Can a DOOR heat your workshop for free?

In this Instructable, I'll show you how I built a passive solar door to help heat my garage workshop using the sun!

Here's the video overview.

Step 1: What Is Passive Solar?

What is Passive Solar?

While there are many forms of Passive Solar, the term is generally used to refer to architectural details, building orientation, and use of windows to allow direct sunlight to enter a building and be absorbed by heavy materials so as to heat a building.

This usually means large south-facing windows with a wide overhang above. The overhang allows sunlight through the windows when the sun is low in the sky (during the winter) but shades the window in the summer when the sun is high in the sky.

This is a very different concept than various active solar strategies, including Photovoltaics, the type of solar which creates electricity. I already designed a photovoltaic system to power my home, my garage, and my electric car (see details here,) but I wanted to be able to also use the sun to heat my garage workshop in the winter!

Passive solar is simple. It mostly relies on building orientation and glazing - a window material which lets light in, but keeps heat from going back out. Glazing can be glass, plastic, or other similar materials. Most of us are familiar with how warm it can be in a green-house, even in the middle of winter!

Even without glazing, humans have used the natural heat of the sun since ancient times. I still remember in grade school learning about Mesa Verde, where Native Americans built an entire village into the side of a cliff. The sun shines on it in the winter, while the cliff shades the village in the summer. (You can now visit it as a National Park.)

When I built my garage, I designed it with a 5 inch thick concrete slab. Under the slab is 2" of rigid foam insulation. When sunlight shines on the concrete, the heat gets absorbed. Because the concrete is insulated below and on the edges, the only place the heat has to go is to re-radiate out to the garage. This acts as a "thermal flywheel" and helps maintain a stable temperature.

My garage doors are fully insulated - filled with urethane foam - about the highest R-value on commercially available doors. My plan was to build a window which would fit in front of the garage door. On sunny winter days, I could raise my insulated door and let in the heat and light. At night, or on cloudy or snowy winter days, the garage door will be left down to keep the heat inside.

I still wanted to be able to move a car in and out of my garage, so instead of a big window, I designed it as a pair of doors, split down the middle, and hinges on the left and right sides.

The best part of solar thermal heating? It's FREE!

Step 2: Tools and Materials

Before getting started, we'll need to know what tools and materials are required for the project.
Essentially, this is a straight-forward wood-working project, so it's mostly tools that should be common to a typical shop.

Tools:

Chop-saw
Table-saw
Clamps - C-clamps, Bar clamps or Ratchet straps
PPE - Personal Protective Equipment - Safety Glasses, Hearing Protection, Work Gloves, etc.
Pencil
Speed-square
Tape measure
Drill, Drill Bits, and Counter-sink
Power Screw Driver
Plane
Router with rabbet bit
Wood Chisels and hammer or mallet
Jig-saw
Caulk-gun
Straight-edge
Utility Knife

The project is primarily constructed from wood and plexiglass. Materials used in the project are as follows:

Materials

Wood Framing - 1"x6" dimensional lumber (.75"x5.5" actual size) 6 x 16' boards
Plexiglass - 2 Sheets of 4 foot by 8 foot, 3/16" thickness
Window Insulation Kit - Patio Door Size shrink wrap plastic
Hinges - 3 pairs of heavy duty door hinges
Barrel Bolt door closures, 2
Screws (1.25" length)
Wood dowels (5/16" diameter)
Glue - Exterior Grade Wood Glue
Weather-Stripping - Nail-on 7' long garage door weather-stripping, 4 pieces
Weather-Stripping - Foam Self-Adhesive, 1/8" or 3/16"
Pressure-treated 2x4
Caulk - Weather-proof, for acrylic and wood use
Paint

Step 3: Framing

To start with, I needed to build the wood frames of the door.

The style of door will be that of a pair, which is hinged on the outside edge, and the two halves meet in the middle. Commonly, this is known as a "French Door".

MEASURE TWICE:
I began by measuring the rough opening of the existing garage door frame. That's 9 feet wide by 7 feet high. For simplicity's sake, we'll use those numbers in this example, although in the real world those numbers weren't perfect. The opening was slightly out of square and the one side had a bit of a curve to it. We built the new solar storm door square and to the original opening size, but planed the outside edges as needed to best match the opening when installing.
The doors would also go over a 2x4 threshold, so 1.5 inches was subtracted from the bottom of the door height.

