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Community and Q&A

Using a smart thermostat to manage vapor drive

tech1234 | Posted in Energy Efficiency and Durability on

I’m looking for input on an idea I had… I have a 16×16 insulated but unconditioned breezeway (fully enclosed room between garage and house, exterior doors on both sides) I am having moisture and condensation issues, likely coming from the mudroom type use. shoes/boots/winter gear/sports gear/air coming in when the house door is opened.

-house is a double stud wall, cellulose, cdx
-garage is 2×6, fiberglass batts, unconditioned

Breezeway is 2×6 walls, 2×8 rafters, cathedral ceiling, vapor diffusion ridge membrane, over vented metal roof, cdx (3m taped), fiberglass batts. (very good air sealing)

NH zone 5

My energy nerd idea to solve my structure wetting issues as well as make it nicer to not have to put on cold boots and jackets was to install a electric resistance strip and hook it to a smart thermostat to heat the room up at peak use times. 8am and 6pm. The cheap yankee in me dies a little to think about heating this space 24/7. I’m thinking set to 40f or 50f base line and then ramp up at peak times to drive moisture out of the structure.

Am I over thinking this? I am missing something? any other thoughts?

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  1. Expert Member
    NICK KEENAN | | #1

    Under most circumstances it would be cheaper to pull heat from the house rather than use resistance heating.

    1. tech1234 | | #2

      The house is a double stud wall "pretty good house" .8 ach50 heated by one 12k hyper heat (and one 9k upstairs for cooling). Can you think of a reasonable way to do this? Seems like doing this would be a nightmare

      1. Expert Member
        NICK KEENAN | | #3

        Vent fan in the breezeway with electrical damper between breezeway and the house. When the fan is on open the damper. At 0.8 ACH50 you're going to need mechanical ventilation of the house anyway.

        That will help warm the breezeway. The question is whether the air that you're bringing in has a lower moisture content than the air going out.

        I don't think vapor drive is going to do it for you, in your climate insulation should have a vapor barrier on the interior side which would make vapor drive difficult.

        1. tech1234 | | #4

          House has a balanced 100cfm ERV. The house has intello. Breezway has no poly as I was trying to keep it vapor open. This damper setup is an interesting idea. A quick load calculation for this 16x16 space comes up with 12k btus which is the same as my whole house! (although prob over sized) so in theory the damper setup would not work (also wouldn't the house need makeup air as well?)

          Does it not seem more reasonable to just add intermittent (or constant) space heating (and possibly balanced ventilation) to the breezeway?

          Do you have any info in regards to intermittent heat "spikes" and their ability to dry a structure?

          1. Expert Member
            MALCOLM TAYLOR | | #6


            The humidity needs to be dealt with. If you heat the breezeway during the day, the warm air will be able to hold the additi0nal moisture. But depending on how much it cools at night, it may just deposit that moisture as condensation again.

  2. maine_tyler | | #5

    I was going to ask about ventilation. As in, have you considered just adding ventilation. It seems like you have. Are you only concerned about the condensation, or are you also wanting the comfort factor associated with heat? If the latter, then obviously you need some heat source. If the former, I wonder if ventilation alone would be enough.

    It might also help to clarify what 'moisture and condensation issues' means. On glass? Walls? IN the walls?

    1. tech1234 | | #10

      Tyler, I am mostly concerned with the moisture and its effects inside the walls. If I didn't know better I would just ignore the window condensation and slow drying and high RH. Although I have not cut the drywall to take a look yet. (also maybe a slight moisture drive in reverse into my double stud house wall???) As far as comfort I thought that could be a nice side effect. I actually was planning to build a freestanding jacket and she cabinet that would be heated for drying and comfort reason but I realized that would just make the moisture issues worst without conditioning the breezeway entirely

  3. charlie_sullivan | | #7

    You could simply ventilate with outside air. If that makes it too cold, you could ventilate without outside air through a small HRV (not ERV). If it's still to cold, you could add some heat. If the heat is sufficient to keep it above freezing, you could use a small desiccant dehumidifier to supply the heat and remove some more moisture. You'll get more heat per unit electricity with that than you would with an electric heater and you'll be removing moisture.

