This would be for the 52 sq ft "cabin" in Gainesville. I need something to knock the high temps down and the low temps up. I want it to use as little electic and human intervention as possible. First the rules:
Low electric usage
Passive as possible
No Fire
Cheap
Easy to build
I need to keep the temps between 90 and say 60. Right now over a year I see inside the cabin from 100 mid day summer to 32 during the coldest nights in the winter. I don't care about day temps as much as I do nighttime temps. Perfect would be nighttime temps inside between 70 and 80. What I come up so far is in the picture below of what I call the Robbie Method.
Cabin in red. South to the left. North to the right. Black is a 6 inch PVC tube that can heat up in the sunlight during the day and thermosyphon into the cabin. Beige tube is same but it stays in the shade for cooling. I would have a simple hinged door to open and close so I can turn the system on and off. Any better ideas or just want to point out how stupid I am?
What temperature is the dirt 1'-1 1/2' down? Edit: Is the cabin in full sun all the time? You got me thinking on this one.
I'd say the odds are low that you make a meaningful difference in the winter with only passive methods unless you're willing to spend some time and money.
In the summer, just open windows. Done.
In the winter, though, you need to bring significantly more energy into the system, and store it for deployment all night. Fire is cheap and easy. Or, maybe take inspiration from the survivalists and make a solar water heater on the roof, then pair it with a big insulated reserve tank and an old radiator inside?
I think you're eventually going to arrive at "open a window when it's hot, light a fire when it's cold" though.
In reply to NermalSnert (Forum Supporter) :
Should be in the high 80s summer and low 70s winter. An earth tube could work somewhat but I'd worry about moisture building up in the tube and causing mold. I'd also like to avoid a permanent install. Yes full sun all the time but I have considered sunshades for the summer.
Lots of heat pumps use the ground as a heat exchanger, and you could use it as a heat sink- bury enough hose to get a good amount of soil at a depth where the temp is always where you want it inside. Roughly. It won't be perfect, but it should hold heat for over night warmth. Basically, a sand battery but you use the soil.
In reply to Tom Suddard :
Good point on winter nighttime. I have thought of a heat storage still using the same system. Like have a passive solar oven heating up a concrete block to then warm overnight. Another thing is body heat affects the temp in such a small space.
Stampie said:
In reply to NermalSnert (Forum Supporter) :
Should be in the high 80s summer and low 70s winter. An earth tube could work somewhat but I'd worry about moisture building up in the tube and causing mold. I'd also like to avoid a permanent install.
Use a 50/50 coolant mix, and then some kind of heat exchanger to heat/cool the air. Like, say, a radiator.....
In reply to alfadriver :
Water was considered but at the cost of electricity to move it.
For summer you might deploy a shade all day. At night a couple of vents at the floor and a couple near the ceiling might help air move at night. A small fan would help. Can you sleep with just a screen door? Edit: If you shade it in the summer you could use brick, old pavers, stepping stones, etc., painted flat black on the South wall to store heat in the winter. Any kind of dense material as a veneer.
In reply to NermalSnert (Forum Supporter) and Tom Suddard :
Yes the opening a door (I have a screen) is the current method for lowing temps after sun goes down. Works decently but I'm trying to be even more lazy.
Don't know if this will help but here's the temps for last week during the cold spell. I just want to smooth out the peaks and valleys.
R20 walls, R40 attic. Weatherstripping. Pretend you live somewhere cold.
Evap cooling wont work down there. And no ground cooling, soooo. Solar panel, battery, fan?
I could show you a home up here that never drops below 57 degrees in winter with the heating system turned off. It uses an earth tube year round, and passive solar for winter heat. The south windows all use solar panels as shades to keep the sun's rays out in summer. It's extremely well insulated and sealed. I would think you could do something similar with a home made solar heater on your south side and some heat absorbing mass inside. The mass could be brick pavers, for example. The earth tube really should have a small fan running constantly .
In reply to DeadSkunk (Warren) :
The home I end up building will definitely be a passive home. Slight differences as we care more about cooling than heat but yeah I'm all for that. To be clear though this is my temp 52 sq ft cabin that's already there. It has decent insulation, R13 wall and ceiling, but I'm just trying to get were I'm not hesitant to go down there because the weather will be too hot or cold. Not looking for pretty boy HVAC that's going to hold it a steady 75 but something I don't have to leave the property in the summer to find AC or have an innie instead of an outtie in the winter.
remembering that the property is "partially protected" and the cabin is "temporary" and on a car frame... I can understand the desire not to dig into the ground. I wonder if there's a way to simulate the ground insulation/regulation system by adding some kind of... material/mass to the bottom side of the trailer. I think you're going to need a fan to move hot-to-cold, one way or the other... but there's some pretty low-watt fans out there these days, and you've already got some solar going.
aside from that, from purely a heat perspective, here's a video I came across a while back:
otoh, I dunno how that design would go well in combination with a tiny house, on a trailer, in hurricane season. thinking about it, it's an interesting & challenge design space you're considering.
