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Q&A Spotlight

Does Passivhaus Work in New Orleans?

Or does superinsulation make it harder to cool a house?

New Orleans Passivhaus:

This award-winning Passivhaus home in New Orleans by sustainable.TO helped kick off a debate about insulation in a hot, humid climate.
Image Credit: sustainable.TO

It is now a given that high-performance houses have high levels of insulation. It is not uncommon for a new cold-climate home to have R-40 walls and an R-60 roof, as builders do their best to lower a home’s energy requirements.

But is this premise in favor of thick insulation weighted toward houses in cold climates, where heating is a higher priority than cooling? Does it make just as much sense to insulate houses as heavily in hot, humid climates? Or does a lot of insulation actually make it more difficult to keep the house cool?

Those are the intriguing questions raised in a recent Q&A post at Green Building Advisor, and the topic for this week’s Q&A Spotlight.

Reading a long thread on a website called Design By Many about a Passivhaus project in New Orleans, John Brooks had come across this comment: “In a cooling climate, the delta T is much smaller than in a heating climate, and due to the internal heat gains insulation actually starts to work against you at a certain point. I think anything up to R-30ish is doable/defendable for all components, including roof and suspended floor. Beyond that it will only add to your cooling problem.”

Brooks gets the conversation rolling with this: “I understand how insulation can ‘work against you’ in a building without air conditioning….

“Or when the conditions outside are ‘better’ than conditions inside. The problem is that most Southerners DO air condition their buildings in order to be comfortable …. and for many of the hours during the cooling season the conditions outside are ‘WORSE’ than conditions inside.”

The issues is humidity

High humidity makes air feel uncomfortably sticky. Even…

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  1. Mike Eliason | | #1

    Au contraire! I'm definitely
    Au contraire! I'm definitely arguing for less insulation - at least, less than R-40/R-60 for New Orleans. Even with a large surface area-volume ratio - R-40/R-60 is complete overkill once you are achieving Passivhaus there. That Passivhaus can be achieved w/ slightly more than code minimum assemblies, double pane glass, smart orientation/design means that a 1-ton mini-split may be overkill (depending on size of building). It's actually a relatively cost-effective method of construction, especially over the lifecycle of the building.

    On the project we worked on, jumping the insulation from R-22 to R-28 resulted in negligible space cooling decrease, and space heating demand went from 3.14kBTU/ft2a to 2.72kBTU/ft2a. Since our TFA was 999sf, this means that another inch of insulation only saves 420kBTU/yr, or 123kWh/yr. as the price of electricity in New Orleans is fairly inexpensive (~$0.095/kWh) this results in annual savings of at most $11.70 for the extra insulation.

    You'll save more than that if the costs of additional insulation are dumped in PV (while also significantly reducing your CO2 emissions). It also means that the walls can be slimmer - which can mean a world of difference on a build-able lot width of 32' or less.

  2. homedesign | | #2

    High R-value walls ARE expensive.
    I agree that High R-value walls are expensive and hard to $ justify in New Orleans and Dallas.

    I think it is wrong to say that High R-value (30+) can work against you in a hot climate.
    A vented Attic is affordable and buildable at levels beyond R-30.

  3. user-788447 | | #3

    heating energy vs cooling energy
    This might be more properly titled combustion energy vs. compressor heat transfer.

    Although the delta T for heating climates is often far greater than the delta T for cooling climates it seems easier to produce heat than coolth. Living in Minnesota I've been disappointed to see air conditioning become the rule instead of used by very few people (and by noone before the heat pump technology became commercially available).

    Does it take more energy to reduce the temp by a degree than to increase the temp by a degree? Air conditioner technology also seems to me less durable than heating equipment. Window unit air conditioners are probably replaced every 5 years and refrigerants eventually escape into the atmosphere.

    Here is an Einsteinian thought experiment: what is going to happen if a good chunk of population in China and India form the same expectations as the affluent west and increase the use of air conditioning five fold?

