One of the features that sold us on our house was its long, narrow roofed porch facing south, toward the street. We had a small enclosed porch on the east side of the previous house that we’d had remodeled into an 81 square foot three season room with casement windows on all three sides. Since we didn’t have central air conditioning for that house, we used that room all summer. We closed the windows to keep rain out in bad weather, leaving them open otherwise so we could enjoy a breezy, shaded sitting area. During sunny but cool conditions in late spring and early fall, we’d leave the windows closed and spend time in the room once the sun warmed it. From October through April we closed up the room and kept subtropical container plants such as citrus trees, bay, and rosemary on it. We moved the citrus trees into the house for three to six weeks in midwinter because the room became too cold for them. Still, the temperature in the room never dropped below 20F even when the outside low dropped to 0F or colder. I considered the room a success but envied people who had the south-facing attached porch I knew would make a more successful solar-heated greenhouse and sunspace. Buying this house put my dream within reach – but first I had to determine what was possible and reconcile what I wanted with the limitations of what we had. Follow along and you might find that you can have a space like this too!
Two books from the appropriate-technology era of the 1970s and early 1980s helped me determine what was possible for our situation and will help you figure out what you might be able to do. These are The Solar Greenhouse Book, edited by James C. McCullagh (1978), and The Homeowner’s Complete Handbook for Add-On Solar Greenhouses and Sunspaces by Andrew M. Shapiro (1985), both published by Rodale Press. The first book discusses solar greenhouse theory (Part 1) and how it was expressed in various working greenhouses from the 1970s (Part 2). Most of these were detached, but one chapter of Part 2 focuses on attached greenhouses. Part 3 discusses crop management in solar greenhouses. Because the second book deals only with attached greenhouses, it includes much more detail on design considerations and helped me to better understand the limitations of our site and thus the potential performance of our porch. That book describes how to do a good-enough solar survey, essential to predicting the performance of a site being considered for an attached sunspace, using only a protractor with a weighted string attached, a pencil, knowledge of your latitude, and the sun path chart for your latitude found in an appendix. It also has lots of pictures of sunspaces to give you an idea of the range of possibilities. I recommend working through the design process in both books in order to better use what you already have or to design the best addition for your situation. Another book worth reading is Building Your Own Greenhouse by Mark Freeman (Stackpole Books, 1997). This last book addresses both freestanding and attached greenhouses.
In the photo below you can see the front porch as it was when we bought the house. The entry door is six steps up from floor level. Wrought iron columns held up the roof.
The porch itself is 28.3 feet long and 6.8 feet wide for a total area of 192 square feet, about 50 square feet of which is occupied by the staircase, and it faces slightly west of south. The height from the concrete floor to the roof is about 11 feet. This would allow for a glass wall of about 311 square feet in area facing a bit west of south with only about 70 square feet of east and west facing walls to deal with and the north wall shared with the house. It seemed like an ideal situation for a solar greenhouse that might add some heat to the house as well. Except for the trees …
Two very large pin oak trees are located in the neighbor’s yard to the south and west of the front porch, with more large pin oak and silver maple trees in the front and back yards of the houses across the street to the east, south, and west. In order to understand how they might affect the porch I performed a solar survey following Shapiro’s instructions and using the sun path chart for 40 degrees north latitude, which is close to our latitude of 39 degrees north. The solar survey indicated the porch would be 50% or more shaded by the trees during all of the critical 9 am to 3 pm time slot from November 21 through January 21. From October 21 through November 21 and from January 21 through February 21 the porch would receive nearly full sun from about 11 am to about 1 pm and be 50% or more shaded the remainder of the critical period. From September 21 to October 21 and February 21 to March 21 the full-sun period would extend from around 9 am to around 1 pm.
Using this information I then considered how well the porch might perform each of the three functions that an attached solar greenhouse can be designed for, according to Shapiro. These are as a heat collector for the rest of the house; a place to grow food and overwinter tender plants; and a place for people to enjoy a few hours of sunshine and warmth on sunny days during the cold months. While some of the requirements are the same for each function (good solar exposure being the primary requirement), some others are not. For instance, the most effective heat collector would not include much heat storage on the porch because the house it is attached to is intended to accept and store the sun-generated heat, while a greenhouse necessitates heat storage to damp swings in temperature from night to day and to keep the nighttime temperature from dropping too low for the plants. Any space given over to tables and chairs for lounging is space not available for growing or overwintering plants or for heat storage.
Our porch is small enough that any seating would be minimal and portable, perhaps a folding chair and table brought from the house that I could squeeze into the area reserved for walkways when I wanted to spend a little time in the sun. Then I considered how well the porch might perform as a greenhouse and as a source of extra heat for the house. Since plants would live on the porch from about October 1 through mid April, the solar survey suggested that the porch would provide sufficient sunlight to keep them alive. However, it was clear that the porch wouldn’t collect much heat during late fall and winter due to the shading from the trees. Shapiro says large deciduous trees like the oaks and maples can block 50 to 70 percent of sunlight even without leaves, the situation we faced with the trees in our neighbors’ yards. It appeared that the porch was best suited to be a winter home for the citrus trees, herbs, and other subtropical container plants that I’ve collected. For this use I needed to figure out how to fit enough heat storage onto the porch to keep temperatures from dropping much below freezing during the winter.