I think, ultimately, you want more insulation on the cabin, and some kind of thermal mass in the floor that gets heated in the winter, but is shaded in the summer.
I've got a relative that used to teacher architecture at FAMU SEAT (including iirc some sustainable/passive design), so I thought about having you contact the school. but, looking at their site, I don't know if that really oriented towards: passing information on to the public, as I understood land grant schools to be setup originally.
I've seen window sized solar collectors that generate enough heat, but you'll need a storage medium of some nature, some sort of Trombe Wall to retain some heat through the night. As for summer cooling, I'd look at having a canvas shade over the cabin to keep it from heating up. That won't do much if your night time temperatures don't drop significantly though.
Is this Gainesville, Georgia or Gainesville, Florida? For 52 square feet in Georgia, bury it underground, and you'll get a lot closer to the average temperature.
Florida is another matter since underground is likely to be underwater too. The above ground equivalent would be a huge amount of thermal mass and insulation.
A ground source heat pump would be a high upfront cost - and I don't know if they make them that small - but would have lowest electric use. This taps into the almost right underground temperature as a starting point so it uses less energy than an air sourced heat pump. Suggesting one for a building that small may be this thread's equivalent of reading your requirements like $2000 Challenge competitors read a budget sheet.
You could try a candle and copper strip in a metal bucket of sand for heat in the winter. I haven't tried it yet, but want to. Might be enough to take the edge off in Florida. A bunch of videos on YouTube if you haven't seen it.
In the summer, a battery powered fan works wonders.
I think shading that decreases light in the summer but allows more light in during the winter will be the ticket.
Ive lived off grid for a few years now and it's fire in the winter and shade/open windows/battery fans in the summer for me. But I live in a colder place than Florida.
First recommendation would be to plant a live oak 30 years ago then stick the cabin under that.
From: https://www.energy.gov/energysaver/landscaping-shade
Shading and evapotranspiration (the process by which a plant actively moves and releases water vapor) from trees can reduce surrounding air temperatures as much as 6° F. Because cool air settles near the ground, air temperatures directly under trees can be as much as 25°F cooler than air temperatures above nearby blacktop.
Second would be to find a used, roof mounted, DC solar panel, a couple of 12v car batteries and a fan or two. Mounting the panel on the roof will allow air to flow between the panel and the cabin, having a cooling effect.
To my eyes a nightly low of 32F seems like the easier issue. Wouldn't fully insulating the cabin and buying a Mil-surplus sleeping bag solve the issue?
Some good suggestions already. For the black PVC pipe heaters, you can buy something similar but more efficient commercially - they're black pipes inside vacuum-evacuated panels with a glass opening that lets solar heat into the pipes but doesn't let heat radiate back out of the pipes. These can work as solar water heaters even in cold environments, which is what they're most commonly used for.
Thermosiphon plan makes sense. I need to go find Robbie's post for a refresher.
Position your solar panels for shading. Kiddie pool for evaporative cooling (and Bob). Trombe wall for winter- gonna need a big window though to let the sun's radiation hit the mass.
R13 roof is gonna let you cook in the summer. Can you find any restaurant demo companies who what to throw out some walk in cooler panels?
No Time
UberDork
1/26/25 10:17 p.m.
What about something like Pool heater solar panels?
Maybe a couple outside and one inside set up so the hot water passively flows up to the inside one and back down to the outside.
Wow lot's of great ideas. Some/most of them I'll do when I build my full time house. It is in FL in a less hot area. The temps at night are low enough to cool down in the summer. Using shade will help summer a lot also as pointed out. I think I'm ok with my thoughts on summer going forward.
In reply to CyberEric :
Now that idea peaks my interest. It does violate the fire rule but a bucket of sand with copper to keep the sand temp even might work if I can heat it up during the day in a solar oven type thing. I'm thinking keep two buckets. One for the night and the next in the oven to warm up during the next day. Just switch them in the evenings. I think I might gather parts to build one with a candle and experiment with my kitchen since it's about the same size as the cabin. Thanks.
If there is airflow under the cabin, block that off in the winter! Nothing worse than the floor being the cold area.