CUT ONCE:
Once I had the proper measurements, I cut the 1x6 boards to length for the verticals. I temporarily set the uprights outside to begin to imagine what the finished doors would look like and realized that a pair of 1x6s down the middle looked too bulky. It would also block more light than needed. I decided that for the central vertical piece of wood, I would rip one of the 1x6's in half on the table-saw. This gave the doors better proportions and maximized the glazing.

The width of the plexiglass is 48 inches. The total door width is 108 inches. The plexiglass also needs to overlap the wood, and I decided to overlap by a full inch on each side. 46" x 2 = 92 inches. 108 - 92 = 16 inches. The three vertical 1x6 boards makes up the 16 inches needed to totally fill the width of the doorway. The verticals were cut to the height of the doorway, minus the height of the threshold.

I cut four horizontal boards to 46 inches each. That's the width of the plexiglass minus an inch of overlap on each side.

DOWELING:
To join the boards, I used wood-working dowels and glue.
All the boards were laid out flat on the floor of my garage. We marked the boards with pencil for where the dowels would go, drilled precisely placed 1/8" holes, and then drilled the 5/16" holes for the dowels. These were then dry-fit for testing and alignment.. In place of dowels, a person could also use wood-working biscuits or pocket-screws.

MAKE IT SQAURE:
Once all the holes were drilled and dowels tested, we needed to glue the frame and ensure that it's square. A simple square can be placed at each corner to check for square, but an even better way to test is using a tape measure. In a rectangular shape (such as these doors,) measure from two opposite corners. Then measure again from the other two opposite corners. The measurements are the hypotenuses of two triangles. If both measurements are identical, they are RIGHT triangles - perfect 90 degree corners.

CLAMP IT:
We glued the dowels, assembled the frames, and did the triangular measurements. They needed just a little adjustment to be square. This was done simple by pushing on the opposite corners of the longer dimension. We then screwed down a piece of scrap wood to hold it in position. I didn't have long bar clamps (which are commonly used in larger wood-working projects,) so we used ratchet-straps. Those are usually used to secure loads on trucks and trailers, but have many uses, including clamping large parts while glue sets.

We allowed the frames to dry overnight to set the glue.

ROUTING:
To make space to insert the plexiglass, I would need to cut a groove in the wood. This is easily accomplished using a router with a "Rabbet" bit. The router bit has a bearing on the tip to allow it to follow the shape of the wood. Which diameter bearing is in place determines the width of the cut. The maximum this bit could do was a 1/2" wide cut. I tested the depth of the router bit by making it just a hair deeper than the thickness of the 3/16" plexiglass. I then routed a scrap of wood and confirmed that the plexiglass would lay in it properly.

Next, I routed the inside of both wood frames.
To get my full one inch groove, I switched to a bit without a bearing on it, clamped a board on the frame to prevent the router from going past one inch, and routed a second time.
(I am not particularly experienced with routing. If I were to do this again, I might simply decide to make the width of the plexiglass groove whatever the maximum is that I could get in one pass with a rabbeting bit. There are also other ways to cut a rabbet, such as with a special blade on a table-saw.)

Because the rotating bit of the router cuts circular, the corners of my cut were rounded, instead of square. For the plexiglass to fit, I needed to finish the corners. I did that with a traditional wood chisel, cutting into the corner by pounding the chisel with a hammer.

After that, the wood frame was ready to receive the plexiglass.

Step 4: Glazing

First, the plexiglass needed to be cut to length.
I looked up how to cut plexiglass, and most of the information covered using very particular styles of power saw blades at certain speeds. It seemed to me that it would be easier to use the "Score and Snap" method.

SCORE!
Since the garage doorway was only 7 feet tall and I also needed to account for the framing and threshold, the acrylic would be cut of approximately 6 feet. The 2 foot cut-off will be saved for other future projects. I measured where my cut would be, marked it and clamped a drywall square in place as a straight-edge. While I had a utility knife, I found that a carpet knife worked well. The carpet knife has a large handle and the blade didn't catch or dig into the drywall square the way the utility knife could.

