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Community and Q&A

Tips for Passive Solar Design

Green_Newbie | Posted in Green Building Techniques on

My designer is finishing up our floor plan and elevations, however, I am very late to the energy efficiency platform. I have been trying to read & watch as many videos as possible, but it is like drinking from a fire hose. We will be building a 2200 sf ranch home with some vaulted ceilings and a walk-out basement, facing the South in Missouri. I would like to know where to get the most bang for my buck, using passive solar concepts. Is anyone willing to look at my plan and tell me where to concentrate my efficiency efforts? I am meeting with a couple of builders to discuss our plans, but I live in a smaller community and very few builders are versed in energy efficiency, most just do what they have always done. Thank you so much, in advance.

PS mr_hugh_reference, can you help me?

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Replies

  1. walta100 | | #1

    Hi and please tell us your name.

    I built my home in east central Missouri.

    I build a BEopt computer model of my home I ran the model with my home facing every direction it made almost no difference in the total energy usage for the year. With more of windows on one side to capture the view.

    In my opinion passive solar is a quaint idea that sounds like it could work but turns out not so much in real life. It turns out the very best windows make terrible wall if you spend a fortune, you might get R5 any uninsulated wall is almost that good.

    From an energy point of view build the smallest house possible with the fewest corners ideally 4 minimize the size and number of windows and doors and keep the ceiling flat.

    Seems to me you are in a dangerous point of your build it is all too easy to allow your designer to draw beautiful plans that you fall in love with but are 2 times your budget.

    Consider talking to AAROW building they work in MO
    https://www.aarowbuilding.com/

    Walta

    1. Green_Newbie | | #3

      Is running a BEopt computer model something I could do for my home or a professional is needed? Is this a special software? Thank you.

      1. Expert Member
        DCcontrarian | | #6

        BeOpt is free software that you can download, just Google it. It's meant to be used by someone who is somewhat knowledgeable in construction but not necessarily a professional.

        I generally agree with what Walta said. In addition, for a comfortable and energy-efficient house you have to design it in from the start. If you wait until the house is under construction you're going to realize too late things you should have done differently.

        By doing energy modeling throughout the design process you can quantify the impact of various changes. Most likely where you are going to end up is lots of insulation, very air tight and a modest number of windows.

        1. Green_Newbie | | #8

          Thanks for sharing! I will look into BeOpt. Do you know if the Department of Energy offers any free services? I will research, but thought someone might know. Thank you!

    2. Green_Newbie | | #9

      My name is Joyce.

  2. Green_Newbie | | #2

    Thank you so much! Why do you ask for my name, just curious?

    1. Expert Member
      DCcontrarian | | #7

      So we can have a discussion amiably.

    2. brendanalbano | | #12

      It's a cultural thing. The culture on a lot of web forums is that everyone is anonymous and uses a screen name. The culture on Green Building Advisors skews a little bot more towards people using their real names as their screen names. Not everyone does it, and it's not required, but I think the idea is that it makes things a little friendlier. Whether or not that's true, I don't know, but GBA is generally a relatively friendly place, so perhaps its working!

  3. GBA Editor
    Kiley Jacques | | #4

    Here is a link to several articles on passive solar design that might help inform some of your decisions.

    1. Green_Newbie | | #5

      Thanks Kiley!

    2. Expert Member
      DCcontrarian | | #10

      Note that most of the articles that come up are skeptical of passive solar. When passive solar first was big, almost fifty years ago, a lot of what was written was decidedly non-quantitative -- as was the fashion at the time, the kind of modeling tools we have now just didn't exist then.

      Joe Lstiburek famously said super-insulation defeated passive solar. I look at it differently: energy modeling defeated passive solar.

  4. walta100 | | #11

    Joyce

    BEopt is about a 20 + hour commitment to watch the training videos, enter the data and come to grips with what it is telling you.

