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Community and Q&A

Dehumidification vs. ventilation in an existing house

Timothy Godshall | Posted in Energy Efficiency and Durability on

I have done significant air sealing of our 1930s cape cod house in Virginia (climate zone 4) and it only recently dawned on me through reading various GBA blog posts that this air sealing is probably the cause of our high indoor winter humidity levels (ranging from 55% to 70%)

I understand that adding ventilation is the correct way to go, and the bathroom exhaust fan approach seems the most cost effective. However, before doing this I have a few questions.

1. I wonder if this humidity level poses a threat to the structure of our house because of moisture build-up inside the walls. The original wood lap siding was covered with a foil-type house wrap, which was then covered with aluminum siding at some point. I assume the foil would inhibit drying to the exterior. However, since the humidity level seems to be caused by lack of air flow, I’m guessing that there isn’t an overly large amount of air passing through those walls to cause such problems.

2. Is there any possibility that running a dehumidifier to deal with the high humidity could be more energy-efficient than running an exhaust fan? A dehumidifier burns a lot of electricity, but it also puts off heat that would help in space heating. I am installing a somewhat over-sized PV system, so that electricity would be mostly, if not totally, covered by the PV output. By contrast, bringing in more cold air would make our oil furnace run more, and oil is not free. (I hope to at some point switch over to a heat pump, but that is not the case as of yet.) I know that there are other reasons to introduce fresh air, but purely from an energy efficiency perspective, how might dehumidification compare with ventilation in this case?

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  1. D Dorsett | | #1

    During the heating season converting the latent heat (humidity) to sensible heat (temperature) with a dehumidifier will be pretty efficient, and probably more energy efficient than exhaust only ventilation, not as efficient as heat-recovery ventilation.

    An RH of 70% is a mold hazard even in the conditioned space any time of year, and if it's usually that high during the winter presents a significant long term risk to the sheathing. (The dew point of 70%RH 70F air is about 60F, so whenever the sheathing is below 60F it will be adsorbing moisture, and won't release it until the sheathing is above 60F.) Ideally you would control the wintertime moisture to something like the mid-30s, no more than 40% RH during the coldest weather, and keep it under 60% during the summer (under 50% if anyone has dust-mite allergies.)

  2. GBA Editor
    Martin Holladay | | #2

    You may want to buy another hygrometer to verify that the one that reads 70% RH is accurate.

    If your indoor RH is really that high, then (a) you definitely need to lower your indoor humidity, and (b) your house is certainly at risk for mold or rot, especially in light of the fact that someone installed "a foil-type housewrap" on the exterior side of your walls.

    I have never seen a monitoring study that shows that running a dehumidifer is a cost-effective way of lower indoor RH during the winter. Theoretically, if your electric rates were low enough, there may be some locations in the U.S. where this approach makes sense. But conventional advice has always held that wintertime ventilation carries a much smaller energy penalty than running a dehumidifier during the winter.

  3. Charlie Sullivan | | #3

    Conventional advice, as reported by Martin, says ventilation carries a smaller energy penalty than a dehumidifier (in winter), whereas DD guesses the dehumidifier will be better. Sounds like we need to consult a psychrometric chart! I'll try that later today unless one of you beats me to it. In any case, the dehumidifier as a heater will be a better option than electric heat, because you get all the waste heat, plus the latent heat of the water you take out. You can think of it as COP >1 heating with the dehumidification as a bonus. So for the poster's situation, with expensive oil and low-carbon low-cost electricity, it seems that it would be a clear win over ventilation with no heat recovery + oil heat. I'll try to quantify that later today.

  4. GBA Editor
    Martin Holladay | | #4

    When a dehumidifier is located indoors, with no outdoor unit, the heating COP of the appliance always equals 1.

    With any indoor electrical appliance, the watts of electricity into the appliance = watts of heat that are added to the interior of the house.

    [Later edit: This answer is wrong. See Charlie Sullivan's correction in Comment #6.]

