless you are able to significantly tighten up the structure.

  2. GBA Editor
    Martin Holladay | | #2

    It's certainly possible that your house is under negative pressure. One way that can happen: If some of your ductwork is located outside of your home's thermal envelope -- for example, in a vented crawl space or a vented attic -- and if your ductwork is leaky (which is certainly possible), then your house can be pressurized or depressurized by your forced-air heating and cooling system.

    If supply ducts located outside of your thermal envelope are leaky, your house will be depressurized.

    If return-air ducts located outside of your thermal envelope are leaky, your house will be pressurized.

    If you are curious about this issue, hire a home performance contractor experienced at pressure diagnostics to test your home's air leakiness and duct leakiness with a blower door and a Duct Blaster.

  3. Jon R | | #3

    Another way is improper room balance. Stick a supply duct with no return in a closed off room and it will be pressurized and air will exfiltrate. This will cause the remainder of the house to be at a negative pressure. I'd measure the indoor/outdoor differential pressure - in every room.

    Consider radon and humidity measurements, a HRV and check for mold on those basement walls.

  4. Jeff Watson | | #4

    - My HVAC ducts are sealed for the most part (e.g., the stuff I could get to) with mastic. Every single window in the house has soil buildup at the bottom of the windows. Keep in mind this dirt is outside the window glass - pretty much right where the window screen meets the window frame area.

    - I never understand this thermal envelope stuff - my HVAC ducts are in the basement's ceiling & come up through the main floor subfloor & floor. My basement is finished with drywall walls & ceilings, but no insulation separating basement from main floor. I've got caulk on all baseboards on the mainfloor. There are no supply/return vents in the basement. Are my ducts within the thermal envelope?

    - So if I see a lot of dirt, it's because of supply ducts outside the thermal envelope? Ok, I'll focus on that.

    - How do you measure the differential pressure? I'm actually doing a short-term radon test now after reading about its connection to lung cancer & the fact the people before me died for that reason. I don't think I can do an ERV/HRV because the outside air is humid. My basement walls are finished, but I run a dehumidifier 24/7 in the basement.

  5. Jon R | | #5

    I'd say that dust outside of the glass is just a sign of dust in the outside air settling out - not negative house pressure.

    A professional will have a differential micro-manometer. Lower cost DIY methods is a good question. An iPhone 6?

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