    If you commit do yourself a favor and watch all 16 videos in this link before you open the BEpot program. Note it is Windows only.

    https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d

    If you can invests some more time on video watch every Joe Lstiburek video you can find. Serious building science with large helping of comedy so you will not fall asleep. Many of us think of him as a god. He will change the way you think about building if you give him half a chance. This link is a good one to start with.

    https://www.youtube.com/watch?v=IaozbiujnWM&ab_channel=RooflogicLimited

    Walta

  5. brad_rh | | #13

    There's some unfounded anti passive solar attitude on this forum. In general Joe Lstiburek is wrong about passive solar. It's probably because it doesn't work where Joe, and most of the experts here live. I had a house built in sunny, but cold, central colorado, and can vouch that it works. I modeled it with energyPlus (I think BeOpt uses energyplus under the hood), and it was clear the south facing high solar heat gain windows are a net energy saver.
    I have an all electric 2300 SF house with only 4.4 kw of solar panels and generate a surplus of energy annually.

    That said, I don't know if it makes sense in southern Missouri or not. If you like a sunny house in the winter put some windows on the south side, but make sure that you have roof overhang or awnings to shade the windows in the summer.

    Make sure you're designer is cognizant of energy efficiency. A simple way, is to build according to a fairly recent energy code. My county is still using 2007, which most around here would agree is inadequate, but more recent ones would result in a 'pretty good house'

  6. walta100 | | #14

    Brad, it sounds like you have built the model please take the time and run the optimization tool with the house facing different 12 directions for a full year and post the graph.

    Walta

  7. mr_reference_Hugh | | #15

    At the risk of not answering your specific question, passive solar heating should be considered after taking care to ensure that the different control layers have been understood and addressed.

    I recommend this Youtube video by Joe Lstiburek.
    https://youtu.be/_JPkfIEYBBc

    Even if Joe S. is wrong about passive solar, few would dispute his perspective on building control layers.

  8. gstan | | #16

    Here's the most bang for your buck!
    1. Design your house to look (NORMAL) like the majority of homes in your area (remember
    you won't live there forever - you will want to sell it someday) - designing for passive
    and/or super insulation does not require weird or dramatic looks.

    2. Put in a little more (or better) insulation than the so-called experts think you need or
    than code requires - WHY? because the future is uncertain, nobody knows what energy
    prices will be tomorrow or 20 years down the road. More and better insulation is cheap
    insurance plus it reduces your yearly heating and air conditioning bills for the rest of your life.

    3. Design your house to have overhangs that shade the south facing glass from about
    the end of May to about the beginning of November - this prevents overheating in the summer. Overhangs can be the edge of the roof, awnings, porches, etc. - if your architect
    or designer does not know how to calculate the overhang required at your latitude either
    find a new one or hire an engineer who does (this is really quite important for passive construction).

    4. The single most cost-effective thing you can do is to put in insulated shutters for
    all windows! This is very uncommon - in fact you will have some trouble finding pictures
    of houses with these - the shutters can be either external or internal and you will end
    up building them yourself or having them built locally. Internal shutters are much more
    convenient of course (you close them at night in the winter). The reason this is so cost-
    effective is that heat loss through windows (even really cheap windows) with insulated
    shutters over them is dramatically less than the heat loss through the most expensive
    windows you can buy without shutters over them. And, the cost of just average quality
    windows plus the shutters will be dramatically less than expensive hi-tech windows by
    themselves (even if you have them built by your local cabinet maker). Have them built with
    1 to 1.5 inches of polyisocyanurate insulation in the centers. The shutters can, of course
    have any look or style you can think up - in my houses I designed to allow room for interior shutters to fold flat against the walls during the day.

    5. Last and about equally cost-effective is requiring the builder to be really diligent about
    sealing all possible air leaks (caulk, tape foam or whatever) - this one is tough - generally
    requires blower door testing and extra labor.

    What makes me feel qualified to give this kind of advice? I have designed, built and lived
    in three of them (I was the designer, the general contractor and actually hammered most
    of the nails - I lived in them for over 26 years). This was the way I did it and boy did it pay
    over the years.