  5. Aj Builder, Upstate NY Zone 6a | | #5

    I think someone has to explain a bit more about what the dehumidifier is doing with "moving" or "removing" heat. The water condensed should be cold, colder than the room and if it is then piped continuously out of the home, then cold is being pumped out of the home. Right? So besides the watts of running the machine it is moving heat into the home via moving cold water out of the home. Straighten me out here people. The humid air has to give up X btus to phase change to water. The pints of water would equal the btus delivered on top of the btus from the watts of the motor. So COP over 1?

    Winterizing a camp today and it's 10 degrees out, speaking of water... phase changing... freezing in my fingertips, ice crystal damage... vascual damage, one finger loves to turn green, I keep an eye on that one now.

  6. Charlie Sullivan | | #6

    The net heat a refrigerator produces in the room it is in is exactly equal to its electric consumption (not counting what happens when you add or remove things from it or make ice, etc.) But a dehumidifier is different, because of the latent heat in the condensation process. To understand this, consider that when an air conditioner "has to work harder" because of the dehumidification it is doing, there is less sensible cooling going on (for a fixed compressor power), but the heat rejected outside remains about the same as if it were cooling already-dry air. A dehumidifier is the same thing, but with condensor and evaporator coils in the same space. The ratio of how much sensible cooling it is doing vs. how much heating it is doing changes when it is doing dehumidification.

    If you run a dehumidifier in dry air, the net heating is exactly equal to the electric consumption. However, if it is condensing water out, there's less sensible cooling going on internally, so the balance shifts and you get more heat out than the electricity you put in. That might seem to violate conservation of energy--hence AJB's question about whether one is getting that energy by rejecting cool water out of the house. A little bit, yes, but cooling water from say 68 F to 33 F involves much less energy than the phase change going from vapor to liquid. So the phase change is the main story in the question of how it is possible to get net heating larger than the electricity input.

    It's a little like a condensing boiler--you get more heat out if you condense the water rather than sending it out as vapor.

    I did do the calculation, and for my sample conditions and dehumidifier specs, I got an effective sensible heating COP of 1.74 [***Edit--just found a mistake in my calculation--mixed up pints and liters. To be fixed in the next post!]. Better than I expected! I'll report more detail on that in a separate post. Here I'm just trying to explain the phenomenon.

    AJ, take care of those fingers. We need you to be able to type with them to participate here. Among other uses for fingers.

  7. Charlie Sullivan | | #7

    Here's the calculation. Energy star dehumidifiers range from 1.85 liters/kWh to 4.2 L/kWh. But that's rated at unrealistic conditions of 80 F and 60% humidity. (At 80 F you need A/C, not a dehumidifier.) So I looked at the test results for other conditions at . I picked an Ultra-Aire 70-pint per day model, and decided to look at 68 F, 55% humidity (incoming air = room air). The energy factor there is about 70% of what it is at 80/60, which comes out to 1.62 L/kWh. And it runs at 160 CFM.

    Air that is at 68 F and 55% humidity is 0.8% water vapor. To get 70 pints per day at 160 CFM, you need to remove about half of that water vapor, which is about all you can remove without freezing the water you are removing. To find out how much latent heat is involved there, we need to compare enthalpy of 68F air at 0.8% water content and at 0.39% water content. The psychrometric chart (actually an online calculator at says that's a difference of 4.47 BTU/lb of air. At 160 CFM, that's 3186 BTU/hr, or 934 W.

    The electric consumption of this unit can be found from the 70 pints per day, which google tells me is 1.38 liters/hour. That means the power consumption. at 1.62 L/kWh, is 1.38/1.62 = 0.851 kW

    So the total heat output is 934 W plus 851 W = 1785 W. The effective heating COP is 1785/851 = 2.1

    That's pretty attractive--certainly if you were heating with straight electric, that would be the best option. If you were heating with a COP 2.1 heat pump, it would still be the best option. If you are heating with cheap oil or with a COP > 2.1 heat pump, then we would need to analyze the other options more carefully to see what is really best.