    Or you could just run a desiccant dehumidifier you are able to keep it above freezing, but in a NH winter I think the HRV would be a better bet for dehumidification.

    If you wanted to get super nerdy, you could install a system that would move heat from the house to the breezeway without depositing any of the high-humidity house air there. That could be a small HRV system installed wrong, so that one air stream goes from the house back to the house, and another from the breezeway back to the breezeway, with heat that it stole from the house added. That would be in addition to a second one for outside ventilation. I'd do one for outdoor ventilation first, and then if you need an energy-nerd project you can set up something to steal heat from the house.

    Here's a small $700 HRV system. I don't know if there are smaller ones available. Hmm, I have a few tiny ones stashed away in my garage--the Bionaire window mount ones that are no longer made.

    1. tech1234 | | #9

      Charlie all great points! I spaced the HRV advantage in this situation.

      Also I was unaware of electric desiccant dehumidifiers until you prompted me to do a google search. Cool stuff.

      Your wrong way HRV idea is pretty trick!

  4. tech1234 | | #8

    Malcom, that is a good point and the more I think about it I agree ventilation has got to be part of the strategy (or dehumidification). I guess I was thinking if I heated the air it would "hold" the moisture and then drive it out through diffusion because the winter air here is so dry in comparison and it would try to find an equilibrium. Either I don't fully understand the physics or maybe it just wouldn't do it "enough"

    Would you recommend a small *HRV or a dehumidifier in this situation?

    1. Expert Member
      MALCOLM TAYLOR | | #13

      I don't think relying of diffusion would work if it is heated intermittently. The dew point in the wall is going to move a lot.

      I'm not entirely sure of the physics involved either. I'll defer to Charlie and others as to what the most effective solution would be.

    2. maine_tyler | | #14

      One way to think about the drive is via temperature of the wall surfaces. Heating for a short time may evaporate more water (especially water near the heat source) and thereby increase the vapor pressure in the room (which does induce an outward vapor drive) but the question is will the amount of heat you're adding significantly warm the wall temperature (throughout it's profile)?

      So the options appear to be: lower the dewpoint of the interior air, or move the wall temperature gradient outward so that the dewpoint location is not a problem. (whether it's a problem is a complex question). Or a combination of both.

  5. user-6975738 | | #11

    If you are worried about thermal drive of moisture into the walls/ceiling structure, then the more temperature fluctuations that occur the faster the moisture will build up in your insulation. But I think you are missing the point that the source of moisture is the warm air from your house, which is condensing on the cold structure of your breezeway. In commercial construction we prevent building humidity from intruding into the roof insulation by adding a vapor barrier.

    To control condensation, you need to reduce the moisture, or raise the temperature. There are some good ideas above with the HRV. Another green approach would be to add a solar panel, which stores heat via an electric blanket on the slab.

    First and foremost, prevent mold now by opening the door between your breezeway and garage to allow the moisture to escape more readily, and try to keep the door to the house closed.

    Good luck, Wells

    1. tech1234 | | #12

      Wells, do you think the house air or the snow covered boots (etc) are the main source of moisture? It may help me consider my plan to remedy. My first thought was that it was probably 80% the boots and whatnot, but I am also uninformed on how much moisture is escaping the house when opening the door.

  6. maine_tyler | | #15

    I'm curious if those more familiar with HRVs/ERVs think a Lunos e2 (or similar through-the-wall- ventilation product) would be a good fit here?

    1. Expert Member
      NICK KEENAN | | #19

      I think you'd need a heat source too. Ventilation by itself doesn't usually help with humidity issues. Ventilation plus heat works. So does mechanical dehumidification.

