Last winter I followed advice in Elliot Coleman's book (Winter harvest handbook) and had great luck in *harvesting* fresh vegetables all winter. They did most of their growing however in the fall (I started them all in late September or later) and then later in the very early spring as in the winter months they barely grew (although they did a tiny bit). My set up: Hoop house - It was an existing (built by me and my husband a handful of years ago for season extention) 12x16 plastic covered hoop house. We made it out of plastic conduit and the only thing we modified for the winter is that we swapped out the flat 4 way fittings at the roof peak with adjustable angle fittings. I found that 45 or 90 degree fittings didn't work (too sharp an angle) and I didn't want to not have an angle for the snow load so I went with manually setting the angle as steep as I could with the adjustable ones. (You can find them by googling "PVC 1" Adjustable Ell Fitting 3 Way" or "4 way"). We never removed snow off the top of it - By design or by luck the snow would shed off the side. (Although I hear brooming it off is good practice.) I sealed the window and door with weatherstripping. I also added ~ 800 gals of water - some large tanks (which were a royal pain to get in) and lots of single gallon containers. against the north wall. I also added 1" foam insulation along the north wall - including covering 1/2 the "ceiling" on the north side (taped to the conduit frame in some areas). I dug the foam insulation ~ 6" deep around the perimeter. I then planted in two 18" rows separated by a small 1ft step path - all under a 4ft wide floating row cover (the row cover hovered on smaller plastic conduit hoops). Under the row cover in the small 1ft step path I put as many gallon jugs as fit leaving me with step spaces. I also put a few 1L and 2L bottles on the outside edges under the row cover. I also put one strand of old-fashioned (ie not LED) christmas lights suspended from the conduit as my "emergency" heat source. I put my thermostat for my lights outside my row cover and set it to turn on at 28*. I also grew a few earthboxes full of veggies but I found that anything in a box didn't grow nearly as well as things in the ground. I think the roots get too cold and they stop growing much, much earlier (but they didn't die - just didn't grow as much) even though they were under the same row cover but they didn't die. The last thing I did was put a small muffin fan attached to a tiny solar panel under the row cover for air circulation. I had my window/door always closed (ie I disconnected my autovents) for the winter months (Dec-Mar). I also never went in to harvest except during the day (almost always a sunny day) when opening my row covers wouldn't release any trapped heat that was vital for the night. I weighed down one side of the row cover with cylindrical bags of sand that I could easily remove and then the other side with soil/rocks piled on top. Watering was mostly a non-issue, but I'd carry in my water in jugs on sunny days and water by hand. I think I had to do this maybe 3-5 times all winter (ie monthly). With this set up I successfully grew: kale, leaf lettuce, carrots, beats, chard, mache VIT, bok choi, spinach, etc. For most plants I would harvest outter leaves and leave the inner leaves to continue growing. With that set-up each plant continued to provide until the late spring heat caused them to bolt (which was fine since I needed the space for the next round of crops). The temperature inside the row cover was much more stable than in the hoop house in general. The temp in the row cover never dropped under I think 25 - even after multiple cloudy/stormy days. In addition my hoop house is drafty so perhaps with a more sealed-tight environment no extra energy would be needed and/or maybe more ventilation. Oh and my hoop house is on the north side of my property and has mature trees ~ 50 feet away from it on the east side which did shade some in the morning (but not so bad because they didn't have leaves) and then we have evergreen trees that would shade the late day sun as well. I still think it's worthwhile but it depends on how much sun you get. I'm planning on building a "real" greenhouse (twinwall, etc) and am spending this winter scouting for the right location (best sun). I plan to use an earthen floor due to my experience of growing in pots vs in the ground, but am still doing my research for that as well. I've read that wherever possible you want to insulate in the ground to keep from losing heat that way. I plan to do that with my final greenhouse. Also wherever you are realize that both snow and a kneewall (if you have one) will shade part of your greenhouse floor in the winter due to the low sun angle so you may lose "significant" growing space that way.

Don't know about water, but I do recall reading about some ancient terraced agricultural plots in the mountains of, I think, South America. The scientists studying the site estimated that the rocks used for building the terraces also acted as heat sinks and probably increased their growing season, adding at least one "zone" to their climate. Coincidentally, I had been using large flat stepping stones in my vegetable garden, and had noticed a slight difference in the growth of plants near those stones. You must have a short growing season there in Zone 5. I would imagine, though, that air temperature is a real killer. Have you already tried a floating row cover or a plastic-covered tunnel? It will be interesting to see what kind of answers come back from others who have experience in your colder climate.

