The Order That Works
Almost everybody buys these in the wrong order. An air purifier bought before the moisture is fixed is money spent on the symptom, and it will run forever without changing anything. The hierarchy that actually works, in this order:
- Control the moisture sourceEverything else is downstream of this. A building with a wet basement has an air quality problem that no filter can solve.
- Ventilate, and verify it goes outsideBath and kitchen exhaust that terminates through the roof or a wall, not into the attic. This is the most commonly wrong item in Rock County housing.
- FilterA MERV 11 to 13 filter in the air handler, changed on schedule, does more than a portable unit in one room. Check the furnace can handle the pressure drop.
- Then, if you still want one, a portable unitTrue HEPA, sized to the room. Useful for a bedroom, irrelevant to the building.
Air that never quite feels right
Tell us your humidity readings if you have them, and whether the house is new and tight or old and leaky. Those two point to opposite corrections, and one of them costs very little.
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Humidity Targets Through a Wisconsin Year
This is where a cold climate needs different numbers from the generic advice, and where whole-house humidifiers do a lot of quiet damage.
| Season | Target indoor RH | Why |
|---|---|---|
| Deep winter, below 0°F outside | 25–30% | Higher than this and the moisture condenses inside walls and on windows. Turn the humidifier down. |
| Winter, 0 to 20°F | 30–35% | The comfort floor. Windows tell you when you have gone too far. |
| Shoulder seasons | 35–45% | Humidifier off. This is when people forget. |
| Summer | 45–50% | Dehumidification, particularly in the basement. Do not open basement windows to "air it out". |
| Basement, April to October | 45–50% | A dehumidifier plumbed to a drain, running continuously, is the single most effective purchase in this county. |
Extremely common here, and a reliable cause of window condensation, sill rot and mold in exterior corners. If you can see water on the inside of the glass on a cold morning, the humidistat is set too high for the outdoor temperature. Turn it down and see whether the glass clears before you go looking for anything else.
What Actually Affects Air in a Janesville House
- The basement, whatever the rest of the house is like. Stack effect means air moves upward through a building in a cold climate, so a substantial fraction of what you breathe upstairs started in the basement. A damp basement is a whole-house air quality problem, not a basement problem.
- The air handler. A coil and condensate pan that are permanently wet, dark and dust-fed, sitting in the airstream that feeds every room. See HVAC cleaning.
- Bath and kitchen exhaust. Either absent, undersized, terminating in the attic, or run for two minutes when it needs twenty. A timer switch solves the last one for about thirty dollars.
- Attached garages. Doors between a garage and the house are rarely well sealed, and the pressure difference in winter frequently pulls garage air inward.
- Radon. Not a mold question, but the largest indoor air risk in Rock County by a wide margin, and one that shares its entry routes with moisture. This is an EPA Zone 1 county.
Ventilation Hardware, and What Each Type Is For
| Device | What it does | Right for |
|---|---|---|
| Bath and kitchen exhaust | Spot removal at the source | Every house. The single most commonly wrong item here. |
| HRV — heat recovery ventilator | Continuous fresh air, recovering heat from the outgoing stream | Cold climates. The usual correct choice in southern Wisconsin. |
| ERV — energy recovery ventilator | As above, plus transfers some moisture between streams | Useful where winter dryness is a complaint; slightly less common here than an HRV. |
| Whole-house dehumidifier | Removes moisture from the whole volume, ducted | Houses with a persistent summer humidity problem beyond one basement. |
| Portable dehumidifier | One space, typically a basement | The standard answer here. Must drain to a drain, not a bucket. |
| Portable air cleaner | Filters particles in one room while running | Comfort during remediation, or for an allergy sufferer's bedroom. Not a remediation. |
If the house has an HRV and nobody knows what it does
Common in the newer subdivisions. Check three things: that it is switched on and not just powered, that the core and filters have been cleaned within the last year, and that the intake and exhaust hoods outside are not blocked by snow, vegetation or a dryer vent discharging next to them. A unit that was commissioned once in 2016 and never touched since is usually doing very little.
What to Measure, and With What
- Relative humidity and temperature — a $15 hygrometer in the basement and one on the main floor. Log it, do not spot check.
- Surface temperature — an infrared thermometer, around $25, to compare a cold wall against the calculated dew point.
- Radon — a short-term test kit. Rock County is an EPA Zone 1 county; this is the highest-value measurement on the list.
- CO alarms — not a mold matter, but if you are auditing a house, combustion safety belongs on the same walk-round.
Measurement changes arguments into facts, and in a rented property it changes a complaint into a documented condition.
Newer, Tighter Houses Have the Opposite Problem
Recent subdivision construction in and around Janesville is built to modern energy codes, which means very low air leakage. That is excellent for heating bills, and it means the water vapor a household generates — cooking, showering, laundry, breathing — has no route out unless one is provided mechanically. Without a working HRV or ERV, humidity climbs and shows up on the coldest interior surfaces: north-facing exterior corners, behind furniture on outside walls, inside closets on exterior walls. A humidity complaint in a five-year-old house here is almost always a ventilation deficit rather than a defect.
Frequently Asked Questions
No, and this is worth being blunt about. A purifier removes some airborne particles from the air in one room while it runs. It does nothing about the wet material that is producing them, and the moment you switch it off the situation is exactly as it was. Purifiers are a reasonable comfort measure for a bedroom during remediation or for somebody with allergies. They are not a remediation strategy, and buying one instead of fixing the moisture is the most common money-wasting decision in this field.
Lower than most people think, and it should move with the outdoor temperature. Around 30 to 35 percent when it is 0 to 20°F outside, and down to 25 to 30 percent in a deep cold snap. The windows are your gauge: persistent condensation on the inside of double glazing on a cold morning means the indoor humidity is too high for that outdoor temperature, and the same moisture is condensing inside your walls where you cannot see it.
Yes, and plumb it to a floor drain or the sump so you are not emptying a bucket. Set it to 45 to 50 percent and leave it from April through October. Also keep the basement windows closed in summer — opening them brings in air that is warmer and wetter than the basement, which then condenses on the cold masonry. Airing out a basement in July makes it damper, not drier.
It does if there is something in the ducts, and it does very little if there is not. The parts that actually matter are the evaporator coil and the condensate pan, which are permanently wet and are where growth establishes. Ductwork itself is usually dusty rather than contaminated. A cleaning that does the ducts and not the coil has cleaned the pipe and left the source. See HVAC mold cleaning.
In Rock County, by the measures available, yes. This is an EPA Radon Zone 1 county, roughly half of homes tested come back at or above the action level, and three Janesville ZIP codes are among the highest in the state. Radon is a recognized lung cancer risk with a defined action level; indoor mold has no comparable numerical standard. We mention it because the two share entry routes and because a radon test costs a fraction of anything else on this site.