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Dehumidifying a High-Performance House in Winter

Carl_W | Posted in GBA Pro Help on

I am a builder and have just moved into a wonderful net negative house we built. Most everything is great, but we can’t seem to knock the humidity down. It lingers around 55%-65% RH, at 68F – 74F (varies mostly based on when we have fires in the wood stove), and we have 3″ of heavy condensation at the bottom of our windows in the morning. Outside conditions are certainly not the bone dry of January, but definitely much more dry than inside. I’m wondering if we need to install a dehumidifier, install a second/larger ERV or HRV, or take other measures. Any suggestions are greatly appreciated! Here are the relevant house details:

-2,000 SF 4-bedroom with 3 adults, 2 cats, and minimal guests
-CZ5 in western MA river valley
-ACH50 came in at 1.0
-Walls are R-49 DPC with double stud, roof is R-70 combination DPC and polyiso.
-Windows are Pella Impervia, fiberglass, double glazed, U-value is 0.24. No window coverings on most windows.
-Wood stove is currently providing 95% of the active heating. It’s a Vermont Castings Aspen with sealed ducted outdoor combustion air.
-“Backup” heat is mini splits with 1 ducted unit and 4 ductless ceiling cassettes. Only one ceiling cassette is being used at all right now, for a somewhat isolated suite off the first floor.
-ERV is the Panasonic Intelli-Balance 100 set at 100/90CFM running 100%.
-100CFM direct exhaust fan over the shower and we use it.
-1,200CFM down draft range hood and we use it.
-Laundry is in its own room, door stays closed, and the room has an ERV return in it.
-Wet trades all finished over 3 months ago, save a small amount of tile work that I poke away at every other week.
-No houseplants (cats…)
-Minimal indoor wood storage, just a couple arm loads of well seasoned wood.
-No home beer brewing hobbies or other notable lifestyle moisture sources

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Replies

  1. DCContrarian | | #1

    The standard answer to high humidity in winter is more ventilation. 100 cfm for the ERV sounds low.

    1. Carl_W | | #2

      Thanks DCContrarian. 100CFM seemed sufficient on paper, but given the performance we're witnessing, my inclination is also to increase ventilation. We have a reasonable opportunity to add a second ERV/HRV in a crawl space, rather than just replacing our current unit. Do you have an opinion on adding a second unit vs increasing the size of our current unit?

  2. Walter Ahlgrim | | #3

    With a tight home it often will take over a year to get the humidity under control.

    Almost everything the house is built out of is somewhat wet before it was rained on and needs time to dry out.

    I think running a dehumidifier or 2 for a few months is a good idea.

    I have to ask with such a tight house where the air going out the flu is getting in to replace it. If you open a door or window does it seem to affect the draft on the wood stove?

    Walta

    1. Carl_W | | #7

      Hi Walter,

      Thanks for your response. We have been expecting higher humidity this year than the future, just not as high as it has been. We used native green lumber for the framing and subfloors, and checked it all frequently with a moisture meter until we were satisfied before closing up the frame. All told, the frame had nearly three months of open air drying before insulation began. All that said, I agree with your point here.

      The wood stove has outdoor air supplied directly for combustion via a sealed duct. With the door closed, the whole stove is tightly sealed against indoor air from any source. Now that range hood fan on the other hand...

  3. AlbertoArriaga33 | | #4

    Condensation on the windows leads me to think that the installed u value is below what Passive House Institute recommends in your climate to avoid condensation. I believe your climate calls for a u value of 0.15 in windows. So I think your best bet is to reduce your indoor humidity enough so that the dew point of your windows is reduced. Try to measure the temperature of your windows to see if there is a noticeable delta compared to indoor temp.

    Curious, where exactly in your window is the condensation occurring?

    1. Carl_W | | #9

      Hi Alberto,

      Thanks for your reply. You are correct, we do not follow Passive House standards. As far as the windows in particular, reaching the 0.15 value was not cost effective when we priced it out, taking well over 100 years to pay off. On paper, I had figured that a RH <45% at 70F should keep the dew point at the windows low enough to avoid condensation for 95% of the year, which seemed like enough. However, our challenge has been reaching that level of dehumidification.