    2. charlie_sullivan | | #24

      Lunos provides partial moisture recovery, kind of halfway between ERV and HRV. So it's not the best fit for the application although it would do something.

  7. Expert Member
    NICK KEENAN | | #16

    Second the idea of a dehumidifier. Dehumidifiers are highly efficient space heaters, all of the latent heat that is removed when humidity is removed is returned to the room. So as heaters dehumidifiers tend to have a COP of around 3. So right off the bat you'd be getting three times as much heat for your money as with resistance heating. They also have humidistat control so they only run as much as necessary.

    The downside is they don't work when it's cold. The cool side needs to be substantially below room temperature, yet above freezing. So most shut off at around 50F. If the room is cold for long stretches that may be a problem.

      1. tech1234 | | #20

        actually this looks to be sized more correctly for this room

        1. charlie_sullivan | | #25

          Those Ecor dehumidifiers don't collect the water as liquid, but expel it in a high-humidity exhaust stream, that has to be sent outdoors. That allows it to work at temperatures below freezing, but it also means that you lose the high efficiency heating that it provides. At that point you'd be more efficient with an HRV and a small electric heater.

          Here's a cheap one on Amazon that does collect the water in a reservoir (and does need to operate above freezing). I'm a little skeptical of cheap stuff on Amazon, particularly if it has an electric heater in it and doesn't have a UL listing, but it could be something to try, and maybe get something higher quality if it works out for you.

          1. tech1234 | | #26

            Charlie thanks for that link. Upon further research last night I did catch that the Ecor was pumping out the steam (and the heat with it) out the wall.

            Here's a link for anyone else interested in the info:

  8. Expert Member
    NICK KEENAN | | #17

    Reading the whole thread, I don't think vapor drive is going to work as desired. Unless the inside dew point is below the outside temperature the vapor isn't going to make it through the wall, it's going to reach a spot in the wall that's at dew point and condense there. That's going to cause a mess, it's the reason walls in heating-dominated climates have vapor barriers on the interior. If the interior dew point is below outside temperature your interior humidity is probably fine.

  9. Expert Member
    BILL WICHERS | | #21

    If you cycle the temperature in that room, and it's an insulated room relatively sealed off from outdoors, you're likely to make the condensation issue worse. Warm air will hold more water (as Malcolm mentioned), and when the temperature of the air rises and falls, some things within the room will lag that temperature change and that's where problems can arise. I've seen this happen before, usually on mechanical equipment that is in a room that is not fully conditioned. Large metal objects have a large thermal mass (sorry DC), so they tend to be the things most likely to show condensation as they'll stay cold in the warmer air.

    If you have a lot of ventilation with the outdoors though, you probably won't have any problems. I have a heated doghouse I built for my dog that is a good example, and I have a temperature/humidity sender in it that ties into my monitoring system. I can see it's warmer in that doghouse because of the heater, and I can see the humidity level as %RH tends to be VERY low in the winter, because the air is warmer than outdoors, but there is a lot of communication (fancy way to say "ventilation") between the air in the doghouse and the outdoors since the doghouse is intentially not super well air sealed. The result is that the air in the doghouse has the same total moisture content as the colder outdoor air, but because the air inside is warmer, the %RH is lower since the warm air would be able to hold more moisture if there was a source of moisture. Your breezeway, if vented to the outdoors, would behave similarly in terms of humidity levels.

    The reason the doghouse is intentionally not super well air sealed is so that the dog inside has a supply of breathable air :-) The door is a simple flap with a relatively large gap around it, since I found it was futile to try to find a a chew-proof way to put weatherstripping on the door. I left a small gap around a rafter on the far side as an "out" to make sure there is ventilation all the time.


    1. tech1234 | | #22

      Bill, great info. The dog house example makes a lot of sense

    2. Expert Member
      NICK KEENAN | | #23

      I have a browser plugin that replaces "thermal mass" with "heat capacity" so all is well.

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