Kelly Hart, your host at greenhomebuilding.com, is an avid solar greenhouse gardener and has built many of them in a variety of climates. One he built can be seen on the Grow your Food page. Kelly spent many years as a professional remodeler, during which time he became acquainted with many of the pitfalls of conventional construction. He has also worked in various fields of communication media, including still photography, cinematography, animation (he has a patent for a process for making animated films), video production and now website development. One of the more recent video programs that he produced is A Sampler of Alternative Homes: Approaching Sustainable Architecture, which explores a whole range of building concepts that are earth friendly. Kelly is knowledgeable about both simple design concepts and more complex technological aspects of home building that enhance sustainable living.
Q: Me and my husband are trying to find a way to make a greenhouse. Just to start a little green house. We have scrap wood and also would like to use a window door, for the top of the opening. Any ideas or suggestions?
A: With materials that you have at hand, I might suggest making a small cold frame, with the glass slanted toward the sun, and down low to the bed.
Q: I am wanting to build a pit type (in ground) greenhouse, but can find virtually no real useful info on one. Building it is not the problem, I am a carpenter, but is it a good workable type of greenhouse. I have heard they are very energy efficient and I have heard they are difficult to grow in, so I don't know. What I want to do is to grow carnivorous plants, which require a fairly humid environment, with plenty of sunlight, yet keeping temperatures in the high 80's would be ideal, as well keeping winter temps in the high 40's. I have a traditional attached greenhouse on my home, but heating it at all in the winter is expensive, and summer overheating is always a problem. So, I have heard that pit type greenhouses need very little if any heating or cooling, if so that would be the answer to most of my problems. If possible, I would like to even grow some vegetables in it during the winter as I have heard can be done. Now, I live in southern Virginia, I have full access to the summer and winter sun, and have anywhere on a 5 acre plot to put this greenhouse, and would plan to run water and electric to it. So, if you could PLEASE just give me the rundown on this idea, the good and bad, so that I can decide on what to do I will truly appreciate it, very much.
A: I would think that a well designed pit greenhouse could do everything that you want, although I have never built one. I would say that digging into the ground will save a lot of BTU's and money to maintain your temperature. The main thing will be to have plenty of thermal mass on the inside, with substantial insulation between it and the earth. There is a big differential between 80 and 55 degrees (of the earth), so you don't want to be leaking heat that way. During the warmer months, a good ventilation system will be critical to keep the space from being an oven...but all of this can be done.
Q: You mentioned in your reply to me about thermal mass, okay, so would it be better to excavate the entire pit and build a insulated cinder block wall up to the roof above ground, or,,,, to leave the ground intact under the roof and just excavate a walkway through the pit, leaving the ground on the sides to place the plants on etc? I have seen info on both, but I have no idea which would be the better./p>
A: I would think that the more that the greenhouse is recessed and insulated, the better, since it would then be buffered more by the surrounding soil, so I think that your first suggestion would be preferable.
Q: Can you tell me where to get movable glazing and movable insulation for an attached greenhouse that will be built on a modest home?
A: I'm not sure what you mean by movable glazing. Are you talking about operable? For insulation, there are several possibilities. Rigid panels can be made with rigid material such as the blueboard that is sold for insulating foundations for buildings. This can be cut to fit snugly into a window opening, and then covered with fabric if you want it to look nicer. Another possibility is to make permanently attached fabric shades that roll up by just pulling on a cord. This sort of insulation is available from various sources that supply these for living quarters, or you can make your own with Reflectix brand foil-coated bubble wrap, and again covering it with fabric if you want. There are probably directions available for doing this for residences, if look around your library or the internet.
Q: Would you please advise on how I should proceed with coming up with a roof garden on top of my double garage. I do not intend to use a concrete deck but rather use other materials. What are the materials that would ensure that the garden would not leak and termites free. I have recently built the garage and I hope that it will serve both as a garden as well as a roof.
A: It is not clear what sort of a roof is already on the garage, since it was recently built...or have you not put the roof on yet? I know one person who made a roof garden on a relatively flat portion of her roof that had been sealed with EPDM roofing. She used containers of various sorts for the plants, and this seems to be working out fairly well for her. Some kinds of roofs would not stand up well to being walked on much, in which case some sort of decking might need to placed over it. Whatever is done, the roofing material needs to accessible for repair or replacement periodically.
