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Where is the humidity in my apartment going?

I live in a 680 sq ft concrete construction high-rise rental apartment in Redwood City, CA, with electric heat only, and I have a problem with dry air in the (fortunately short) heating season. I have to run my drug store vaporizer nearly 24/7 just to keep humidity over 30%. I go through several gallons of water a day, and my sinuses really don't like it when humidity goes much below 30%. This is crazy. Where is all that moisture going?

I'm pretty sure I'm losing humidity by air exchange, not condensation. It's just not that cold outside. I've looked around for obvious leaks. The walls, floor, and ceiling are concrete, and the windows, while enormous, aluminum framed, and single pane, do at least seem to be decently sealed.

My chief suspect now is the bathroom vent. It's just a register connected to ductwork with a central fan somewhere. It's always on and it seems to be sucking mighty hard. I'm thinking that if it's the culprit, I can just cover the register and take the cover off when I shower. So how would I go about measuring how much air it's sucking out of my apartment? Without expensive equipment?

Asked by Jacob Weel
Posted Feb 24, 2013 6:05 PM ET


5 Answers

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I bet you're right about where it's going--it's leaving with all that air being sucked up into the ventilation duct, and you're not generating enough to replace it.

The usual tools for measuring the actual flow are on the expensive side. I would simply experiment with covering the grille with a piece of cardboard. It will probably just stick to the grille by itself, and you can remove it as needed.

If everyone in the building starts doing this, then the last few tenants will lose their cats, small dogs, and wigs into their ducts due to the extreme airflow that will result.

Answered by David Meiland
Posted Feb 24, 2013 6:46 PM ET


It looks like your February average outdoor temperature is about 50F with about 70% relative humidity. Warming this air within your apartment to 75F, reduces the relative humidity to just 30%. A combination of enclosure air leakage and bathroom ventilation is likely the culprit of the dry air through extra air changes. Unless you have other reasons (like new furnishings, paint, stored chemicals, hobbies, etc) that may require extra ventilation, I'd suggest sequentially reducing the rate of mechanical ventilation, and watching to see if the humidity level responds positively.

Answered by Mike MacFarland
Posted Feb 25, 2013 12:32 AM ET


Thanks, guys! I did indeed go ahead and block the vent off with a simple sheet of paper, and it seems to do the trick. No poodles have yet been reported missing. The negative pressure is plenty to hold the paper in place, but apparently it's intermittent, since it invariably ends up on the floor as soon as I stop watching it. This I fixed with two fridge magnets. I know Walmart has magnetic vent covers that are like those big flat fridge magnets you sometimes get with advertisements on them, but shaped like a vent; I might pick one up next time I make a pilgrimage to the House of Everlasting Discounts, but for now, the paper seems adequate, and I get to feel all Japanese and minimalist. At the moment, with the baseboard heater in front of the big sliding glass doors set to 65F, I measure 69.5F and 38% humidity in the middle of the room, with no vaporizer running – just me taking a shower in the morning and breathing for a few hours, and I did take the makeshift vent cover off while showering. No cooking on weekdays, usually; they feed me at work. The air leaking around the edges of the sheet of paper appears to be plenty to prevent any obvious stuffiness.

The sad thing is that I completely understand why the building management would run the fans that hard. Tenants cannot be relied upon to turn on a fan or open a vent when showering, and moldy bathrooms are expensive to fix. Much easier to just run the blower at top speed all the time.

It beats the approach taken by my old Dutch landlord, a student housing organization that ran the kitchen exhaust fans on a timer based on what times of day a 1950s Dutch housewife might have been boiling potatoes. They were quite unimpressed by my protestations that the schedule, which involved adequate ventilation between 4:30pm and 5:30pm or some such, had no relationship whatsoever to actual tenant behavior.

Answered by Jacob Weel
Posted Feb 26, 2013 5:16 AM ET


Humidifiers are great if you find your home’s humidity levels are too low and the air is too dry. You will find portable humidifiers filled with water and have a fan that blows the evaporated water into the air.

Answered by Jenny Belman
Posted Feb 26, 2013 9:15 AM ET


I strongly disagree. Humidifiers are not "great." They are dangerous. They threaten the integrity of a wood-framed building.

In almost all cases, low indoor humidity during the winter is caused by air leaks. The solution is air sealing, not humidification.

Answered by Martin Holladay
Posted Mar 4, 2013 1:34 PM ET

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