We have a passive solar home, on a radiant thermal slab with very tight insulation doors and windows. The house DOES NOT HAVE forced air ventilation (attic or basement). Our climate in western North Carolina is warm humid summers and cool dry winters.

In the summer, we open the windows at night and close them during the day and the humidity equalizes with the ambient outdoor humidity (75%). In spring and fall the windows are open during the day and closed at night. During the winter the house is closed up tight with no ventilation. 

Question1: Do we need to dehumidify in spring summer and fall when the windows are closed?

Question2: Should we dehumidify, ventilate, or both in the winter when the house is sealed up tight?


J
Pic: the batcave.

 

1. 75% humidity inside a non ventilated tight house is a recipe for mold and rot.
2. ventilation is a necessity in a closed tight house for health reasons if nothing else.

Posted By Bob I on 19 Jun 2014 12:44 PM

1. 75% humidity inside a non ventilated tight house is a recipe for mold and rot.
2. ventilation is a necessity in a closed tight house for health reasons if nothing else.

Thanks Bob:

We are already seeing mold and rot, especially in the winter. I suspect I need an ERV for ventilation in the winter, and a dehumidifier any time the windows are closed, just to dry thinks out.  Are there ERVs that also dehumidify or should I expect to have two separate units with separate controls?

J

You should run the ERV or HRV whenever the windows are closed.

To prevent mold, you need at least occasional dry conditions. For example, 4 hours per day at 50% RH. Exactly how far you could push this (3 hours @60% every other day?) is unclear.

Fans help prevent pockets of high humidity.

Posted By jonr on 19 Jun 2014 01:35 PM
You should run the ERV or HRV whenever the windows are closed.

To prevent mold, you need at least occasional dry conditions. For example, 4 hours per day at 50% RH. Exactly how far you could push this (3 hours @60% every other day?) is unclear.

Fans help prevent pockets of high humidity.

Good information, thanks. 

All the literature and advise I get on HRV vs ERV adds to the confusion.

In our case, we would use the ventilator in summer and winter when the windows are closed, since the Fall and Spring have lower Humidity and the windows are usually open 24/7. 

Ventilating in winter with dry outside air ranging from 20-40 and summer with humid outside air 65-85 would require HRV or ERV?  Note: I will add a separate dehumidifier also.

J

the rule of thumb is that ERVs should be used in hot humid climates.

I have a friend who sells Santa Fe dehumidifiers. Excellent commercial grade units - I've used one for drying out a wet basement - made in the USA. Pretty quiet and efficient use of electricity.

At NC type summertime outdoor dew points you really can't use nigthttime ventilation schemes for cooling without introducing a LOT of humidity.

The % relative humidity is a meangingless number without the temperature to which it is relative. The same body of air will have a different %RH at different temperatures. If you want to keep it under 60% RH at an indoor temp of 75F, the ventilation air mus have a dew point of 60F or lower, or a"wet bulb" temperature of 65F or lower. These are measures of the absolute humidity of the outdoor air.

If there is a weather station near you with reasonably accurate dew point reporting you can look it up to make informed decisions about whether to keep the house closed up and air-condition/dehumidify mechanically rather than ventilating with outdoor air. The summertime AVERAGE dew points for most of NC are in the mid-60s F, which translates into a ~70% relative humidity at 75F.

Insert your zip code and pull up a dew-point graph to see both history and current dew points, with projections for then next several days:

http://weatherspark.com/#!dashboard;q=Asheville,%20NC,%20USA

An ERV won't magically dehumidify when the outdoor moisture levels are too high, but it will somewhat lower the amount of moisture that needs to be removed with dehumidifiers or air conditioners.

Thanks Dana, more good information. Yesterday, (June 21), the temperature inside was 78 and the RH was 74 which puts my dew point at over 69. Besides being uncomfortable, an ice cube dropped on the concrete floor left a puddle that stayed there all day, the air is saturated. I think I will go with one of the ventilating dehumidifiers,Since there is no ductwork in our 1800 sq ft, I will feed dry air into the two bedrooms and circulate it through the living room. I can bring in fresh air in the winter when it is dry, and use the window option in spring and fall, then just dehumidify in the summer. This has been very helpful. J

Yep, 69F dew points are downright sticky when it's hot, clammy when it's cooler, uncomfortable at any air temperature. That's in stark contrast to 55F & lower dew points are pretty comfortable, though I know people who feel "cold" at 70F when the dew points are sub-50F.

A dehumidifier will add a considerable amount of heat to the house, which will be counterproductive during the summer. The compressor and fan motors will add heat, plus the phase change from water vapor to liquid will add heat. An air-conditioner has essentially the same pieces as a dehumidifier, but it is designed to transfer the heat from the house to the outside. You might consider an air-conditioner for the summer and portions of the spring and the fall.

In the winter, the outside air should naturally have a lower absolute humidity (lower dew point) than the indoor air. An HRV that brings in the dry outside air and transfers moisture, CO2, and household odors outside would add to the comfort and health of the indoor air.