Q: Would you please advise on how I should proceed with coming up with a roof garden on top of my double garage. I do not intend to use a concrete deck but rather use other materials .What are the materials that would ensure that the garden would not leak and be termite- free. I have recently built the garage and I hope that it will serve both as a garden as well as a roof.
A: I know one person who made a roof garden on a relatively flat portion of her roof that had been sealed with EPDM roofing. She used containers of various sorts for the plants, and this seems to be working out fairly well for her. Some kinds of roofs would not stand up well to being walked on much, in which case some sort of decking might need to placed over it. Whatever is done, the roofing material needs to accessible for repair or replacement periodically.
To make a wooden roof structure that also can serve for a garden involves several considerations. It has to be impervious to water, engineered to drain the water that does fall there, flat enough to easily walk around on it, and strong enough to support what might be rather heavy planter boxes, or in the case of a living roof, all of that soil. What I might suggest would be round log rafters, big enough to easily support the weight (given whatever the span is), and spaced closely enough together to do the same. You may need the advice of an experienced carpenter for this. There should be enough of a slope of these rafters to provide the necessary drainage.Over these logs would be placed the decking material, perhaps 2X6 T&G decking. Over this would be placed a very heavy-duty moisture barrier (something like EPDM membrane), which is contoured at the perimeter to contain the depth of soil that you plan (if you are going for the living roof concept). If you are going to do container gardening, then you still need the moisture barrier, but also, ideally, a second deck over it for protection. All of this can be rather tricky and costly, but do-able if you are resolved.
Q: I was hoping to build an attached greenhouse on the south wall of a home to reap the benefits of the food grown and the energy efficiency and passive solar benefits. Do you know of any studies with an attached greenhouse where the year round performance was examined?
A: There have probably been such studies, but I am not familiar with them.
Q: Also do you know of any information on what climate zones can be achieved in this application. I am asking to see if it would be possible to grow food which is not standard for my area.
A: This should be easily accomplished. I grow all manner of warmer season vegetables in my attached greenhouse (peppers, tomatoes, cucumbers, eggplants, etc.) at over 8000 ft. in the mountains of Colorado, with only about 3 months of frost-free weather.
Q: I am trying to help my daughters school in Long Island, NY. I am trying to find a source of help to possibly provide their school's greenhouse with a passive solar heat system. Maybe even tied into a hydroponic system. The greenhouse was built as a memorial to local victims of the 9/11 tragedy. They were turned down for a heat system due to lack of funding. Do you know if any such system exists and is already being used? I think it would be beneficial to teach the children about alternative sources of energy. Do you know of any corporations who help with such projects? The school is a public elementary school.
(Dan Chiras) A greenhouse is by its very nature a passive solar
heating system. So, I assume you are looking for a way to provide
back up heat to the greenhouse. If that's what you are asking, the
best way is to include some thermal mass inside the greenhouse that
absorbs heat during the day and radiates heat into the greenhouse
at night. Some people put large barrels of water, painted black,
inside the space. They absorb quite a lot of heat. You an also build
a simple solar flat plate collector that pumps heat into the ground
underneath the greenhouse or into planters during the day. This
heat will then flow into the greenhouse at night, keeping it warmer.
I would also recommend that you look into Eliot Coleman's Four Season Harvest techniques. There have been several articles on this subject in Home Power magazine and Mother Earth News within the last year that you should read. Basically, in this technique, gardeners install hoops inside a greenhouse over their planters or beds. They then put plastic over the hoops, essentially building a greenhouse within a greenhouse. The plastic is removed during the day, but put back over the plants at night, keeping the temperature from falling. This technique keeps nighttime temperatures about 20 degree F higher at night, according to those who practice it. Why don't you check out those articles and see if this might work for the school.
Q: I am working in NW China, designing a solar model house, using sustainable technology. We'd like to include an attached greenhouse (double-paned glass). If we have active ventilation from the greenhouse into the living quarters, will that cause the greenhouse to reach unacceptably low temps at night in the winter? We'd like to grow veggies there. Night lows in the winter get to -20 degrees Celsius or less (winters mainly clear and sunny, high altitude).
have an attached solar greenhouse on our house, and it commonly
gets that cold in the mountains of Colorado during the winter. It
has never frozen in that space, which includes not only the growing
beds but also a bathroom, utility room and spa room. If your greenhouse
has sufficient thermal mass to store the heat gained during the
day to keep the plants comfortable at night, there may not be much
heat left over to vent into your living space. This can be a delicate
balance, depending on the thermal mass/solar gain quotient. If your
highest priority is to grow veggies, then have lots of mass in there...if
your highest priority is to provide heat for the rest of the house,
then have less mass and vent the extra heat into the living quarters.