  4. user-788447 | | #4

    in the thick of it
    In Minnesota for the entire week we are forecasted to have temps in the 90s and dew points in the upper 70s and lower 80s.
    I live in a turn of the century home recently insulated with dens packed cellulose. With average windows and a ACH50 of ~3.5 the interior of my home can maintain about 4 degrees lower temp than the outdoors.
    My wife and I are accustomed to sweat out weather like this though typically in Minnesota you only get about 3 days in a row. We now have a small dog so I bought a dehumidifier to bring down the RH in the home.
    So now we run a dehumidifier and fan and live with internal conditions of 85 degree temps with ~60 RH. This is just fine with us.

    My neighbor (from Louisiana originally) lives outside and wood fire grills just about everyday in the summer. I've pondered that if I lived in a Passivhaus with mechanical ventilation (note my home has no mechanical ventilation) I might be drawing in my neighbors grill smoke (don't know how good the air filters are) and outdoor humidity. Knowing now that an ERV won't keep the humidity out of the home (according to Martin) I wonder if I would still have to run a dehumifier anyway.

    PH probably makes sense when air conditioning is employed. But people can live without air conditioning. In this case does an airtight envelope make sense for cooling conditions where air conditioning is not used? Again in my case during the summer I seem to be saving energy by completely closing up the house during the day with no mechanical air exchange. This helps keeps the RH down in the home by minimizing the amount of outdoor air coming into the home. Early in the morning we typically introduce exterior air when the outdoor air temps are at its lowest.

  5. GBA Editor
    Martin Holladay | | #5

    Keeping your pet comfortable
    Let me get this straight -- you and your wife don't mind sweating out the occasional spell of humid weather, but you ended up buying a dehumidifier to keep your dog comfortable?

  6. user-788447 | | #6

    cooling via sweat
    I'm not an expert but here is my understanding that was repeated on today's local news.
    Infants, old people and small pets do not have the same ability as healthy people to cool down via perspiration. Small pets can only sweat through their noses and by panting. We get to sweat over most of our biggest organ, skin. My wife and I can determine when too much is too much. Since I don't know when 'too much' is for the dog I make some allowances for his health and well-being.

    OK, and it helps us sleep a little better at night also. Currently 89 degrees @ 65 RH inside ; )

  7. 5C8rvfuWev | | #7

    @ jchesnut
    You say the typical heat spell lasts about 3 days ... just wondering if you think you'd feel good about doing w/o AC if you were in the same hot/humid climate every day (except two) since, say, early May ... and scheduled to continue until sometime in September.

    Any thoughts? Do you feel you would sustain your comfort over the long term?

    RE: equipment durability -- the heat pump on this apartment has been the source of both A/C and heat since 1992. As an fyi, the current indoor numbers are 77 degrees w/53% humidity.

  8. gusfhb | | #8

    I know we like to beat on ac
    I know we like to beat on ac use but:

    yeah, I know, lies, damn lies statistics......

    a state of the art ac probably uses less power than the comic room of fans we have seen on tv ads

    Funny, my first thought[re: j chesnut] was about the increased heat load from the dog, I must be hanging out here too much...........

  9. homedesign | | #9

    Concerning PV
    I don't think it is realistic to compare the price of enclosure upgrades to the price of PV.
    Believe it or not... there are many deed restrictions in Texas that do not allow PV to be visible from public areas.
    The other problem is "the public" ... the HomeOwner/HomeBuyer
    99% of my clients have strong feelings about the "style"/fashion of their homes.
    Usually the "style" is not PV friendly.

    I would rather compare the cost of Enclosure Improvement to the cost of spray foam.
    Use the cost of a decent(min code R-value) spray foam Enclosure as a starting point for cost comparison.

    It is very costly to upgrade a sprayfoam enclosure beyond minimum code R-value.
    Not so costly with some of the non-spray-foam stratagies.

  10. homedesign | | #10

    Peak Cooling Demand
    I like this study concerning Cooling and Peak Cooling Demand
    I think BEOPT would be useful for considering projects with or without PV

  11. user-788447 | | #11

    @ JoeW
    I'm not as familiar with the range of AC/heat pump equipment available. Based on my reading of GBA I am looking into Air to Air heat pumps as an option for Passivhaus type designs. I'm also aware that my dehumidifier is similar technology. Scrutiny over whether these technologies are necessary and how they can be most prudently used is a good discussion and I appreciate your comment on the heat pump unit functioning since 1992.

