Hello Thanks for all your help. My flood zone project house in North Jersey has reduced its consumption by more than half so far. With 2/3rds of the basement spray foamed, we have been able to stop running the basement dehumidifier...'til now. The hot and humid weather has kicked in the past couple days and I am curious if you think ventilating the basement to the main floor is prudent? It is a two story L shaped house with 900 sq ft on each level for a total of 2700 sq ft inside the thermal barrier. There are no ducts in the basement, it is not heated and the Rinnai combi is sealed. The basement door needs to remain closed. The past couple days the basement has been 75-85 RH at 71 degrees. The living area has been 53-59 RH at 77 degrees with the Central AC. The basement is divided into three rooms which creates a little ducting issue. Are panned joists OK for this purpose? What fan style and cfm would you recommend? Is it possible to do with no fan? Can I locate a duct under the kitchen stove? Under radiators? Gratefully John

I would leave the basement sealed off from the main floor and measure humidity in each room without ducts between rooms (but with the dehumidifier on). Add small ducts (very little flow is needed for humidity control) if the hygrometer indicates the need for it.

What would you guess the flow couldn't be more than? I want to purchase a fan once. 50, 100, 200 cfm?

Hard to say without knowing how much moisture you are getting, but my guess is that 50 CFM (~.6 ACH) blowing into the other rooms would suffice if everything (exterior walls and ducts) is well sealed. Partly because this will increase the other rooms pressure slightly and reduce infiltration (the primary cause of humidity). If it doesn't suffice, then you should fix the leaks instead of throwing more fan and dehumidification at it.

did you do anything to reduce the moisture that is coming up through the floor?

50 cfm for 7000 cf of space or each room? Do you think discharging the slightly warmer dryer are closer to the floor may be important? 1/3 of the floor was dirt. It was covered and sealed with poly tarp and sealed with the foamed walls. 1/3 of the floor is tile. 1/3 is bare concrete. suggestions?

Sorry JonR. I see you meant for the entire area. My concern is that the way it is divided. A sheet-rocked man cave separating the other rooms, there won't be enough convection to spread the love.

The 1/3 of the rim joist/wall that is not foamed is behind the sheet rock. Any way to seal from outside? Looks don't matter, it is under a covered porch. Vinyl siding.

The basement being coolest place in the house, it's pretty common for the basement RH to run high during the more humid summer months.

At 71F/80% RH the dew point of that air is about 64-65F, and according to Weathespark data sets, the outdoor dew points in Newark have been running in the mid to high 60s- ANY amount of air leakage into the basement from the outdoors would be adding to the humidity issue. In the air conditoind upstairs, 55% RH @ 77F corresponds to a dew point of ~60F, which if simply moved into the basement would yield about 67% RH & 71F- still high, but not a unhealthy mold-hazard the way 75-85% RH air is.

Giving even a tiny amount of AC duct & return would most likely take care of it. The air coming directly out of the AC duct is much drier than the upstairs average, so as long as you don't have such a high cfm of air into the basement that it brings the temp down, it'll be mostly handling the latent load. It just has to be enough volume to do a complete air exchange over 3-4 hours, since the sensible cooling load down there is zero- it's the air exchange with the extra-dry air that you're after, not the sensible cooling, and it doesn't take a whole lot of air-volume to handle the latent load. You can probably get there with 4" round trunk ducts and 3" branches, the way HRV systems are run. A panned joist bay would be ridiculously oversized for the volumes required, but ducts that size can fit easily into joist bays. You'll probably want to have a vane-valve on the supply trunk to throttle back flow if the flow proves high enough to be cooling the basement significantly.

50 cfm total. How many gallons per day do you remove from the basement on a hot muggy day? I wouldn't count on the house A/C to provide dehumidification. There are too many days when it doesn't run but basement humidity is still high.

At any flow rate simply using a simple air exchange with the upper floor air is going to be a problem, since it's not dry enough.

Ideally you'd be able to keep the basement under 60%RH, which you WOULD be able to do with very modest air flows from the AC when it's running, flows low enough to avoid too much sensible cooling.

Dana, adding a duct from the attic air handler would be difficult. I was hoping just to jumper the basement to the first floor if possible? I realize the air won't be as dry, thus the cfm question. Maybe 2 small supply AND return fans?

Just to be clear, my comments were about running ducts from one dehumidified room in the basement to the other basement rooms. Moving upstairs air to an often cooler basement is usually a bad idea.

