Relative humidity is a crucial factor in determining the comfort and well-being of both humans and the environment, and it varies with temperature. In summary:
- Relative Humidity Defined: Relative humidity (RH) is a measure of the amount of moisture or water vapor present in the air compared to the maximum amount of moisture the air can hold at a given temperature. It is expressed as a percentage. When the RH is 100%, the air is saturated with moisture and cannot hold any more, leading to condensation.
- Ideal Humidity and Temperature Relationship: The ideal relative humidity level for comfort and health varies with temperature. Generally, lower temperatures are associated with higher ideal RH levels, while higher temperatures require lower ideal RH levels. For example, an indoor environment at 68°F (20°C) is typically comfortable at around 40-60% RH, while a hotter environment at 86°F (30°C) is more comfortable at 30-50% RH.
- Effects of Low Humidity: In colder conditions, low humidity can lead to discomfort and health issues. Dry air can cause skin and respiratory problems, as well as increased static electricity. It can also lead to dehydration because the body loses moisture more rapidly through respiration.
- Effects of High Humidity: In warmer conditions, high humidity can be uncomfortable and pose health risks. When the air is saturated with moisture, sweat evaporation is less efficient, making people feel hotter than the actual temperature (as discussed in the previous response). High humidity can also contribute to the growth of mold and mildew, which can be harmful to both health and property.
- Ideal RH for Specific Activities: The ideal relative humidity can vary depending on the specific activity. For instance, indoor humidity for preserving wooden furniture and musical instruments may be different from that for human comfort. Certain industrial processes and laboratories may require precise humidity control to maintain product quality or experimental conditions.
The ideal indoor relative humidity is 30-50%. For some people the range is 45 to 55%. Ideal humidity is expressed as a range since people have different comfort levels. Sometimes the range is quite big (usually 30 to 60% relative humidity).
Relative vs Absolute Humidity
Humidity is expressed in either absolute humidity or relative humidity. In HVAC, we use relative humidity. But what is the difference between relative humidity and absolute humidity?
Absolute humidity is a measure of the actual amount (mass) of water vapor in a given volume of air regardless of the temperature, expressed in grams per cubic meter (g/m3). Absolute humidity tells us the mass of moisture in a given volume of air.
Relative humidity is the amount of water vapor in the air, expressed as a percentage of the total amount of moisture that the air can hold at that particular temperature.
The potential for air to hold moisture increases with an increase in temperature. That is why relative humidity is at its highest during the hot months of summer.
When temperature drops, relative humidity levels drop as well. Again, that is why low humidity is usually a concern during winter.
Relative Humidity vs Temperature
I have mentioned that relative humidity is a function of temperature. In fact, relative humidity is directly proportional to temperature.
Apart from that, relative humidity also affects how our bodies perceive the temperature to be. For instance, during the summer, the air feels hotter than it actually is while during winter the air feels colder than the actual temperature is.
Why is that though? Why does relative humidity affect how our bodies perceive temperature?
You see, our bodies have a natural method of cooling themselves which is perspiration. Tiny pores in the skin release sweat from the body and when the sweat evaporates it leaves a cooling effect on the skin.
Perspiration is however highly affected by relative humidity. When the relative humidity is high, it means that the air is concentrated with moisture.
High moisture in the air means that the air is not likely to accept the extra moisture on our skin (sweat) and that is why our bodies feel sticky during summer. If the sweat cannot evaporate, our bodies cannot cool off and the temperatures therefore feels hotter than it actually is.
The situation is the exact opposite during winter, especially in arid states which have the lowest humidity. Low humidity means that the air is dry and as a result it pulls away moisture from our skins and a result it feels colder than the actual room temperature.
This different temperature that our bodies feel as a result of the relative humidity level is commonly known as apparent temperature. Apparent temperature is also known as “feels like” like temperature because it explains what the temperature feels like even though it is not the actual temperature.
For example, the apparent temperature when relative humidity is 80% is 97 degrees while the actual temperature is 85 degrees. However, the apparent temperature is 86 degrees when relative humidity is reduce to 40% at the same actual temperature.
Outdoor Temperature vs Recommended Indoor Humidity Chart
Controlling indoor humidity is tricky. You don’t want the humidity to be too low when the outside temperature is at its highest. You also don’t want it to be too high when the outside temperature as at its lowest.
The table below will help you understand the recommended indoor relative humidity depending on the temperatures outside.
| NPT | BSP |
|---|---|
| North America | Europe, Australia, New Zealand, Asia, Africa |
| 60 degrees thread angle | 55 degrees thread angle |
| sharp crests and roots | rounded crests and roots |
| tapered threads | parallel threads |
| Thread sealant needed | Gasket/O-ring needed for sealing |
| lower thread density (per inch) | higher thread density |
| bigger pitch | smaller pitch |
Although I mentioned that the ideal indoor humidity is between 30 and 50%, in some instances you may need to go lower than that as shown in the table. Also, based on your comfort levels, you can increase the indoor humidity to as high as 60% if it doesn’t affect you.