We’re all familiar with the fact that glassed-in spaces like cars and enclosed porches gain heat, sometimes quite a bit of it, on sunny winter days. However, as soon as the sun sets, the heat source turns off. Because cars aren’t well sealed and don’t have much in the way of heat storage materials in them, they don’t hold onto the heat that they gain on a sunny day. If you already have an enclosed sunny porch on your residence you’ve probably noticed that it also warms up on sunny winter days but cools off rapidly once sunlight stops entering the porch. A greenhouse does the same thing if it doesn’t include material that can absorb some of that heat during the day and slowly release it at night. A greenhouse also needs to be sealed against cold drafts that can carry away stored heat. Curtains that cover the windows and any glazing on the roof will prevent heat from radiating out through the glazing at night. To convert a glassed-in porch to a solar greenhouse requires adding materials to store heat and, if the windows and doors aren’t already sealed against drafts, sealing them as well as possible. Roofs should also be insulated or have insulation added if what is there is inadequate, to prevent heat from escaping through the roof. The glass windows should have some type of insulated covering placed over them at night and perhaps on cloudy days as well. Alternatively, the plants in the greenhouse could be covered at night or during cloudy periods, just as we cover tender outside plants to protect them when a frost is predicted. Eliot Coleman’s book Four-Season Harvest describes his use of row covers over the beds in a greenhouse or hoophouse to provide extra protection to his crops. Without including any heat storage materials, he finds that the climate underneath the row covers is equivalent to that found two USDA zones to the south of his location.Assuming we are in Zone 7 as I discussed in this post, two zones south is Zone 9, with the coldest winter temperatures expected to be from 20 to 30F.
The best materials for heat storage based on availability and cost are water, concrete, brick, and stone. Of these, water has the advantage of the highest heat capacity (it can absorb about twice as much heat as an equivalent volume of masonry, according to Shapiro) and it also conducts heat better so it will give up its heat faster at night when it is needed by the plants. The disadvantage to water versus the other materials is that water is not a structural component. It must be contained in some way and, if the containers are stacked, they cannot be stacked so high that the weight of the containers above bursts those below. Concrete, brick, or stone can be used as structural materials, but they will not absorb as much heat as water and they do not give up the heat they have stored as rapidly as water does. The most practical designs use water for primary heat storage and masonry or rock walls as structural materials and secondary heat storage. In our case we already had a concrete floor and the concrete block staircase that could double as the masonry heat storage component. The question thus became how to add water for heat storage.
Determining how much water storage we could add meant balancing the needs of the plants for heat storage and the space available for them against our need to retain easy exit to the outside from the front door and the local water utility’s need to access the port to read our water meter, located just west of the staircase on the north wall. After pondering on these competing needs I found that we could fit five 55-gallon drums of water onto the porch. Of these, three sit against the north wall to the east of the end of the staircase. They can be seen in the photo below, taken from the top of the staircase. This leaves enough room to allow us to exit from the house and also provides space on top of the drums for starting seedlings on a heat mat and for a few small container plants. There is enough room in front of the drums for several container plants, including an olive tree, and still leave room to walk to the east wall.
The other two drums are on the west side, oriented parallel to the width of the porch, far enough from the water meter port so the water meter reader can squeeze in between the drums and the staircase. This leaves a roughly square-shaped area west of the drums for our sweet bay and citrus trees. I prune them so I can fit all of them into this space. I could keep another citrus tree or two in the area left open for meter reading as long as I moved them out of the way when the meter reader is expected to arrive. You can see this part of the porch in the photo below.
The area in front of the staircase is narrow, but I can fit some plants there as well. In the photo below you can see pots of lettuce seedlings in front of the stairs. In February and March I keep flats of seedlings in this space.
To predict how well our porch might perform as a solar greenhouse we need to know how much heat storage is required for our climate. Table 5.5 on page 108 in Shapiro’s book recommends the amount of water and masonry to be used in different climatic regions for winter growing and for the less demanding season extension for each square foot of floor area. According to the map on page 95, St. Louis is in the moderate climate zone, with 4,000 to 6,000 heating degree-days during heating season (for comparison, the St. Louis NWS says that St. Louis averages around 4,400 heating degree-days). For moderate climates, Shapiro recommends 2 gallons of water and 2/3 cubic foot of masonry per square foot of area for season extension. That translates to 384 gallons of water and 128 cubic feet of masonry since our porch area is about 192 square feet. We have about 275 gallons of stored water in the drums and about 112 effective cubic feet of masonry (the effective width of masonry for heat storage is only 8 inches) in the concrete floor and the staircase. This isn’t quite enough for season extension according to Shapiro. On the other hand, St. Louis isn’t too far from being in a warm climate (less than 4,000 heating degree-days) and our winter features about 60-70% of possible sunshine, so we might be able to squeeze by with heat storage suitable for season extension in a warm climate. For that we’d need 1 gallon of water and 1/3 cubic foot of masonry per square foot of the porch area, or 192 gallons of water and 64 cubic feet of masonry. We have more than enough water and masonry in this case. Thus I predicted that our front porch, once glassed in and with the five 55 gallon drums of water placed on it, would collect and hold enough heat to allow us to overwinter subtropical plants and raise cold-tolerant vegetable seedlings without supplemental heating.
