First Passive Solar Project

Hi there - i'm hoping to work on a small low-stakes project that will serve to grow my construction skills and my understanding of some permaculture principles.

The top idea i'm drawn to is to construct a small solar-heated shed, that i could sit in on chilly but sunny days to bask in the additional warmth. I have no formal building plans yet, but i think it boils down to 4 walls, a door, and a transparent roof.

Principles i'm aware of, and would try to put into action:
- transparent roof at fairly steep angle, to allow low winter sun to enter as directly as possible
- foundation that insulates the structure from the ground (with simple air gap, or real insulation?) to prevent heat being sapped down into ground
- dark colors inside the shed to absorb heat

What other principles can i take into account to make this project even more successful? I welcome any and all feedback! Sort of brainstorming-out-loud here.

My house is passive solar so your post drew my attention.

What are your plans for transparent roof at fairly steep angle?

Liam Hession wrote:Hi there - i'm hoping to work on a small low-stakes project that will serve to grow my construction skills and my understanding of some permaculture principles.

The top idea i'm drawn to is to construct a small solar-heated shed, that i could sit in on chilly but sunny days to bask in the additional warmth. I have no formal building plans yet, but i think it boils down to 4 walls, a door, and a transparent roof.

Principles i'm aware of, and would try to put into action:
- transparent roof at fairly steep angle, to allow low winter sun to enter as directly as possible
- foundation that insulates the structure from the ground (with simple air gap, or real insulation?) to prevent heat being sapped down into ground
- dark colors inside the shed to absorb heat

What other principles can i take into account to make this project even more successful? I welcome any and all feedback! Sort of brainstorming-out-loud here.

Not sure about your setting and/or surrounding trees, hills, etc., but I would recommend considering a south facing glass/greenhouse wall, with stone, water barrels, etc. to capture that energy. I did this at my old place in New Mexico, and not only was it cozy, but it allowed me to grow greens through the winter, and not have to burn wood as much.

I am going a different direction to the same end goal.  This plan is for a completely passive solar heater than I want to use on my shop:  BuildItSolar site - PDF

If you want the sunlight to sit directly in as well as to light the shed, I would put in a window as well as one or two or a dozen of these units, depending on the size of the shed.  If you want a heated shed, insulation in the roof and walls is key.  I personally wouldn't use a transparent roof, I would only have solar heat being captured on the south wall of the building and insulation in the roof.

See now i'm more on the right track, with just a few replies. Thank you guys!

So my idea of the transparent roof was to use something like the transparent corrugated plastic that's used in the solar heater plans Trace shared. But it sounds like the benefits of the more direct solar incidence that i would get from having a 45º transparent roof could easily be eaten up by how it would be harder to insulate well.

I definitely need to think about what thermal mass would be inside the shed.

Trace - do you have any idea why corrugated panels are used in that solar heater project? Also, wondering if you have any qualms about using plastic, and whether you could share how you rationalize its use in projects such as this. Not judging, not "should"ing, just curious because i feel strongly anti-plastic but realize it certainly has its draw in a lot of specific cases. Very long-term it does seem like BPA can leach from typical polycarbonates. But using glass doesn't seem very practical!

Hi Liam,
I have a few questions. How big are you planning to make this solar retreat?  Given a constant size window, the bigger the space, the slower it will heat.

Are you just wanting to go in there when the sun has warmed it?  Or hoping to be able to warm up after dark?  You wouldn’t necessarily need thermal mass, insulation and lots more windows if it’s for sunny afternoon use.

What else are you planning to do with the space?  What’s your latitude?  How cold is your winter, and how humid?  Do you have unobstructed access to sunlight?  How often do you have cloudy days?  Are you going to be dealing  with building codes and inspectors?

I don’t see why a glass window on a sunward wall is impractical.  If you look around, you can often find a sliding glass door or windows someone doesn’t want anymore.  

