ok, my fellow energy geeks, i need some help here. i just visited a house built just last year. it has condensation and frost on every single window. There is even frost along the top plates in some areas. they have a continuos running bath fan and their humidifier settings were at 1 out of 10. what in the heck is going on here?

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How about turning OFF the humidifier entirely? If that's not the case I'd look for water leak(s) and/or wet crawlspace.

Hi Chad,

Whenever dealing with humidity and moisture issues you need numbers.  What is the temp and RH inside the home.  Then we need to know how tight the home is.  If it is well sealed, the air exchange will be low and the natural internal moisture/RH will be high.  Also, who lives there and what are their activities.  6 daughters and the shower would never stop running.

Next is the humidifier.  A better setting would be OFF, but we will get to that.

Bud

Bud is right, you need numbers. But not just on the RH -- what's the blower door reading? Is that continuous fan actually moving any air??

Condensation is either "too humid" or "too cold" or both. You often need to manuipulate both side of the equation to get good results.

First -- kill that damn humidifier. The last thing this house needs is a humidifier!

Too humid?? The continuous fan should be pulling a lot of humidity out. I'd strongly suspect it's not doing it's job -- that probably means the exhaust duct is way too long and has too many bends. Check it out.

I see you are in Minnesota -- when it gets well below zero up there, you are likely to get a little condensation on anything but the very best windows unless the indoor RH is 25% or lower. And that's too low to be comfortable. Homeowners sometimes think that ANY condensation is terrible -- but it costs a fair amount of money to get windows efficient enough to defy the laws of physics. So, a little condensation for a week -- not much of a problem. Wet window sashes for several months -- not good. Measured RH will tell you where you stand.

At sub-zero, you start seeing lots of bad things if the RH gets much above 30 or 35%. But condensation (frost?) on top plates SUGGESTS the RH is at least that high. If the RH is high -- what's the source? If this is the house's first winter, it might be just construction moisture in all that framing. Jack up the fan for one winter (after making it work) and get the humidity out. If there are six teenagers, all taking 43 minute showers, that bath fan had better be working VERY WELL. Lots of plants, open fish tanks that should have covers -- all need consideration.

Too cold -- little areas of the shell that are RELATIVELY under-insulated (framing and bypasses down low in the house) will condense first. The best solution is too warm them up. Your description suggests the top plate is way too cold. Your code requires energy heel trusses and windwash barriers for the eaves, so the top plate SHOULD be well-insulated. But you still have to check -- an uninsulated top plate could get cold enough to frost up. Make sure there's insulation all the way out over the top plate.

If the house is generally working well and the humidity is reasonable, but there are still a few bad spots, the only solution for this winter may be to get better air mixing. If the furnace is oversized, the air handler doesn't run long enough to mix the air and get warm air into all those corners with lots of framing. Turning the air handler on continuous-low may be enough to get them through that first winter until all the framing dries out.

 

Good luck.

As others are saying, you need to check all possible sources of moisture. Bath fans not vented outside, bath fans not used during showers, dryer dumping moisture into the house, water leak, wet crawl space, damp basement slab, humidifier malfunction (I would shut the water off to the unit), lots of occupants, lots of cooking without the range hood, etc etc. Last few situations I've looked at with high indoor RH were due to not using the bath fan while taking lots of showers. In MN in the winter you should easily be able to keep the interior dry.

Nice Replies, particularly Don's.

Remember, it might not be just one thing, might be a combination.  

  • SF, 
  • Occupants, 
  • Blower Door, 
  • Constant Fan Measured CFM, 
  • Indoor Dew Point, 
  • Thermostat Settings, 
  • Signs of Bulk Moisture, 


You'll probably get pointed in the right direction (or directions) with that information.  

BTW:  Communicating furnaces solve the "humidifier setting" problem.  They pay attention to outdoor temp and indoor rh, and manage accordingly.  Might want to learn about them - once you begin to understand all they do you are unlikely to sell anything else.   At the very least - always offer/recommend - it's a powerful CYA if they select a cheaper option that results in unanticipated problems.  

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