Passive solar house, radiant heat, and hydronic solar
Wasn’t sure exactly where this fit sorry if I categorized it incorrectly. I am in the planning stages of a small well insulated home build. The house will basically be a modified A-frame, technically it will be an extra tall gambrel on 4 foot knee walls so that I can have counters and furniture up against walls that are 90 degrees. Footprint of 24×36′ Full ICF basement as well as the two long 4′ knee walls and the rear 24′ knee wall. Building will be south facing and the south gable wall will be primarily glass. The first floor deck I believe I am going to do with an ICF decking system so it will be poured concrete with a ton of thermal mass I believe around 14 yards of concrete or roughly 56,000lbs. There will be an open loft in the back with half wall front (south) half will be open cathedral ceiling. Open floor plan, I believe I will do a spiral staircase roughly centered that goes from basement to loft continuous. Insulation will be r-49 Minimum in walls and (hot) roof.
Ok now that you have the gist of the build a couple questions.
Am I crazy to do radiant heat with 20′ high cathedral ceilings? I will have some ceiling fans of course to circulate heat down but will I be making heat at the floor and losing it to the ceiling or will this work out fine?
I would really like to add hydronic solar for the radiant but house is already going to be passive solar, so on days where the sun is shining in winter (I am in Vermont and there are less of those sunny days than we would like but its always cold) Anyway, on days the sun is out I really should not be heating the slab at all the house with all the glass should really heat well on its own. Could I way oversize a basement water storage tank to use as additional thermal battery? There will be a poured patio outside so that could become a dump zone if I needed to get rid of excess heat.
If this is a solid plan how would you size the solar array?
I have about 1000 more questions for this build but lets leave it here for now.
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Replies
This is just a quick sketch-up. Glass design was just thrown in and is far from finalized.
The biggest mistake you can make with passive solar, or any heating system for that matter, is to assume it's just going to work without being engineered. The landscape is littered with failed attempts at passive solar where it would have been obvious from the get-go that they weren't going to work if any engineering had been done.
The place to start is with an energy model. The software I would recommend is BeOpt, it's free.
I don’t want to be mean but everything on your wish list is straight out of the 1970s.
Over the last 50 years the passive solar ideas have been tried and mostly failed in the real world they simply just don’t work not for the lack of money and effort by the smartest people.
I say the few passive solar success stories work not because of the passive solar but in spite of the oversized south facing windows.
What does work in real life is reasonable insulation good windows with air sealing but don’t go crazy. flat ceiling covered R60 of cheap fluffy insulation. Keep the window area to no more than 20% of the walls double pain windows are fine in zones 1-4 triples in 6&7. Radiant heat will at least double your HVAC budget and in a tight well insulated building the floors will mostly be lukewarm not hot the way they were in the drafty uninsulated building you remember. Almost everyone needs cooling today and that has you installing a second system.
Read about the “pretty good house” I think it has the right balance.
Walta
I was trying to nudge him to do the calculations to get there on his own, but you're right.
Thanks DCcontrarian I will check out that software for sure.
Walta, I guess I don't fall under 'almost everyone' because I don't have cooling and I do just fine without it Id guess here in Vermont there are less than a dozen nights a year where I wish I had it but even on our hottest days its usually in the 60s or low 70s at night. Get the cool air in at night and close up the house in the morning works well for me. But if I really wanted cooling I'd add a mini split they are easy enough to install.
I wont disagree low flat ceilings would be better but I am hoping I can be "pretty good" with the cathedrals because they work well with the rest of the design.
I helped a friend build a passive solar house about 15 minutes from here. His designs not mine. and much different design than this build. But its very well insulated and very tight. On a single digit sunny day in February his heat (hot water baseboard) will not kick on and the house will climb from 62 to 72 degrees by late afternoon. His propane bills are incredibly reasonable across the season.
I do understand the floors wont 'feel' hot. I don't care about this. I want the house to stay a stable 68 degrees with minimal energy input.