    With a dehumidifier that just meets the energy-star minimum of 1.85 L/kWh, with the same 70% derating for conditions, the power consumption is 1.07 kW, and the effective COP comes out to 1.87--still great.

    I'm starting to think that bathrooms in cold climates should have dehumidifiers instead of exhaust fans.

  8. Timothy Godshall | | #8

    Thanks to all of you who have taken the time to respond to my post.

    I have a follow-up question about summer time humidity: We have central air, but we only use it a few weeks out of the year. Usually opening windows at night and closing up the house during the day is enough to manage keeping the house below 80 deg. F. I have assumed that the only cost to this frugal approach is personal comfort, but is this problematic otherwise? Is it just plain bad for a house to be humid, even if there isn't a significant temperature differential between the indoor and outdoor air?

  9. GBA Editor
    Martin Holladay | | #9

    Thanks very much for correcting my error, and for sharing your calculations.

    After I posted my answer yesterday, there was a nagging worry in the back of my head that my answer was wrong, but I was too busy with other matters to return to the question. You're right, of course, that the phase change from vapor to liquid releases heat.

  10. Charlie Sullivan | | #10


    High humidity indoors in the summer is in one sense less damaging than high humidity in the winter, because there aren't the same cold surfaces where water can condense. And if you start to have problems with mold, it is likely to be on visible surfaces, not hidden in walls, so you can take corrective action when you see it. An exception to this is a basement, where the walls are cool because of the cool earth they are in. So basement dehumidification can be especially important.

    I would recommend monitoring the humidity in the summer and considering using A/C if it is high. One problem with the strategy of opening windows in the evening is that the outdoor humidity can be especially high in the evening. In terms of absolute humidity, it's generally lowest just before sunrise. A fan on a timer could help you take advantage of that.

  11. Yestermorrow School | | #11

    Hi Tim!
    Glad to hear you're putting your Yestermorrow classes to work on air sealing projects on your house in VA...
    Thanks for posting the question as this is very similar to the issue I've been dealing with in my 1876 Cape in Vermont. I have been running a dehumidifier in the basement nearly year-round to try to control the humidity level, but balk at the fact that it adds about $40/month to my electric bill (I've been using a Kill-o-Watt meter to confirm this, and this is with a relatively new Energy Star rated model). This whole discussion makes me feel a little better about running the dehumidifier, but makes me think I need a second one to run upstairs as well since we keep the basement door closed at all times (it's unheated down there). I think the first move for me will be to invest in a better hygrometer since my cheapo hardware store version does not give me a ton of confidence. I also want to test other areas of the house, since we usually keep it in the kitchen and it shows 60-70% most of the time even with generous use of the range exhaust fan.
    I'm especially concerned about what seems like an excessive amount of condensation happening on the interior of the aluminum storm windows that seems to stick around all winter.

  12. Aj Builder, Upstate NY Zone 6a | | #12

    Kate, just a guess but your home is not the same as Timothy's and since your home has high humidity you most likely have a source that could be corrected. I sealed up a damp cellar with basic block sealer a few years ago and it made a huge difference.

    Yestermorrow folks may know much more about all than I do but who knows...

    -old basements

    No home should need constant use of one dehumidifier let alone two On a good summer I run my basement one for a week or so and on a bad summer maybe two to four weeks max. And it is not on only about half the time. $10-20/year for electric.

  13. Tim O'Brien | | #13

    Residential refrigerant dehumidifiers (typically) consume electricity, receive a stream of air, convert water vapor (from the stream of air) to liquid water, and exhaust a stream of air which is warmer and contains less water vapor than the entering stream. The dehumidifier converts the electrical energy it consumes and the latent heat of vaporization of the water it collects into sensible heat which increases the temperature of the air stream leaving the dehumidifier. You can confirm this by placing your hygrometer near the dehumidifier's air inlet and air exhaust. When running, the exhaust air temperature will be warmer and the exhaust relative humidity will be lower than the inlet air.

