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Questions on Building a Passive House in a Humid Climate


It’s hard enough to control humidity in humid climate homes when they’re just built to code these days. The air conditioner traditionally has handled the dehumidification, but increasing requirements for insulation and air tightness affect the sensible load more than the latent. Then, when you go all the way to superinsulated houses like Passive House, you’ve got almost no sensible load. But you also have limits on energy use to get the certification.

It’s hard enough to control humidity in humid climate homes when they’re just built to code these days. The air conditioner traditionally has handled the dehumidification, but increasing requirements for insulation and air tightness affect the sensible load more than the latent. Then, when you go all the way to superinsulated houses like Passive House, you’ve got almost no sensible load. But you also have limits on energy use to get the certification.

Superinsulation and Passive House came out of cold climates. The Saskatchewan House in the 1970s, for example. Humidity is a concern there, but for a different reason. It’s condensation in winter from indoor humidity that’s the problem. With Passive House in hot-humid or mixed-humid climates, it’s indoor and outdoor humidity that can bite you.allison-bailes-texas-pin-bolo-tie-300.jpg

Today the inaugural Humid Climate Conference is happening in Austin, Texas. I’ve got my bolo tie and Texas lapel pin and am ready to be the moderator for a panel of experts. They are Dr. Joe Lstiburek, Katrin Klingenberg, and Robert Bean.

The topic:  Achieving Passive House Performance in a Humid Climate.

Maybe I’ll see you there. If not, you can still contribute. I’ve put together a list of starter questions, but you can send me any that you think would be appropriate. Here are the ones I came up with:

  • What do you see as the biggest challenges with superinsulated building enclosures in mixed-humid and hot-humid climates?
  • How do we cool and dehumidify a Passive House or other superinsulated house in a mixed-humid or hot-humid climate without blowing the energy budget?
  • How important is it to hit the Passive House airtightness threshold of 0.6 ACH50 or 0.05 cfm50/square foot of building enclosure?
  • What are the biggest challenges with mechanical systems in humid climate Passive House projects?
  • Can mini-splits handle all the cooling and dehumidification or will we always need supplemental dehumidification?
  • Can a split system dehumidifier do all the cooling and dehumidification?

 What would you ask?


Related Articles

It’s Called an Air Conditioner — Not an Air Cooler! 

The Mystery of the Ventilating Dehumidifier

The Tale of the New Mexico Dehumidifier


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This Post Has 45 Comments

  1. Is adding a dehumidifier to
    Is adding a dehumidifier to the ERV and dehumidifying the incoming air enough?

  2. Would the Cromer cycle be a
    Would the Cromer cycle be a solution for dealing with low sensible loads and high latent loads in a Passive House? It is claimed to use less energy and extract more moisture than traditional solutions such as dry mode minisplits or standalone dehumidifiers.

  3. What we sometimes fail to
    What we sometimes fail to take into account is the amount of sensible load we generate ourselves. Cooking, bathing, or simply breathing adds considerable moisture to air along with the gulps of moist air that we bring in every time we enter or exit the structure.

  4. Can you post some answers
    Can you post some answers here after the conference? Not all of us can travel to Texas!

  5. In my experience in Europe
    In my experience in Europe (southern England to be precise, which is marine climate — not often hugely hot but prone to high humidity when it is warm) conventional mini-splits don’t quite do the trick. What is extremely effective as a solution are the top-end Japanese makers units which feature hot gas reheat and a resizable surface area cool in the air handler. The second part of gives a laypersons overview.

  6. I would be curious as to the
    I would be curious as to the role heat pump water heaters can play in a hot humid climate. I personally advocate for locating them within conditioned space or at least indirectly conditioned space within thermal and pressure envelopes.

    Free cooling and dehu in Florida…what’s not to like?

  7. I wonder if you meant to
    I wonder if you meant to remind us of occupant activity-generated latent load, rather than sensible.

    If so, point well taken. Houseplants and dogs add yet more.

  8. Great question, Curt! I don’t
    Great question, Curt! I don’t know how I forgot about that. (And welcome back! Haven’t seen you here in a while.)

  9. What if, during the summer,
    What if, during the summer, you’re willing to live with higher indoor temperatures than the “average” American, so you open the windows. So what if it’s a little muggy? But if you’ve built to PH, or even high-performance, standards, do you need to modify the latest energy-code wall assembly requirements, especially regarding permeability and drying potential? (Maybe those fat walls will be slower to dry when fall comes and the windows are closed and the ERV is turned on again.)

  10. 2 Canadians and a German on a
    2 Canadians and a German on a panel discussion for hot-humid Passive Houses? What’s up with that?

    Just kidding…wish I could be there to listen in. Have fun, Alison!

  11. why are rigid foams
    why are rigid foams discouraged by many Passive house activists. 🙂

  12. With the required higher
    With the required higher levels of insulation what are the best details for inhibiting the infestation of termites and carpenter ants?

    If a homeowner can’t obtain a termite bond what’s the point of spending a premium for a PH in these climates?