If this is active, then you can choose whether to send the heat
or not. This might take some experimenting once you get it all set
up; it is possible to add more mass later if necessary. You should
also be sure to provide insulated curtains to cover the glazing
at night to help hold in all that heat.
Q: I live on a farm on the western part of South Dakota. On this farm is an 10 X 10 building that was once used for restrooms at a local bible camp. It has a solid cement floor and is fully insulated and covered with drywall inside. There are existing plumbing and electricity sites throughout the building, and it has a shingled peak roof. I have not noticed any major weather leaks throughout the different seasons, but I'm sure it is not sealed completely. Would this building be able to be transformed into a greenhouse, and if so how would you suggest we go about doing that?
A: You should be able to convert the building into an adequate greenhouse. Depending on how the building is oriented, I would suggest that you remove some portion of the roof that faces the most southerly direction and replace it with a material similar to Lexan (an insulated plastic material made for this purpose). This, along with placing glass or the same material to cover most of the southern-most wall above the growing beds, will give the plants enough light to happily grow. Keep the rest of the building the way it is with its insulation. The other thing you might do is provide plenty of thermal mass (stones, bricks, barrels of water, etc.) inside the greenhouse to hold the heat from the day...and provide insulated curtains to cover the glazing at night.
Q: We have plants for sale every spring and have built a greenhouse in Finland, the weather is a lot like in the middle-south of Canada but much less sunny. It gets 40 Degrees celsius cold in winter and 40 celsius warm in summer. The sun goes up at noon and down at 3 o´clock PM; the rest of the day is almost pitch black. We live close to the polar circle. Sometimes the sun only gets up 1 hour a day in November. When the sun says goodbye the temperature can drop from 15 degrees minus to 30 degrees minus in 2-3 hours. We usually start in February but it would be beneficial for the plants to start in January except that the costs for heating and extra light are too high. A lot of other professional growers reflect the light in the greenhouse with mirrors or white plastic. Although the sun shines on the snow so incredibly much in January we do not get enough light into our green houses. Now we have a small greenhouse 20 square meters. We are planning to build a big one 200 square meters and then later a 2000 square meters at least. Is it possible to somehow get the light from the big snowy area reflected into the greenhouse and not just reflecting the light that actually enters the greenhouse? And how do we do that? Materials available are any professional greenhouse material, white plastic black plastic, mirrors and the possibility to build big snow piles, slants and small "snow hills" up to 4-6 meters high. Let me know if You have any ideas.
A: The conditions that you describe
seem about as difficult to deal with as I can imagine in trying
to create a truly solar greenhouse at that time of the year. Even
if you can get enough light in there, how can you keep it warm enough
without resorting to other fuels? I am afraid I don't have an answer
for you except to create a space that is
1) Dug into the ground to take advantage of the relative warmth that it might provide over the outdoor ambient temperature.
2) Insulate all of the walls, ceiling and floor extremely well.
3) Insulate the glazing extremely well when the sun is not shining.
4) Provide plenty of thermal mass (water, masonry materials, etc.) on the inside to help hold the heat that you do collect.
5) Use mirrors or shiny material in some way to reflect the light from the snow into the space.
6) You might experiment with "light-tubes" to bring more light into the greenhouse, such as are pictured at greenhomebuilding.com/Products/lighting
You might take a look at the diagrams at the bottom of this page for some ideas about how to reflect more light into the greenhouse: dreamgreenhomes.com
Q: What can you tell me about the BeadWall system for insulating a greenhouse?
A: I don't know much about it, except that the void between two panes of glass is either filled with styrofoam beads or they are sucked out of the space, depending on whether you want the insulation or not. I understand that plans and specifications for the beadwall insulating system ... seven pages of blueprinted diagrams and instructions... are offered by Zomeworks Corporation, P.O. Box 712, Albuquerque, New Mexico 87103 for $15.00.