    For those wishing to perform calculations:

    About 1,000 Btu of heat energy is required to convert one pound of water from vapor form to liquid form. One pint of liquid water weighs slightly more than one pound, so you may multiply the pints of water collected by your dehumidifier by 1,000 to determine how many Btus of sensible heat were created by your dehumidifier.

    In addition to the latent heat converted to sensible heat, your dehumidifier converts the electrical energy consumed into sensible heat. One kWh of electricity produces 3,412 Btu of sensible heat energy.

    If you measure the electrical consumption and collect the water removed by your dehumidifier you can make a reasonably accurate calculation of the sensible heat added by your dehumidifier as well as the COP/EER of your dehumidifier as a heater.

    If you compare a dehumidifier to electrical resistance heat, the dehumidifier will be a more efficient heater when it is running and converting water vapor into liquid water. Dehumidifiers typically respond to relative humidity levels and will cease operation once the relative humidity level in the room drops below the dehumidifier control set point.

    If you want to increase the temperature and lower the relative humidity of a room, a dehumidifier is a good choice. This is often the case in spring and fall seasons. Operating a dehumidifier may be less expensive than mechanical ventilation and heating in the winter, but this depends on your cost of electricity and the cost of your heating energy source.

    Note that most residential dehumidifiers have a small heating capacity compared to typical boilers, heat pumps, and furnaces. They will not replace a dedicated heating system in a typical house.

  14. GBA Editor
    Martin Holladay | | #14

    Thanks for your clear summary of the issue we are discussing.

    For anyone tempted to install a dehumidifier for space heating, I'd like to emphasize one more point: in most of the U.S., indoor air tends to be quite dry during the winter. This problem is worse in leaky homes (the majority of U.S. homes) than in tight homes. The problem is so common that it has spurred the development of the humidifier industry -- the bane of energy experts.

    If you hope to scavenge some free heat by condensing moisture from your indoor air, think again. During the winter, your indoor air is probably too dry for this idea to work.

  15. Charlie Sullivan | | #15

    Adding to what Martin says---if your winter air isn't dry yet, it will be soon if you are heating with a dehumidifier! But if do have a need to dehumidify, it's nice to know that you get some bonus heat out of the process.

    And Tim, your calculation approach is much simpler than mine. Thanks.

  16. Aj Builder, Upstate NY Zone 6a | | #16

    Charlie I agree with you. I don't think anyone is ever thinking heating, but it is neat to know that the COP is plus 1 if a humidifier is used for some reason during a heating season.

    Last night for a rare day my home gobbled up 60,000 btu/hr without blinking a sleeping bears eye. Try installing that size dehumidifier along with enough moisture from maybe an inside lap pool running with no cover, to heat, with, a dehumidifier? NOT.

  17. Timothy Godshall | | #17

    Good to run into you on this forum, Kate!

    Regarding the basement humidity issue, we do run a dehumidifier in our basement all summer, set to 60% (which I am realizing might be too high?) and don't notice any problems. It is a conditioned space, but unfinished and only certain portions are insulated, but it is quite well air sealed. Much of the below-grade portion and the floor is exposed concrete. Usually once or twice a year we get some water seeping into the basement after a major rain.

    Interestingly, though, in the winter our basement humidity drops to the low 40s on its own. I'm not clear why this is the case when it tends to be very humid in the summer. The basement is probably the best air sealed part of our house, so I don't think it has a lot to do with air infiltration. And I would think that the underground moisture level would be about constant year round, or if anything wetter in the winter because the cold outside air doesn't have the same capacity to pull moisture from the ground as hot summertime air. I'm not complaining about the dry winter basement, but I am curious to know why it is that way when it's not in the summer.

  18. Charlie Sullivan | | #18

    Even if the basement is theoretically well sealed, stack effect in the winter wants to make air come in any leaks in the basement, and exit at the top of the building. So you'll have outside air coming in. If that air is cold, when it comes into the basement, it warms up to a higher temperature, at which point the relative humidity is lower for the same moisture content. Even if it's only 55 F in the basement, and 32 F outside, that temperature difference will result in low basement humidity.