I scored the full width of the plexiglass by dragging the knife over it 50 times.

BEND.... AND SNAP!
Next, I lined up the scored line over a hard edge. We used a spare 1x6, but the square edge of any table or work-bench would be fine. With steady pressure bending down, and the score on the edge of the wood, the plexiglass snapped cleanly.

FIT AND CAULK:
I test fit the plexiglass into the wood frame. I found that there were a few places where I hadn't routed wide enough. I marked those with pencil, then came back and routed slightly farther. I then test fit the plexiglass again until it fit. Once it fit, I ran a bead of caulk all the way around inside the routed groove and finally placed the acrylic into the frame. I also placed a scrap of 1x6 right in the middle of the frame to prevent any possible sagging of the acrylic. I also peeled back the adhesive wax paper from the edge of the plexiglass.

BLOCKING:
Next, I cut scraps of the 1x6 to about 9 inches long. I screwed each of these into the four corners of the frame. This strengthens the corner joint AND holds the plexiglass in place. I also cut smaller pieces and screwed those into the center sides of the frame in the longer direction.

OTHER OPTIONS:
While I chose sheet acrylic for this project, there certainly are other options. Glass is heavy and fragile, but has great light transmission. Twin-wall polycarbonate is very durable and less expensive than plexiglass, but also less transparent. The material is commonly used for greenhouses because of its combination of strength and affordability.
I chose plexiglass partly because it transmits 92% of the light hitting it. Twin wall polycarbonate tends to be about 85%. While that doesn't sound like much, it's DOUBLE the loss. I want to maximize the heat gain through my windows.

Step 5: Mounting

With the glass in place in the doors, it was time to mount them in the opening of the garage door.

We first installed the right-hand half of the door. We knew that the garage door frame was a bit out of square on the left. By installing the right one first, we would have something to place the other door half against, and figure out and trimming or other work needed.

We placed the right-hand half in the frame and propped it up on 2x4 scrap blocking and shims. I screwed in two hinges - one top and one bottom - once we leveled and plumbed the door half. The back side of the door would more or less stop against the trim that was already in place for the original garage door. A middle hinge was added once everything was otherwise in place.

We then positioned the left-hand half of the door. That half needed to be planed down on the bottom outside edge. We used a power-planer and trimmed the edge of the door until it fit. Once in place, held up by blocks and shims, with an even 1/8" gap between the doors, we installed the hinges on the left half of the door.

Mounting the doors was really just a matter of making sure that everything was plumb, level, and square. Where the doors didn't properly meet the wall, they were planed down until they did. 3/16 foam weatherstripping takes up any spare space between the edge of the door and the garage door opening.

Step 6: Threshold

A threshold is a piece of stone, metal, or wood that forms the bottom of a doorway. It is a raised surface which allows the door to be higher than it otherwise would so that the door can clear the floor or ground. It is also a surface for the door to make a weather-tight seal against.

In my case, I needed something for the the bottom of the door to seal against, but it also had to be rather tall.

Unfortunately, we had some SEVERE frost heaving, causing the concrete apron of the garage and the blacktop of the driveway to pull apart and rise almost 2 inches. To be able to open the doors, they would need to be able to clear the top of the pavement.

Before building the wood frame of the doors, I took measurements with a tape measure and level to see what would be needed to clear the bulged pavement. It happened that the measurement was very close to one and a half inches, which is the thickness of a common 2x4. So, the threshold would just be a 2x4, laying flat on the concrete.

I measured the width of the garage door opening, which is roughly 9 feet, and also noticed that I would have to notch around the existing garage door weather-stripping.

I cut a pressure-treated 2x4 to length, then cut a notch on each end with a jig saw.

Next, I put the 2x4 in position and hammered it down, friction-fitting it in place. A bead of caulk prevents air infiltration below it.

The doors swing in right over the top of the 2x4.

With the doors in place, I could now work on the weatherstripping. Unfortunately, my assorted pieces of blocking didn't lend themselves well to straight seamless pieces of weather-stripping which I wanted to use. To remedy that, I installed interior framing which matched the outside of the door. (See Next Step.)

