Air humidity is one of the key factors for comfort at home, and can sometimes be unexpectedly high in a brand new “green” build. I spoke to Positive-Energy‘s Kristof Irwin and found out one method he uses – psychrometric testing – to help identify issues with discomfort in the home.
This little gadget logs temperature and humidity
The type of comfort we’re talking here is thermal comfort, and the following factors which are most frequently controlled by your HVAC play into that:
- Air temperature
- Relative humidity – we lose heat when we perspire, and the amount of water vapor in the air determines how fast we can cool
- Air movement – moving air feels cooler
A side effect of your HVAC system is the reduction of humidity as it cools air. This is great as keeping the humidity down within an acceptable range is important for you to feel comfortable.
The challenge for builders of energy efficient homes is that building envelopes are getting tighter – there are less places for water vapor to escape. So if you’re cooking and generating steam, or taking a shower without an exhaust fan, your traditional HVAC will have to expend a great amount of energy to dehumidify the air.
There’s a sweet spot at which air temperature and air humidity feel comfortable for most of us. The interesting thing from an energy efficiency standpoint is that you don’t need to run all of your HVAC to dehumidify the air. You can simply add a lower power dehumidifier to your home, and use less energy on HVAC operations to achieve a feeling of comfort.
So what goes wrong? If your relative humidity remains high (even at a lower temperature) the combination doesn’t feel right. And if the relative humidity stays above 60% for too long, fungal spores can germinate. The EPA advises maintaining a relative humidity of 30-60%
What starts off as feeling uncomfortable can end up being unhealthy. So that’s where Kristof gets called in. He uses psychrometric testing:
“By logging humidity and temperature over time, you can learn where the air moves,” says Kristof. From there he can understand your home and figure out what needs to be addressed.
Little data loggers can be placed strategically around the home to test theories. I’m adding two in my home just to see, and I’ll report back what we learn.
 |
 |
Garreth Wilcock is an Austin EcoBroker®
Specializing in: New Green Homes in Austin.
(512) 694 8873
{ 2 comments… read them below or add one }
Two elaborations:
1. It’s not just that the tight house doesn’t allow water vapor to escape, it is that the cooler surface temperatures of a well-insulated house allow the water to condense into liquid form. There is less surface heat, and less heat generally, available to keep the water in vapor form. This is unfortunate b/c the air conditioner needs the water to be in the air, in vapor form, in order to effectively dehumidify.
I have used the loggers to discover indoor humidities above 70% – even in newly built green homes. They can also be placed in the supply and return plenums of HVAC systems to quickly diagnose whether a system is functioning properly.
2. The 7 factors for human comfort are:
1. metabolic rate
2. activity level
3. clothing (lack of, or too much)
4. temperature
5. humidity
6. air velocity
7. MRT (mean radiant temperature)
Numbers 4-7 are associated with the environment, the first 3 are considered personal. MRT is an average of the radiation to/from surfaces around you, based on their temperature and emissivity. MRT can be a big deal – as surfaces around you either radiate heat too you, or absorb heat from you. This can cause you to feel uncomfortable even in environments where the temperature, humidity and air velocity are all in safe/comfortable ranges.
Thanks for the elaboration Kristof. It seems that the additional challenges of a well insulated home mean that getting the HVAC right is critical.