We've been in our new ICF home for about 6 months now and just love it.  I think my favorite part is how quiet it is.  The wind is whipping outside but you could hear a pin drop in my living room.  My least favorite part is my current inability to control our relative humidity.  The summer months were fine (high 30's, low 40's) because we were running the AC all the time.  The fall was terrible because it was quite wet, so the humidity was consistently in the 50-55% range.  The winter (so far) has been a little bit better because it's so dry outside, the HRV is actually doing it's job.  We're back down to the high 30's and low 40's, but I still have a ton of condensation on my windows in the occupied rooms (kitchen, bathrooms, bedrooms).  Do you feel that the period of time where the humidity was in the 50's consistently (maybe 8-10 weeks?), I could have experienced any sort of mold growth in my walls or duct work?  Also, I'm hoping some of you guys can comment on what you see in your own homes and identify what kind of equipment you are using:

- Using an HRV or ERV?  If so, how do you control humidity in the spring/fall?
- Using mechanical dehumidifiers?  If so, individual units strategically placed or some whole house model? 
- Using any other special filtering systems? 
- Condensation on windows?  How are you dealing with this? 

I'm located in Illinois, zone 5.  Thanks so much for the replies. 

What type of mechanical ventilation are you currently running?

With your location, I believe a dehumidifier in your home would be mandatory with an ICF home. I believe an ERV system would be better for your application.

gainer - how long does your HRV actual run?
How often during the summer did you inside rh exceed your outside rh?
I'm a little surprised that you are getting "a ton of condensate" on the windows with the cooler and not cold temps.
Bob

Are you sure that your numbers are accurate? Mold should be OK at anything under 60% but condensation might require around 30% in the middle of the winter. Are you getting enough circulation to even out the humidity around the house?

I'm wondering about the numbers as well. 50-55% is not that bad, especially during the fall and spring where outside RH is high, and the HVAC system isn't running. In fact, I think that's the recommended range for RH is about 45%.

Condensation occurs when moist air contacts a cold surface. Cold air cannot hold as much moisture as warm air, so the warm air condenses when it hits a cold surface (dew point).

I also can't see where mold will be an issue. Concrete and foam cannot mold, since they are not a source of food for the mold, but if your RH is over 60%, everything else could possibly mold (ie wood, drywall paper, cabinets, furniture).

I have just recently moved into my house which is a full ICF structure.

I would recommend if you can to run your HRV constantly at a low setting with full operation above setpoint over the 20 on -40 off settings some units offer. I have not had any problems controlling the humidity, currently set at 45%.

Are you seeing condensation on specific windows? Only in the early mornings? How much condensation? A small strip at the bottom of the window about 1" wide? or the full length? What kind of windows? Double pane? triple pane?

Thanks for the replies. Right now the HRV is running off a humidistat set at 30ish on a dial. We debated the HRV/ERV at the time and ultimately I went with my HVAC guy's recommendation. I know that Clark lives relatively close to me and he ended up putting a separate dehumidifier for the spring and fall. I'm guessing I'll need to do the same thing.

In the summer I'm confident the indoor RH was typically lower than the outside RH. I didn't keep track to see if/when it might have been higher than outdoors but it is common for our humidity to be in the 80% range and up during the day all season long.

I am not positive the RH numbers are correct. I'm using a combination thermometer/hygrometer I got several years ago from Radio Shack. Do these things go bad?

Yes, only seeing condensation on specific windows - basically all the rooms we're using. We've got no condensation in the formal dining room or the unused bedroom on the main floor. The condensation is on the bottom inch or two of the window and is occasionally wet enough to drip down onto the bottom of the sash. It is definitely worst first thing in the morning. We have started opening the drapes/blinds when we get up which has helped a little. The windows are Jeld Wen Custom Wood casements, double pane, energy star, etc. I know they're not top of the line but I think they're pretty good.

Again, many thanks for your input and the discussion.

Condensation on the window is a symptom of the window being colder than the dew point of the interior air.  For copious condensation to occur on the windows with the conditioned space air at 35% RH/70F interior temps that means the interior surface of the window has to be well under 40F. (How cold is it outside, and what type of windows have you?)  It's possible that if it's a few rooms that are affected, those rooms aren't getting sufficient ventilation air. With better windows, better sealing/insulating around windows usually takes care of window condensation in zone 5 until it's quite cold outside if the space is kept under 35% RH.   Kitchens and bathrooms tend to be moisture source areas, and  using exhaust ventilation during bathing or cooking is usually required to keep those areas at or near the average conditioned space RH.

