I ‘m looking for an easy way to figure out wether the outside air, or the air in my home has more humidity in it. Can anyone refer me to a chart? For instance, right now outside it is 32 degrees farenheit and 91 percent humidity. Inside it is 68 degrees and 42 percent. I am trying to keep the house in the 30% humidity range by using my heat recovery ventilator with the humidistat set to 35 or so. But I am not sure at what point I am just pulling in air from outside that is more moist than the inside air.
I’m living in a house I just completed in the Hudson Valley of New York. I used Joseph Lstiburek’s book “Builders Guide to Cold Climates” as my guide for much of the construction details. Consequently, I have 2×6 framed walls with damp spray celulose. Outside of that I have 1/2 inch plywood and 1 1/2 inch extruded polystyrene.
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http://www.martin.amedd.army.mil/wet_heat_web.html
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
This may not be the best chart but it should give you an idea. http://www.tis-gdv.de/tis_e/misc/klima.htm
This chart gives absolute humidity which does not change with temperature. It is simply the weight of water in a given volume of air. From the chart, the absolute humidity at 32F (0C) and 90% relative humidity is 4.4 grams per cubic meter of air. At 68F (20C) and 40% it is 6.9. In those conditions, you have over 50% more water in the indoor air than the outdoor air. Your heat recover ventilator should be reducing your indoor humidity under your conditions.
That army link does not apply.
You need a simple psycrometric chart like this:
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/picts/9511p12.gif
Go to the indoor setpoint temperature on the bottom, follow vertically upwards until you reach your desired indoor RH curve, follow the horizontal line to the left until you intersect with the outdoor temperature, and the RH curve at (or near) that point will indicate the outdoor RH that has the same moisture content (or humidity ratio).
"I am trying to keep the house in the 30% humidity range by using my heat recovery ventilator with the humidistat set to 35 or so."
Better to keep it at 40% for a healthy indoor environment, but you need a dehumidistat, not humidistat, to limit the RH.
Under your stated conditions, running a HRV at 0.35 (recommended) ACH, assuming a 2,000 sf (16,000 cf) house, you'd be eliminating 0.99 pounds (0.12 gallons) of water an hour and would have an indoor RH of 36%.
Solar & Super-Insulated Healthy Homes
Edited 12/24/2008 2:25 pm ET by Riversong
Edited 12/24/2008 2:27 pm ET by Riversong
Edited 12/24/2008 2:36 pm ET by Riversong
"Better to keep it at 40% for a healthy indoor environment, but you need a dehumidistat, not humidistat, to limit the RH."Neither a humidistat or "dehumidistat" will change the humidity levels in the house in the slightest amount."
Main Entry:
hu·mid·i·stat Listen to the pronunciation of humidistat
Pronunciation:
hyü-ˈmi-də-ˌstat, yü-
Function:
noun
Date:
circa 1904: an instrument for regulating or maintaining the degree of humidity"" 2.Main Entry:
1ther·mo·stat Listen to the pronunciation of 1thermostat
Pronunciation:
ˈthər-mə-ˌstat
Function:
noun
Date:
1831: an automatic device for regulating temperature (as by controlling the supply of gas or electricity to a heating apparatus) ; also : a similar device for actuating fire alarms or for controlling automatic sprinklers"They are both devices for meauring and controlling equipment that can change temperature or humidity.And when you want to control a air conditioner you connect it to a thermostat not a "dethermostat".What you need is to determine the type of equipment that you need to change the humidity level and that may be a humidifier and/or it might be a dehumidifier. Then you need to see what kind of input that equipment needs (24v AC, 120 AC, 0-5v, active on rise, active on fall, linear, etc) and then get a humidistat that will control it..
William the Geezer, the sequel to Billy the Kid - Shoe
Neither a humidistat or "dehumidistat" will change the humidity levels in the house in the slightest amount.
Not if they're not connected to equipment which will either remove moisture or exchange moist air.
But the OP said his "humidistat" was controlling his HRV.
A humidistat is a close on fall controller that is designed to maintain a minimum RH level.
A dehumidistat is a close on rise controller that is designed to maintain a maximum RH level.
This is a dehumidistat: http://www.energyfederation.org/consumer/default.php/cPath/39_766_376 designed for controlling ventilation equipment.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Robert,
What are the economics of using a dehumidifier instead of an hrv?
After fresh air demand has been met there is an energy penalty for bringing in additional outside air even if you recapture 74% heat.. you are loosing 26%.