    I lived in South Korea for a couple of years in small apartments without AC. South Korea on average is hotter and more humid than Minnesota and may be similar to southern climes in the US. Because I was in a small apartment in Seoul with only a couple windows on one wall it was more difficult to stay comfortable. At the time I was younger and really only slept at the apartment so I got by.

    Of course I can't speak to the experience of living in hot/humid climes in the US but I do believe with good housing design AC would either not be necessary at all or its use could be reduced significantly. I do like to think I could get by without it but am open to arguments for a modeled energy standard that could incorporate AC while maintaining a low overall annual energy consumption (like PH).

    [soapbox alert]
    Among the people I know most everyone has an aversion to sweating. Comfort is an personal expectation on one hand but it has also been formalized and semi-quantified as a concept by ASHRAE for the commercial workplace to determine temperature setpoints and ventilation rates. We should view this critically at least in part as determined as a factor of productivity. There are examples of past cultures that slowed down during the heat and did not maintain a homogenous standards during all seasons in order to stay productive.

    I do wish I knew more people who maintained their health; ate, dressed and acted appropriately during hot periods; and had a healthy mindset that can take perspiration in stride. I know this comes off as 'holier than thou' but I think I'm a down to earth person. Most people I observe too easily default to refrigeration with little concern about their energy consumption. I also think day to day experience is more nuanced and interesting when you find no/low-tech methods of living within local climatic conditions. In architectural school most people romanticize over 'vernacular' architecture but wouldn't advocate for regional differences in no/low tech methods of staying cool/warm.

  12. jgant61 | | #12

    One reaction to Passive Haus in Europe: "ACTIVE HOUSE"
    The issues of Passiv Haus to reduce daylighting and natural ventilation, perhaps excessively, for ever-increased air-tightness has led to a thoughtful response by others - see this new movement: "ACTIVE HOUSE" seems to be a very sensible evolution.

  13. gtmtx | | #13

    Living in a Hot-Humid Climate
    I lived in upstate NY for 22 years, where the occasional hot-humid spell (2-3 weeks a year) is bearable without a/c, but living in Houston without a/c, which can be so humid you might need to have a/c at Christmas (at least for dehumidification), sweating it out is not a practical solution. That is, unless you don't need a good nights sleep. Extended humidity is also detrimental to various personal belongings, photos for example. During the last hurricane, we lost power for 14 days in September, and yes it was bearable, but to argue we should live without it, is not a practical solution. That and living in the City, the joys of traffic, barking dogs, etc all night also help when your windows are open.

    For 4-5 months of the year, the outside air is more humid and warmer than the inside air. For example, the low tonight may be 80D/60RH at 3:00 in the morning, at no point does this really provide any cooling benefit.

    Technology has created solutions that eliminate common sense vernicular architecture, but that doesn't mean we should not use it, when appropriate. Would I argue you should where a down parka inside your house in MN so you use less heat? Of course not.

    From my perspective, the main goal in this climate is to seal the house as air tight as possible with a reasonable amount of insulation and control infiltration of outside air, by some means. For my money I would use foam (open or closed) for air sealing with blown-in walls for the remainder (as applicable) with a ventilated rain screen walls and heavily insulated (at least R38) "ventilated" attic.

    A few years ago I insulated the crawl space under my house (previous uninsulated) to R11 (open-cell foam) and cannot say there was a noticeable difference in my cooling bills, but actually helped more in winter to keep the floor warm.

    As far as the internal heat loads, cooking is the only thing that I have noticed can be a problem in the summer months. Moving this heat outdoors is not easy tasking since you need to bring in air (that generally is not wanted) inside to get it out. That said, I don't think over insulating a house is a real concern, except to ones pocketbook. has several designs for this climate, which pretty much hit the sweet spot for balancing the costs/benefits based on my experience.

  14. B_Carr | | #14

    @ j chesnut
    I would totally agree that “regional differences in no/low tech methods of staying cool/warm” should be employed whenever possible to achieve desirable results for energy efficiency. I would always advocate for a passive or mechanical solution to a design problem versus an active solution, which often tend to fail for reasons of over-complicated technology and/or user neglect or lack of maintenance. I would, however, say that the introduction active cooling technology into the market in the middle of the last century basically sent this country down a path that cannot be reversed. People have an expectation when it comes to comfort in their living spaces that is far higher than that of only 50 years ago (part of that expectation is interacting with coworkers or house guest and not smelling their “funk”). That expectation has been quenched (and fueled) by ever greater technology to fill the gap between comfort derived from passive design and our ever-greater expectation.