One kilogram of water holds 1.16 watt-hours of heat energy per degree difference in Celsius. That tells you nothing about how fast that heat will be released though, just how much heat energy the water can store when it's at its warmest point. So for example, if the daytime and nighttime temperatures are 30 and 10 degrees Celsius, 1 kilogram of water should be able to hold 2.32 watt-hours of heat that it can release later. That assumes that the water is actually able to heat up to 30 degrees Celsius. If you have a lot of water in there, the temperature of the water may never be able to rise to the ambient temperature inside the greenhouse. The other issue is that as the inside of the greenhouse begins to cool, the water is already going to be releasing energy even when you don't need it. Suppose the temperature inside the greenhouse reaches 32 C and then is wanting to cool down to 25 C. The water is already going to be releasing heat during that time to prevent the inside of the greenhouse from going down to 26. Now suppose the temperature of the water has reached 25 and the temperature inside the greenhouse is reaching towards 17.5 C. The water is still going to be trying to keep the temperature inside the greenhouse from going down to 17.5. That is heat energy that's not going to be there much later in the night when it could be going down to 2 degrees. Depending on the rate of heat transfer, it might not flatten out daytime-nighttime temperatures as much as delay the temperature drop by two or three hours. That might mean if it has dropped down to 3 degrees C by 11pm, the warmest the water is going to be able to keep the greenhouse by 2am (the coldest part of the night) is 3 degrees.

Based on your screen name, I assume you're building in Texas. Unless you're way up in the panhandle somewhere - unlikely since you mention you have a lake place and there aren't many lakes up there (LOL!) - you're in an area where the number of degree-cooling-days per year is 2 to 2.5 times the number of degree-heating-days. Or, to put it more simply, you normally need to run you air conditioner 2 to 2.5 times as much as you do your heater! Be aware that much of what you will read regarding passive solar was written with cooler climates in mind. For those of us in hot dry or hot-humid climates, passive-solar must be far more about keeping excess heat OUT of the house (or moving it to unoccupied areas) than it is about using the sun's heat to warm our homes on a few cold days in the winter. So the very first rule is, you want to select windows with low solar-gain and low emissivity numbers. In colder climates they need high-solar gain glass. The low solar gain windows are sometimes called "Low E 2" windows so be sure you get the right kind. I love bright light-filled rooms and wanted every major room in our house to get light from two or three directions but I also didn't want to pay thru the nose to keep temperatures comfortable. So I did a lot of research on passive solar principles when designing my home. As a central Texan, I basically decided that, given that we can and do often have temperatures rising into the 80s in December and January and only very seldom have temperatures that drop much below freezing, it would be better for me to avoid daytime solar gain as much as possible pretty much YEAR ROUND. While we might have 20 or 30 days each winter when I'd want to have the sun help heat my house, we're just as likely to have 20 or 30 days each winter when excess heat from the windows could easily force me to have to turn on the AC. This meant putting most of my large windows on north facing walls. These windows provide plenty of natural light without direct solar heat gain. On east facing walls, you can have large windows if you have overhangs deep enough to begin shading them completely by around 9AM. Downstairs I have a wrap-around porch and trees that shade my large east-facing windows. I do have large unshaded east-facing windows upstairs in my master-bedroom and in one of the secondary bedrooms. In the summer I keep these windows covered with insulated window coverings that are opened for only a little while each morning as we get dressed. In the wintertime, I usually leave the drapes open to allow the sun to warm those rooms a bit because I know that even on the warmest winter days, by the time any heat really builds up, the sun will no longer be hitting those windows. On south facing walls, you should generally limit the number and size of windows and have deep overhangs that will shade them completely in the summer. Small windows set high on the walls with overhangs deep enough to keep them fully shaded in the summer but shallow enough to allow some direct sunlight to enter in the winter are best. But note that in hot climates - even in the wintertime, you DO NOT WANT direct sunlight entering a bunch of large windows. If you have too much unshaded south-facing glazing, your home will get too hot on those balmy winter days we enjoy. If you decide you must have large south-facing windows to take advantage of views, then put a deep covered porch on the south side as well so that, even in the winter, direct sunlight is blocked from all but a very small portion of your south-facing windows. I do have a bank of large south-facing windows in the breakfast nook of my home but they are almost 100% shaded year-round by a second floor deck and by a grove of nearby live-oaks which are green year-round. On the second floor, the only south-facing windows I have are in bathrooms and, in addition to double-paned, low E, low solar gain windows, they have glue-chip glass which cuts