The mold hazard at 50-55% RH is low at any reasonable conditioned space temps. The dew point of 55% RF 70F air is ~55F, so if it's in the 50s outside there isn't much risk of condensation/mold in any building assemblies (attic, roof framing around windows etc.) but if it's in the 40s or lower there can be if there's an exfiltration path from conditioned space air through that assembly.

To control humidity in very tight houses when the outdoor dew points are above 55F requires lowering the ventilation rates and mechanical dehumidification (dehumidifier or mini-split with dehumidification mode, etc.)  In tight houses it's often possible to keep the whole place reasonably dry with a single 70pint room dehumidifier tucked away in the cooler parts of a house (typically the basement).  Whole house dehumidifiers are expensive and usually overkill unless you really need a high ventilation rate. For the same or similar money a mini-split with a dehumidification mode will get you excellent efficiency on dehumidification and provide more function (sensible cooling/heating.)

While water is a major component of the chemistry of making concrete, the quantity of excess water in freshly poured concrete is significant, and it can take a year or so to actually purge it from an ICF home.  The drying rates through EPS are slowed somewhat.  If there isn't a capillary break between the footing and stem wall and there is a lot of ground moisture (or poor perimeter drainage) a lot of water can wick up into the walls too.  What you're most likely seeing is a combination of the original excess moisture in the concrete plus moisture originating in conditioned space.

galnar-

You said:
"I am not positive the RH numbers are correct. I'm using a combination thermometer/hygrometer I got several years ago from Radio Shack. Do these things go bad?"

Since humidity is of concern to you, and since humidity is hard to measure, I suggest that you get a sling psychrometer to get a good measurement. I have four devices for measuring humidity around my house, two like you described (one inside and one outside), and one on the high-quality, new thermostat, and a static wet-bulb/dry-bulb instrument with two "mercury-type" thermometers. The readings vary widely. For example, the indoor cheap temperature/humidity sensor is reading 67 F/LL%, which means roughly 10% or less RH. The thermostat is reading 65 F/31% RH, and the static wet-bulb/dry-bulb is reading 65 F/39% RH. The cheap outdoor sensor is reading 25 F/52% RH, while two nearlby weather stations are reading 20 F/96% RH and 25 F/84% RH, and it has been lightly snowing outside. An engineering colleague who does design work for humidity control says humidity sensors are notorious for changing calibration.

I draw the following conclusions:
1. Humidity is hard to measure, and a single instrument should not be relied upon.
2. The cheap temperature/relative humidity sensors can be worthless for humidity measurements. My two cheap units always measure much lower RH than any other readings, and lower than believable by local conditions (e.g., a reading of 55% RH outdoors when it is rainy, water is dripping off everything, and it "feels" really humid).
3. A wet-bulb/dry-bulb instrument for humidity measurements is still probably the most reliable, although a sling psychrometer would be more believable than my static unit.

Lee, thanks very much for your recommendations. I'm going to invest in something more reliable.

Using the windows as your crude sling psychrometer, condensation on windows when it's not really all that cold out is always going to be a sign of high/very-high interior RH. If you have copious condensation on a double-pane window when it's 20-25F out, it's a problem. If it only happens when its 0F, maybe not.

If those windows are covered by curtains or non-air-tight blinds you may have increased condensation due to lower window temps, but I'm assuming that's not the case here.

An outdoor measurement of 55% RH is a meaningless number- 55%RH @ 35F and raining is an extremely different level of humidity than 55% RH @ 60F and raining, but either/both would normally rise much to higher than that over the duration of a rainstorm. Mounting the instrument where it gets heated by proximity to the house or exposure to sun can also skew the measurement badly, since "relative" humidity is only relative to the temperature of the instrument. I've had cheap humidity meters designed for indoor use go way off calibration (and permanently) after only a few weeks of outdoor (ab)use. The same cheap meters when used indoors tend to track more precise measurements within a few percent RH, at least in the middle range (25-75%) and haven't drifted appreciably. YMMV.