    With that being said I can tell you that here in the desert southwest (Phoenix area), you’d be hard pressed to find a person that doesn’t rely on some sort of active cooling method (either refrigerant or evaporative) in the summer time (and the “shoulder” seasons, even though we really don’t have them). So, how do you then resolve the issue of ever-increasing energy demands when passive design approaches don’t reach our comfort expectations? With more technology of course! I am a firm believer, however, that the technology we should be employing is that of the passive kind, such as better envelopes and increased insulation, to reduce the cooling load in our buildings. Taking that to the next step would be to reduce latent heat loads when possible as suggested by other responses here. I often grill outside during the summer, but doing so in 110-degree weather isn't pleasant either. There is obviously a point where too-much insulation doesn’t make financial sense, but I’d hate to continue to build the loose, under-insulated homes that have become the norm around the desert southwest because we’re worried about the heat not being able to escape the house. The benefits of reducing cooling load from heat generated by outside temperature and solar heat gain far outweigh the cost to remove latent heat loads from the house, in my opinion.

  15. servant74 | | #15

    Living in the 'Heat Belt'
    I have lived in the 'Heat Belt' my entire life. From ElPaso TX to Houston TX to Nashville TN

    The humidity is the main 'killer' in all these areas. With 'swamp bucket coolers' (evaporative air conditioners that add humidity to air entering a house) in ElPaso cooling was inexpensive and only overly oppressive about 2 days a year, when humidity was 'high', but turning up the volume of air with the cooler and other fans made it tolerable. During the dry days (about 362 days a year) a temperature swing of close to 40F high to low for the day was not unusual. Cooling season is really about 6 months of the year. And yes, it does get to 0F sometimes in the winter but typically is still above 32F during most of the 'winter'/heat season. ... Rather than more insulation (which does help) I liked wide awnings all around the house, open-able windows (painted shut windows are death traps in that area).

    Houston has a cooling season of about 9 months of the year and cooling of about 2-3 with only a couple of weeks in transition on each end. For well installed HVAC buildings, yes insulate it more than you think you need. We had a 'well built' (for the day) home (4 over time before we left Houston) and found that adding 'Attic Blanket' (non-backed thick voice over either batts or blown in insulation. It does help both in summer and winter to make the houses more comfortable, and we did notice a difference in HVAC bills typically noting less than a 1 year payoff (who wouldn't like a 8%/month ROI that continues after the bill is paid off). In 3 of the 4 houses we either had ceiling fans or installed them. They helped considerable in making life more comfortable year round, and kept me from turning the temp down so helped keep the bills down. ( I worked in a computer room, and wife kept the house typically at 83-84F, I did turn it down to 82F once arriving home. That was just at my 'level of tolerance' of the temperature. It went back up in the morning when I went to work.

    In TN, we live in a rented house (so we are not making changes), near the Cumberland River (a tributary runs within 50' of the house and the river is less than 4 miles from the main channel of the River near Nashville. We find this area to be as humid as Houston. It gets a lot colder here, and this 100+ year old farm house is uncomfortable year round. We seldom use the fire place in the winter as it really just seems to cool the house (even though the room with the fire place does get warmer). We do use the worst of all heating devices, portable electric heaters, to do spot warming even though the entire gas has propane forced air heating installed. And yes, we do use that too.
    This house has no opening windows (all have external sold storm windows and are painted shut anyway). ... Oh yes, the house and paid utilities come with my wifes job, since we are required to live on property as a term of employment.

    In both Houston and TN it is unusual to have more than a 20F daily swing in temperatures.

    IMHO, In general, for cooling use the 'old school' passive cooling techniques (broad eves, planting deciduous trees appropriately, have opening windows available, proper curtains/drapes and use them. But better insulation is always appreciated and often economic.

    For a house, I would not mind paying an extra 2 to 3% for much better insulation.

    If/when we move again, we want to build a super insulated house. Current plans are a dome, one level, external foam insulation over a concrete 'shotcrete' shell. Part of the concern is tornado's. We have always lived in 'tornado alley', and have notice even a higher incidence of tornadoes after moving to TN. The architecture probably drives design affectionado's crazy, but we like it.

