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Solving Humidity Issues

If you live where it's hot and humid, an air conditioner will draw off moisture from inside air. If you live where it's cold and humid, heating will reduce the relative humidity of inside air. See: thermodynamic concept of relative humidity. But if the temperature outside is ideal or only slightly cool, then a heating or air conditioning system is unnecessary.

With Outdoor Ideal Temperature

If the temperature outside is ideal but overly humid, is a dehumidifier needed? Before reading this section, read Whole House Ventilation,

Sealed Environment

If you have a mostly sealed environment, such as most modern living spaces, even a low powered and relatively silent peltier dehumidifier can have an impact. A few percentage points can keep dew and mold from forming.

Door sweeps, door shoes, or draft seals can be used to seal the gap between the bottom of the door and the threshold.

Build tight, ventilate right.

Open Environment

In an environment that isn't sealed, such as a room with a gap under an exterior door, then a low powered peltier dehumidifier can't reduce the humidity significantly, because outside air will be continuously mixing with whatever moisture is withdrawn. Water content per volume of air will remain relatively constant.

A compressor-driven dehumidifier will have sufficient power to affect interior air and surfaces, while also producing more heat and consuming more power. To reduce noise, the unit can be placed outside with ducts for processing air.

To reduce power consumption and to have better control of humidity, sealing a house is highly desirable.

Supply Ventilation

Supply Ventilation is covered in the article. Providing a ventilation fan for positive house pressure can be used in conjunction with air filtration.

I read somewhere that 15% of air should arrive from outside to keep internal air healthy. A supply ventilation system benefits from the same flow direction as passive ventilation. Outside air must flow in, certainly when air flowing out of the uppermost house exhaust is more humid than outside air.

Having ductwork and a ventilation fan lends itself to air filtering. See HEPA filters on Wikipedia.

Passive Ventilation using Tall Ceilings

Hot air and humidity rise, so a tall ceiling with a small vent at the pinacle will reduce humidity in the living space. Sun shining through windows will create flow to pump out humidity. Porous internal walls are a buffer to humidity during the night.

In a sealed house, air flows from the intake-vents to the uppermost vents. Intake-vent are placed strategically to provide outside air of lesser humidity.

Automated Windows

In a living space where there is a constant source for increasing humidity, such as a shower or aquarium, less humid outside air can help draw out moisture.

Low cost bluetooth temperature and humidity sensors are available to keep track of indoor and outdoor readings. Their hygrometer function accuracy is limited but still useful. For example, to automate opening and closing of windows, or to turn on bathroom or ventilation fans, based on the indoor/outdoor humidity differential.

Air flow directional control is also important. Warm air, or moist air, or warm and moist air, they all rise because they are less dense than cold dry air. Having a high window or duct can pump humidity out.

Dessicant Dehumidifier

Not anything I've seen, but an idea. For a supply ventilation home, have a dessicant such as calcium chloride in the air intake duct. Air drawn from outside will have its moisture reduced. As the dessicant saturates and becomes liquid, have a drain for the liquid to flow outside into a sun facing receptacle, where it can dry for reuse. The receptacle could be a solar oven.

Mario S on YouTube explains how to use calcium chloride as a dehumidifier. “For basements use 5 gallon pails (of calcium chloride) for best results. Expect these to last 60 days or so.”

The dessicant will slightly increase the temperature of the air: “Physical adsorption of moisture is typically exothermic. The strength of the adsorptive bonds can thus be measured by the heat of adsorption. The higher the heat of adsorption for moisture on the desiccant, the stronger the bonding and the less easily that moisture can be subsequently removed.”

Unfortunately, in Peru calcium chloride is a controlled substance, I think because it gets used in the processing of coca into cocaine.

Waterproofing Walls

There are products for wall interiors and products for wall exteriors. Some are waterproof and/or make the wall vapor impermeable to varying degrees.

Not sure which products to use:

YouTube video by Apple Drains showing a sealant and drainage solution. This situation can be avoided by a foundation well above ground level.

… air & rain tightness are more deserving of attention (than a vapor barrier). In my opinion, a vapor barrier is worth consideration if you're going to strive for active humidity control, as then you have an energy-cost associated with vapor migration, and you have consistent vapor delta across the envelope assembly.Is Interior Paint the Real Vapor Barrier? mrd999 at

Vapor barriers should be placed on the warm/humid side of a wall, so that the wall insulation does not build up condensate. In a temperate climate, that would be the interior. Vapour retarder latex primers, Does a latex primer keep mold from forming on the walls by not providing an attachment for mold growth?

Extraneous notes

Avoid Basements

A basement can also increase living space humidity from the surrounding earth if the floor and walls are porous. Even if the basement is treated to prevent water vapor flow, the walls and floors of the basement will likely be cooler than the outdoor temperature, and this coolness may cause the floor and walls to become damp.

Air close to the basement walls with will be higher in relative humidity, and may reach the dew point. See: thermodynamic concept of relative humidity.

On rare occasion there is a geothermal gradient.

With Outdoor Slightly Cool Temperature

Heat absorbed by the surfaces in a room will reduce the local relative humidity. Radiant heaters heat surfaces with infrared radiation. Surfaces can also be heated by solar energy to reduce humidity:

Passive Solar Heating

Choosing an infrared lamp type, Not affiliated with any company, just the best info I was able to find for now.

With Outdoor Slightly Warm Temperature

“Well water is around 55 degrees F …” Gary Ahlers at Quora. This could be channeled through a heat exchanger in the ventilation system. You can also channel the water through a floor or wall, but in a humid climate, anything besides an arid desert, there would be condensate. In the ventilation system the condensate can be trickled outside.

Air Ducts

The following paragraphs are for a supply ventilation system with air recirculation through a HEPA air filter, where outside air comprises a dynamic percentage of conditioned air to the house.

The percentage of outside air let into the system depends on:

  • outdoor air quality
  • desired interior temperature
  • the extent to which the home can be sealed.

In order to be able to transition from passive ventilation to supply ventilation, the direction of air flow should be based on passive flow.

Each floor or major space should have a floor-level conditioned-air-inlet, and a ceiling-level-exhaust either to a stairway/atrium or a dedicated exit to outside the home. The intlet and exhaust preferably on opposite ends of each interior volume.

Duct work can be built into the walls, or travel along the house exterior for easier servicing. Less exposed duct surface within the home means less surfaces to clean.

The ducts should be internally free of seams to allow easy cleaning. A furry ball pulled by a string can be used for duct cleaning.

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engineering/hvac.txt · Last modified: 2024/05/19 02:27 by marcos