      Do you have a suggestion for measuring the temp of the windows? I've tried before with a contactless thermometer, but it does not seem to work on glass.

      The condensation is occurring almost entirely in the bottom 3" of the windows, and extending in a thin line up the sides.

      1. Charlie Sullivan | | #14

        That surface 4 coating is why you can't measure the temperature with an IR probe. You could use a contact surface probe, but I think you are correct in wanting to reduce the humidity as a higher priority than reducing the condensation per se. In fact, the condensation is providing dehumidification. The problem is that to take advantage of that, you would need to soak it all up with a rag every morning and then dry the rag on a line outside.

        1. Carl_W | | #22

          Also an interesting point about the IR probe not working. I don't have any sort of contact surface probe (other than what we use to check our fever, would that work?).

          The more I think about it, the less I am actually concerned about the humidity on the windows, other than it being a potential symptom of interstitial conditions. Maybe at some point I'll wipe all the windows one morning and carefully wring out the rag into a measuring bucket to see how well they work as a dehumidifier!

  4. Greg Smith | | #5

    Achieving a U.24 in a dual pane configuration requires using a surface 4 coating on the glass. And while the S4 coating does improve window energy performance by reflecting interior radiant heat back into the room, by doing so it also lowers glass temperature and subsequently condensation resistance of the glass surface versus glass without a S4 coating.

    Lowering of glass surface temperature is significantly more pronounced at the edge-of-glass (approximately 2 1/2” from the sash/frame) than at center-of-glass where condensation is much less likely to occur.

    Unfortunately you may find it difficult to keep dew point below glass surface temperature in order to eliminate condensation even when using a dehumidifier or more aggressive HRV/ERV ventilation when the outside temperature begins to drop. It’s doable but will require keeping indoor humidity levels lower than would be required without the coating.

    1. Carl_W | | #10

      Hi Greg,

      Thanks for this response. I was not aware that the S4 is how they achieve that relatively low rating, or the effect it has on the glass temperature. That is very good to know, and helps me understand the full picture better. I've known that we would see condensation at these humidity levels, but have been surprised at how much we are seeing. This makes more sense now.

      This is also good to know, because it makes me a little less concerned about trying to completely eliminate the condensation. Ultimately, I'm not so concerned about the windows themselves, especially since they are fiberglass and very moisture tolerant. My main concern is that it is an indicator of risky interstitial conditions. My second concern is that is a little bit noticably uncomfortable, though just barely.

      Knowing that the window temp will be lowered by the S4 coating is helpful for adjusting my expectations of how they will realistically perform in optimum conditions.

  5. Expert Member
    Zephyr7 | | #6

    New homes tend to have higher than usual humidity levels as the lumber (and other materials) gradually dry out. You'll probably have better results next season, so I wouldn't do anything drastic right now like spending lots of money on a big central dehumidifier. Use additional ventilation over the winter as DC mentioned, which will help bring down humidity levels. Don't worry too much about the lost heat caused by this ventilation -- you'd be using energy to run a dehumidifier too, and at least with ventilation you don't have to buy any new equipment.

    I'd only look at adding a dehumidifier if increasing ventilation didn't help.

    Bill

    1. Carl_W | | #11

      Thanks Bill for your reply. Regarding the building drying out in its first year, I agree and refer to my response to Walter above.

      As a short term solution, do you have a recommendation for increasing ventilation? Our current ERV is maxed out already, and we use our other direct exhaust ventilation as appropriate. The only thing I can really think of to increase ventilation is to add a second ERV or HRV, since we do have a relatively easy place we can do that in a crawl space under the eave. Of course, even doing this myself, we're spending at least $1,500 on equipment and a good long weekend of "free time". Not exactly a short term solution.