    Now, I would LIKE to have (outside my expertise range) a spreadsheet with reasonable rule of thumb of the economic breakdown on how the balance between the cost of energy and cost of insulation. IMHO, energy prices will not do anything but going up in the long run, and insulation is typically a 'pay once' insurance policy against the increasing utility bills. Even doing a full building load analysis is not out of line, but most analysis I have seen focus on either heating or cooling and not doing 'life cycle analysis', which is my interest, not just the 'next season' but for at least 2 to 5 or more years so the cost and value forecast can be done.

  16. Mike Eliason | | #16

    ugh, summer in osan korea was
    ugh, summer in osan korea was terrible. worse than alabama by almost every measure except for food and culture.

    active house isn't really aas much an answer to passivhaus, as it is niedrigenergiehaus, a lesser energy standard.

    activhaus is passivhaus w/ double pane windows. it still requires airtightness, super insulation, mechanical ventilation and proper orientation. it also requires a significant investment in mechanical systems. i really don't see how it's any cheaper than PH in the long run or even first costs. it seems more a system set up by velux et al.

    i know of several passivhaeuser that have little usage of MHRV outside of winter, incorporating natural ventilation for nearly 10 months out of the year.

    Also, passivhaus in and of itself, doesn't reduce daylighting. this is part of the reason why i take umbrage with the 'living in a thermos' analogy.

  17. user-723121 | | #17

    Better buildings

    The software you are looking for to do a "life cycle anaysis" does not exist and probably never will. There is no money in designing such software, the market would be tiny. You will have to do your own analysis and I would suggest an energy simulation program to help with design and performance decisions.

    Harold Orr defined superinsulation as heating degree days (hdd) divided by 180 for the wall R-value. Denver, CO with 6,128 hdd would have an R-34 wall for example. In an area where the cooling season is dominant I'm not sure how you define superinsulation. I would say R-30 walls and R-60 ceiling insulation as a minimum should be considered. This coupled with an ach50 of 1.5 or less will make for a low energy home.

  18. GBA Editor
    Martin Holladay | | #18

    Mike and J
    Mike and J,
    I never ate better than during my 8 months in Korea.

  19. user-788447 | | #19

    great posts
    Glen, Brad and Jack I read your posts with interest. I don't get much of a perspective from the cooling climate other than John Brooks frequent and good input.

    Mike E,
    Did you eat the boiling hot chicken soap (sam gye tang - with the whole chicken) that Koreans eat during the hottest days of the year? The paradigm being beat the heat with sweat.

    My mother-in-law lives not far from Osan in a small 4 room slab on grade concrete structure without AC.
    My brother-in-law and sister-in-law live in that region also in high rise structures. The layouts of all the high rise apartments I've visited in Korea have windows on two opposite sides of the unit (meaning no single or double loaded corridors). They get great cross ventilation; a breeze you can actually feel most of the time particularly the higher units. However I'm sure my brother-in-law's generation, as they've become more affluent then the generation before, are using AC now.

  20. user-788447 | | #20

    korean food
    Something we all can agree on ; )

    Recommended for people who have tasted the pleasures of Korean cuisine is the Korean TV drama series Dae Jang Geum.
    Its a drama that depicts the life of an orphaned daughter of a royal court kitchen cook who becomes one of the few females in history to attain court status based on talent rather than bloodline. The early part of the series is a great historical piece recreating the workings of the royal kitchen with a lot of great discussion about qualities of food accompanied by delicious visuals.
    Later in the series the drama gets a bit overkill but the content transitions into background on oriental medicines which is quite interesting and appropriate in the context of food and diet.

  21. Mike Eliason | | #21

    never had the sam gye tang.
    never had the sam gye tang. sweating it out isn't limited to korea. i really miss cheap beef bulgogi, mondu and sitting on hypocaust floors in winter.

    all the military apts (at least the ones on/near osan AB) were double loaded corridor and had no cross ventilation. the air quality was pretty horrendous as well - though i seem to recall locals blaming the chinese.