Because Mike and I are not confident builders, we contracted with a patio room installation company to build the porch enclosure. Since we didn’t have enough space for permanent seating or enough sun for the best solar heating, we chose a three-season room rather than the more expensive option of the all season room. We also chose to use the existing roof and floor to keep costs lower. Since I wanted the sun to reach to the floor we chose a company that installs sliding glass doors rather than one that constructs a knee wall and puts windows on top of it. The porch was finished in December 2010 and has been in use since then. You can see it in the photo at the top.
So how does the porch perform compared to how I expected it to perform based on the analysis above? There’s a little more sunlight in late afternoon than the solar survey suggested and the sunlight enters a little later in the morning than suggested, but for the most part the times and amount of sunlight entering the porch was well predicted by the solar survey. The porch warms the most in late winter and early spring, as the survey suggested. Only when the outside low drops into the single digits Fahrenheit does the porch temperature drop below freezing. Even then it drops only a few degrees below freezing; 25F is the lowest temperature I’ve recorded since the enclosure was completed. Later we had a different company add insulation to the roof. The porch seems to stay a degree or two warmer on the coldest nights with the porch insulated. Interestingly, the more noticeable effect of insulating the roof is that the porch feels cooler in the summer than it did before.
As a place to overwinter plants, the porch has proven itself. Sweet bay, rosemary, olive, fig, and other plants that can withstand temperatures into the teens to low 20sF need no protection at all. The first winter I killed a lime tree by moving it and the other citrus trees from a warmer basement as soon as the porch was enclosed, forgetting that they would not be acclimated to such a rapid shift to temperatures near to or slightly below freezing. Covering the citrus trees with row cover kept the rest alive. Since then I’ve covered the citrus trees when the outside temperature is predicted to drop low enough that I suspect the porch temperature will drop below freezing. I’m not sure that a properly acclimated citrus tree really needs to be covered, but I will continue to do so since I don’t have night curtains on the windows. I keep a large clivia plant on top of one of the 55-gallon drums on the west side, covering it during mid-winter, and it is doing well there. So far I haven’t risked my seven-year-old geranium plants to below-freezing temperatures, bringing them into the house from December through early March. But this year I’ll try covering them with row cover too and hope I can bring them through that way. Plants that flower in cool conditions, such as rosemary, ‘Goodwin Creek’ lavender, and calendula, offer blooms from November through March when kept on the porch. When kept inside the house they do not experience the bright, cool, moist conditions they need to bloom. I also start and grow amaryllis on the porch, bringing the pots inside if the porch drops below freezing.
Because the hours of full sun and thus the porch temperature increase when it’s time to start seedlings, the porch has also proven itself to be an excellent place for this purpose. I put flats with seedlings that need warm temperatures for germination on the heat mat on top of the barrels on the east side. Supplemental heating from the heat mat is only needed for about two weeks or so when I start those seeds in early March. All the rest of the seedlings are started on flats that occupy space on the floor against the staircase. As weather conditions permit all seedlings are moved to a cold frame outside.
While I grew some lettuce and green onions on the porch last winter and am growing some again this year, the porch isn’t large or sunny enough to grow serious amounts of food. If I weren’t overwintering container plants there would be more space for growing winter vegetables. However, the amount of shading in fall and winter limits vegetable growing more than the lack of space does, I suspect. I’ll continue to grow some lettuce in the winter and I hope to get a larger crop once I figure out when to start it. Last year I sowed my winter lettuce too early (August 5). Much of it bolted by December. This year I fear I started it too late (September 20) since the plants are still small and may not grow much more. The last week of August may be right for sowing winter lettuce in order to have full-size heads by December and I will try that next year.
The porch offers some space for enjoying warmth on sunny days. While I sometimes sit in a folding chair, more often Mike and I sit on one of the steps and eat lunch, drink a cup of tea, or read for a bit. The limited time that direct sun enters the porch along with the space constraints do not permit extensive use of the porch for this purpose, but even a half hour or so of warmth is greatly appreciated in midwinter! In late winter and early spring we enjoy more time on the porch as more sunlight enters then.
Because we keep the house at 60F during the day and the porch heats to 70F or more on a sunny midwinter day, the porch does heat up enough to allow some of that heat to enter the house. While in past winters we’ve used a pedestal fan to move some of the warm air into the house, these days we prefer to open the front door and the two windows that open onto the porch and allow the air to move on its own. In later winter and early spring more sunlight means a warmer porch and more heat that can be transferred to the house. We’ve never used supplemental heat on the porch other than the heat mat I mentioned earlier and don’t expect to need it, especially with the trend to warmer winters that I noted in this post.
All that tells you how well the porch performs. What it doesn’t tell you is how much I enjoy having April available on the other side of a door during the coldest months of the year. If you can modify an existing space or add a space like this, you too can open the door to spring all winter long!