With the interior framing in place, I then had a single-thickness continuous surface to seal against. I cut a piece of 3/4" cedar about an inch wide and the width of either door half. I screwed that to the 2x4, allowing a small gap for the foam weatherstripping. I then applied the self-adhesive foam to the front face of the cedar strip. Moving to the second half of the door, I realized that it might be better to install the foam on the door side. That would prevent the foam from being damaged every time a car rolls over the threshold. Both sides of the door seal well. In the future, I'll compare the two and see if there are advantages to one particular installation location or the other.

Step 7: Interior Framing

In all of my projects, I'm always learning while building. Projects often change from the beginning to end because of budget, materials available, or other limitations.

I originally planned to simply use 1x6 lumber for the exterior frame. I would use cut-offs from it as blocking to strengthen the inside corners. Lastly, I already had quite a bit of flat wood trim from another project. I would use that trim to cover the inside edge of the acrylic.

However, the trim didn't match the thickness and width of the corner blocking and it suddenly became complicated to add the weatherstripping.

For a good sealing edge, I really wanted the entire back side of the door to be a single smooth surface. Because of that, I decided to completely do away with my original plan of the blocking and trim and instead just use 1x6 lumber on the back of the doors. It would add some weight and cost, but the doors would also be more rigid and have a better finished appearance.

I purchased the additional lumber from the lumberyard and again cut the boards to size. This time, I shortened the vertical boards and lengthened the horizontal boards so that at the corners, the inside and outside boards overlapped. This gives a very strong corner joint.

I removed the corner blocking and screwed full lengths of 1x6 boards in their place.

For the center board, I again ripped it in half on the table-saw, but this time made sure it was an off-center cut. This creates a lip for the one door to press against, holding them both shut. It also creates a nice location for a thin piece of weatherstripping.

The doors are held shut by a pair of barrel bolts installed on the west half of the door. This door overlaps the other one in such a way that both doors are held shut by only securing the one. One barrel bolt goes up into the door frame and the other goes down into the threshold.

Step 8: Weather-Stripping

A very important part of this project is that it is AIR-TIGHT! The sun shining in doesn't help if COLD AIR comes in too!

I purchased a type of weatherstripping commonly used for garage doors. It comes in seven foot lengths. The material is a rigid trim board with a wide rubber flap. The trim is nailed or screwed to the side of the garage door opening with the rubber flap pressed right up against the closed door.

I bought 4 pieces, one for each side, and two for across the top, which is more than 7 feet long. I wouldn't use this on the bottom of the door as the rubber flap would get torn up from driving a car over the top of it.

This material happened to be a little wider than that of the existing garage door weatherstripping. I cut it to fit by narrowing it on the table-saw. Although it looks like wood, the trim is actually a type of plastic and cuts extremely easily with any saw.

I cut the weatherstripping to length, pressed the rubber lip against the solar door and the "wood" trim side against the garage door opening, and screwed it in place.
I repeated that for other side and two pieces for the top.

In the top corners, the rigid part of the weatherstripping can butt right against each other. That means that the rubber flaps overlap, fully sealing all the way into the corner.

On the bottom of the door, I applied self-adhesive foam weatherstripping to the front of the 1x1 screwed down to the pressure-treated 2x4.

Once all the weatherstripping was in place, I could feel that there was no longer any air-movement through the door. However, just the plexiglass itself was pretty cold. Not only that, but moisture in the air could fog up and freeze directly on the interior of the plexiglass.

Since I had the full framing on the back side of the doors, it gave me space to install a window insulator kit. I purchased a patio-door-sized kit, which included shrink-plastic and double-sided tape. I placed the tape around the edge of the door, pressed the plastic against it, and then shrank it until taught with a hair dryer.

The extra layer of plastic is very clear and adds insulation value compared to just the plexiglass.

Step 9: Energy Production and Budget

How much heat can a window or door like this really add to a building? It's generally accepted that sunlight has energy of about 1,000 watts per meter square*.

Using that information, we can calculate how much heat energy can be brought into the building.

