Build a Solar Attic2008-07-05 by Rob
Around July of 2007, 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.
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:
- 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.
- 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.
- 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?
- Would it stand up to the wind? I was more worried about this. We get some fairly severe winter storms here. Time will tell.
- 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.
Update Jan, 2009: 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.
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 40Â°C peak with outside temperature around 10Â°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.
Still To Do
- Renovate space under east facing roof for an indoor greenhouse.
- 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).
- 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.
I will update this article as work progresses. – Rob