      1. Expert Member
        Zephyr7 | | #12

        Cracking a window is a good way to add some extra ventilation :-D Sure, you lose some heat, but you're going to be using energy anyway to run an HRV or dehumidifier. Cracking open a window or two doesn't cost anything up front, so it's probably your cheapest option -- and you'll only be doing it for a few months or so anyway, if even that long.

        Bill

        1. Carl_W | | #20

          You might be right that it would be the cheapest option, but not by enough that I'm willing to go that route! It might be the principle, it might be the localized comfort issue, or it might be the inconvenience of having a system that requires frequent attention, but I'm just not wild about cracking a window when it's <40F outside. I'd be curious to do a real cost analysis on the impact of cracking a window, but not so much that I'm going to attempt that dive. If anyone else has, I'd love to see it!

  6. Deleted | | #8

    Deleted

  7. Walter Ahlgrim | | #13

    Green lumber is the big bongo as your moisture source.

    3 months is only a good start as far as drying your lumber goes.

    The general rule of thumb is one year for each inch of the board thickness with good ventilation. Your meter was only checking the surface moisture.

    I think you will be running dehumidifiers for two years.

    I like Bill’s idea in the winter whenever the humidity gets over 45% crack open a window for an hour or two. The cold air will be very dry and once warmed will dry the house.

    Walta

    1. Carl_W | | #19

      I'm sure the green lumber is a contributor as it finishes drying, but I don't think it is as much as you might think. One year/inch is a conservative metric for hardwood (large, stickered piles harvested at a high initial MC). Softwood coming off the mill at 22%-25% and installed 16" OC dry a lot faster. We measured the lumber with a high quality EMF moisture meter that reads down 3/4" below the surface; not quite to the core of a true 2x, but pretty useful as a reading in our experience. I'm interested if anyone here has had experience framing a house with green lumber and tracking the MC empirically. I'd love to see that data.

  8. Charlie Sullivan | | #15

    I agree with the consensus that you shouldn't invest a lot in more ventilation equipment for what is likely a short-term problem. The simple solution is to crack windows and/or run the bath fan continuously for at least part of the day. A dehumidifier might seem like a waste of energy, but it actually acts sort of like a heat pump heater with an effective COP about 2. The heat you get is both the electric energy input and the heat of vaporization of the water you pull out of the air. You could get a basic energy star rated one for not a lot of money, or you could get the best, high-efficiency model from Santa Fe.

    You might find it beneficial in the long term disability a high performance dehumidifier, for the late summer and early fall when you don't really need to heat yet but you are no longer cooling and getting dehumidification from that process.

    1. Carl_W | | #21

      Thanks for the input Charlie. We have, in fact, decided to go with a cheap dehumidifier. We bought a nice little 22 pint Danby for $100 and set it up in the laundry room, which is both a source of humidity and has one of our ERV returns in it. After running it for 2 days (probably ran 20-22 hrs/day), we seem to have stabilized at right around 40% RH at 72F. It seems like a very workable solution at an estimated cost of $1.50/day; and that doesn't account for the savings of an extra "heat pump".

      The biggest determining factor for me was ultimately recognizing that we will likely use a dehumidifier occasionally in the future, as you point out. In addition to the shoulder season, it also seems like it will be useful for summer days where we let the humidity get high in the house (via bad window management or otherwise), but it's not actually hot enough inside that we feel like using the AC. Having a small, cheap unit with a good warranty that fits in the back of the closet seems like a worthwhile investment.

  9. Expert Member
    Peter Engle | | #16

    You might want to consider replacing the ERV core with an HRV core if your unit allows for that. The ERV is recycling moisture back into your house, and you don't want that right now. For your current needs, an HRV would be more appropriate. If you can replace the cores, that would provide an easy and inexpensive short-term solution that would be reversible if you find the house running too dry in the future.

    1. Carl_W | | #23

      Thanks Peter. This was my preferred option, for all the reasons you mention. Unfortunately, Panasonic does not appear to offer an HRV core and I could not find any sort of after-market alternative. I briefly considered buying the closest HRV core I could find and "making it work", but then I thought better of it.