    How does one reach PH standards in these climates when building codes generally prohibit sub-slab and exterior slab insulation that extends underground?

    Feel free to modify, clarify, condense.

  13. Another great question that I
    Another great question that I’m embarrassed not to have thought of. And I’ve just been dealing with termite damage in my bathroom remodel. Thanks, Kris!

  14. Hey, it’s not just rigid foam
    Hey, it’s not just rigid foam. It’s all foam.

  15. Yeah, that does look a little
    Yeah, that does look a little odd. Next year we’re probably doing it in Atlanta so we’ll see if we can get some homegrown humidity experts.

  16. Thanks, Clive! I’ll check
    Thanks, Clive! I’ll check that out.

  17. That’s my plan, Leigha. Look
    That’s my plan, Leigha. Look for something next week. We may be bringing the conference to Atlanta next year, so maybe you’ll be able to make it to that one.

  18. Internally generated humidity
    Internally generated humidity is definitely an issue. (As Curt already said, your first sentence should say “latent load” rather than “sensible.”)

  19. The curmudgeon in me thinks:
    The curmudgeon in me thinks: Their climate affords them the luxury of doing so and the typical euro-PH owner has the financial wherewithal to use more expensive products (ex, ICF, Mineral Wool).

  20. Thanks, Bruce. I’ve never
    Thanks, Bruce. I’ve never heard of the Cromer cycle, but maybe someone on the panel has.

  21. That’s one of the biggest
    That’s one of the biggest questions. Thanks, Elizabeth.

  22. Regarding heat pump water
    Regarding heat pump water heaters – in general, the sensible/latent “cooling capacity” split on these is pretty high…I suspect well over 80/20 in the summer (NREL testing of “older” models showed sensible fractions as high as 99.7% at 68F/50% RH indoor air – The manufacturers are optimizing these appliances for water heating capacity and efficiency…not dehumidification. To get better hot water capacity/efficiency, they will try to move more air across the heat exchange coil (boosting water heating capacity and efficiency), at the expense of dehumidification.

    In houses with already very low sensible cooling loads, HPWH’s may make the sensible/latent load split “slightly worse”.

    1. Good point about sensible
      Good point about sensible heat ratio relative to efficiency of HPWH. In Florida the prevalence of 68F/50% RH indoor are is rare…75 / 55 is common much of the year.

      Both my HPWHs used to fill a 5 gallon bucket fairly regularly depending on time of year until I got ’round to running drain lines into a utility sink.

  23. I’ve had clients install ERV
    I’ve had clients install ERV systems that tie into all of the bath fans in very tight and PH houses. Even with the booster mechanisms, they have sometimes had difficulty controlling humidity. Any advice on this? Is anyone looking at creating ventilation systems with separate mechanical dampers for different humidity zones within the homes?

  24. I would still use a HPWH in a
    I would still use a HPWH in a PH in a hot-humid climate. I just wouldn’t expect it to help much with the latent load.

  25. Great questions, Helen! If
    Great questions, Helen! If you use an ERV exhaust vent for all of your bathroom exhaust, you aren’t removing the humidity from the house, so that could definitely make things worse.

  26. I haven’t looked at any
    I haven’t looked at any numbers on it, but I wonder if in some projects you could do all the cooling and dehumidification you need with an HPWH and a split-system dehu.

  27. Broadly speaking, to me the
    Broadly speaking, to me the PassivHaus goal is using the least energy feasible, consistent with health of occupants and the structure itself. In practice, we’re pretty constrained by commercially-available technologies, whether for shell or equipment. Given that, it seems to me that it’s appropriate to reconsider the energy budget specification from a universal one to a climate-sensitive one. Maybe the PassivHaus spec for hot and hot-humid climates should accommodate the best current systems that assure high indoor environmental quality – and encourage multi-function approaches such as heat pump water heaters that also cool and dehumidify. Is it morally wrong to think that the best feasible house in Mobile might need a different energy budget than one in Minneapolis, at least in the near future? We don’t think that working Clydesdale draft horses take the same amount of feed as show horses, even though they’re the same species.

  28. Helen – we’re the humidity
    Helen – we’re the humidity problems in winter or summer? or both?

  29. My concern is not with single
    My concern is not with single showcase houses but with the production houses in a hot, humid climate. The Florida home builders, for example, have successfully changed the Florida building code to require a minimum of 7 ACH 50 beginning next year. What impact will that have on dehumidification requirements and energy consumption?

  30. PHIUS and PHI disagree on
    PHIUS and PHI disagree on this in terms of heating demand (PHIUS uses a climate-dependent metric), but both agree that sensible/latent cooling demand targets should be climate-specific.

    However, if you’re thinking about whole-building source energy use – both PHIUS and PHI use metrics that are universal….the rationale being that we have a global atmospheric “commons” to protect. A household in Halifax doesn’t deserve to be “allowed” to use more energy than one in Houston (or vice versa depending on what your bias is!)

  31. Larry, thank you for your
    Larry, thank you for your focus on production housing. While I think the Passive House standard and approach is admirable for new construction, it is the millions of existing housing stock units, and replacement for same as older stock decays or is ripped out for something bigger, that is also my primary concern.