    1. Expert Member
      DCcontrarian | | #18

      #3 above is where passive solar usually falls apart: "Design your house to have overhangs that shade the south facing glass from about the end of May to about the beginning of November"

      The beginning of November is 51 days from the winter solstice. The day in the spring where the sun traces the same path is February 9. The end of May is 21 days from the summer solstice, the fall day where the sun traces the same path is July 12.

      On average, the hottest day of the summer is August 1 and the coldest day of the winter is February 1. Each is about six weeks after the respective solstice. Because of that offset, there are days in the fall that are in cooling season, while the day in the spring with the same sun path is in heating season. So it's impossible to build a structure that is shaded in cooling season and unshaded in heating season.

      1. gstan | | #19

        You're right you can't get 100% coverage year around
        just from overhangs - BUT-
        It really doesn't matter - you can mitigate most of it AND
        notice I said "ABOUT". It's up to the designer to pick the exact
        dates and overhang distances to optimize shading for the
        latitude and the height of glass and vertical distance of the glass
        below the overhangs. IN ADDITION, it's possible to install movable
        shading to cover the few days per year when the overhangs don't.
        The idea that solar heating can't be utilized in energy efficient
        homes is deceptively appealing but largely incorrect - it just requires
        incentive and a little competent engineering.

      2. brad_rh | | #20

        Correct,
        It's a tradeoff. My house has full shade on the windows around the solstice. I don't have a problem with overheating in the late summer or fall. If I were to do it over again, in my climate, I'd do a little less overhang so I had more sun in the spring.
        Note that I don't dispute the passive solar (sun tempered is probably a better term) pessimism for a house in southern Missouri. I just get annoyed with the generalizations, when it would work for a large portion of the US.

    2. Expert Member
      DCcontrarian | | #22

      What I would recommend instead is energy modeling the house, and then change each aspect of construction and see how it affects annual energy usage. I'm more quantitative-minded.

    3. Expert Member
      DCcontrarian | | #23

      So let's take a look at shutters.

      Where I am (DC) code minimum u-factor for new windows is 0.3 or R-3.3 equivalent. We get about 3600 degree days of heating a year. Let's say you have a house with 500 square feet of windows and code-minimum windows, the heat loss through those windows will be about 3600 BTU per degree day, or 13 million BTU per year. A therm of natural gas is 100,000 BTU's, so that's 130 therms of gas. Right now it's about $0.92 per therm, let's call it a dollar to account for combustion efficiency. So $130 worth of energy per year goes out the windows.

      Let's say I make a shutter that adds another R-6.6, or triples the insulation of my windows. Let's also say it's closed about a third of the time, but since it's colder at night than during the day, it's closed for half of the heat loss. So half the time heat loss is reduced by 2/3, or overall it's reduced by 1/3. So instead of spending $130 per year I spend $87 -- a savings of $43. My heating season is about 130 days, so that's about 33 cents a day savings for opening and closing all of those shutters -- remember, I'm talking 500 square feet worth. That's not counting the capital cost of making all of those shutters.

      I could get the same savings by improving the U-factor of the windows by a third -- going from U-0.30 to U-0.20. That might get pricey, although it might be cheaper than shutters. Or I could get the same savings by cutting the square footage of windows by a third, to 333 square feet. The most practical solution is probably a combination of slightly better than code windows and a modest decrease in fenestration.

      1. andyfrog | | #24

        Skipping the shutters also retains the qualitative benefit of waking up to sunlight, which feels somewhat more humane than waking up in the dark every day.

      2. davidsmartin | | #25

        Instead of shutters, I installed honeycomb shades in my Vermont house that has an excessive number of windows to take advantage of the northern view. I don't know if they are cost effective, but I do know that in the winter they are closed about 15 hours a day and that they make sitting near a window much more comfortable. So for me they are worthwhile.