Q: We purchased a solar heated home last fall that has a bead wall in the greenhouse area. We are excited about it but need to add more beads to it and don't know where to purchase them. The previous owner has died and in his notes says he got them from Zomeworks in New Mexico, but they don't sell them any more. Any help would be appreciated as to where we can get 1/8"-3/16" polystyrene beads.
A: (Paul Shippee) Great to hear about a working Beadwall... the mother of all window insulation! Where do you live? ... I installed several Beadwalls in Colorado, and may have built yours? I used to get my beads from Advance Foam Plastics, formerly of Broomfield, now located in Denver, and now specializing in R-Panels, made from compressed beads and laminated between plywood sheets. The phone number is: 303-297-3844; my sales & info contact there is Dave Nelson X18. Also: some users have found that water spray will dissipate the static electricity, almost as well as the glycerin we used earlier.
Q: Can one use mirrors in a greenhouse to provide light to the side of a plant that would not receive light otherwise?
A: I don't see why this wouldn't work; it would be easy to experiment with the idea.
Q: I was wondering if you knew of anything I could do with ashes from a fireplace or burning barrel? Possibly in making brick type items or something else. I burn a lot of yard waste and paper garbage.
A: I throw wood ashes in my compost pile; they are full of potassium, which is one of the main requirements of most plants.
Q: I want to build green houses on 42000 square meters desert land in Egypt. I was wondering if I can build them out of wood and glass roofs ...can it work out? If yes, what type of wood shall I use in building?
A: Because the harsh desert environment is very hard on wood as well as the fact that it would have to be imported, I would suggest that you consider metal for the framework of your greenhouses. Steel is often recycled, so this might be a good choice.
Q: I am building a greenhouse in Connecticut, so the winters are cold. Can I grow all kinds of veggies in the greenhouse all winter? If so what kind? I'm thinking that plants need pollination.
A: You can grow an amazing assortment of veggies all year round in a well-designed passive solar greenhouse; some plants require more daylight during the Northern winters than they might get naturally, so you might have to supplement the light for some things. In general plants only need pollination if you plan to save their seeds, and this can be done by hand if you don't want to introduce pollinating insects into your greenhouse.
Q: I am interested in attaching a greenhouse and would love some advise. I have enclosed the patio area and now am looking to attach a greenhouse to the back side of this improvement. Mostly I'm looking for advice on structural strength needed for the materials. This will come off the existing structure and span about 8 or 9' to the outside wall. It will be about 12' or more wide. The main part of the house will be closed off from this structure by a window and door, though I may draw in excess heat when the time/heat is appropriate.
My plan currently calls for a concrete foundation supporting a rock or concrete planting box at the outside of the structure, with a 5' walkway in between the existing structure. I'm considering using recycled glass or extruded plastics to panel the structure. Naturally, this faces south with no tree obstructions.
I look forward to any advice you can offer. I'm hoping to use this (in our very mild climate) to grow some winter crops, and possibly to offset heating costs. I put a ripple plastic greenhouse over my pond and this kept the plants healthy and the pond (mostly) free from freezing, so I think our mild climate could do well with this sort of structure.
A: It sounds like you have the right idea about making an attached solar greenhouse, and are just not sure about the specifics of construction. Unfortunately it is difficult to advise you without looking at drawings and evaluating the specifics. In general, what you want in such a greenhouse is plenty of glazing (including some overhead) to allow the sunlight in, plenty of thermal mass on the inside that is insulated from the outside, good ventilation from a low point on one side to a high point on the other, and thermal curtains or shutters to keep the warmth in at night when it really gets cold. Framing for the glass or plastic is more thermally efficient with wood, and it can be rather minimal, since it is not supporting much weight.
Q: I recently lost the green home I designed and constructed (divorce/finance) and ended up in a Keyland home in a bedroom community to Minneapolis. We are in Zone 4 for growing. I am a permaculture, organic gardener and would like to try my hand at retrofitting this house as much as possible. My garage faces south and has 3 stalls. We have 1 car. I would like to convert the 3rd stall to a greenhouse area using light tubes and possibly replacing the door. I know it will need to be super-insulated on the sides that are not glazed or shared with the heated house, and am thinking about using straw bales to serve this purpose. Suggestions or resources?
