Hey Okie. First off I wish you well in your upcoming treatments!!

Honestly I think the most important thing for a hoop house is ventilation. On a sunny day mine can exceed 90 degrees fast. a vent on one end and a door on the other will help especially if the door is on the south.. I use a fan and thermostat that kicks on at 85
Mine is 10x12. I used 2 10' pieces of 1" gray PVC and that gives me almost an 8' center height. I used gray because 1) im and electrician so i had it on hand, 2) its UV treated to protect it from the sun.
Corner bracing will be your best friend when the wind kicks up and putting T's and X fittings at the center peak to tie the bow together really strengthened mine. That with the hip boards and wood framed and sided end walls it solid as a house.
If you can afford it get the UV treated plastic. The plastic at lowes will get you through a winter but that is just about all it will do. I have had it break down in just a few months. I should get at least 5 years out of mine now.

I wish I had made mine longer than 12. Im building a new one this year and Im going for at least 20' long. You will be amazed how quickly you can fill one.

mike

Okie, if you are intending to start your seeds inside the greenhouse, I suggest that you build a plant starting bench that can be covered with plastic and a heavy quilt on very cold nights. My DH made me one many years ago that I used on the sunporch for years and now have in the greenhouse. Seeds like to be kept much warmer than it is practical to keep the entire greenhouse. I also have an old meat cooler that I bought for $10 from a grocery store many years ago. I also once knew a woman who started seeds in an old refrigerator that she heated with a 100 watt light bulb. Or for fewer seeds you could turn a large styrofoam cooler over your trays and heating mat on really cold nights. As your plants get bigger you may have to heat the whole house, of course.

Old Okie, I hope your radiation treatments are successful.

I agree with Mike that good ventilation is the most important thing. Heat builds up incredibly fast in a closed greenhouse, but with the use of doors and vents to facilitate ventilation, you can manage the heat fairly well.

My greenhouse is 10' wide by 24' long. (I had to go out and measure it to make sure because I never can remember if we built it 10 x 24 or 12 x 20.) The center ridge pole is 7'6" high. The short sides are east- and west-facing and the long walls face north and south. Each short wall has a nice walk-through door and two vents that I can open manually for ventilation. The vents are 30" tall and 30" wide and sit just above ground level. So, when the door and vents are open on each end, we have wonderful air flow. On a really,really hot day I drag out a big round fan from the garage and turn it on, placing it just outside the west door and letting it blow air out through the east door. On a cool winter day I might not open any doors or vents at all, but usually I have at least one door open as long as the outdoor air temperatures are above freezing.

I also use shade cloth to keep my greenhouse at a halfway reasonable temperature. The first few months that I used it without shadecloth that first year, the temperature could reach 145 degrees by 9 a.m. on a cold winter day if I didn't go out there shortly after sunrise and open the doors and vents. With the shade cloth on it, it doesn't hit 145 degrees until about noon if I forget to open the doors and vents on a sunny winter day. I like having the flexibility of opening doors and vents as needed to increase ventilation. On cloudy and cool winter days when the temperatures are barely above freezing, I might open 1 door or I might not open any at all. It depends on how cold it is outside and on how warm I'm trying to keep it inside.

The kind of shade cloth we have on our greenhouse is Aluminet, which is a woven, shiny aluminum material. It is pretty pricey, but I wanted it because it not only keeps the greenhouse cooler on sunny days but warmer on cold nights. It helps hold in the heat, so I use that knowledge to my advantage on cold days.....making sure all the vents and doors are close by 3 pm in order to allow enough heat to build up to keep the greenhouse warmer at night. The shadecloth runs from the ground level on the north side of the greenhouse all the way over the top of the structure and down to the ground level on the south side of the structure. The east and west ends are not covered with shadecloth because I want for that sunlight to come into the greenhouse on those sides in the winter. You can buy shadecloth that blocks various percentages of sunlight, and the one we have blocks 50%. There are times I wish we'd bought 60% for the summer, but I didn't want to block too much of the winter sun for the sake of the plants in there in the winter. Normally, even with shadecloth, the greenhouse is too hot to use from sometime in May through at least the end of August, so I don't have plants in there during that time.

Our greenhouse is covered with 6 mm clear greenhouse plastic purchased from a garden supply company. I think it is in its 4th year now and is in great shape. Because it is covered with the shadecloth, I expect it to last a long time.

