Hi all, I'd like your advice and counsel on the following situation.  New construction, about 1 year old, climate zone 6 (Minneapolis), ~4000SF, <3ACH, basement radiant slab and forced air on main and upper levels, typically one person in the house, avg windows, 90+ furnace with AprilAire humidifier, HRV, Honeywell thermostat.  Currently global humidity set at 35% and window control at 3 (was at 5).  System tuned up and HRV intakes cleaned mid jan.  (I can get more detailed specs if needed).

Anyway the problem: Humidity level and resulting window condensation.  I've been watching this house/system on and off all winter during periodic visits (no I don't have web access to the T-stat) after customer experienced (and was alarmed by) major window sweating on initial cool down in December, followed by large amounts of that water turning to ice under the lower sashes (these DH windows have a design that includes a trough on the sill that interlocks with the bottom of the lower sash and can hold water).  As I mentioned since the problem was reported had the system "maintained" by the installing HVAC contractor and reset the thermostat to 3 for window control (per HVAC contractor advice), dried out the sills manually with a towel during a warmup in January and all thru the rest of February generally noted that the system could not achieve humidity setpoints in the morning when it was colder so therefore allowable humidity level lower (i.e. actual humidity above setpoint) and therefore dry out the house to the lower humidity level that the window control was calling for, and therefore the windows continued to sweat (thereby continuing annoying the customer).  I've since learned that the HRV runs it's typical cycle based on ASHRAE62.2 and no more say to lower the humidity, therefore the only way the house can "lose" humidity is via the HRV exchanging lower humidity outside air with higher humidity inside air, and of course leakage and/or manual point source fan operation.  However during typical day if it warms up enough outside that the window control algorithm allows a higher humidity level sometimes the humidifier comes on an adds humidity to the house, might even happen each day.

Theorize that due to the tight house there is no way the house can actually "lose" enough humidity to get down to the lower levels required to keep windows from sweating (may not create a real problem but does create customer annoyance). 

Is this typical system performance that you have observed in a similar house?  What other questions would you have?

While this winter is over for the purposes of solving this problem what would you do to address?  Would you lower global humidity?  Turn off the humidifier?  

NOTE: I realize that the condensation on the windows is just annoyance, the larger concern is the condensation that could be present in the walls and ceiling, but it's what the customer notices, any other thoughts like have you used the T-stat to drive the HRV to run continuously when the humidity level is above setpoint?

I appreciate your thoughts and hope my description above is useful enough to describe the situation.

Thank you!

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Sorry this will be short but what are levels throughout house - i.e. could it be concrete slab still drying out, customer use bath fans properly, exhaust hood, etc... Just how well were those windows installed - air leakage? 

Fix the issues that can be - use bath fans on timer, dehumidifier in basement if needed for slab, etc...

You will always have condensation if the temps outside are cold enough http://thehtrc.com/2015/faq-window-wall-condensation-iced

Sean, thanks, I have not measured levels at other areas of the house and I suppose I could even question how accurate the Thermostat readings are, but the condensation tells the story on it's own.  As far as using the bath fans that's a fair question which I haven't asked enough but in this person's case if the bath fan would run on it's own when needed I think that would be best vs. counting on the person to turn it on.  Also, window install done by my company but unsure of the quality.  Good link.  

The window glass / sash intersection is a weak point for condensation. Whether or not the walls (structure) are at risk depends on the ratio between exterior and cavity insulation.

One thing you should mention to the homeowner is that window coverings work to isolate/insulate the windows from interior ambient air, thus increasing the potential for condensation. When it gets cold, most folks tent to draw draperies to better insulate the chill effect of windows. Leaving the drapes open during the coldest nights, at least for the windows that tend to sweat, will mitigate condensation, but there will be an energy penalty.

Increasing the ventilation rate above the ASHRAE level would likely impose an even greater penalty. The phrase 'shooting oneself in the foot' comes to mind. You could accomplish the same thing by turning down the humidity -- without an energy penalty.

David, thanks for the additional details/thoughts, just shows again how effective exterior insulation is in so many ways. 

Other than one window in a daughters bedroom there are NO WINDOW COVERINGS in the house!  But I get the point.

Appreciate the comment on increasing the ventilation rate, appears it is an HRV as well so we can't count on that to help this situation.  Lower global humidity is going to be a change I make.

Hi Steve,

             Check out my website plusaire.com.  I had a house like yours in 1982 but there was nothing that was available to fix it.   "Experts" said an HRV would work but when I did the math I found that it didn't come close.   I designed a Plusaire and went into production in 1992.    There are 5 sizes to suit homes upto 10,000 sq.ft.   The whole problem with the homes is a bad lack of ventilation.

Tony Baptist

Plusaire Inc.

Tony, thanks for the link and thoughts.

If shades or blinds are drawn in the evening, that is the primary culprit, as David noted. Beyond that, I recommend testing. I have been using Acurite's Smarthub, now the Access, with several remote temp./RH sensors, which allows you to monitor those parameters throughout the house.. You can also look back at trends. I always include an outdoor sensor as well. I had a client in a similar situation, with 2.5 ACH50, 3200 sq.ft. of conditioned space, radiant heating with 4 zones, and chronic problems with condensation, primarily in the Master Bedroom.