  22. user-154536 | | #22

    ERV in hot, humid climates
    I have to chime in on Martin's comment on ERV impact on humidity. The use of an ERV in a hot, humid climate can be very significant. If the enthalpy efficiency of an ERV is 60-65%, that translates approximately as follows:
    Outside humidity = 80%
    Inside humidity = 50% (assuming air conditioning/dehumidified air)
    The result would be the air coming in would be approximately RH 60%. Although the air would have elevated RH relative to the inside air, it would be significantly less than using a HRV or opening the windows and bringing in that 80% RH air . In extreme conditions, this has been tested to result in a reduction of cooling loads of up to 45%, based on the fact that it takes more energy to cool humid air than it does to cool dry air.

  23. user-788447 | | #23

    @ Barry Stephens
    This is how I originally assumed an ERV would function, basically exchanging moisture from the higher moisture air content to the lower moisture air content regardless of the direction (outside to inside or inside to outside).
    Your response doesn't conflict with Martin's explanation-

    “If you are ventilating, running the ERV can only lower the indoor humidity when it is dryer outdoors than indoors — in which case an HRV or an ordinary fan would do the same thing,” Holladay writes. “In most summer situations, the ERV raises the indoor humidity — but not quite as much as the humidity would be raised if you were ventilating with an HRV or an ordinary fan.”

    It seems to make more clear that with AC/dehumidification the ERV's impact can help keep most of the humidity out when it is more humid outside than inside. But, in the case of trying to avoid or minimize the use of active AC/dehumidification, there maybe is not enough of a humidity differential between outside and inside air for the ERV to have much of an impact keeping the humidity inside as low as possible.

  24. user-1005581 | | #24

    Poor planning
    "Peak Cooling Demand
    by John Brooks

    I like this study concerning Cooling and Peak Cooling Demand"

    Although the concept of this study is intriguing, I believe they made some stupid mistakes in the planning of the model home.

    The study makes clear that its goal is "... to determine the least-cost pathway to the development of new homes with zero peak cooling demand." That's great.

    But if you are planning energy efficiency, the choice of your lot and building orientation are the most important first decisions. It takes one ton of air conditioning to remove the sun's heat that comes directly in 50 square feet of single-pane glass. That means that east and west facing windows bring in a lot of heat in the mornings and afternoons. The report clearly states: "The study was limited to one orientation (back facing west), allowing full exposure of the windows on the back of the home to afternoon sun." Remember, this study was meant to reduce peak cooling load. Why would you place big "solar panels" (windows) facing west when your goal is to reduce afternoon peak cooling load?

    Assuming that various vacant lots are available for the same price, purchasing a lot that has the rear of the home facing north or south is great way of reducing cooling costs at no cost. And that was, after all, the goal of this study.

    Another strange idea is planning equal amounts of glazing on all four walls with the homes to either side just ten feet away. So either you would be looking at a wall ten feet away or into the neighbor's home. As a realtor I have seen homes with windows directly opposite the neighbor's windows. What do people do (unless they are voyeurs or exhibitionists)? They keep the curtains or blinds closed all the time which means that the windows serve no purpose.

    I find it difficult to believe that researchers go to such lengths to test an idea and then plan it so poorly.

  25. homedesign | | #25

    Neil I agree
    I was disapointed that they modeled a "dumb design"
    a sort of generic worst case

    I would rather know the "what-ifs" based on a good or at least a decent design as a starting point. The "design" reminded me of the HERS reference house

    I like the concept of the BEopt software and the concept of considering peak cooling demand.

  26. fi5cAAjbSD | | #26

    The warmer the climate, the more likely I would be to have all my windows face the north, away from direct sun. This would reduce internal heating loading during daytime hours and would be a boon to the cooling system. The heating system would have to work a little harder, the same way the cooling system does in northern Passivehaus design.

  27. Mike Eliason | | #27

    one of the problems we

    one of the problems we discovered on the N.O. house we worked on was that the northern windows still were picking up quite a bit of solar gain in summers/shoulder seasons leading to increased cooling costs. you're better off just shading southern/western windows (as we did) w/ deep porches. this also allows the opportunity to still get decent solar gain in winter, which - as you said - would be a struggle if all windows faced north. plus, northern light is great for certain tasks/spaces, but i'd hate to live in a house wholly lit by northern light.

    I was wondering if they modeled a 'dumb' house for a number of reasons - most likely spec builders taking the same poorly designed house and then flip/rotate/repeat the hell out of it... starting from the worst case scenario might give a better sense of how bad things are, not how bad things should be.