      The issue has made me consider scenarios in which we might install two units; one ERV and one HRV. There seems to be a good application for this sort of tandem installation in high performance homes in 5a (or similar climate zones), where we are trying to keep humidity out in the summer and PUSH humidity out in the winter. Also, in the summer we have the air quality benefit of opening windows and, arguably, don't need as much active ventilation. I do plan to do some cost/benefit analysis on this soon for a house we will building next year. I'll probably start a new thread then!

  10. qofmiwok | | #17

    I didn't go with R7 windows to save money, it was to avoid this type of condensation issue. My ERV is 214 cfm for 2 people in 3800 sf. You have a lot more people in less space. Plus I am in climate B (dry) and you probably are A. But I still think a large part of your problem is construction moisture. I am planning on industrial dehumidifiers for the first year or two to handle this.

    1. Carl_W | | #24

      Yes, we're 5a here and I'm definitely inclined to agree at this point that we are simply dealing with construction moisture. The fact that our recently acquired 22pt dehumidifier has quickly brought us down to a pretty ideal condition has alleviated my longer term concerns about significant seasonal condensation.

      For future builds, I'm going to have to dig into this question more about the relative cost of triple-glazed windows vs. higher ventilation and dehumidification requirements. In our area and for the types of homes we are building, I suspect that it is still hard for us to justify the $$ on triple-glazed. However, this added condensation issue is a factor we haven't really been considering so far. Probably a good topic for a future post.

  11. Brian P | | #18

    I have some experience with this situation for a "high performance" home we built in NH. I don't think you're seeing anything unusual given your info: new house, green lumber, and minor tile work. It's also been a warmer (and more humid) fall in the Northeast.

    My recommendation would be to just buy an inexpensive portable dehumidifier (+/- $200 for an Energy Star model) and start running it 24/7. Just keep it simple with one you empty when the tank is full (and empty when you go to bed). See where this gets you over this fall season. Maybe add another if one unit isn't cutting it. Maybe you'll only need to run this until the real cold/dry winter air sets in, or maybe you'll need to run over the winter. After the house dries out this winter, I think (or hope) you'll find your ERV to be capable enough.

    On a separate topic, where is the makeup air coming for the 100cfm bath fan and certainly for the 1200cfm range hood?

    1. Carl_W | | #25

      Thanks for the input Brian. I agree at this point that we are mostly dealing with short term construction humidity. Especially, as you point out, since we are having a pretty warm and humid fall. We have gone ahead with a $100 (on sale) 22pt Energy Star dehumidifier that we have draining into the washing machine stand pipe. It's working great and has brought us down to our approximate target of 40%RH at 72F.

      As for makeup air, I have been thinking quite a bit about this recently. For bath fans, we have made no accommodations and have not found them necessary. For the range hood, we unfortunately did not have a plan during construction for makeup air as we initially planned to not have a range hood (for reasons). This changed shortly before plaster began (for more reasons), but we only really had opportunity at that point for a downdraft unit. Given the relatively poor performance of downdraft units, we opted for a high capacity 1,200 CFM in-line blower to make sure it worked. The setup has a variable speed down to ~300 CFM and we figured we could size a makeup air system after doing some cooking and seeing what speed we needed to run it at. What we have discovered is that we operate in the top 75% of the range to get good capture, and it does work remarkably well at that speed. That is some long-winded context to say that we are basically just now ready to install appropriately sized makeup air.

      The plan is to install a 10" automatic make-up air damper with a pressure sensor installed in the exhaust duct. We will run the duct through the ceiling to a diffuser above the cooktop. Since we have a downdraft range hood, we hope that this configuration will create a relatively effective "curtain" draft that minimizes how much conditioned air we are exhausting at high rates, and doesn't create too much discomfort when the cold air starts being sucked in. We will pull the air from a well-vented large space above a porch ceiling that has a black metal south facing roof, which we hope will give us a little "pre-warming". We are lucky that we should be able to do all this with no plaster patching, given the joist direction and relative locations of all the equipment.

      Until then, we crack a window... (I know)

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