I do plan to use insulated window dressings at night I know, I know, another 70s trick...
The absolute worst way to gauge the energy efficiency of a house is how it feels inside.
In my experience, houses built to "passive solar principles" often use more energy than ones built according to modern building science. In some cases, a lot more.
I will also add here that just because your friend's house seems to work, doesn't mean that yours will. Maybe your friend just got lucky with his design. That can and does happen sometimes, but it's much more likely you'll have problems down the road if you don't do the proper design work up front. Usually passive solar houses either overheat, or are not heated evenly. You have little control over heating with those designs.
I'd avoid the gambrel roof. I think you complicate a lot of things going that route (structurally, and the insulation/air sealing details). It's easier to build a box. That said, I do personally like cathedral ceilings, so maybe try a really wide one, with the walls not so high, but a large, open expanse through the middle of the space? Modern materials (thinking LVLs and glulams here, but also steel) can help to make long, unsupported spans a reality. Don't limit yourself to framing things with only dimensional lumber here if you want that feeling of soaring open spaces.
For solar, you are MUCH better off going with solar voltaics. The electricity produced can be used to heat the space, to cool the space, or to run gizmos in the space -- or even all three at the same time! Maybe heating and cooling at the same time isn't the best idea, but you get my point :-) Use some minisplits in that same well-insulated home and you'll have much more control over your indoor temperatures, with a lot less complexity and a lot less risk of unforseen problems down the road. You can use the extra power in a peak shave grid connected system to cancel out some of your electric bill too, regardless of if you have a favorable net metering program available or not. On top of all of that, I'm pretty sure solar voltaics are cheaper now than the old hydronic solar panels were, and those solar water heating systems are very much a thing of the past now -- I'm not even sure anyone is making them anymore.
Bill
WardBrookWoods,
If you want to take a deeper dive into discussion on the subject here on GBA, you may find these articles useful:
https://www.greenbuildingadvisor.com/article/reassessing-passive-solar-design-principles
https://www.greenbuildingadvisor.com/article/revisiting-the-passive-solar-debate
https://www.greenbuildingadvisor.com/article/solar-thermal-is-really-really-dead
Be creative, don't be dissuaded by the days of yore talk. Daylighting, window placement means everything in a new home. Go for some soaring spaces, why live in a cave? The energy penalty for vaulted ceilings and a few extra well-placed windows is tiny. Make the building envelope superinsulated and airtight and you will not look back.
Doug
I actually agree that you should build the house you want to live in.
There's no energy penalty to high ceilings, interior volume doesn't affect energy consumption, it's all about exterior surface area and what it's made of. I think Walta's point is that it's harder to make a cathedral ceiling energy-efficient than a flat one.
I'm not a fan of high windows. You can't see out of them and they're a maintenance nightmare. Generally windows are a net loser when you do energy modeling, so you want to keep the glass area down to ones that you can see out of.
Again, tight well insulated building will work in spite of oversized south facing glass.
I encourage you to build a BEopt model of your friend’s home spin the model in every direction my guess is the energy usage will not change by more than 15% from best to worst.
No mater how you slice it a cathedral ceiling is 10 pounds of bull dropping in a 5-pound bag there is never enough room to fit in a reasonable R value at an affordable price point.
The radiant heat choice gives you few heat pump options and propane pricing are unregulated and very volatile.
You should consider marble window sills because the insulated window dressings will lower the surface temps of the glass and they are going to be wet and lower the indoor humidity.
https://www2.nrel.gov/buildings/beopt
Walta
WBW,
Build your house as you see fit and use all of the tools available today. Spend a lot of time planning, build the house in your mind before you break ground. If it does not work on paper it will not work in the flesh. My first build in Minneapolis/St. Paul in 1988.
https://www.redfin.com/MN/Eden-Prairie/10481-Bluff-Rd-55347/home/49888384
Doug
Thanks all I do appreciate the feedback. I have started messing around with the BeOpt software but I have a lot of learning to do to understand how to utilize all the features. It also seems to be a bit limited to a lot of the design features of this particular build.
