How to Build a Solar Attic for Heating, Cooling, and Growing Plants

Few years ago, I decided it was time to re-roof my home. I had been interested in experimenting with solar heating for some time. There is large section of south facing roof on my house over top of an uninsulated attic crawlspace. I wanted to make use of this surface in some way for a solar heating project. I thought about what I might do for a long time.

Most rooftop solar heating installations involve mounting flat panel collectors to the outside surface of a finished roof. I thought since I was going to put on a new roof anyway, I would build my solar installation directly into the roof, offsetting some of the roofing costs.

Being a tinkerer at heart, I did not want to commit the entire space to any one type of solar heating solution. I wanted to be able to experiment with different solar collectors. The ideal solution, I figured, was to create a transparent roof. Then I could experiment with whatever type of collector I could dream up simply by mounting it under the roofing, between rafters.

Initial Thoughts

Skylights are commonplace, but I was thinking about something on the order of 8ft x 20ft (a little bigger than your average skylight). As it turned out, this was fairly easy to do for no more cost that that of the roofing material it offset. The key was that the space below the roof was not living space. It was not insulated, and it was already vented directly to the outside. Thus, the transparent roofing installation didn’t need to be air tight or offer significant insulation for the space underneath. It just needed to shed rain.

There is a product called Suntuf which is a corrugated transparent polycarbonate roofing panel often used for greenhouses. I selected this product for my roof. There are cheaper transparent PVC panels that are commonly used in do-it-yourself projects, but PVC has too low a softening temperature and tends to fade with time. PVC also tends to shatter when impacted. Polycarbonate panels don’t have these drawbacks and are worth the additional expense.

Not having done this before (and not being aware of anyone who had) I did have some reservations about installing transparent polycarbonate roofing on my house:

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  1. Would it last? I was hoping to get at least 10-15 years out of it. The rest of my new roof has a 50 year warranty. Given that I didn’t expect the polycarbonate material to last nearly that long, I made sure the panels could be easily removed and replaced without disturbing the surrounding roof.
  2. Would it warp or fade? I had seen a few backyard greenhouse installations using transparent PVC that looked horrible (warped, faded, and mildewy) after a few years. Polycarbonate has a much higher softening temperature so I was hoping it would not suffer from warping, and I had read that it would not fade. We’ll see about the mildew/mold problem, but even if I have to go up and clean the panels once a year it’s not that big a deal.
  3. Would it stand up to the snow load? Actually, I was not too concerned about this. In the past 10 years I haven’t seen more than 6″ of snow. The roof was designed for the appropriate snow load for our region, but it still seems a bit flimsy?
  4. Would it stand up to the wind? I was more worried about this. We get some fairly severe winter storms here. Time will tell.
  5. Would it leak around screw heads? The installation instructions called for drilling oversized screw holes to allow for thermal expansion of the panels. The result was that the rubber washers on the screws barely covered the holes. I was a little worried that with thermal expansion and contraction of the panel, leaks might develop. Normally these types of panels are used for greenhouses and small leaks probably aren’t that big a deal.

I decided to accept the risks, confident that I’d be able to find solutions to any problems that arose. Not only that, but I decided to build two transparent sections of roof, the other being in an east facing roof over my attached garage. This roof has a large enough space underneath it that people can walk around reasonably comfortably. I thought the space was ideal for an indoor greenhouse and sunspace. I also thought it was a good way to test out construction methods since the outside of this roof is barely visible from the ground unlike the south facing attic roof which is very visible.

Most of my concerns have been laid to rest. We had more snow this winter than I’ve seen in the last 30 years (a little over 2 feet at it’s peak) and the polycarbonate roofing held up just fine. We’ve also had some severe wind storms that have had no noticeable effect on the roof. As for leaks, some of the screw holes (about 1 in 50) did leak, but on closer inspection it was the result of burrs left over from the drilling process. After deburring the leaking screw holes, the rubber washers sealed properly and I haven’t had any problems since.

Building the transparent roof

I hired my friend, Steve, to do the re-roofing of my home. The main roofing material you’ll see in the following photos is a product called Enviroshake which is made from recycled tires. Re-roofing took several months of part time work. The installation of the polycarbonate panels was the interesting part so that is what is shown in the photos below.

Solar Attic - east roof 1 The finished east facing roof over the garage. This section was done first because it’s barely visible from the ground.
Solar Attic - east roof 2 A closeup of the flashing used for the east roof. This flashing was made by cutting up sections of an old steel garden shed.
Solar Attic - east roof 3 A mock up showing how the flashing fits with the shingles on the east roof.
Solar Attic - east roof 4 Underneath the east roof. This is where the indoor greenhouse will eventually be located. It’s amazing how much more cozy and inviting the space feels already.
Solar Attic - flashing 1 For the south roof, I decided to try my hand at making custom flashing. This is the roll of aluminum flashing I started with.
Solar Attic - flashing 3 I cut pieces off the roll and bent them between an improvised sheet metal brake made from two pieces of aluminum bar clamped together.
Solar Attic - flashing 4 Bending the front. This material is light enough that it can be bent by hand. The improvised brake is required only to give a nice straight bend line.
Solar Attic - flashing 5 Bending the back.
Solar Attic - flashing 6 Flashing clamped in preparation for the final bend.
Solar Attic - flashing 7 After the final bend.
Solar Attic - flashing 8 Viola! Finished custom flashing. Note that not all bend lines are parallel. The “hump” is smaller at one end to account for the thickness of the shingle.
Solar Attic - south roof 1 Construction progressing on the south roof, looking north.
Solar Attic - south roof 2 Closeup of construction on the south roof, looking west.
Solar Attic - south roof 3 Closeup of construction on the south roof, looking east.
Solar Attic - south roof 4 A continuous sheet of flashing covers the top of the polycarbonate panels. We were careful not to cover any screw heads so the panels can be easily replaced if necessary.
Solar Attic - south roof 5 A view of the flashing at the top of the panels prior to installing the final rows of shingles.
Solar Attic - south roof 6 Steve driving the last spike.
Solar Attic - south roof 7 A view of the completed south roof with 23ft x 8ft of polycarbonate panels.
Solar Attic - south roof 8 A view from inside the attic crawlspace. I added 3 sheets of plywood for a makeshift floor while the roof was open.
Solar Attic - south roof 9 A view of the house looking northeast after completing the roofing project.

