Replies

1. GBA Editor
   Martin Holladay | Apr 03, 2011 03:46pm | #1

   Ben,
   A new building often requires a high ventilation rate during the first winter to help get rid of construction moisture. If your bathroom fans are now set to come on at the highest humidity level, why not set them to come on at a LOWER humidity level?

   If your outside air duct is now programmed to come on for 20 minutes every 4 hours, why not increase the programming to 40 minutes every 4 hours? If you know how to program your ventilation equipment, just increase the ventilation rate.

   After one full year, you can always adjust the ventilation rate back down. But as long as you have condensation on your windows, you should increase the rate of ventilation during the winter.

2. 
   davidmeiland | Apr 03, 2011 04:15pm | #2

   I agree with Martin that more ventilation is needed and that you should experiment. 1112 CFM50 is not particularly tight for a 1100 square foot house. Using your stated numbers, you have 12,650 cubic feet, and at 1112 CFM50 you have 5.27 ACH50, which is average new construction in my experience. It's pretty close to the .35 ACHn you gave.

   Curious to know more details. When was construction completed? Is this on a slab or a crawl space? How many occupants, and how long has it been occupied?

3. 
   skylarswinford | Apr 03, 2011 06:39pm | #3

   What type of foundation is this building on? If it is built over a poorly sealed crawl space this could be a hidden source of humidity...judging by the poor airtightness on this project I wouldn't be surprised if the crawl space has some issues.

4. 
   hHPTbL7FKm | Apr 03, 2011 08:51pm | #4

   Thanks for your help...I'll try to fill in a few blanks.

   The unit is on a crawlspace. We used superior wall frost walls beefed up with with 8.5" of EPS. The rim joists are insulated with 3" of icenyne foam The crawlspace has a limestone gravel floor with a 3mil vapor barrier buried about 3" below the surface. (A few months ago I found a mumified mouse carcass in the crawlspace which seems to me to indicate a dryer environment.

   The combinination of SIPS, sprayfoam and E-star windows and doors makes me think the structure is very air tight. and thus not getting enough fresh air.

   My hvac guy thinks that it is so tight that the Aprilaire ventilator can't push the air in because of the pressurization. That is why I turned up the sensitivity on both bath fans in order to decrease the pressure..

   The homes (2) were built in 2009 and we did installed a fantech ventilator (260cfm) in the crawlspace controlled from a switch in the living area to mitigate the left over humidity from construction. I recommend to the tenants to run the crawlspace ventilator on very damp days. The tenants (2) were able to manage the humidity (stay below 65%) by cracking a window or two but the locksets continue to sweat. The E-Star report estimated an annual energy bill that was pretty close to the actual bill for the all electric house (about $100/mo average).

   If as you say, the unit isn't as air tight as I imagined, why so much humidity? You can get more info at http://www.elane.biz..

5. 
   davidmeiland | Apr 03, 2011 10:09pm | #5

   Sounds like time for more detective work. Not sure what to think about the crawl space--dry mouse carcass vs. ventilator installed, so is it dry or isn't it? Stick a thermo-hygrometer in there and find out. Your HVAC guy thinks it's tight, but if your blower door result is accurate, it's average. Any chance there's a humidifier on the heating system?

6. 
   user-723121 | Apr 04, 2011 12:16pm | #6

   At .35 achnat your ventilation/infiltration rate is 74 cfm, this house should require very little if any mechanical ventilation once it dries out.