down on the solar gain even more. Total unshaded south-facing glazing in the winter is probably less than 3% of my square footage. And total unshaded south-facing glazing in the summer is as close to zero as I can make it. As for west-facing windows, in addition to large overhangs, I very deliberately positioned our two-story semi-detached garage on the west side of the house so that it almost totally shades the entire western facade from the setting sun. The three upstairs west-facing windows get direct sunlight for about an hour a day. The one west-facing window downstairs gets no direct sunlight at all. All four windows however receive reflected sunlight from the side of the garage for a good portion of every day so those rooms are all still nice and bright. (Reflected sunlight carries a lot less heat than direct sunlight.) BTW - even though both the walls and the roof are insulated, our garage stays toasty warm year round and in the summer can get downright hot...evidence of the heating due to west facing facade. The upper room over the garage was originally intended to be storage so we didn't bother to put in HVAC. It then turned into DH's man-cave but he uses it only when the guys come over watch football or play poker. On those occasions we use a couple of portable AC units to cool the place down. They're not terribly energy efficient but since used only occasionally, its better than paying to keep the whole space air-conditioned all the time. In addition to optimally sizing and placing your windows, the more "thermal mass" your building materials have the better. Thermal mass absorbs excess energy when it is warm outside and releases it slowly when it is cooler. Whether in a hot or cold climate, thermal mass helps to "even out" temperatures. Good insulation in the walls and attic is also important to keep excess solar heat from penetrating to the interior of your home. Also, keep in mind that light colored building materials reflect more sunlight and therefore don't absorb as much heat. So, light colored brick or siding will help keep your home more comfortable. Light colored roofing shingles or a highly reflective metal roof are also helpful. We painted our hardiboard siding a light cream color and, because the budget wouldn't stretch to metal roofing, chose the lighted roofing shingles we could find - Owens Corning Amber. You also want to provide a way to move excess heat OUT of the spaces where you actually live. In warm climates, high ceilings provide a place for excess warm air to go. And, if you build a two-story house, the downstairs rooms will stay much cooler during the day as the warm air rises to the upper rooms. So, if your daytime living areas are downstairs and your bedrooms are upstairs, you'll be in the coolest part of the house when it is warmest outside and in the warmer part of the house at night when it is cooler outside. This should help reduce the amount of heating and cooling you need to do year round Maximizing ventilation also helps to reduce your dependence on air conditioning. Operable windows on opposite walls allows for cross ventilation. This is especially good if you have windows that can be opened near the ceiling on the warm (south & west) sides of the house and opened near the floor on the cooler (north and east) sides of the house. Even on days when there is no breeze, rising warm air will escape from windows opened near the ceiling and will draw in cooler air on the other side of the house creating natural ventilation. Additionally, ceiling fans work wonders. They use very little energy but can easily make a room feel 5 degrees cooler. So, instead of needing to set the thermostat at 73 degrees, you can feel perfectly comfortable at 78 degrees. Just remember fans cool people, not rooms, so you should turn them off when you're not in the room. Finally, design your HVAC system so that the units and all ductwork is INSIDE the heated/cooled envelope of your house. Do not let your builder put ductwork and your HVAC units into your attic. Consider that most codes in Texas call for at least R-30 attic insulation (the equivalent of 9 inches of the pink stuff) but flexible ductwork has only about 1.25 to 2.5 inches of insulation around the central core that carries cooled air into your home. (R 4.2 to about R 6), Even when the duct work is brand new, isn't "leaking" or crimped or otherwise improperly installed, consider how much harder your HVAC unit has to work to cool your home when it must first push the cold air it generates thru 10 or 20 or 50 feet of ductwork that has only R4.2 worth of insulation between it and attic temperatures that may reach 150 degrees in the summer! And, flexible ductwork almost always starts leaking after a season or two which means everytime you do run your air conditioner, you're paying to dump cool air into the attic. Far better to run smaller ducts with less insulation through open web trusses within the heated/cooled envelope of your house. And, if the ductwork starts leaking, well, the cooled (or heated) air is going INTO your home where you want it instead of into the attic. Insist that your architect draw up the HVAC plan requiring the HVAC ductwork run inside the heated/cooled envelope of the house and incorporate that plan in your building contract. Otherwise your builder will probably insist that having the HVAC unit in the vented attic and having flexible ductwork running throughout the attic like a giant spiderweb is the "way we always do it" and therefore the only right way to do it. Do your reading and you'll learn that it is not the best alternative. Hope some of this is helpful...