      3. gstan | | #26

        DCContrarian:
        Took a quick look at your numbers and assumptions - most places don't require
        a u-factor that low (which ups the window cost) in most areas the common
        contractor installed window will be double pane with a u-factor of o.5 (much
        less expensive) - right now (2022) the national Ave. for of a therm of nat. gas
        is about $1.18 - building shutters with 1.5 inches of polyiso in them will produce
        a window with a combined R-value at night (shutter +window) of at least 11
        (u-value 0.09) - All of which adds up to a whopping big increase over your guess
        of 33 cents per day - then I suggest you take a look at current prices for windows
        with a u-value of 0.2 or lower (most of them are made in Europe ) - talk about
        EXPENSIVE! Another thing, most highly efficient houses to be built will be all
        electric (more expensive for elec. resistance heat than nat. gas) - in fact some state & local governments have already passed legislation forbidding natural gas in new construction.
        So, you tell me what the cost of heat is going to be in 5 or 10 years.
        I stand by my statement "The single most cost-effective thing you can do is to put in insulated shutters for all windows!"

        1. Expert Member
          DCcontrarian | | #27

          If shutters work so well why doesn't everyone use them?

          The 2018 IECC requires a max U-factor of 0.30 for most of the country:
          https://www.windowanddoor.com/article/2018-international-code-requirements-windows-doors

          Note that the map shows California as 0.32, they've adopted their own code which requires 0.30 for the entire state.

          The Pretty Good House Folks -- who seem to know a thing or two about value engineering -- recommend a window with a U-factor between 0.13 and 0.20.

          1. gstan | | #29

            There are, no doubt, many reasons why working insulated shutters are so uncommon - one is because so far in history the economic need just wasn't there.
            Another biggie is that they are aesthetically unusual and most home
            buyers place a much higher value on aesthetics than heating efficiency
            (I don't expect that to change during my lifetime).
            As far as recommendations, keep in mind that they are based on
            current prices and preferences not what's to come
            - This house is to be somewhere in Missouri are they
            governed by the 2018 IECC? What' their heating degree day number?
            What's their electric rate and/or therm cost? - if the state goes all
            electric in the future how high will heating costs go?
            Better to go a little over the top now than suffer economically latter!

          2. Expert Member
            DCcontrarian | | #33

            Under the 2018 code the requirement for Missouri is a maximum of 0.32.

        2. Expert Member
          Akos | | #28

          "all electric, way more expensive for heat than nat. gas"

          That is completely wrong.

          If you look at equipment cost, a two stage gas furnace and bargain basement AC unit is about the same cost as a hyper heat modulating heat pump. Installing an AC or a heat pump is exactly the same amount of work. Now if you take out the cost of natural gas piping, combustion vents and CO detectors, a heat pump is cheaper. Around me, lot of new smaller construction is going with heat pumps for this reason.

          If you factor in gas meter fees, with a properly sized heat pump hitting a seasonal average COP of 3, operating cost is less.

          P.S. Around me U0.5 windows have not be allowed for a LONG time. Code min is 0.28 to 0.32 depending on zone.

          1. gstan | | #30

            I agree if you go with heat pump technology electric becomes
            competitive or even cheaper than gas - not sure about windows
            in Missouri - not sure what building departments would have to
            say about u-values with insulated shutters (my guess would be
            that they would give no consideration to them)

          2. Expert Member
            DCcontrarian | | #31

            In DC, just repairing the street for a gas hookup is $30K.

          3. StephenSheehy | | #32

            Heat pumps coupled with rooftop solar make the most sense. Electric rates here in New England are set to double from just two years ago, negating much of the financial benefit from air source heat pumps. Put enough solar on the roof and electric rates don't matter.

        3. big__o | | #44

          Gstan I wholeheartedly agree with this. Window insulation is the elephant in the room we are not talking about.

          I have 364 sf of South facing glass with a u factor of .60

          With 2500 hdd I lose about 13 million btus through those windows in the heating season.

          If I add window insulation and bring the combined u value to .1, the loss would drop to about 2 million btus . A savings of 10 million btus in the heating season. That's a far bigger savings than going from code minimum construction to passive house, for my location. Window insulation is something we need to talk more about.