I don't know yet how many gallons per day. This was a basement that required constant year roun dehumidification. The entire house was airsealed last fall and humidity in basement just rose above 70 for first time this week. I think the blower door tested out around 2500 at 50? 22500 cubic feet.

Ahh! Thanks JonR. Why?

Posted By jw on 28 Jun 2013 02:00 PM
Dana, adding a duct from the attic air handler would be difficult. I was hoping just to jumper the basement to the first floor if possible? I realize the air won't be as dry, thus the cfm question. Maybe 2 small supply AND return fans?

Like I said at any reasonable volume of air exchange using just the conditioned space air you'll be averaging 67% RH in the basement given your stated temp & RH on the first floor.   That is on the high side of what's healthy & comfortable for humans (ASHRAE sez 65% RH max, health professionals say 50% max for folks with dust mite allergies, 60% otherwise.) 

At an extremely high air exchange volume you'd get there, by pumping sufficient volume of 77F air into the basement to raise it's temp several degrees, but your dehumidifer would use less power.

I'm a bit surprised that the AC isn't pulling the first floor RH down a bit more- is your system also pulling in ventilation/makeup air? If yes, lower the ventilation air volume. If not, it's likely that air leaks on either the ducts, the house or both are creating an air-handler-driven infiltration issue.   If you dry out the upstairs more, the basement will follow. If you get the upstairs under 50% RH, then sharing that air directly with the basement would get you where you want to be. (50% RH @ 77F air becomes 61% RH @ 71F.)

Ahh! Thanks JonR. Why?

I get lots of weather where I don't run the A/C and open the upstairs windows, but the %RH is high. Take 78F air at 78% RH and blow it into a 70F basement and you get condensation (admittedly not much in terms of quantity). Moving small amounts of air down there will add moisture but not bring the basement temperature up. Independent humidity control is best for a basement.

On a related subject, I am considering going to timed dehumidification. For mold prevention, higher humidity is apparently OK as long as it goes low for part of the day. So it may work to set a dehumidifier to 80% for 20 hours/day and 50% for 4 hours/day (2am to 6am appears to be optimal).

Jonr- I was planning on at least one humidistat. I understand what your saying. So far, the conditions have only come when the AC is needed.

Dana- You maybe right about the make up air. Is it required at all? The system was also oversized(4ton). And now with the envelope improvements...hah! 5 minutes on 20 minutes off. It is due to be replaced next year. What do you guys think about burying ducts in the attic?

Dana- I am looking at some of the dc fans. 20 watts for 200 cfm. Dehumidifier = 300watts? Granted, run times might be different but I'm not so sure about your claim?

i think Dana is on to something again. I run my dehumid to 70% and have my HRV terminal is the basement bath. This is how we ventilate most of our basement remodels as well. Ducting a basement of cooling rarely works out as intended and why we don't recommend it.

Posted By jw on 28 Jun 2013 03:02 PM
Dana- I am looking at some of the dc fans. 20 watts for 200 cfm. Dehumidifier = 300watts? Granted, run times might be different but I'm not so sure about your claim?

But the fan doesn't remove the humidity with any efficiency- it just spreads it around so that the central AC can handle it (if it's running), and the 20 watt fan would literally need to run 24/7 to have any appreciable effect on basement humidity. 

An Energy Star dehumidifier converts the humidity load to a sensible cooling load fairly efficiently, while raising the temp of the room modestly (thus lowering the RELATIVE humidity.)  Most of any size (60 pints/day) pull about 500watts while running, but only run when needed, since they're under dehumidistat control. 

A 300W dehumidifier manufactured before 2013 will almost always be running substantially lower efficiency than it's slightly bigger sibling, but a kilowatt dehumidifier, while efficient given a large enough load, is almost certainly going to be oversized for the load and lose efficiency to short-cycling, etc.  To get an Energy Star label on a 2013 manufactured unit a sub-75-pint/day dehumidifer has to remove 1.85 liters per kwh under some standard test condition. IIRC in prior years the cutoff point was 65 pints- units smaller than that were allowed 1.3liters/kwh, with those larger than 65 had to deliver 1.8 liters/kwh.  There is lots of older stock still in distribution, but somewhere on the label or fine print there will be a l/kwh number.  Anything 1.8+ is pretty good compared to what was out there just a couple of years ago.