A: You should be able to convert that portion of your garage to a greenhouse without too much trouble. Strawbales are indeed good insulation, and could be used to surround the interior of the garage space that you want to use. An insulated partition between the greenhouse part and the rest of the garage would probably be a good idea, and this could also be done with strawbales. The bales can be plastered with earthen or lime plaster if you want. Don't forget that the ceiling will also need to be insulated, which would likely be difficult with straw, so something more convention might need to be used.
If I were doing this, I would probably replace that garage door with a bank of glass down to the height of the planter on the south side. For greatest conservation of heat in the winter, you can make insulated curtains to hang next to the glass at night.
You will also need some ventilation provided, low on one side and high on the other to exhaust hot air when necessary.
C: Here's a greenhouse built with used windows. It might give readers ideas about using salvaged materials. It's got a solar powered fan for air circulation. Also, a a gutter that directs rainwater into a 200-gallon barrel (visible at extreme left). The rear wall is adobe bricks made with a Cinva-Ram from soil left over from digging a well.
Q: We are wanting to build an attached greenhouse or sunspace to the cob/strawbale home we are building and I am wondering if it is too moist of an environment for that type of building - particularly a greenhouse. We would not build with a vapor barrier if that makes any difference. I am hoping that if the walls are breathable than it should not be an issue.
A: Cob has the ability to absorb quite a bit of moisture from the air without ill effect. I would think that it would suffer less from a highly humid environment than strawbales would, although it would be important that they both remain completely breathable. Any sunroom or greenhouse needs to have excellent ventilation capability, so that would generally help the situation. The biggest danger would be during the cold season, when you would naturally close up the space to keep it warmer, and at the same time the colder air will condense moisture more readily.
Q: I am curious about simple, low cost flooring options for the 7'x30' greenhouse/mudroom/bath addition I'm building on the south side of my 1930's Virginia home. My climate: daytime winter temps are F40-50's and F20-30's at night with drops to single digits several nights/year; rainfall is 40"/year; winters are mostly sunny; summers are hot and humid, though big shade trees cool the house so I rarely use air conditioning. Status of the addition: The doors and windows (vertical) are framed in and sit on a block stem wall with 4 piers which abuts the main house concrete foundation. I plan straw bale walls on the east and west sides, a double roof (to capture some additional heat,) remote thermosiphon hot air panels connected by an insulated underground pipe to heat the main house which has only a wood stove, and 6" of insulation in the ceiling. Currently the floor is dirt. I have about 12 inches to bring it up to the level of the existing house. On site, I have gravel, some sand, a pile of Virginia red clay and numerous large 1 1/4" flagstones. I was planning to pour a 4" stained concrete floor (over gravel, plastic, 2" of rigid insulation) for the mudroom/bath floor, but I'm open to any and all ideas. I'd love to avoid the concrete if I can and just mortar the flagstones for the greenhouse portion or the whole addition onto something else. Are earthbags an option? Do I need to insulate? How about lightweight concrete? How about moisture? Your thoughts will be greatly appreciated.
A: This sounds like a very worthwhile project you have going there. In a passive solar greenhouse/mudroom addition you would do well to have thermal mass for flooring, and this would be provided by either concrete or mortared flagstone. If it were me, I'd opt for the flagstone, since they make very nice flooring and require very little cement. With 12" to fill, you have a variety of options for doing so. Some insulation under the floor surface is a good idea to preserve the interior heat, and this could be provided with rigid insulation (as you suggest), crushed volcanic stone (if available), or lightweight concrete. When I put flooring in a similar situation in the last house I built, I used local flagstone in the greenhouse and mudroom, and then poured concrete in the bath, over which I placed tile. You can actually see some of both of these in the second picture on this page. The flagstone was set on about 6" of crushed volcanic stone that was available locally. If you want to go this route, but can't get the volcanic material, you could place some sand over the rigid insulation to seat the stone, or even place the flagstone in a wet lightweight concrete...but this is rather tricky to get it all arranged and level. Then a regular grout can be used in the gaps between the stones.
Q: I am looking in to a glass dome molded from a single piece of glass, if its green, or even possible.
A: That seems like a tall and expensive order to have manufactured. Theoretically it is probably possible, but it doesn't seem worth the trouble to me. Similar concepts can be achieved with framed geodesic and plastic glazing, such as with the grow domes ( http://www.geodesic-greenhouse-kits.com/ ) But even these are not particularly efficient as greenhouses because there is too much unneeded and uninsulated glazing.