  9. gstan | | #17

    One last thing I forgot to add : *** GOOD LUCK***

  10. rockies63 | | #21

    You can use this free overhang calculator.

    https://www.susdesign.com/overhang/

  11. hughw | | #34

    Interesting discussion....I now live in the passive solar home we completed on Martha's Vineyard year round. For it's time, 1982, the house was way ahead of things in terms of energy. The house was originally designed as a combination active-passive house. Most windows are south facing. Some window are shaded by 4 foot overhangs. Other windows with large expanses of glass are in "sun spaces" that can be isolated from the rest of the house by interior French Doors. The original conception was that warm air from the sun spaces would be pushed by a fan on the 2nd floor down a duct to below the "basement" ground floor slab where it would circulate through a system of mini-runners created by holes in concrete blocks, warming the slab. The idea was that excess heat during the day would be stored in the slab where it would then warm the house at night.....The fan system never really worked...instead, we would keep the sunspaces closed during the day and open them in the evening.

    Additionally, the house was considered "super-insulated" compared to normal houses 40 years ago. All non-sunspace windows were triple glazed. Walls were 2x6 with R19 batt insulation at a time when most homes were 2x4 with R13. Additionally, a 3/4" layer of foil-faced polyisocyranate was installed on the inside of studs with 3/4" strapping on top and then sheetrock. This provided additional insulation, an air space with a reflecting surface, and stopped thermal bridging in the studs.

    There was no heating system....just a wood stove. For 30 years, the house was mainly a summer house. When we would come up on a winter weekend, the ambient temperature was never below 55 degrees on arrival. We would use the wood stove as needed if there was a cloudy day. About 10 years ago we installed a heat pump system as the summer days were sometimes just too hot and muggy, and when we used in winter, the wood stove was just a pain in the ass.

    For the last 3 years, we've been living in the house full time. Generally, we keep the heat pump thermostat set at 66 degrees...If it's a sunny day, we might turn the heat up a bit in the evening if we're home. Until perhaps around 5pm, the house will be toasty warm.If it's cloudy, we'll turn it up when we get up in the morning. Is the house perfect? Of course, there are things I would do different today, but all-in-all, the care (and money) we put into the construction in 1982 was well worth it.

    Note: little structure to right is unheated, uninsulated, summer studio.

    1. user-723121 | | #37

      Nice to hear about a 40 year old well thought out build still performing. Hard to put an exact value on comfort but you sure know when it is missing. This is what I like to call inheritance quality real estate.

  12. Expert Member
    DCcontrarian | | #35

    "The original conception was that warm air from the sun spaces would be pushed by a fan on the 2nd floor down a duct to below the "basement" ground floor slab where it would circulate through a system of mini-runners created by holes in concrete blocks, warming the slab. The idea was that excess heat during the day would be stored in the slab where it would then warm the house at night.....The fan system never really worked."

    The big difference between now and 40 years ago is we have computer modeling that is relatively easy to use. With a model it would have been apparent that system wasn't going to work.

    Other than that, it sounds like a pretty good design -- thick insulation, triple-glazed windows strategically placed, big overhangs. Modeling helps a lot with optimizing those things but it sounds like you got it pretty good.

  13. gstan | | #36

    As far as I'm aware, local building officials have complete authority to
    change, modify, and adopt any version (including none) of building codes.
    Therefore, it's up to the homeowner to contact their local building
    department to discover what's legal and what's actually enforced in their
    jurisdiction. Local official often means someone as far down the political
    chain as a county or small-town employee. There are some states where the
    building codes are set by and enforced at state level - I know of none where
    it rises to federal level. This is why quoting something like "2018 IECC" for
    a question in this forum may or may not have any relevance.

  14. Robert Opaluch | | #38

    Sorry to be so late responding to this Q&A...

    Good tips that Akos, brad_rh and hughw noted above. They are familiar with or have designed and built homes that are solar-oriented and know something about how to design them and their tips could be useful for your design.