The windows are covered by regular old 1.5" horizontal wood blinds. Some of the windows in the kids' rooms are covered by those blinds as well as room-darkening curtains ('eclipse' is the brand name). The condensation is definitely worst in the morning (after the blinds and curtains have been closed 12 hours), and definitely worst in the kids' rooms. I'm wondering if something is wrong with the HRV or our HVAC setup. Even with this thing set to run constantly I can't get under 35% on my cheap meter. I've ordered a couple of new meters so I'm anxious to see how they read.

I have the solution to your problem. Move to Colorado where you have to run a humidifier in the winter to get your relative humidity up to 30%!

You have taken the correct approach of ordering some new instrumentation to measure humidity. Hopefully you have ordered a sling psychrometer, which is probably the only fundamentally based accurate way to measure humidity. Another fundamentally based instrument is a cooled mirror to measure dew points directly, but those are probably prohibitively expensive.

Your window condensation trends sound reasonable to me, with more condensation where you are using window coverings. Consider my situation with relatively dry indoor air. During the last week, we have had several nights where the temperatures reached around -10 F (-23.3 C) for lows. The thermostat is set to 67 F (19.4 C) during the day and early evening, and 60 F (15.6 C) at night, and the humidity control is set to 30%. (There is an additional control on humidity to reduce window condensation, but let’s ignore that.) Typically the indoor room temperature will drop to about 62 F (16.7 C) before the heat comes back on in the morning. I observe a thin layer of frost on the lower surface of the lower pane of the double hung windows. Is this reasonable?

At an indoor temperature of 62 F (16.7 C) and 30% RH, the partial pressure of water in the air is about 0.57 kPa (4.3 mmHg). The triple-pane windows where I see the most frost are R=3.2 (American units) and the room-darkening shades with side seals have been measured by me (http://www.residentialenergylaboratory.com/r_value_cellular_shades.html) to be about R=2. Therefore, the average air temperature between window and shade with an outdoor temperature of -10 F (-23.3 C) is 34.3 F (1.3 C). However, there is a considerable gradient in air temperatures between the window and the shade, with the middle of the upper window typically 3.2 F (1.8 C) warmer than the middle of the lower half over the range of cold temperatures where I took measurements. Certainly the bottom of the lower window pane is the coldest area (although I have not measured it), and the air in this region is probably at least 3.2 F (1.8 C) colder than the average, or 31.1 F (-0.5 C). Now this is the AIR temperature in the gap between window and shade, but there is a temperature drop across the air film next to the window, and this can be estimated to be at least 2.7 F (1.5 C), so the inner window surface temperature is about 28.4 F (-2 C). At this temperature, the saturation vapor pressure of water is about 0.53 kPa (4 mmHg).

Since the saturation vapor pressure at the window temperature is less than the vapor pressure of the room at 62 F (16.7 C) and 30% RH, it is reasonable that the water should condense, and since the window temperature of 28.4 F (-2C) is below freezing, it is reasonable that it should freeze. So for these windows and shades, it is reasonable to expect frost even though the room humidity is low. It is reasonable to expect the frost to disappear when the shades are raised, which is the case.

So my house is not overly humid, but with excellent insulating shades, I still get condensation and/or frost on triple-pane windows at cold enough outdoor temperatures. You are on the correct path to investigating your situation by getting better instrumentation to measure your humidity.

Hello galnar,

Lee makes some really good points.   Some additional things to consider:

---  I have seen this problem occur with wood drying in the conditioned space  (particulary with wall studs, wood floors, cabinets,  or wood paneling and even sheetrock).  Wood saturation is about 25% and not really considered dry until its below 19% or so.  Wood is extremely dry (like kiln) at about 10%.  Your wood will need to dry out one way or the other.  That said, I believe it should be dry by now (6 months).   Anyway, I have had to use a dehumidifier for homes with lots of wood -- e.g. wood floors.   

--- I don't think any measurable humidity can be coming from the concrete external walls since your walls are down to about 2% water after a month or so. The heat of hydration process will continue to chip away at the remaining moisture in the walls for many many many years but it's essentially trapped in the wall.

--- I believe an ERV swaps humidity between the airstreams and an HRV swaps sensible heat only?  This is from an earlier Dana post if I recall.   Having it running all the time, particularly if its not working could cause your HVAC to short cycle.  If it's dumping air in without doing its job your AC will absolutely short cycle.   If your AC short cycles it will not dehumidify -- it will never reach this work cycle -- most units need to run for 15 20 minutes or so to start pulling out humidity.   You might check that -- see how long it's running.