POWER PRODUCTION:
I measured the glazing area and converted it into meters square. That gives me 4.15 square meters.

Next, we have subtract system losses. I know from the spec sheet that the transmissivity of the plexiglass is 92%. 4,150 x .92 = 3,818 watts.

There would also be additional loss from the interior heat-shrink layer, although I do not have any specific data of that particular material. If it did happen the same transmissivity as plexiglass, that would be another 8% loss. ( 3818 x .92 = 3,513. (Rounding to the nearest watt.)

That means that the sun shining through the window is equal to 3,500 watts. That's more than two 1500 watt electric heaters running full blast!

Note that with passive solar, losses are pretty minimal. There's no losses to pumps or fans or other equipment. While I love photovoltaics, common PV solar panels can only capture and convert 15-20% of the sun's power. It's interesting to know that in the middle of a sunny winter day, my PV panels can produce over 4,000 watts of electricity, but using a MUCH larger surface area, and at a much greater financial cost!

At this point, we know how much POWER can be produced, but we also need to know average hours of sunlight at that intensity to figure out the total ENERGY.

A great tool for solar calculations is the PV Watts calculator from the National Renewable Energy Lab. It's typically used for Photovoltaic (PV calculations) but in this case, we will just use it to check for total winter solar radiation in my area.
Using those numbers, I ran some multiplication in a spreadsheet to find that my Passive Solar Garage Door could produce the equivalent of 1.8 mega-watt-hours of energy over the heating season. If I had to pay for that electricity (which is how I would otherwise have to heat the garage) at a cost of $.013/kWh, it would come to just over $240.
This door should save me about $240 per winter, but how much did I spend to build it?

BUDGET:
My total out of pocket cost was just over $500. See the budget image for details. The biggest expenses were the wood and the plexiglass. Cost savings could have been realized by using less expensive materials. The lumber I used was a premium material. It was pre-primed and perfectly straight and square. However, a person could just as easily use carefully chosen plain wood. Instead of plexiglass, I could also have chosen twin wall polycarbonate. It's durable and about half as expensive, but also doesn't let in as much light. The plexiglass also offers a better view than the polycarbonate.

Money spent versus heating cost savings means the Passive Solar Door will pay for itself in just over two winters. Had I used less expensive materials, I probably could have made it pay for itself in one winter. However, I like that the exterior of the door immediately had a nice finish, and I didn't have to try doing painting during the winter

I also have to say that the daylight is priceless. I really can't tell you how nice it is to be indoors, working on a project with the winter sunlight shining right on me. The diffuse light bouncing up off the concrete floor and white walls fills the entire garage workshop, eliminating the need for electric lighting.

So far, I've unscientifically studied the temperature swings in the garage. On a sunny winter day of 18 degrees F. outside, it reached 48 degrees inside. If I choose to build a glazed door to go over the OTHER overhead garage door, I could double my heat gain.

AUTOMATION?
My system relies on me opening the overhead door in the morning of a sunny day. It's a manual door, but the other side features an automatic garage door opener. On that side, I could open the garage door remotely from inside the house. Some garage door openers now also interface with whole house automation systems. It wouldn't be hard to make a garage door open in the morning automatically based on a timer or even from weather data polled from the internet.

GREENHOUSE SEED-STARTING
Just inside the passive solar door would also be an ideal place for starting seeds in the spring. The sun would warm the potting soil to encourage germination, while the seedlings would be protected from wind and low temperatures.

SHINE ON!
Solar energy is an amazing thing. It can be used to create electricity, make hot water, or heat a home. With well established weather records and location information, we can accurately predict how much energy we can create.

Using simple and low tech materials, like a pane of glass, we can harness the sun!

This project is only one part of my total DIY Garage. To learn about the Solar Photovoltaic system, please visit: https://www.instructables.com/id/DIY-Solar-Garage/

You can also learn about my projects at my clean transportation blog, 300MPG.org.
If you enjoy my YouTube videos, please subscribe!

Until next time,
Stay Charged Up!

-Ben Nelson

* At sea level, on a perpendicular surface, air mass at 1.5, etc. Standard test conditions. Your mileage may vary.