Temperature vs Relative Humidity Chart
| Nominal Pipe Size (Inches) | Inside Diameter | Outside Diameter | Wall Thickness | Pressure Rating (PSI) |
|---|---|---|---|---|
| 1/8 | 0.249 | 0.405 | 0.068 | 810 |
| 1/4 | 0.344 | 0.540 | 0.088 | 780 |
| 3/8 | 0.473 | 675 | 0.091 | 620 |
| 1/2 | 0.602 | 840 | 0.109 | 600 |
| 3/4 | 0.804 | 1.050 | 0.113 | 480 |
| 1 | 1.029 | 1.315 | 0.133 | 450 |
| 1-1/4 | 1.360 | 1.660 | 0.140 | 370 |
| 1-1/2 | 1.590 | 1.900 | 0.145 | 330 |
| 2 | 2.047 | 2.375 | 0.154 | 280 |
| 2-1/2 | 2.445 | 2.875 | 0.203 | 300 |
| 3 | 3.042 | 3.500 | 0.216 | 260 |
| 3-1/2 | 3.521 | 4.000 | 0.226 | 240 |
| 4 | 3.998 | 4.500 | 0.237 | 220 |
| 5 | 5.016 | 5.563 | 0.258 | 190 |
| 6 | 6.031 | 6.625 | 0.280 | 180 |
| 8 | 7.942 | 8.625 | 0.322 | 160 |
| 10 | 9.976 | 10.750 | 0.365 | 140 |
| 12 | 11.889 | 12.750 | 0.406 | 130 |
| 14 | 13.073 | 14.000 | 0.437 | 130 |
| 16 | 14.940 | 16.000 | 0.500 | 130 |
| 18 | 16.809 | 18.000 | 0.562 | 130 |
| 20 | 18.743 | 20.000 | 0.593 | 120 |
| 24 | 22.544 | 24.000 | 0.687 | 120 |
From the chart you can now see how the apparent temperature changes with changes in relative humidity.
- At 85 degrees and 0% RF, the apparent temperature is 78 degrees.
- At 85 degrees and 10% RH, the apparent temperature is 80 degrees.
- At 85% degrees and 30% RH, the apparent temperature is 85 degrees.
- At 85% degrees and 75% RH, the apparent temperature is 95 degrees.
Effects of Low Humidity
Low humidity as I mentioned is just as bad as high humidity and will be a problem during the months of winter. Low indoor relative humidity affects people in arid areas like Utah, Nevada, New Mexico, Arizona and Colorado more than other areas.
Basically, states that are far from water bodies have the least levels of relative humidity (both during summer and winter). That is because moisture in the air is as a result of evaporation of water from water bodies. You now know why Florida is so humid.
So, what are the effects of and signs of low indoor humidity? The following are the signs of low humidity:
- Dry and itchy skin
- Dry/irritated eyes
- Nasal congestion
- Respiratory infections
- Cracking wood/furniture
- Static electricity
- Dehydration
- Nose bleeding
- Worsening asthma and allergy cases
Installing a humidifier is the best way to increase the relative humidity in the house. But how else can you increase the humidity in the house?
- Boiling water in the kitchen
- Have pots of water in the house
- Leaving a bucket of water close to the furnace
- Having house plants
- Showering with the bathroom door open
- Leave the water in the tub for some time after a bath
- Air dry laundry indoors
- Vent you dryer indoors
Effects of High Humidity
High humidity is a usually a big problem for folks in humid states like Florida. The following are the causes of high levels of relative humidity in the house, apart from your home’s location:
- Oversized air conditioner
- Setting fan to ON instead of AUTO
- Rising damp
- Poorly maintained AC unit
- Water leaks in the house
- Open crawlspace vents
- Showering
- Cooking
- Drying laundry indoors
- Poor ventilation
The effects of high humidity are:
- General discomfort
- Mold growth in the house
- Increase in allergies and worsening asthma
- Musty smells in the house
- Damage to property
The following are the different ways to reduce humidity in the house:
- Install a dehumidifier
- Have a properly sized AC unit
- Setting the AC fan to AUTO
- Install vapor barriers in crawlspaces
- Close crawlspace vents
- Use exhaust fans in bathroom and kitchen
- Line dry laundry outside
- Fix water leaks
- Take cold showers
- Clean and dry rugs and carpets
- Use charcoal briquettes
- Consider a rock salt dehumidifier
For more information on how to reduce indoor humidity check out this post.
Wrap Up
And basically that is everything about relative humidity charts and how it relates with actual temperature and apparent temperature. I hope that this guide was helpful.