Initial Temperature Data

In February of 2008, before experimenting with any solar thermal collectors, I decided to measure the temperature in my solar attic. I measured at 3 hour intervals over a 24 hour period using a digital weather station with logging capabilities. I measured once on a cloudy day and once on a sunny day for comparison. The following plots show the results.

Impressive though this data may seem (about 40°C peak with outside temperature around 10°C), it’s not particularly useful. It only indicates the maximum stasis temperature of the attic (with no solar thermal collectors in place and no heat being extracted into my home). It gives little indication of how much useful energy I may expect to extract. So what good is it? Well… it was easy to measure and rewarding to see. Sometimes you need a little boost to the morale when tackling large projects like this.

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Still To Do

  1. Renovate space under east facing roof for an indoor greenhouse.
  2. Renovate space under south facing roof for easy access to install flat panel collectors. I need to install a plywood subfloor so I don’t need to walk on the rafters. Before doing that, I want to pull up all the insulation and check that the building envelope is well sealed everywhere. I’m also considering installing additional insulation (currently there is only 6″ or about R19 which is well below the recommendation of R49 for my region).
  3. Build and install solar collectors under south facing roof. I’ve already experimented with building a flat panel solar collector for water heating. See the results from that experiment here: How to build a simple solar water heater. However, I don’t trust that design for permanent installation since it is too prone to leaks. Further experimentation is warranted.

Polycarbonate panel longevity

The polycarbonate roofing has held up great. Over the past 7 years the panels have been exposed to 70 km/h winds, 2ft of snow, and temperatures ranging from -20 to +55 degrees C (inside the attic). There has been no mold or mildew. The panels are as clear as when first installed and have required no maintenance. One commenter has suggested that I can expect a lifetime of over 20 years.

The one issue I have experienced is leaks at screw heads. I do an inspection at least once a year during a heavy rainstorm and I usually find 2-3 screw heads that are dripping water into the attic. I think things can shift around a lot due to thermal expansion and contraction of the panels. I have been contemplating more definite solutions to this problem (if you know of any please leave a comment), but so far it hasn’t been so bad to just adjust the couple screws that start dripping each year until they stop dripping.

Solar attic for heating

My priorities have shifted over the past 7 years and I have yet to install any type of mechanism for transferring heat from the solar attic to the home below. However, I have experimented with simply installing a fan in the attic access hatch and blowing attic air down into the home when the attic is warmer that the house. This does a good job of overheating the top floor of my house while leaving the bottom floor cold. This was no surprise but was worth the experiment. A true solution should be a closed system (not drawing outside air into the home) and should circulate heat down to the 1st floor. Unfortunately there isn’t a convenient place to run an air duct to move hot air all the way down to the first floor, so my solution (if I ever get around to it) will likely use water to circulate the heat.

Solar attic for cooling

One unexpected benefit of the solar attic is passive cooling of my house in the summer. Let me explain how that works.

Because I was worried about the attic overheating in the summer, I installed two awning windows at opposite ends of the attic. I can open these windows in the summer to provide additional ventilation and close them in the winter to capture more heat.

The first summer, when I opened the ventilation windows and returned to the attic access hatch to descend from the attic, I was struck by the strong draft rising through the hatch from the house below. I immediately recognized that what I was experiencing was a solar chimney effect. That is, the heated air in the attic was rising and exiting through the open ventilation windows. This in turn was drawing air through the access hatch from the house below. This in turn was drawing cooler outside air into the house through open windows. It’s like having a whole house fan but with no fan. The system is powered passively by solar energy.


Using a solar heated attic for cooling a house

I will admit it is only anecdotal evidence but summers certainly do feel cooler in the house now compared to before installing the solar attic. However, it’s possible I could have achieved much the same benefit without the solar attic simply by adding extra attic ventilation and opening the access hatch in the summer.

In theory I can improve the cooling capability of the current setup by adding some thermal mass (ex water barrels) in the attic. Then the thermal pump will operate 24/7, drawing cooler night air into the house, even when the sun isn’t shining.

Note that we do not use air conditioning or the above strategy would not work.

Solar attic as a greenhouse

In my lower attic (above the garage) I’ve experimented with growing lettuce, tomatoes and peppers. Lettuce does well in the early spring, and late fall when it’s too cold to grow outdoors, but does not do well in the heat of the summer. Tomatoes and peppers like the heat, unless it gets really hot. I installed a solar powered exhaust fan under one of the existing attic vents to lower the temperature a bit. This helped a lot.

Growing tomatoes in the solar attic, July 2009

Even though tomatoes and peppers like the heat, it takes a lot of effort to keep them well watered. These days I don’t grow anything in the attic through the summer. I just use it for early lettuce and starting seedlings before transplanting outdoors.

Growing lettuce in the solar attic, May 2012

Note the solar panel in the above image. It is directly wired to a 12V exhaust fan under one of the attic vents. Even in May it can get a little too hot for lettuce without the fan.

That’s all for now.

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by Rob