  28. user-651098 | | #28

    Thermal mass
    The blog and the responses have dealt with insulation, ventilation, humidity levels and Korean cuisine. I'm surprised no one has discussed the benefits of thermal mass in reducing the heating or cooling loads. How effective would the use of thermal mass be in making the homes more comfortable in "cooling" climates?

  29. Mike Eliason | | #29

    in regards to


    in regards to passivhaus, i'm not sure... our model performs slightly better with high thermal mass, but the difference is pretty insignificant. difference in cooling load between lightweight construction and a heavy thermal mass is 0.17kBTU/ft2a, a 3.5% decrease. percentage of overheating also drops, but again a fairly insignificant 4.5% decrease.

    in training, kat hammered home that in a passivhaus, 5/8" GWB was considered enough thermal mass - so i'm thinking that the capacity to store or buffer the heat just isn't there when everything inside the thermal envelope is at the same temperature, plus or minus a degree or two.

    however, i have noticed a significant benefit to thermal mass in heating-dominated climates - especially in regions that can get cold in winter but receive ample sunlight - like calgary, denver and albuquerque.

  30. Mike Eliason | | #30

    I was thinking about this a
    I was thinking about this a little more... a number of passivhaus projects - especially institutional utilize thermal mass in conjunction with night flushing. so while it might not make sense or work out on our model, thermal mass is being utilized. however all the projects I'm familiar with in that regard are still in mostly heating-dominated climates.

    The passivhaus institut did a report on achieving passivhaus in Mediterranean, and I think results were pretty consistent regardless of construction type, that thermal mass didn't really change the numbers much. additionally night cooling can't always be done in hot/humid climates.

  31. user-651098 | | #31

    Response to Mike E
    Mike, Thank you for your observations.
    I've read about the use of thermal mass and other features to reduce household overheating in the Mediterranean region. Of course, these ancient features and building design are not used in conjunction with insulation. - Jim

  32. user-1005581 | | #32

    No night cooling in a hot, humid climate
    @mike eliason: "...night cooling can't always be done in hot/humid climates."

    This is certainly true here in southwest Florida. It can still be 80 degrees at midnight and only get down to 77 a few hours later. Even worse, the relative humidity (which is called latent HEAT) goes up to 80-100% every night year round except in winter when a dry cold front goes through. So you'll need a high heat tolerance if you want to open your windows in the evening and during the night. Turning on the air conditioner then during the day will involve not only cooling the home but also removing the previous night's humidity in the air and absorbed by all the furnishings.

  33. user-1005581 | | #33

    Water as thermal mass
    The Florida Solar Energy Center did research that showed thermal mass is helpful in reducing energy use in a hot, humid climate. That, along with hurricane resistance, led me to want to build with solid concrete and exterior insulation. However, as a realtor I experienced a huge drop in my income and no money to build this type of new home. So now I'm going to start a total rebuild of my single-wide trailer.

    I have two top priorities: hurricane resistance and hopefully a net zero-energy home.

    There are to be all new 2x6 studs along with metal hurricane connectors, plywood inside and out and closed-cell foam in the walls to greatly reduce heat gain and increase strength. Unfortunately this will provide little thermal mass.

    My plan all along has been to use radiant cooling in the ceiling with a water-based system. The ground water temperature year round is 78-80 degrees which is fairly close to the interior temperature that is comfortable for me. I'll need a water chiller to drop the water temperature a bit further in order to cool adequately. How much added cooling will be necessary is something I still have to calculate but my goal is to power the chiller with photovoltaic panels. There will be an insulated water tank similar to a boiler but containing cold water instead of hot water in the home. That way the heat it absorbs will be from inside the home instead of outdoor heat. By running the chiller as long as the solar panels are producing electricity and storing the coolth in the water tank, there would be cooling capacity during the hours of no sunshine.

    Since radiant cooling systems only cool the building, there would be a dehumidifier to remove moisture from the incoming ventilation air.

    My thoughts are that this is centrally stored thermal mass which can be easily moved around with a small electric water pump. Water can also store more heat than an equal amount of concrete and more quickly respond to changes in temperatures.

    I would like to hear other comments about this idea.

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