    Passive solar implies not only that you have sufficient south-facing solar glazing to heat the interior in winter, but you also have some “thermal mass” heat stabilizing material that will absorb some heat gain when the room air is warmer than the thermal mass, and radiate it back into the interior when the room cools overnight or the following day, which reduces interior temperature swings. A basement slab floor is a good example of a useful thermal mass or “thermal battery” that can charge and discharge some interior heat to keep temperatures more stable despite daytime peaks in solar gain and cooling during nighttime. So your walkout basement could be considered passive solar if its well-insulated, airtight and has the optimal amount of south-facing glazing. It should function well in your climate. (Note that windows must not be shaded by adjacent buildings, trees etc for direct solar gains.)

    Solar tempered buildings lack sufficient thermal mass heat tempering, so interior temperature fluctuations are greater, so less south-facing glazing is used. Typically, this means no need for space heating during daylight into evening hours. However, overnight or the next morning, your HVAC system would likely be used to keep desired interior temperatures. Not as much south-facing glazing is used, otherwise the space could overheat during fully sunny winter days. (Note that during summer, the sun is overhead midday, so south-facing windows get far less solar gain until Fall.). This might describe the upper floor(s) of your home. There are many articles on GBA in which owner-builders are happy with the results of incorporating more south-facing glazing in order to benefit from that daylighting and interior space heating. Plus lower utility bills and a more resilient building in case of power outages. Hundreds died from hypothermia in Texas power outages in 2021.

    In order for passive solar or solar tempered heating to work effectively, the building needs to be fairly airtight and well-insulated. Otherwise, the winter interior heating load is too high for solar gains to match, especially since the sun shines only about a third of the day in winter, and less when overcast. Passive solar is a sustainable heating system integrated into the building, not a substitute for insulation and air sealing.

    This is the passive solar home I built in Colorado with some information about designing a passive solar home and how they perform.
    https://www.greenbuildingadvisor.com/green-homes/a-passive-solar-home-from-the-1980s
    Colorado has a great climate for passive solar heating, since winters are cold but very sunny. Missouri not quite as sunny in winter, but similar to CO and much better than many other US locations. The more southerly location than locations farther north, means more powerful solar heat gains mid-winter. Kansas City MO is sunnier than St Louis, MO, and you can look up your location on weatherspark.com to get many detailed, useful charts in addition to the posted samples below.

    Note that about a third of the time for St Louis, it appears you would get useful solar heat gains, and with an airtight, well insulated home, likely require little or no supplementary heating for those rooms. However, half the time, skies are too cloudy so your solar gains will be more modest, and on mostly cloudy days, you may lose as much or more heat through the windows as you gain during daylight hours. Kansas City is better with a third of winter daylight time is sunny, and about 40% mostly or fully sunny, and less than half overcast or mostly cloudy.

    At night, window coverings help improve your window’s resistance to heat loss. As already noted by others, cellular shades or insulated shutters would be better than curtains that are open at the top and bottom. Two-thirds of the day its dark mid-winter, so you might as well cover the windows to reduce heat losses. Modeling your heat losses is a must, as others noted.

    The following article says more about the climate and the BTUs of solar gain that you can expect in various climates. Kansas City MO is one of the cities examined. Other locations with overcast winters are NOT candidates for a passive solar design. However, if you have unshaded south-facing windows in any of these climates, you will get more solar heat gain from south windows in winter, and get far less solar heat gain in summer, compared to west and east facing windows. So typically, a south-facing orientation is helpful.
    https://www.greenbuildingadvisor.com/article/a-quantitative-look-at-solar-heat-gain

    Best of luck with your home design and building project!

  15. walta100 | | #39

    I think the unanswered question comes down to.

    It is clear hughw house is a success but is it because it is a pretty good house that is well sealed with lots of insulation and the good windows?

    Are the large south facing windows the key feature that makes it work, or does it work in spite of the windows?