There was NO regular ventilation and the problem was exasperated by the fact that the client kept the Master Bedroom about 10-12 degF cooler than the rest of the house. The house had 2 occupants, 2 dogs, and a few plants. No other moisture source. Bath fan was run during a shower and for a short time after. I scanned the floors with my IR camera and found notably less radiant tubing in the area in front of Master Bedroom windows. I measured the flow of the 3 bath fans. All three running at the same time moves about 225 CFM. After much cajoling and explanation, I finally got her to run the fans 24/7. It took several days to reduce the RH in the house from upper 40s to below 30% RH. I replaced the bath fans with Panasonic 2 speed bath fans, with slightly higher flow rates (I could not talk her into an HRV).

After months of monitoring, I concluded that the RH had to be kept lower to avoid the dewpoint in the Master Bedroom, except for the very coldest nights. If there were several days of warmer (30s & 40s) and then the temperature dropped to the single digits or lower, the RH in the house was too high. It was difficult to balance that RH to avoid the dewpoint, with such temperature swings.

You have to monitor the temp/RH and disconnect the humidifier.

I look at the ASHRE standard as the capacity of the ventilation system, and NOT the rate at which you must ventilate. Control ventilation to control RH to avoid the dewpoint on cold nights on windows.

Measure flow rates of the HRV and be sure that they are balanced.

We should be using RH, CO2, VOCs and particulates to control ventilation, but that is for another day.

Excellent post, Brad. Your comment about master bedroom setback is an example of a topic every high performance practitioner should discuss with clients. I love to sleep in a cool bedroom at night... I even open the window in the dead of winter if solar gains warm the room above 70F. But I live in a dry climate so excess RH is the least of my concerns.

In most parts of the country, I advise clients when they build a super tight home, they can't close off rooms or otherwise allow areas to cool off more than a few degrees below the setpoint, depending on how high RH is allowed to get.

In most cases, indoor RH can be well managed with bath, kitchen and laundry spot exhaust, without having to amp up continuous ventilation. Stop moisture at the source. And never rely on an ERV as primary exhaust in a heavily used bathroom. It will just kick a good portion of that moisture back into the house.

Brad, WOW, I believe that's similar situation to what's happening here and I appreciate the detailed explanation.  Per other replies I've made, for now I'm going to reduce the global humidity setting from 35 to 25.  I'm also going to verify and setbacks in use.  The time for experimentation is over for this year (the latest snow and cold not withstanding).....!

You may not be able to make the problem go away completely in Zone 6, but bringing it under control shouldn't be hard. This house still has tons (literally) of construction moisture to lose, in the concrete, the framing, the drywall, etc. (Most builders are getting better about keeping materials covered on site, but one bad thunderstorm after framing but before the roof deck goes on is all it takes to saturate everything.) The last thing this house needed this past winter was a humidifier. But I'd bet the problem will get better as the house dries out.

In the meantime, I don't think we know enough to be definitive.

There's only one occupant -- they CAN develop a lot of humidity with exactly the "wrong" lifestyle -- long, hot showers, lots of plants or open aquaria, lots of cooking (especially gas ranges) without the range hood. But you don't report any of that -- so it's hard to see that the "operating load" is that high. That's why I'd bet it's construction moisture. 

Is there a thermostat setback involved, and is window condensation influenced by time of day related to the setback? It sounds like it's a 24/7 problem, but deep setbacks (when the air handler doesn't run for hours) and insulating drapes can really affect window condensation.

I hardly ever trust off-the-shelf humidistats -- my calibrated, NIST-compliant psychrometer hardly ever agrees with any of them. I always measure, always with my trusted, calibrated device, in many spaces.

What are "average" windows in a 4,000 SF house? The u = 0.30 windows (wood frame, double pane with argon) newly installed in our house are fine at 0 degrees outside and 35% RH inside. But my place is 100 years old, so I think most of the construction moisture is gone! The window closest to the bathroom fogs some just after a shower, but it's dry in 15 minutes. 

What is the ventilation system? Double-check the specs. If the the ventilation system is actually an ERV, rather than an HRV, running it more won't help dry out the winter moisture.

In this climate, I'd just run an exhaust-only system at moderate rates 24/7, and turn off the humidifier. It's hard to believe that one person could develop enough humidity to create this problem.

Don, windows are as you mention u=0.30 (Performance Information: U-Factor 0.30, SHGC 0.27, VLT 0.50, CPD PEL-N-35-00259-00001, Performance Class LC, PG 30, Calculated Positive DP)

Appreciate your comments about the time it takes to dry out building materials, that could be a very significant factor.  I don't know if I will have access to this home next year, but if so it will be interesting to see what changes occur.  Per above I think the air exchanger is a Honeywell 150cfm HRV, checking with HVAC contractor as final proposal only says: ERV150 air exchanger.  If this is the case I can't expect this to reduce humidity, which makes complete sense and speaks to not adding humidity to begin with.

I agree with you on it being hard to believe one person could do enough to cause this issue so the other factors help explain it.  

The thing is my employer sells the humidifier based on keeping the humidity up to protect the wood floors/etc. Need to do education there as well.

I've appreciated all the advise and info from this forum!  Happy spring everyone from snowy MN.

I have been using inexpensive digital temp./RH meters from Acurite for 8-10 years. I give them away when to energy clients and in the past few years I have been using dozens of their remote reading meters. I buy 6-10 at a time, put batteries in them and put them on an interior book shelf. I have found that over 90% of them read within 1 degF and within about 2% RH, which is about the same standard as a calibrated meter, which I also use.


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