We chose not to run electricity to the greenhouse so it doesn't have any sort of heater in it. I reserved the right to change my mind and add a heater later, but so far haven't done so. I'm not trying to overwinter much in there. I only have 2 citrus trees and they are easy to overwinter in the sunroom that sits on the southwest corner of the house. If I wanted to overwinter them in the greenhouse, I'd have to heat it.

I do have some good solar collection going on in there. I filled the floor of the greenhouse with molasses feed tubs lined up in rows. There's about 40 of them in there, each of them roughly the size of a whiskey half-barrel. I filled them with water well before the first ice storm arrived, and then I lined up cat litter buckets buckets as well to fill up most of the rest of the floor and filled those with water. Every square inch of floor space is occupied with molasses feed tubs or cat litter buckets filled water or with molasses feed tubs and other planters filled with plants. The water heats up during the day, and releases the heat at night. With those solar collectors in there, my greenhouse stays roughly 9-15 degrees warmer at night than the outside air. How warm it stays depends on how warm the daytime highs were and also on when I closed the greenhouse doors in the afternoon to capture all the heat possible before the sun set. Last night we dropped down to 20 degrees here, but the greenhouse only dropped to 29 degrees.

Usually, I can keep my fall tomato plants and pepper plants in containers alive until at least Christmas unless the first really cold blast of Arctic air arrives really early. This year, the first Arctic blast, which dropped us down to 11 degrees, hit in early December and pretty much everything in the greenhouse froze or at least dropped all its leaves. I do still have a few green plants in there....some mint in a container, a couple of lantanas and some Laura Bush petunias.

We use the greenhouse to extend the growing season a few weeks in both spring and fall but its main purpose was to give me a place to harden off transplants grown from seed indoors on my light shelf. Because the light shelf is in a spare bedrooms upstairs, carrying plants outside every morning and inside every night was getting pretty old. Now, I just watch the temperatures carefully in late winter and early spring when I am hardening off plants and throw frost blankets over the flats of plants in the greenhouse if there is any chance the temperature will be colder than the plants can tolerate. Between the shade cloth, solar water collectors and frost blankets that give 10 degrees of protection, my unheated greenhouse stays as warm as needed by the time I have plants at the hardening off stage.

I did break down and buy a space heater this winter that I could use as a temporary measure, if needed, to keep the greenhouse warm at night once I have plants hardening off out there. I don't know if I'll use it, but there is a certain degree of peace of mind that comes along with having it.

What I learned very quickly the first year was that I needed a Min-Max thermometer so I could keep track of how hot and cold the greenhouse was. I bought an inexpensive one at Walmart and it has worked just fine for several years. We have had to replace the battery once.

I don't know what kind of base/floor you have in mind, but if you are building the greenhouse directly on the ground, it helps to use black plastic to block light to the grass so you aren't growing a wild jungle of grass iand weeds n there year round. Often, people put in a gravel floor, or use brick or stone pavers, but a plain dirt floor is doable as long as you use something to keep the weeds in check.

I love, love, love my greenhouse. Some years I have overwintered lettuce in there, but it almost gets too hot for lettuce on sunny days inside the greenhouse in the winter, so it is easier for me to grow the lettuce in a cattle trough by the barn. I just throw a frost blanket weight floating row cover over the lettuce if the air temperatures are going to drop below about 20-22 degrees.

Our greenhouse has had to deal with winds in the 50-55 mph range a couple of times and that much wind hasn't bothered it at all.

Someday I hope to build another one, and I think Tim feels the same way. I'd like to have a permanent one, built low-tunnel style with side walls we can raise and lower for excellent air flow. I'd put it right out there in the new back garden and use it almost year round. If we put one back there, I'll ban the cats from it and grow plants directly in the ground.

One thing to remember when you are raising your own seedlings in the greenhouse is that they are going to be somewhat sheltered from the full impact of the wind and, if you use any sort of shade cloth at all, they also are not hardened off to sunlight just because they are in the greenhouse. I still have to carry flats outside from the greenhouse to the nearby patio to harden off home-grown seedlings before I transplant them into the ground. I do the standard one hour per day the first day, two hours the second day, three the third day, etc. Even though they have had some sun exposure while in the greenhouse and some wind exposure when the doors and vents are open, I still am careful to give them the full, proper hardening-off experience because it protects them from having issues once they're in the ground.

Dawn

Definitely get a min/max thermometer. I have 3 in mine, one in each starting bench and one in the open. When you are looking for them be sure they are calibrated for cold temps. There are some on the market that the minimum temp only goes to 32 F. Lowe's has one that goes to 0 F.