    Short of modeling the home facing different directions we are all just guessing.

    All I can say for sure is I was very surprised at how little difference the direction I faced my house made when I modeled it! My house has the big windows and large overhangs and failed to change the numbers in my location.

    Walta

    1. Robert Opaluch | | #40

      Walta,

      You mentioned that you built your house in MO.
      I listed Kansas City, MO in the second GBA article at the end of my reply (#38) above yours. Please look at Table 2 in that article.
      If you have four unobstructed, equally sized window glass 1 SQFT each, U=0.2 (R-5), one each facing North, South, East & West....and you subtract out the glazing heat losses for an average January day (assuming 16 hours with shades drawn when dark, making them U = 0.15 for 16 of 24 hours)... then 69% of the solar gains from the one SQFT facing south are retained after subtracting out the heat losses from that south-facing glazing. North facing is a net loser obviously with zero direct solar radiation. Slightly more than all of the solar gains for East and for West are lost due to heat losses through that glass, approximately no net gain or loss for glazing in those directions. This isn't guessing, its modeling the average January day for Kansas City MO.

      Your program may not model solar heat gains from windows, or your home might have about equal amounts of glazing facing each direction? Or your windows might be shaded by other buildings or trees? Generally passive solar designs have the majority of glazing on the south side, part of habitable rooms located on the south side, and partly due to a longer East-West axis rectangular building shell with longer south and north sides. That's how the solar heat gains add up, despite cold January temps and heat losses.

      As the articles notes, for Alaska, the glazing heat losses are so huge and gains so meager that there's no net solar heat gain after subtracting glazing heat losses. And other locations are more overcast on average, so the gains are far less. Only in Alaska are there no net gains for south-facing windows for an average January day. That's what Table 2 shows.

      UNOBSTRUCTED south facing glazing is a source of heat gain, even after subtracting glazing heat losses, even in the dead of winter. If shaded, it doesn't work of course. And not all days are average, some sunny days, some average, some overcast, so the gains vary depending upon the weather.

      1. andyfrog | | #41

        It seems like one major factor for relying on solar gains is the temporal distribution. Net average gain can still be rather uncomfortable if distributed extremely unevenly.

        It makes sense to keep in mind solar design, but I'm not sold on passive solar mostly because of climate change. I don't believe we know how things will turn out with that, and it's hard to ensure a passive design will continue to perform with what could be two zones worth of climate shift over the next 20-40 years.

        1. Robert Opaluch | | #43

          I look at BTUs of solar heat gains on fully sunny days, average days, and overcast (15% x full sun) days for January. And look at heat losses for average January temps and design temp. Typically coldest day is clear nights and day, so that combination is worth evaluating. Overcast day at average Jan temps is likely the worst performance realistically, and looking at those days for 3 to 5 days in a row to see interior temp drops is useful IMHO. Experienced only one 3-day overcast period in 5 years (dry sunny climate), and interior temps dropped to 59F. (I never used supplementary heating.)

          I also look at hourly heat losses and gains for 24 hour cycle.

          (Most people would have backup heating for unusual events, like a broken window or extreme weather events.)

          Climate change is warming. The problem with the house I built would be inability to continue overnight natural cooling during the summer. AC would be needed now or the future.

          Being in an area that thunderstorms and occasional intense rainstorms might be problematic, and worsened by climate change.... the quarter-acre yard is designed with a dry creek bed to drain flash flood water from west (uphill to foothills) across the south side of the house (20' away) to the street (downhill and out without nearing the house). Snow is Colorado powder, feet at a time but lightweight. Winds are notoriously high but I built fairly low to the ground vs. nearby properties, and used extra bracing in the attic and hurricane clips for truss and roof joist connections to top plates. Now I know ways to have done more for wind events.

          What would you do re climate change?

    2. Expert Member
      DCcontrarian | | #45

      Yeah, if you're not modeling you're just guessing. BeOpt lets you put in "thermal mass," when you model you can see it's not the panacea some seem to think it is.

  16. Expert Member

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