    The battle for reducing energy footprint (and by turn carbon footprint and reliance on non-renewable energy sources) won’t be won by select implementation of Passive House construction alone. It will be advanced by scaling up improvements to existing housing stock on a massive scale. This is just beginning to happen, but it needs to happen to make any significant difference. Otherwise we continue down the same road as we’ve been on for decades.

    I assume Robert Bean will speak on “exergy” during the Humid Climate conference. This is not an easy concept to grasp (I’m still working on it myself) but from what I’ve grokked so far it is the difference between energy used and energy wasted, having combustion as a basis for creation of the energy. To narrow the exergy gap, Passive House is one but many team players needed for engagement. Existing housing stock, and my own field of energy management in institutional/commercial sectors, renewable energy transport, etc. Personally I’m seeing positive movements on all these fronts, but I think the less discussed ones, such as existing housing, need much more airplay.

  32. Larry, Florida homes having
    Larry, Florida homes having to hit 7 ACH50 or lower is a big issue. Combine that with increased ventilation requirements because of changes to ASHRAE 62.2 and it’s clear that supplemental dehumidification becomes pretty much mandatory, especially homes.

    Also, Passive House isn’t just “single showcase houses.” In fact, those are a minority of projects certified through PHIUS (and PHIUS certifies most US Passive House projects). Most of the Passive House projects getting certified now are multifamily homes of various sizes and styles.

  33. Yes, Cameron, fixing existing
    Yes, Cameron, fixing existing homes is huge and necessary. (Ask me what I found in my bathroom remodel!)

    Robert is in the last 20 minutes of his talk and hasn’t broached the topic of exergy yet, so I think he’s not going to include it today.

  34. I was wrong, Cameron. With 10
    I was wrong, Cameron. With 10 minutes to go, he raised the issue of exergy.

  35. Yes, indeed, Charlie. That’s
    Yes, indeed, Charlie. That’s exactly the one I was thinking about when I mentioned split-system dehumidifiers.

  36. After reading all of the
    After reading all of the comments on attic fans I still feel that I have something to contribute.
    The ceiling in the second floor of my condo got very warm in the summertime. The association had brilliantly re-shingled with darker shingles and we could immediately feel the difference. I complained, and the roofer said that it wouldn’t make any difference… The roof was properly vented with screened eve vents and six eyebrows. There was also had 8 inches of insulation in the attic. So I measured the temperature of the inside ceiling at 3 o’clock in the afternoon. I than installed a fan that forced air out of one of the large eyebrows. I then measured the temp again, in the same place at the same time, same weather, a day later. The ceiling was 30 degrees F cooler and the upstairs was very noticeably cooler all day and night. I suggest that the heat from the hot air in the attic was being conducted through the insulation and the ceiling (obviously). Do the experiment. Insulation will conduct heat. It slows down conduction but does not stop it. It is pure physics; If one side of the insulation is hotter than the other, heat will flow in an attempt to cause both sides to have the same temperature. Insulation is actually better at blocking radiation than it is a blocking conduction even though blocking conduction is it supposed purpose. And why in the heck anyone would use dark shingles in California is beyond me. If you want the best results use white shingles, or better yet silver, or even better yet, cover the whole roof with solar panels… The idea that conduction is unimportant in energy transfer from attic into living space is easily proven to be false. Again; don’t just talk about it, do the experiment. The structure of the radiative surfaces has a great deal to do with how easily heat moves from one environment to another.

  37. I agree — products like the
    I agree — products like the Ultraire range should always be considered. The only proviso is that they require ducting / forced air distribution and smaller footprint PH residential builds might not want to specify this. It will always be difficult to get a forced air system to be as efficient as the best of the mini splits and if you’re aiming for the lowest energy consumption per sq. ft. (regardless of air tightness) the mini split(s) might get selected for their heating performance, thus ruling out the presence of duct runs — and Ultraire or similar.

  38. True and good points. I like
    True and good points. I like mini splits but they are not easy to to find in the hot humid gulf coast of the US. Then again passive home don’t work well here.

  39. I’d like to understand
    I’d like to understand solutions that meet both the passive house/tight house needs in a hot-humid climate (need for dehumidification, often need for heat, low energy use so ductless preferred, etc.) while also addressing the aesthetic desires of the typical homeowner. Mini-splits are a tough sell and usually a no-sell.

  40. What about using a dessicant
    What about using a dessicant wheel system like what is used in commercial buildings? Don’t they use less energy to remove the humidity?

  41. Allison, would it be possible
    Allison, would it be possible to see a set of notes or video from the panel discussion? Wish I could attend but currently out of state. Also do you have any recommendation on a good source of information for designing to passive house standards in humid climates? Many thanks

  42. The discussion session was
    The discussion session was possibly the best part. Without the engineer’s daughter none of this would have happened. Canadians are American’s too. I was surprised and delighted by the number of people from outside the local area. Now, we need to build and have actual people live in houses in humid climates as well as more build everywhere.

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