Humidity buildup
I have a net zero post and beam SIP house in coastal Maine (Zone 5). The house is two post and beam frames tied together with an 8×12 stick built connector. South facing, passive solar with cement slab floors. The main house which is 22×26 and includes a 10×14 loft includes kitchen and bath. It has a Fujitsu heatpump and a Panasonic Whisper Comfort ventilator/heat exchanger. NO HUMIDITY problems on this side, the Panasonic does a great job of keeping humidity in the 35-45 % range.
The 16 x16 post and beam framed bedroom is another story. It is heated with a ecotile (we like a cold bedroom). There are two 7’x2′ closets along the north wall, otherwise open. Humidity consistency in the 50-65% range. Finally, after using a dehumidifier (noisy, energy hog) last winter, we installed another Panasonic Whisper Comfort energy exchange ventilator (good for 900 sq.ft) set at 20 cfm. It has been a week, the best it has done, is bring the humidity down to 48 % one day, the rest of the time it is 51-52 %. I would have thought it would make a dramatic difference in this 16 x16, 14/12 pitched cathedral ceiling room.
What am I missing? Thanks in advance for shedding some light on this issue.
Valli Geiger
BAC Graduate in Sustainable Design
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Replies
If you want to reduce humidity, use a HRV, not an ERV.
Accounting for latent energy, a dehumidifier can be fairly efficient in Winter (depends on temp and dew point).
Hi Jon, thanks so much for your reply. It doesn't make sense to me. This ERV is bringing cold, dry air into a warm, moist space while expelling warm moist air. This suggests to me that mixing cold dry air with moist warm air should decrease the RH in the room. The larger main living space, despite kitchen and bathroom, has much lower humidity using the same ERV.
Thoughts?
Valli
Unlike an HRV, an ERV transfers much of the moisture you are trying to get rid of into the incoming air stream. So the incoming air is moister than it would otherwise be.
Setting it to 40 CFM will help some. In cold weather, I'd maintain 30-40% RH@70F.
A timer on the dehumidifier is an option for noise.
The elephant in the room question is. Where is the moisture coming from?
I know sum of my questions are silly but I have to ask sometimes you get silly answers.
Do you have and run exhaust fans in your kitchen and bath when you are making moisture?
How many house plants do you keep?
Do you have a damp basement or crawlspace?
Did you say what temperature you are keeping the bedroom?
Is this your first heating season in the house?
Walta
Hi Walter, good questions.
We run exhaust fans in the kitchen and bathroom when. making moisture
We have a huge rubber plant in our bedroom and 4 plants in the stick built connector
Cement slab in the bedroom is on 4" of bluebird over dirt with crawlspace wall
We are keeping the bedroom between 55-60 degrees, main living area is 68-72 degrees
This is our 2nd heating season.
Thank you for asking!
Valli
Valli,
Might be just the temperature you keep the bedroom at. If your main area is 70F 45%RH the same air in a bedroom that is at 65f would be at something like 55%RH.
I think your idea of adding in ventilation to the bedroom is a good step. if you reduce the ventilation in the main space and increase it in the bedroom, it should help lower the humidity there. An ERV removes moisture, not as fast as an HRV, in the wintertime (moisture transfer ratio is something like 40 to 65%), running it more should help dry the room.
Hi Akos, you might have the issue, the bedroom is kept at 55-60 degrees while the main living space is 68-72 degrees. I will lower ventilation in the main space as you suggest. It just may take some time to remove moisture in a bedroom that has been too moist for over a year.
Luckily I have fans in both spaces at the peaks and have had no issues with condensation on the cathedral ceilings, but I have had mold and condensation on the double pane windows.
Valli
Valli, because warm air can hold more moisture than cool air, the absolute humidity in both of your spaces is actually similar. 70°F air at 35% RH is about the same as 60°F air at 50% RH, or 55°F air at 60% RH. If you raise the temperature in your bedroom, the RH should be similar to the main house. Unless I'm missing something.
As far as I know the Panasonic ERVs have an intake damper that closes below about 20°F. I live in Maine as well, and we have had plenty of sub-20° days in the last month. So your ERV is not actually bringing in fresh air.
I have heard of problems with trying to use ERVs in tight homes in our climate. The experts say that's what we are supposed to use, but using them sometimes results in elevated moisture levels. One reason I like Zehnder ventilation is that it's easy and affordable to change the core from ERV to HRV. I tend to lean toward HRVs as a safer choice, unless there are other systems in use to control humidity. I definitely prefer HRVs when they are being used to exhaust bathrooms in a busy household.
You can confirm the effect of more ventilation by cracking a window open.