    Hot outside air cooling when it gets to the basement has the opposite effect and is a key cause of high basement humidity in the summer.

  19. Venkat Y | | #19

    Thanks for the info on calculating the COP for the dehumidifier. I am curious to understand what COP can be expected from a typical HRV/ERV.

  20. GBA Editor
    Martin Holladay | | #20

    The most common use of COP (Coefficient of Performance) is to describe the efficiency of a heat pump. It's the ratio of energy input to heating capacity.

    HRVs and ERVs are not heating appliances. Nor are they dehumidifiers. They are ventilating appliances whose main purpose is to bring some fresh air into your home. So there is no meaningful way to apply a "coefficient of performance" number to an HRV or ERV.

    For more information on this issue, see Misconceptions About HRVs and ERVs.

  21. Venkat Y | | #21


    Thanks for the info. I understand it's a different animal. But I was wondering about COP, *if* we looked at it as a Heating device in Winter since it does "add" some heat to the fresh air. In googling this, I found a couple of attempts at a COP:

    "For comparison, heat pump Coefficient of Performance (COP) ratios are fairly well known. A COP of 3 or over is considered very good, if actually achieved. A Passivhaus certified MVHR with 80% efficiency could achieve a COP of 15."

    "The Brink range of high efficiency units are designed to use rigid ducting. The Brink range is made and tested in the Netherlands, and is one of the most efficient range of heat recovery units on the market (quoting from Brink). It has a Coefficient of Performance of (CoP) up to ± 10.

    CoP is a term normally used with heat pumps and is a measure of efficiency, but can equally apply to heat recovery units. This means that a Heat Recovery unit can recover up to 10 times more energy than it uses. i.e. In using 1kW of electricity, it will recover at least 10kW of heat. "

  22. Charlie Sullivan | | #22

    I too was stumped at how you could use COP to describe an HRV, but now I get it. It confusing and has potential conceptual problems, because if you run the HRV vs. turn it off, you actually lose heat by turning it on. So you have less heat and more electricity consumption. That could be considered a negative COP! But I think the way your references are looking at it is to assume that the ventilation rate is a given--you are going to ventilate at a certain rate, either with a fan or a an HRV. In that case, you could consider only the extra electricity consumption of the HRV above and beyond the fan, but Panasonic fans run at low speed use such low energy that I think it's OK to neglect that.

    The calculation would then be
    COP = (heat loss from ventilating at the given rate) x (heat recovery % of of the HRV) / (electricity consumption of the HRV)

    The heat loss number depends strongly on the temperature difference between inside and outside. So when it's very cold outside, that type of COP goes up, whereas when it's mild, the COP is lower. But that could be a misleading concept, because the ventilation process still is a net loss, and the loss is bigger when the temperature difference is big. So it's not like the higher COP of the HRV when it's cold out helps make up for the lower COP of the heat pump you might be using.

    Coming up with a number is hard. The typical range includes at least 65% heat recovery at 1 CFM/W to 90% heat recovery at 3 CFM/W of fan power. So that's a 4:1 range even for the same indoor/outdoor temperature difference. I suppose we could call it 2 CFM/W, 80% heat recovery, and then a 25 F temperature difference? Then we have:

    heat loss = 25*1.08*(flow in CFM) = 27 BTU/h/CFM = 8 W/CFM.
    Recovered energy = 80% of that = 6.4 W/CFM.
    Energy consumption = 0.5 W/ CFM
    Ratio = COP = 6.4/0.5 = 12.8

    That makes it sound pretty awesome, and of course the naive question would be "gee, can I heat my house with an HRV if it's so much better than a heat pump?" And the answer is no, you can't because it only loses heat and never supplies it. The only question that can answer is whether you'd be better off turning off the HRV and opening your windows for ventilation when it's 45 F outside and the answer is no, but we already knew that.

