Indoor Relative Humidity and ERV Conundrum

Indoor Relative Humidity (RH) and Temperature Reading using only ERV

UPDATED: 05-12-13

These past seven days have been very humid.  As I write this, it is currently 61F outside with a relative humidity of 98%.  Earlier today, the RH was 100%.  Dew points have been in the mid to upper 50s pretty much all week.

These conditions have ultimately had an impact on our interior relative humidity levels.  For the past five days the indoor relative humidity levels have remained consistently elevated--ranging for the mid to upper 50s.

While the operation of an ERV typically serves to reduce indoor humidity levels, we are at a specific point where both the indoor and outdoor conditions are such that our ERVs are not reducing the interior humidity levels at all and may actually be contributing to even higher RH levels. 

Under a more typical situation, the higher humidity intake air supplying the ERV would have some of its moisture transferred to the exhaust air (generally of lower humidity) resulting in dehumidified fresh air coming into the house.  However under the current conditions, the indoor relative humidity levels are sufficiently elevated--as a result of occupant activity--to create a circumstance where our ERVs have little or no means of transferring humidity out of the supply air into our exhaust air. We have essentially reached an equilibrium point.

To make matters especially challenging, since the interior temperatures remain quite moderate (in the low 70s), it is not preferable to use our ductless mini splits in dry or air conditioning mode as their set temperatures would have to be in lower to mid 60s for their dehumidification mode to effectively function and we would prefer not to have to actively cool the house just for the sake of achieving lower humidity.

So we have an interesting situation here.  Indoor temperatures are fine.  Indoor humidity levels are elevated.

Interestingly enough, I have been sharing our conditions with another passive house owner who is experiencing nearly identical indoor and outdoor conditions over the same period of time.

I had been operating our ERVs at 95% capacity to promote cooling and ostensibly lower indoor humidity levels, however given this specific combination of indoor and outdoor temperature and humidity conditions, running them on "high" may actually be exacerbating the situation.

That's what Barry Stephens of Zehnder America believed when I explained my situation.   Apparently as the flow rates of the ERVs increase, the efficiencies of temperature and moisture transfer decrease.  In other words, exchanging higher rates of air may actually hamper the ability of the ERV to evacuate moisture from the interior of the home.

This runs contrary to the notion, under "normal" circumstances, that one should operate an ERV on high during times when interior moisture is created (ie; when taking a shower).  Zehnder's enthalpy exchange efficiencies are maximized at a flow rate of roughly 50% of maximum capacity.  As per Barry's suggestion, I lowered the flow rates back to their standard setting.

Passsive houses, I believe, tend to have higher indoor relative humidity levels than those of conventionally constructed homes and they can sustain these high levels of RH in safe manner.  But since the need for active air-conditioning is significantly reduced, I also feel there may be a greater correlation between indoor and outdoor humidity conditions during these times (swing seasons) of the year.

This morning I tried something new; I opened the window in our bathroom while I took a shower.  My feeling was by doing that, I would reduce the amount of additional interior moisture introduced.  I believe I was correct in doing this as the impact of the shower on interior RH appeared minimal.

Clearly, the operating characteristics of any given passive house is closely tied to its localized climate.  In other words, the indoor temperature and humidity profiles of two similarly designed passive houses would be entirely different if one were located in southern Florida and the other in northern Alaska.

In our circumstance, it appears that humidity control may be the greater challenge than temperature control, at least during these periods of extended high outdoor humidity levels.   There are whole-house dehumidification solutions available, such as those from Ultra-Aire, but they are not inexpensive and these systems consume a fair amount of power to operate.

Our expectation was that we were not going to require much A/C during the warm months.  It would be disappointing if we ultimately needed to rely on it more than we anticipated--not for its cooling, but for its dehumifidication.

At this moment, I am not considering any "corrective" action.  Even during these times of elevated humidity levels, the house is still tending to settle in around 55-56% RH.  Certainly an acceptable level.  My preference would be for the RH levels to range from the mid to upper 40s.

As I write this last paragraph, it is 71.9F and 56% RH inside.   Weather forecasts are predicting a return to much lower levels of outdoor RH by early next week.  I anticipate our indoor RH levels will return to the mid to upper 40s.  Now that the weather is clearing, if the interior temperatures rise from the lower 70s and the humidity levels do not appreciably drop, I will then consider running the mini splits to provide some dehumidification.  I will follow up with another post.

In the meantime, I would be interested to hear from any other passive house owners sharing similar experiences.

UPDATE 05-12-13:  Today turned out to be an absolutely delightful spring day.  Outside temperature is 59F and RH is 33% with a dew point of 30F.  After opening a couple of windows, the interior relative humidity level hit 34% in a manner of minutes.  Since the  house is so air-tight, it doesn't take long at all for the effects of an open window or door to be felt throughout the home.