Posted By Lee Dodge on 23 Jun 2014 04:36 PM
A dehumidifier will add a considerable amount of heat to the house, which will be counterproductive during the summer. The compressor and fan motors will add heat, plus the phase change from water vapor to liquid will add heat. An air-conditioner has essentially the same pieces as a dehumidifier, but it is designed to transfer the heat from the house to the outside. You might consider an air-conditioner for the summer and portions of the spring and the fall.

In the winter, the outside air should naturally have a lower absolute humidity (lower dew point) than the indoor air. An HRV that brings in the dry outside air and transfers moisture, CO2, and household odors outside would add to the comfort and health of the indoor air.

The temperatures in Asheville never typically 65-85 in the summer (night-day) and never quite reach 90, so we can get by without AC since the concrete thermal mass keeps the house cooler than the outside highs.   The RH this week is about 74 and inside temps about 75, outside temps 85.

I am planning putting the dehumidifier in a closet ( we do not have an attic or basement) and vent the closet to the outside when the humidifier is on the same way we do the electric dryer. The same closet holds the electric water heater, boiler for the hydronics and is adjacent to the washer and dryer.

I see that some of the portable air conditioners advertise that they can also serve as dehumidifiers. Ie, I assume that depending on duct connections, they can dump heat outside or retain the heat (either can be appropriate, depending on the weather).

How well humidity and fresh air will distribute in a ductless house is a good question.

Jay - I think what Lee is trying to tell you is that if your house is closed up and well insulated, like you say it is, the dehumidifier will put out a lot of heat that will build up inside the house and make it hot. If you instead use an AC unit, the heat from it will be sent outside of the house. The AC will dehumidify your house without heating it up, and you wont have to worry about emptying it out! :-)

Thanks Everybody.

So, I installed a 70 pt/day whole house dehumidifier and added ducts to the two bedrooms.

I pulled 3 gallons of water the first day and the humidity inside dropped from 80% to 58%. The temperature jumped from 76 to 83. The humidity outside is staying in the 80-90% range (temperate rain forest) with temps ranging from 65-85.

We are still opening the windows at night to drop the temperature, but the humidity is not jumping up past 70% or so. I will add the ventilation option next weekend and tie the humidistat to the bath fans so I can ventilate without dehumidifying.

The added heat is annoying but it will work out well in the winter and we can periodically dry out the house in the summer. So far so good.

J

Posted By Lee Dodge on 23 Jun 2014 04:36 PM
A dehumidifier will add a considerable amount of heat to the house, which will be counterproductive during the summer. The compressor and fan motors will add heat, plus the phase change from water vapor to liquid will add heat. An air-conditioner has essentially the same pieces as a dehumidifier, but it is designed to transfer the heat from the house to the outside. You might consider an air-conditioner for the summer and portions of the spring and the fall.

In the winter, the outside air should naturally have a lower absolute humidity (lower dew point) than the indoor air. An HRV that brings in the dry outside air and transfers moisture, CO2, and household odors outside would add to the comfort and health of the indoor air.

It's not really adding a lot of heat to the house- most of it is changing the form of that heat from vaporized moisture into higher-temperature air. An Energy Star dehumifier delivers more than 1.85 liters of water per kwh.   At 2.2 lbs/liter that's ~4lbs of water/kwh.  The heat of vaporization of water is about 965 BTU per lb., so that 4lbs of water represents 3860 BTU of latent heat converted into sensible heat per kwh.

The only heat that is actuallly added is the 3412 BTU/kwh of power dissipated in the compressor & fan, which is less than half the total (but more than 1/3) of the apparent sensible heat.

In home with low sensible-cooling loads, nighttime ventilation with more humid may add more heat to the house in the form of water vapor than it's worth.  On any given night the amount of latent heat brought in on the humid night air will vary, depending on the absolute humidity of that outdoor air.  (At my house we only open the windows at night for sensible-cooling when the outdoor air's dew point is 55F or lower. YMMV.)

I tell my customers in humid states like MN to keep the windows close and let our cooling systems operate as designed. Like you I open my windows when the outdoor rH is at 50 or below. Today! But this is the first time this year that I have been home and cared enough to save couple of bucks. The Mitsubishi get a rest until noon tomorrow.

I wonder how much moisture building materials store. Ie, if you open the windows and tolerate high %RH for some number of hours, how much harder does a dehumidifier have to work later when the windows are closed?

I wonder how much moisture building materials store.

I do not know about building materials, but having lived in a high humidity house for two years:
1) Any cloth materials like towels, clothes, and mattresses absorb enough to smell musty.
2) Water that spills on the concrete floor does not evaporate.
3) Plants need less watering.
4) Windows collect condensation.
5) Wood floors are swelling (slightly)

I do not know if the drywall, studs, and cabinets are absorbing but I would imagine so, we have no curtains or cloth furniture.

Once mold and mildew started showing up on the window sills from dripping condensation, we decided it was serious enough to start mitigation.

J