As pointed out a lot of the design features are not being made on a strictly most energy efficient model basis, thats fair. I guess I am more looking to build the house I want, and then figure out how to make that house super efficient, rather than figuring out how to build the most efficient house period.
I am not afraid to use LVLs or extra thick foam wraps, non conventional framing etc to get the R values I want in roof and walls. I will be doing to build myself so some of that is easier than trying to convince an outside contractor to do something non conventional especially without breaking the bank in labor.
I want radiant heat not so much to have warm feeling floors, which I understand I wont have in a super insulated home, I like radiant because I love how clean looking it is. No baseboards, and no heat registers, and no blowing air. And I think if doing radiant, doing it with as much thermal mass as possible makes sense to me.
I am struggling with the BeOpt software but I did do some modeling with ChatGPT, this was the first time I had played around with capabilities like that in AI and it was pretty scary how in depth of answers I could get but thats a topic for a different discussion. Anyway you guys were quite right. The differences it showed me in large amounts of glazing passive solar design, versus standard glazing design, was less than negligible. The propane usage was almost identical if one can trust modeling done in this manner at all.
That being said this is going to be a compact home, I want it as small as possible but I dont want it to feel overly cramped, and I like natural light. I want high ceilings and lots of southern glass for those reasons alone. But after your advice and seeing these numbers, I see that I can probably find more of a balance. Rather than trying to maximize every inch of glazing I can out of that southern wall I think it will make more sense to add enough for the natural light and open feel but I don't need to go crazy.
I really like the idea of marble or other rot proof sills, major condensation issues with insulated curtains is a great point I had not thought of.
One other thing while at the end of the day I probably will have a lot more glass on the southern wall than efficient design would otherwise advise, being a modified A-Frame the east and west walls will be all roof and therefor no windows at all, and the north wall will have very few windows, so it really probably does not average out to that much more glazing than any standard home of similar size.
Anyway thanks for the advice I do appreciate all of it.
Whatever you do, please take all that concrete out of your design. It does nothing for efficiency and is environmentally irresponsible.
Don't be afraid to trade R value for window area if you really want a lot of windows in your home. If you have a stunning view, or you just want to have a big "open" area to look through, don't give up on that just to gain efficiency. Windowless boxes are not enjoyable to live in after all. I always try to maximize efficiency without compromising other aspects of the design. Energy efficiency is NOT, and really should not be, the only goal when designing a home. Just be informed so that you can make intelligent choices to get the best efficiency you can from a design that meets your other goals too.
Bill
In post #3, Walta recommended the book "Pretty Good House." I recommend reading it before starting on your journey. It basically describes what a practical house should look like.
Now, this is your house, if you want it to be impractical that's your right. But it helps to know what practical looks like so you can make an informed decision.
There is nothing wrong with utilizing solar gain
Your design, however, has no way of stopping solar gain when there is the most solar energy
In the summer
With careful siting, proper overhang design, high performance glass[say U.16] and excellent insulation you can get good solar gain without the nighttime cold wall of death in the winter.
Another issue : The sun is at the same height in April as it is in August. So, entirely passive solar is really not such a great thing
BEopt is not quick or easy. I forgot to link to the 16 training videos.
If you put in the time the program is very powerful and can help you make logical choices instead of wild guesses based on gut feelings. What U value window is best for my house? U28 at X per SQF or U22 at Y per SQF? R 38 at X dollars a square foot or R60 at Y dollars?
https://www.youtube.com/watch?v=GdMYCuwp0AY&list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d
If you heart is set on a dream by all means build your dream.
You’re A frame will have a cathedral ceiling be sure to read this article and pick one of the 5 options.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
You know, I was looking at a similar concept except I was looking for a way to add a cost effective second floor to a basement by using prefab gambrel trusses. I had no intention of trying to incorporate passive solar though. I had thought of adding solar panels to the southern high sloped roof and see if I could match the angle to sun at the winter solstice though.