Even insulated glass has a low R value, but it’s way better than single pane.  Just make sure it’s not “low e” because low e reflects sunlight and PREVENTS solar gain.  If you are not planning to store cold sensitive things in there, or use it for a greenhouse it won’t matter so much if the shed cools off at night. If you are wanting it to hold it’s warmth over night, then you just put insulating curtains up on the inside, insulate walls roof and floor, and incorporate thermal mass…. And it gets more complicated in a hurry.

A wool rug on the floor might provide adequate floor insulation for sunny afternoon use.

Since it’s your first project, why not keep it very simple as you practice and learn?  Maybe build your basking shed with the idea you are going to add and modify when you see how much heat you get, and how much you need, and what else you want from the shed.

The use of a Trombe wall would be a good experient, it introduces more skills.
Also a solid masonary wall inside to hold het over when the sun goes to bed!

Thanks for all the good questions Thekla, here's my itemized responses:

What’s your latitude?

36.6º

How big are you planning to make this solar retreat?

I was thinking around the size of a phone booth, a bit bigger

Are you just wanting to go in there when the sun has warmed it?  Or hoping to be able to warm up after dark?

I originally pictured this being used the most in the winter, as a place to get warmed up for free if it’s a sunny day, after the sun has been out for a while. Do you think it would take until afternoon to achieve comfortable temperatures? Even if i really prioritize insulation? Maybe it’s more suited for fall and spring, when there’s not as much of a temperature “hill to climb”.

What else are you planning to do with the space?

Now that you ask... i’ve been thinking i’d like a tick-check room, really bright lighting and a mirror or two. Is there good overlap between the two usecases? Perhaps. It would be especially nice to have the residual warmth there during the day's end tick check on colder fall/spring evenings.

How cold is your winter, and how humid?

January’s average low is 21ºF, there are around 107 days per year with nighttime temps below freezing. Average humidity is below 30% throughout the winter. Gets quite humid in summer, ~67% humidity in July and August

How often do you have cloudy days?

Average here is 213 sunny days per year. Got all this from https://www.bestplaces.net/climate/city/missouri/alton by the way

Do you have unobstructed access to sunlight?

Yes this will be in a clearing on the ridgetop with full sun, even in winter. Thinking i will construct it on skids too, so it can be tried out in various sunny spots.

Are you going to be dealing  with building codes and inspectors?

Nope, no building code or permits in my county. Just confirmed that!

I don’t see why a glass window on a sunward wall is impractical.  If you look around, you can often find a sliding glass door or windows someone doesn’t want anymore.

It's funny, going back i don't recognize my thinking on "glass isn't practical!" I like that idea of reclaiming it

Since it’s your first project, why not keep it very simple as you practice and learn?  Maybe build your basking shed with the idea you are going to add and modify when you see how much heat you get, and how much you need, and what else you want from the shed.

I totally agree, that’s how i’m going to frame this project for myself.

Am i right to think that adding thermal mass could actually work against the goal of getting warm temps in there as early in the day as possible? Because the sunlight will be working to warm the mass that was chilled overnight, in addition to the air in the space?

And John, i would love to try out making a Trombe wall! Perhaps version 2.0 of this building, or a really simple version to start

Hi again Liam, those are some detailed answers!  
Your latitude has to do with what angle you would tip your glass if you wanted the most direct sunlight on the shortest day of the year. I don’t think you need to set your glass in an angle. Vertical would probably work fine.

There is a ratio of surface to volume to be considered when you are wanting efficiency in heat retention. I think a phone booth has so much surface area for so much warmth, that you are maximizing heat loss. A sphere is the most advantageous shape, and a cube next. A phone booth size is a rectangular prism.  you have a lot of wall for heat loss and a small surface for heat gain.  I think you might do better to make it bigger, and closer to a cube.  If you are going to make the ceiling 7 feet tall, then a 7 foot cube might be your best bet or since building materials are often 4‘ x 8‘ an 8 foot cube might be your best bet. Making room for two NON low e repurposed sliding glass doors on your solar gain side.  You might not want it that big, but consider the efficiency of the shape itself as you design.