    We could look at a low-performance HRV in milder weather and then the result become maybe a little more interesting: at 65% recovery, 1 CFM/W, and 10 F difference between inside and outside temperatures, the "COP" comes out to 2.06. That's pretty low--especially since in such mild conditions a good minisplit would provide COP = 5 or so. So in that case you would be better off opening the windows for ventilation and turning off the HRV.

  23. GBA Editor
    Martin Holladay | | #23

    The main reason that I object to using COP in this context is that you'll retain even more heat in your house if you turn the HRV off or unplug it. Every time you operate your HRV during the winter, you are sending heat outdoors. The same can't be said for a heat pump.

    Of course, if you would otherwise be using a fan without heat recovery, the HRV is an improvement. But not operating it at all is even better (from a thermal perspective).

    [P.S. Charlie and I were writing simultaneously -- and his comments beat mine. We were thinking along the same lines, as it turns out.]

  24. Venkat Y | | #24

    Charlie and Martin,

    Indeed I was assuming balanced ventilation at some rate to be a given, so then how to determine the COP for the heat that's added to the incoming fresh air.

    I was trying to compare the HRV/ERV to the new Exhaust Heat Pumps' (magic boxes) efficiency in terms of COP. Specifically, are the Exhaust Heat Pumps more energy efficient at scavenging heat from the stale airstream than the highest efficiency HRVs/ERVs such as a Zehnder or a Recoupaerator.

    Incidentally Mr. Audet called me this afternoon to answer some questions I had on the Minotair and he pointed me at the ARE, SHRE specs in comparing the Boreal to other HRVs/ERVs. He warned that the ratings should be compared at the same CFM rate, otherwise it won't be a fair comparison.

    In terms of the hypothetical COP that Charlie arrives at earlier, the hypothetical HRV/ERV would seem to be more energy efficient than the exhaust heat pumps (COP 3 - 4). I would appreciate your insights as into this question as for why the COP and the SHRE ratings don't seem to match up. For example, the Recoupaerator would possibly be at a high COP as Charlie's HRV but its ASE is rated up to 96%.



  25. GBA Editor
    Martin Holladay | | #25

    I think that Charlie has done a good job of explaining why the use of COP for HRVs is unhelpful. Most of the time that an HRV operates, the delta T is relatively small -- but the delta T is all over the place, so making these calculations is tricky (or based on lots of assumptions).

    It's simpler to say:
    1. Ventilation always exacts an energy penalty, so ventilation should be minimized. You don't want to ventilate at a greater rate than necessary.

    2. If you can afford it, choose an efficient ventilation appliance. But note that the capital investment in an efficient HRV is hard to justify (in terms of energy savings) in all but the coldest climates. For more information on this issue, see Are HRVs Cost-Effective?.

  26. Charlie Sullivan | | #26

    When you are comparing appliances with similar functions--for example two heating systems--it's fairly straightforward to pick one number, such as COP, and compare them on that. When you are trying to compare a ventilation system to a heating system, or to a box that does both, it is harder to figure out the right figure of merit. At some point it makes sense to give up on the figures of merit and instead compare total energy consumption for two different designs that achieve the same end result. In this case it might be comparing an HRV for ventilation and a mini-split for heat to a scenario with a magic box to do both, or maybe a magic box with a heat pump to provide any additional heating needed.

    Doing that comparison is complicated for heat pumps and ventilation systems, because their performance varies with weather. So you more or less need to set up a modeling program and run it with a year's worth of data, or pick one operating point that is more or less representative of what you are interesting in. You could pick an outside temperature, look at what the minotair would deliver in terms of ventilation and heat, and then figure out how much power it would take to run a combination of your choice of HRV plus a mini-split providing the extra heat. That would give you a clearer picture of whether the extra heat provided by the minotair is worth the extra electricity cost.

  27. Mateo K. | | #27

    Not to high-jack this thread - but to the OPs post - I have a similar question RE: dehumidification vs. ventilation, etc. I live in Seattle, WA (Marine 4C) - and our crawlspace is unvented, has a rat-slab, and has high humidity (70-75%). I've installed vent fan, and there are 3 old supply registers that open directly into the crawlspace from the main house to supply "make up air" as the vent fan removes air to the exterior. While the vent fan has helped with musty odors and lowered the humidity a bit (it used to be 80%+), I would prefer it to be <60%, ideally 50% (or within a few points of the main living space).