The colder it is outside the greater the loss.
I believe Dr Joe is starting to recommend dehumidifiers for high performance homes in many climates.. especially homes less than 2,000 sf.
The dehumidifier will also contribute some heat.
Just a thought
John B
Edited 12/24/2008 9:55 pm by homedesign
What are the economics of using a dehumidifier instead of an hrv?
A dehumidifier is going to use a lot more electricity. It's basically an air conditioner. And it's not going to provide any fresh air.
After fresh air demand has been met there is an energy penalty for bringing in additional outside air even if you recapture 74% heat.. you are loosing 26%.
Why are you exchanging more than 0.35 ACH? Do you have an uncontrolled system?
If you don't want to invest in an HRV, then use a programmed exhaust-only system using the bath fans with passive make-up inlets in bedrooms and living space.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Thanks for the calculator. I am trying to maintain the humidity level nearer to 30% because I am concerned about condensation on the interior side of my exterior sheathing. I have seen the damage this can do in other wall assemblies, mainly fiberglass insulation with imperfectly detailed poly vapor barriers. I am trying to avoid having my house rot from the inside out by following the book to the letter. I'm hoping that building science has progressed somewhat since the days of fiberglass and poly in mixed climates like mine.
I don't know much about how unhealthy 30% relative humidity would be, but we have found it somewhat uncomfortable, which is why I have been keeping it nearer to 35 or even 40. Part of the reason I have been setting it higher is because even with the hrv capturing 74% of the heat I still hate all that cool air coming in and exhausting all that warm air.
Btw the heating system is radiant.
I am trying to maintain the humidity level nearer to 30% because I am concerned about condensation on the interior side of my exterior sheathing.
If you followed Joe's guidelines, then you shouldn't have a condensation problem at 42% RH. What's the rest of your wall section? Latex paint, drywall, VB?, 2x6 studs with 5.5" densepack, CDX or OSB?, 1½" XPS, housewrap?, siding?, paint?
That cross-section, even just vapor retarder primer, will not experience condensation at the sheathing. At the inside/outside conditions you describe, the temperature of the inside of the sheathing will be 44.3° and 90%RH (above the dew point). And the cellulose has a high moisture storage capacity to safely store, distribute and release any small amounts of condensation.
I don't know much about how unhealthy 30% relative humidity would be, but we have found it somewhat uncomfortable, which is why I have been keeping it nearer to 35 or even 40. Part of the reason I have been setting it higher is because even with the hrv capturing 74% of the heat I still hate all that cool air coming in and exhausting all that warm air.
If you've found it uncomfortable, then it's not healthy. To maintain a healthy indoor environment, the ASHRAE standard is 0.35 ACH continuously (or the equivalent intermittently). You have to accept the necessary heat loss from fresh air exhange. It's a small price to pay for health. I program my houses for 0.25 ACH, because I avoid toxic building materials and my customers tend to have non-toxic lifestyles.
What's the design heat load for your house and what is the HDDs for your area? Did you incorporate any passive solar design? That's a great way to make up for the infiltrative heat loss - by getting free heat from the sun.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Riversong: Can I ask you a question regarding the Psych chart? If he takes outside air at 32F 91%RH and heats it with say an oil furnace (sensible heating process) to 68F, without adding or taking out any water, will the RH be 23%? In other words is this a constant humidity ratio process on the Psych chart?
Thanks
Dave
If he takes outside air at 32F 91%RH and heats it with say an oil furnace (sensible heating process) to 68F, without adding or taking out any water, will the RH be 23%? In other words is this a constant humidity ratio process on the Psych chart?
Yes, though my calculations show 24% RH. Constant at constant barometric pressure.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I like a calculator better.
http://www.uigi.com/WebPsycH.html
You can enter any 2 parameters
Choose 2 Properties + Pressure or Altitude)
Dry Bulb Temperature
Wet Bulb Temperature
% Relative Humidity
Enthalpy of Dry Air
Dew Point
Humidity as Weight Ratio
Altitude (feet or meters)
Pressure (inches or mm) of Mercury (Hg)
Pressure (psia or KPa)
And for this purpose just leave the altitued at zero ft.
In your case Tempeature (dry bulb) and RH will be the inputs.
And you can get outputs in several different forms giving the amount of moisture.
But I like using dew point. It is easier to understand.
The lower the dewpoint the less humidity.
William the Geezer, the sequel to Billy the Kid - Shoe