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Winter humidity with wood stove

John Cleveland | Posted in GBA Pro Help on

Good day. We are renovating a horribly built ca. 1992 Cape in southern Indiana, Zone 4. During the winter we rely on our Vermont Castings Encore stove for heat, and love it! Since the indoor humidity drops quite low, sometimes to 10%, we do not vent the shower steam through the ERV so as to add some humidity to the house. There are only two persons, my wife and I, so there is not that much steam, and it quickly spreads through the house.

There is no interior vapor or air barrier, though the exterior of the house is sheathed with 1/2″ iso-board with a shiny side faced in. They gave that a little thought — very little: The seams are “sealed” with duct tape. I would rather have had some rigid sheathing to keep the house from racking. But that’s another story.

So, keep the steam in: Good idea? Bad idea?

Thank you.

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Replies

  1. Robert Sanders Jr | | #1

    I used to keep a tea kettle on the wood stove all the time. That did more than anything to help.

  2. GBA Editor
    Martin Holladay | | #2

    John,
    If the indoor relative humidity of your home is too low, that's usually a sign that your house is very leaky. If that's the case, you probably don't need to run your ERV very much at all. If your home is leaky, the best solution is to find the air leaks and seal them. A blower door can be very helpful for this work.

    As long as you don't have any condensation on your windows, there probably is no harm in having a shower without running the exhaust fan.

  3. John Cleveland | | #3

    Thank you. I suspect the house was leaky, but that we have plugged many of them with new doors, a sealing rubberized sheet on the back of the electrical outlets, and believe it or not, the warranty on the Anderson windows had not expired so they gave us new sashes for all deficient windows. Good on ya, Anderson.

    The ERV runs on auto, as I said, partly because we live in what most folks would call the basement of the house. I believe they call them now a berm house. We had a problem with RADON when we moved in, so between RADON sealing the slab and the ERV, I hope we are safe.

    The only windows to gather condensation are of course the bathroom and the windows where the argon inside has gone to wherever is goes when it is gone. The center of the glass has collapsed, leaving a fun-house style window for our enjoyment, but the new windows are right again.

    I don't believe for one minute that this house is now tight, but it has been improved upon by at least 500%.

    My prime concern is the health of my wife and myself.

    Cheers, All.

    john

  4. John Cleveland | | #4

    As I re-contemplate your answer, Martin, I must remind that we live surrounded by trees in a valley, or hollar, as they call it here. Most mornings in the winter, the outside humidity is well above 50%. In the summer we usually see well above 90%. i do have an outside weather station.

    As a retired yacht master, I am sensitive to what equipment is running when, so I can say that summer-wise, our dehumidifier (did I mention that we have one of those?) runs about 30% to 50% of the time. In the winter, almost never. The dehum has reduced our air conditioning bill about the same. But then we keep the indoor air temp at 78F.

    Does that change one's perception of our predicament, or lack thereof?

    thanks again

  5. GBA Editor
    Martin Holladay | | #5

    John,
    The only way to determine the leakiness of your house is to hire a home performance contractor to conduct a blower door test.

    The biggest air leaks in most homes are hidden. Weatherstripping around doors and installing gaskets on electrical outlets are two measures that are known to be almost useless. For more information on air sealing, see Questions and Answers About Air Barriers.

    If your ERV is running on "auto," and your indoor RH in winter is 10%, then you are probably overventilating your house. I would experiment with running the ERV for fewer hours per day.

    There is no need to run a dehumidifier in winter. For more information on dehumidifiers, see All About Dehumidifiers.

  6. David Meiland | | #6

    I second Martin's recommendation for a blower door test. You will find all kinds of leaks you weren't aware of, and can probably get your humidity up, comfort up, and heating cost down.

  7. Expert Member
    Dana Dorsett | | #7

    The outdoor relative humidity (RH) is meaningless unless accompanied by the temperature to which it is relative. Think "dew point", which is a measure of the absolute, not relative humidity. The RH of the ventilation air at the INDOOR temp (not the outdoor temp) will tell you something about how over-ventilated you are.

    An indoor RH of 10% @ 70F has a dew point of about +13F, well below the average mid-winter outdoor dew point in say Bloomington, of about 25F. Pull up a dew point graph for your own ZIP code on the weatherspark.com, zoom out to cover a whole winter, and use the cursor to estimate the Jan/Feb mean outdoor dew point:

    http://weatherspark.com/#!dashboard;a=USA/IN/Bloomington

    While the outdoor dew points will vary and most likely drop to 13F and below fairly often, if the indoor dew point is tracking it very closely is an indication of a VERY high ventilation rate, and dialing back the ventilation rate of the ERV may actually improve overall indoor air quality. (Air that is too dry is low-quality air.) Unless you really need a high rate to keep the radon at bay, back off. Running it on "auto" doesn't tell us anything about it's duty cycle, since we don't know if that's just a pre-programmed duty cycle or running on some sort of sensor.

    For reference, in my own New England location the average outdoor dew point is about 13-14F, and though I don't have anything like the tightest house on the block (a 1923 antique, plank sheathed w/clapboards, no vapor barriers of any type) the indoor RH never drops below 30% RH @ 70F. If you have foil-faced goods on the exterior layers you have a very powerful vapor barrier, but it takes copious air to bring the indoor RH down that far in a usually-occupied house with two breathing/bathing/cooking humans inside.

    With half-inch (R3) foil faced iso on the exterior side of 2x4 construction it would be fine to run the interior RH at ~35% RH (~40F dew point), which is a lot more comfortable and healthy for the humans inside. If it's 2x6 construction you may end up with moisture accumulation issues in the wall cavities with indoor dew points that high, but that would have to be carefully assessed for your actual local climate. But if there is no plywood or OSB sheathing, and the iso is only "sealed" with duct tape, and insulated with low or mid-density fiberglass batts, it's probably pretty leaky, and may require quite a bit of air sealing attention to get it to where it stays at 30% RH or higher even with the ERV off.

    The wood stove has (next to) nothing to do with the indoor humidity- it neither adds nor subtracts moisture from the air, and the infiltration driven by combustion air volume requirements are very small compared to the overall ventilation rate.

  8. John Cleveland | | #8

    Thank you for that explanation. I will order up a door test and see what we find.

    For the record, the first floor is 2x6 construction with R-19 fiberglass-- not properly installed, except where I have gotten to it--and the second floor is 2x4 gable-end walls with R-13. Rafter bays, which make up the "ceiling" are R-19.

  9. John Cleveland | | #9

    As I ponder -- in the summer months, it is very humid here in the hollar in the woods. (Feels like Middle Earth) Our first year without a dehumidifier we suffered and the air conditioner ran almost continually. the dehum takes the load now, but left off, the indoor humidity would soon approach that of the outside. A good indication then of a leaky house?
    thanks again.

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