    Living in Seattle, the main house has fairly high humidity (60-65% up to 70%) without a standalone dehumidifier that runs 6-8 hrs per day about 8 months of the year to keep the humidity to around 50%. Usually we'll only get a few weeks of cold enough weather where the furnace is on enough of the time to limit the need for the dehumidifier. The solution I've come up with is a whole house dehumidifier (Ultra-Aire 70H) - but wanted to make sure there wasn't something I'm missing in the equation - most references to humidity and whole-house dehumidifiers always focus on the South East, not the PNW.

    Am I missing something significant in my decision to get a dehumidifier? I am aware of the energy penalty I will be paying, but I don't have another solution to keep the house to a reasonable humidity. Thanks!

  28. GBA Editor
    Martin Holladay | | #28

    There are two questions here. The first question is: What's the least expensive way to make sure that your home isn't too humid?

    The second question is: Should you worry about the relative humidity level in your crawl space?

    Before discussing options for ventilation and dehumidification, you need to make sure that construction defects and occupant behavior aren't contributing to high indoor humidity levels. For more information on this issue, see Preventing Water Entry Into a Home.

    If the outdoor weather is cool or cold -- even when it's rainy or snowy -- you can usually use ventilation to lower indoor humidity levels. Dana Dorsett advises that ventilation will work to lower indoor humidity levels as long as the outdoor dew points are 55F or less.

    The real problem occurs when the weather is hot and humid. Under hot, humid conditions, you need an air conditioner to lower indoor humidity levels. Of course, you should always keep your windows closed under these conditions, or you'll never have a chance of controlling high indoor humidity levels.

    Following this advice makes more sense than relying on a dehumidifier.

    [Last paragraph edited in light of Dana Dorsett's subsequent comment.]
    Since you are venting your crawl space continuously with a fan that draws makeup air from your home, it's important to verify that you aren't ventilating at too high a rate. Remember, by pulling conditioned air from your home into your crawl space, you are also pulling unconditioned exterior air into your home. (Air leaks through cracks in your thermal envelope to replace the conditioned air being pulled into your crawl space.)

  29. Expert Member
    Dana Dorsett | | #29

    There is (almost) no such thing as hot humid air in Seattle, even when it's hot out, and it's definitely NOT the source of the humidity in Mateo's case.

    In Seattle outdoor dew points are rarely above 60F (it usually makes the newspaper when that happens), and the summertime averages are below 50F Today it's in the mid-50s, which is common enough- the peak humidity late-July/early August weeks dew points typically average about 53F:

    Scroll down to the Dew Point graph well down the page, or here's the hot link to Seattle's dew point image:

    It's normal to have NEGATIVE latent cooling loads in that area, which makes Mateo's house an extreme outlier.

    A dew point of 55F @ 70F is 59% RH, and at 75F it's 50%RH. So ventilation alone (HRV preferred) SHOULD be able to keep the RH well bounded year round with only a few rare exceptional days.

    For the house to be running 60% RH+ @ 70F in Seattle year round requires either a lot of internal sources of moisture, or an extremely tight house & low ventilation. I suspect there may be no ground vapor retarders under the rat-slab &/or insufficient waterproofing on the foundation walls, some slow plumbing leaks, roof or flashing leaks or some other constant background source of moisture going on. Nailing down and fixing those sources is far preferable to running a mechanical dehumidifier or over-ventilating the place.

    But an HRV under dehumidistat control would almost certainly be more efficient than a dehumidifier for keeping the indoor RH under 60%.