Q: I am planning a wooden greenhouse with an inside oven made of clay (for baking bread once a week). Do you think this will work out without risking a "greenhouse armageddon"? The advantages are the shelter for the clay oven, a dry place for doing the bakery and an the opportunity to create a kind of special-social-meeting-point ... I worry about the temperature differences especially in winter...the plants? the wooden construction (based on concrete walls)? the glass? the soil?) The greenhouse will have about 18 to 20 cubic meters (3x3x2.3 meters). I will not fix the glass/plastic with glue or screws so maybe this will relieve the tension caused by temperature differences... Do you have any advice or considerations?
A: I think that you are wise to be concerned about the heat from the oven in such a small place. Any greenhouse needs to be very well ventilated, and this would be especially true if you install an oven in there. You have to be able to keep the space at a reasonable temperature for the plants and humans; if not, you might as well just build a roof over the oven and put it outdoors. It might be that if you oversize the inlet and outlet vents, and provide a fan to boost the air movement if necessary, that you can combine these functions. I hope this works for you.
Q: We are planning to add a lean-to greenhouse. Our south side is a deck several feet above the dropoff. We plan to put a layer of insulation and plywood as the floor. Will this be adequate?We are between zone 6 and zone 7 (lows in winter generally in 'teens or 20s) Would painting the plywood black help to heat it in the winter?
A: This insulated floor should help to keep the greenhouse warm, but since none of this is thermal mass (like brick, concrete, or large containers of water) it won't hold the heat built up during the day to warm the space at night. Painting the plywood black won't do much good either. What you need to do is place plenty of thermal mass material inside the greenhouse. The planter beds themselves will help with this.
Q: I would like to build an underground greenhouse in Phoenix, AZ. area. How far do I need to dig in order to sustain a constant temperature around 68'. I'm trying to grow produce in summer.
A: My chart tells me that the stable ground temperature (about 5 - 6 feet under the surface) in the Phoenix area is actually about 73 degrees F., so that is a little warmer than ideal perhaps, but still much better than summer air temperatures would be.
Q: I don't understand what "glazing" is. I want to build a greenhouse addition to a manufactured home. Can you tell me more about the two plans you sell so I can better choose which one is right for my situation?
A: "Glazing" is whatever translucent or transparent material is used to allow the sunlight in. The main difference between the two plans is that one has vertical glazing and the other has slanted glazing. It somewhat depends on whether the purpose of the greenhouse addition is mainly grow plants or whether it will be used mainly to help heat you home, as to which plan might be better for you. Slanted glazing is better for growing plants because it provides light from directly overhead. Vertical glazing with a roof overhang will automatically eliminate much of the light and heat during the summer, if you are using it for heating.
Q: We are considering building an attached greenhouse. The area we are thinking of is south/southwest and is currently a sunk in garden area that has an active man-made pond. Thought this might help regulate temperatures and perhaps let us dabble in aquaponics. We live in South Dakota so the temps get well below freezing for long periods of time. Summer brings 90 ad 100 degree weather. My main question is what kind of precautions do we need to take concerning moisture damage to the wall of the home where the greenhouse will attach? Any other suggestions would be welcomed. The area is a circular shape and roughly 40' by 30'. Can't do Geodesic dome because the foundational wall (four foot high) isn't the correct shape.
A: Any exterior siding material that is designed to handle exposure to the elements outside should do fine within a greenhouse environment...so in other words whatever is now on that wall should be OK. Using your existing curved foundation might be the biggest challenge since the glazing will have to be faceted to match it somehow. You'll need to come up with a design that will allow the foundation to support straight sections of glass, unless you use something that could be curved like some plastics.
Q: I have a log home in Boulder county at about 8500 feet and have long entertained the idea of turning the two car garage into a greenhouse garden. It faces southeast and has incredible sun all year. I am interested in having an attached structure that will capture the sun while withstanding hail and snow. Inside I would like to have an integrated aquaponics and garden. I am not aware of any architects here that are familiar with this concept.
A: I see no reason why you shouldn't be able convert your garage into a greenhouse. You would probably want to put some glazing on the roof for enough light for some plants. You would also want some way to insulate the glazing at night. And you ideally would have some good thermal mass in there to hold the heat generated during the day.