I think it will be wonderfully warm from pretty early on a sunny day, as long as you can sit (or lie) in the direct sunlight.  An upholstered recliner, or a garden chair with a wool blanket, and you’ll be looking forward to a nap.

Insulation is a complicated question.  If you’re not wanting it to be warm overnight, then you might be able to dispense with it.  Likewise thermal mass. Keep drafts to a minimum though.  They steal warmth from your body in a noticeable way.  And if it is windy on that ridge, it will steal your warmth away from your retreat!  (Plant wind breaking plants that you want for some other function). If the wind consistently blows from the same direction, shelter or insulate that side first.  And put your mirrors where they aren’t going to reflect the sunlight back out of the building.

As for the thermal mass, I agree, it is going to slow the daily heat gain, but again if you are able to sit in the sunlight, not in contact with the thermal mass, you might be plenty warm.  You could throw a wool blanket or down comforter or sleeping bag over the mass to protect you from the radiant cooling of the mass while you are using the shed for warmth, leave the covering off during the day while your not.  Then cover it again after you’ve done your tick check.  In a building on skids, movable mass might be simplest.  It saves you having to build the whole building beefier.  (To me that’s a plus because you need less materials, and poorer building skills). A black 55 gallon steel barrel of water, or pebble style mass: pebbles in some kind of container.  Surface to volume ratio of the container as well as conductivity of the material the container is made of are some fun variables to consider.  And you can add the mass after you begin using your sun room.

It’s a perfect project to learn about passive solar!

Good luck and have fun!

Wow Liam, your project sounds much like a "relief shed" I built last year!

I wanted a project to "warm up with" in the literal and figurative sense of the phrase, too.  
I wanted to sharpen some light construction and design skills, and I needed a booth where I could escape rain or cold weather, somewhere to dry off, store gardening tools or start seedlings, check for ticks, change clothing, have a sit for a snack...a place where ladies could do their business if needed.  Here's how it came out:

I wrote: Features:
-Ground screw foundation
-Pallet floor base (potential to be skidded or lifted and moved on pallet forks)
-Clear roofing and salvaged storm door for winter warmth
-Solar phone charger + music (SolarHome 620 by Biolite)
-Small shelving for soap, sunscreen, first aid kit, bug spray
-Nice hooks for hand-tools, overalls, jackets.
-Bucket toilet (removable) with urine diverter running out the back into pine-mulch.

At one point I had water collection off the roof into a rain barrel I salvaged somewhere.  It was surprisingly effective for its size, and it was really nice having water to wash my hands and face or clean up with, or water my growies during drought.  I seriously considered making a little solar hot water shower system, and using the shed to shower in too, but I didn't want to drill holes through the base and I needed to move on to bigger projects.

Issues with my design:

Permaculture zoning. By design, my relief shed is not located in my current Zone 0, but more of a Zone 3; so as a result, weeds build up around it very rapidly.

Not taking into account the wind sector. The rain barrel at one point emptied, and because of strong winds, fell off the logs/pallet  it was resting on, and broke.

Underestimating the animal sector.  Despite what I thought was a mice-resistant design, mice snuck in and made a mess.  The reason they snuck in was...

Plywood. A lot of folks on permies are anti-plywood.  I had never really used plywood in any projects, and it was cheap and available, so I purchased some non-pressure treated stuff for the east and west wall.  Unfortunately, I made the poor decision of having it low to the ground.  As a result, moisture built up near the base, and the plywood warped, and left openings for mice to get in.  Lesson learned -- now I know for my bigger project that I'll avoid plywood!  The back (north) wall, however, is made of corrugated galvanized aluminum, and that has held up nicely and looks rather pretty, I think.

Even so, it served its purpose, and now I'm on to building bigger and better things!

P.s. Note that this is not passive solar due to the clear roofing which allows gratuitous summer sunlight in!