    If the crawlspace has a high RH in winter simply because it's cool down there, the solution is to insulate the crawlspace walls, which also lowers the heat load. Deep subsoil temps vary a lot in that area, but tend to run 50-55F, and a tight insulated crawl space would idle along in the mid-60s or higher in summer. A 55F dew point at 65F air temp is 70% RH, which is right at the threshold of high mold-growth, but the average outdoor dew points are below 55F, even in summer, and below 50 F from September through June.

  30. Expert Member
    Dana Dorsett | | #30

    First, verify that you don't have major moisture issues in the foundation walls. Tape a 10" square of plastic sheeting on a sub-grade section for a week or two, and see if the wall becomes visibly damp there. If it does, some amount of remediation is in order.

    It may or may not be cost effective to insulate the rat slab, but 4" of rigid EPS against the walls meets code min. With closed cell foam it would take 3", which is quicker (most products would need to be installed in two lifts no thicker than 2" per lift), but has an HFC blowing agent environmental issue. If it's impossible to get sheet foam in there easily, that's often the only way. With either, you can usually use intumescent paint on the foam in crawl space rather than a time rated thermal barrier against ignition to meet fire codes (talk to your building department about this first.)

    Air sealing and insulating the foundation sill and band joist with 5.5" of open cell foam (R20) works in your climate zone, but you can't use open cell on the foundation walls without running into problems, especially if there is even a hint of ground water being a problem. That can be installed after the rigid foam, and can be safely installed in a single pass.

    For other details & ideas see:

  31. Steve Knapp CZ 3A Georgia | | #31

    Dana and Martin. What would be the best course for insulating the crawlspace? For DIY, would you use rigid foam on the walls and floor and tape the seams? With a contractor, would it be best to spray the walls and floor with close cell, or would rigid still be the most eco friendly approach?

  32. Mateo K. | | #32

    Thanks for the input, Martin and Dana.

    Martin - As Dana pointed out, Seattle rarely has hot, humid weather. We do not need to run the dehumidifier during the summer (July and August - sometimes September), and the windows are generally open during the afternoon / evening (we do not have A/C). The crawlspace fan is on a programmable switch, so while I do not have concerns about it running continuously during the summer because it is drawing air from open windows. I was hoping that it would lower the humidity in the crawlspace enough to the point where in the winter we could set it on a timer to only run 10min / hour, or for 2-3 hours a day to replace the air a few times and leave it at that.

    Regarding Martin and Dana's points of occupant behavior - we have a single bathroom (small - ~55 sq ft), with a 120 CFM exhaust fan on a timer, which generally runs for 30-45 minutes during/after bathing, we're very good about using this. For cooking we have a hood (200-600 CFM) over a "pro-style" gas range - but every time a burner comes on the hood is turned on.

    For other sources of moisture - we have had some significant crawlspace water issues that we've addressed, but potentially not fully. The year we moved in during a 2.5" rain event we had about 7" of standing water in the crawlspace. We've since installed a perimeter french drain, and surface applied tar water barrier to 3/4 of the house (the side of the house where grading is an issue) and the french drain and downspouts are now carry this water away from the house. We have not had water in the crawlspace since, but I would wager that, to Dana's point, our rat-slab has no barrier under it and is allowing what seems to be a relatively high moisture /water table to let moisture through the slab and into the crawlspace. Would the plastic test described above be the best method for testing this?

    If the rat-slab is leading to excess moisture, it sounds like we will need to go the route of poly on the slab/walls of the crawlspace with rigid foam on the walls only (would you recommend any on the slab itself?). If we're going to do this, I would probably want to use a closed-cell foam (maybe spray?) on the band joist and mud sill correct?

    Also, Dana this comment "...A 55F dew point at 65F air temp is 70% RH, which is right at the threshold of high mold-growth, but the average outdoor dew points are below 55F, even in summer, and below 50 F from September through June." I'm still not clear how exterior dew points influence interior - shouldn't these be based on the RH of the area in question?

  33. GBA Editor
    Martin Holladay | | #33

    It sounds like you realize that your crawl space has issues. I urge you to read these two articles; they both provide relevant advice.

    Building an Unvented Crawl Space

    Fixing a Wet Basement

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