--- You may be "overtonned"  AC guys want to sell tight houses the same tonnage even though the old 450-600 square feet per ton rules no longer apply.  Most of our ICF houses here (foam foamed attics) operate at 1000 square feet per ton or so.  All - foam houses operate (around here) at 800 square feet per ton or so.   I run mine at about 1500 square feet per ton.   This is of course a very basic rule of thumb since windows and home orientation and etc will make a big difference.   The few "problem homes" I've seen with ICF are with overtonning.   If you have two units you can turn one off and run the other hard to see if it helps.  Or you can electronically disable your second stage if you have multiple stage units. 

--- You can also turn off your HRV and see what happens.  

Obviously, you should only change one thing at a time or you won't learn too much.  

If I had to bet I'd say your probably overtonned or you have an HRV that dumping outside air in without doing its job first.    Regards.

Posted By galnar on 27 Dec 2011 11:09 AM
The windows are covered by regular old 1.5" horizontal wood blinds. Some of the windows in the kids' rooms are covered by those blinds as well as room-darkening curtains ('eclipse' is the brand name). The condensation is definitely worst in the morning (after the blinds and curtains have been closed 12 hours), and definitely worst in the kids' rooms. I'm wondering if something is wrong with the HRV or our HVAC setup. Even with this thing set to run constantly I can't get under 35% on my cheap meter. I've ordered a couple of new meters so I'm anxious to see how they read.

The blinds & curtains are contributing to the problem, and may even be INCREASING energy use by charging the convection loop with the now much colder window surface. 

In order to avoid condensation with interior blinds requires a fairly tight air seal along the edges & bottom (difficult to implement on an operable blind even when attempted). Air leakage between room air and the space between blind & window is rampant with curtains or venetian blinds.

This has been studied fairly extensively by the folks in Alaska where window condensation issues are chronic and far more severe. A synopsis of what works, what doesn't and the relative contribution to energy use can be found here:

http://cchrc.org/docs/reports/windo..._final.pdf

Acu-Rite makes a series of small cheap battery powered humidity meters that are sometimes sold at box stores (or even Wal-Mart) that have reasonable mid-range accuracy that don't seem to drift a large amount over time.  I keep  a handful of these  ~$10 units that I'll sometimes place in different places of a house to monitor & track a house over a week or more.  They are not precision instruments, but they'll store the maximum & minimum RH & temps sensed, and aren't way off on accuracy in the mid-ranges.  Having more than 2 is useful for figuring out which one has lost calibration, should they ever disagree by more than a couple of percent after being side by side for an hour or two.

Dana1 said: "The blinds & curtains are contributing to the problem, and may even be INCREASING energy use by charging the convection loop with the now much colder window surface."

But the heat transfer rate equation for energy transfer across the window states:

q = k*A*(T2-T1)/L

q = heat transfer rate (W or Btu/hr)
k = thermal conductivity (W/(m degC) or Btu/(hr degF ft))
A = area (m^2 or ft^2)
T2 = higher temperature (deg C or deg F)
T1 = lower temperature (deg C or deg F)
L = thickness of material (m or ft)

Everything is constant except for the temperature differential and the heat transfer rate, so,

q ~ T2 - T1 = T_inside window - T_outside

If the inside window temperature is lowered by the presence of the blinds and curtains on a cold night, reducing the temperature differential, then, by the equation above, they are reducing the heat transfer across the window. If they were setting up some sort of chimney effect that was increasing the heat transfer across the window, that would require that the inside window temperature to be higher so that the temperature differential was increased (relative to the no shade case). If the air velocity on the inside window surface was high, then you could argue that the boundary layer on the inside of the window was reduced and that would affect this argument, but the air velocities will not be that high.

The CONDENSATION problem is made worse by the curtains and blinds lowering the inside window temperature, but that is a different issue from the heat transfer rate.

The heat of vaporization of the water condensing on the window and the change in emissivity of the water vs. glass are not accounted for in your simplified window model. There's an in-depth paper on the subject at one of the Nat'l Lab websites (Lawrence Berkeley?) somewhere- I'd have to dig to find it.