Saving Heat in a Small Winter Hoop House

hoop-houseOnce again I had no intention to compete in this quarter’s writing contest, but the article from the Mad Scientist (Russell) about his Greenhouse has got me interested in sharing my experiences with inexpensive hoop houses.

Russell’s greenhouse looks more expensive than what I can justify, as my focus is on sharing information about survival gardening and cooking on my public access TV show in Albany, NY. All 130 TV episodes are available online on my webpage crunchtimeprep.com.

Starting with episode 56 you can view the building of the hoop house with about $100 of building materials from Lowes or Home Depot. Today’s emphasis about using the hoop house in the winter, even as far north as upstate New York, is covered in episodes 61 and following. I will share a few things I did to make do with cheap materials and a marginal setting where trees blocked half the day’s sun at the winter solstice.

I will have to admit that the winter covered in the videos was not the coldest on record, like the year after was, but it was more towards the average to colder range of the climate we see here. Certainly if I could keep stuff growing all winter this year, almost anyone anywhere in the continental US should be able to do likewise. Remember we have lots more clouds here than Russell has in Colorado.

I too wanted to use no heat and no energy from the grid to maintain a growing climate inside the hoop house. I have to admit that the hoop house was a success and I even planted sugar beets in December. Most winter crops have to be well established before November (planted in September) and usually only cold weather crops will survive – lettuce, arugula, spinach, kale, and mini-greens. I am testing with root crops like carrots and rutabagas and sugar beets.

As you can see in the videos I also kept scientific records of temperatures and humidity as I too am a scientist, bachelor in chemistry but self-taught engineering and inventing all my life.

Now how did I keep a cheap hoop house warm all winter without a heat source?  Many techniques were employed as you can see.

First we’ll discuss the normal procedures of a commercial non-heated greenhouse that sells their crop at farmers markets. You have to realize that only the space where the plants live has to be kept warm. You don’t need the area up by the ceiling to be warm all night. To accomplish this you have to put your plants to bed at sunset, tuck them in with a warm blanket like a commercial frost cover, which come in different thicknesses. I used the thinnest – I made some covers that were even better out of disposable hospital gowns – which are water repellent but still breath like the commercial frost cover. I like to use two layers on some plants (because they are the thinnest available) but other plants were only covered with one thin layer. Low tunnel hoops are used to keep the frost cover from touching the plants. Of course in the morning at sunrise or shortly thereafter you have to uncover your plants so they get the sunlight and it warms up the soil and the air. If the sun is not going to come out the frost cover is usually left covering the plants to keep the warmth in but still letting some light in through the fabric.

That’s about all the commercial greenhouses do for heat retention, but with a small hoop house I thought I needed to do more. The first thing I added was a half inch thick foil backed insulation board on the north side, and in my case also on the east end, because in my location the east sun is blocked by trees until about 11 o’clock. The insulation value of the panels is obviously much better than the insulation value (nil) of the single layer of 6 mil construction plastic. But the foil backing was also a great advantage because all the sunlight that would have gone out the north side of the hoop house was reflected back in and increased the brightness and warming value of the sunlight that entered from the south during the low angle radiation in early winter.

The next thing I added to my 13′ x 20′ hoop house was about 30 2 liter bottles filled with dark liquid (I could have used 100). My liquid happened to be future fertilizer (urine) with dark food coloring added to make it dark. During the sunlight I placed the bottles upright in the walking rows where they got the best sun exposure. At sunset before covering the plants I laid the bottles down under the leaves of the plants so they would radiate the 110-120 degree heat all night under the frost cover, keeping the soil and the plants warm.

On the side where I used only one layer of frost cover I placed a few five gallon plastic buckets full of water, some white, some dark blue from Lowes, and some painted black. They were placed so as to keep the frost cover up off the plants. The white ones froze quite often and only got up to 35 degrees during the day. The blue and black buckets got up to 55 degrees or more on sunny days and hardly ever froze on top.

The heat saved by the dark bottles was about 320 BTUs per day and the dark buckets saved about 1000 BTUs per day, radiating it back at night. Because of being able to place the bottle where the heat was needed each night I think they were the best device I used to save heat.

The following year I installed a 260 gallon fish tank painted black, with growing bed on top that I insulated at night with sheets of half inc foil backed rigid insulation (videos 86 and following). The temperature of that tank only raised 5-10 degrees but each degree saved 2000 BTUs. But the radiated heat at night didn’t help the crops or the greenhouse because it all radiated inside the tank insulation. So once again the bottles are the best keeper of heat.

After a couple good sunny days raising the air temperature inside the hoop house to well over 100 degrees I had to find another way to save the heat. I installed a 4″ drainage pipe 8-12″ below the surface across the length of the hoop house with an elbow on each end and a short drainage pipe going up into the air. On one end I installed a removable extension that brought the end of the pipe to about five feet above the ground. On top of this pipe I installed a computer fan that pushed the hot air down the pipe and across the underground pipe and up the other end. The computer fan was solar powered with a small 10 watt solar panel tucked up against the ceiling of the hoop house. When the sun shined and heated up the air inside the hoop house, the fan blew the hot air through the underground pipe and the air exiting the pipe was usually only 10-15 degrees above the soil temperature. This banked a lot of otherwise wasted heat into the soil where the plants were growing. If I was to do it again I would install more pipes because the plants growing near the pipe grew twice as fast as those a couple feet away.

So in summary the heat was saved in the hoop house by thin blankets of frost cover, dark two liter bottles (the best device), dark buckets, large fish tank, and an underground heat sink pipe. The temperature outside got down well below zero quite a few times, but the temperature inside the hoop house was never below 32 degrees and no plants died. At the end of the winter I calculated that the average air temperature inside the hoop house was about 15 degrees warmer than the outside average temperature. The soil temperature at the location of the probe, about one foot away from the underground heat sink pipe, averaged 45-degrees which coincidentally was real close to the average inside air temperature.


Thanks to Crunch Time Prepper for participating in the [Grow] Network Writing Contest. We have over $1,500 in prizes lined up for the current writing contest, with more to come. Here is a list of the current pot of prizes:

– A 21.5 quart pressure canner from All American, a $380 value
– A Survival Still emergency water purification still, a $279 value
– 1 year of free membership in the [Grow] Network Core Community, a $240 value
– A copy of The Summer of Survival Complete Collection from Life Changes Be Ready, a $127 value
– 2 copies of the complete Home Grown Food Summit, valued at $67 each
– 3 free 3 month memberships in the [Grow] Network Core Community, valued at $60 each
– The complete 2014 Grow Your Own Food Summit interview series, a $47 value
– 4 copies of the Grow Your Own Groceries DVD video set, valued at $42 each
– A Bug Out Seed Kit from the Sustainable Seed Company, a $40 value
– 4 copies of the Alternatives To Dentists DVD video, valued at $32 each

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  • Jacqueline

    Thank for sharing, you have inspired me to try growing fresh food this winter. I really like the way you are using your hands to make it work!

  • Donna

    Thank you for several great ideas. I live in TN and will not need to use all of these. In fact, I would need to vent a greenhouse a lot of the time. However I stayed down in the teens several days in a row last winter. I look forward to trying these ideas next winter.

  • Tina

    Wow, you did your due diligence and hopefully will have many future successful crops. I never thought of the tube (to keep the earth warm) so it was very helpful, even here in Texas.

  • Julia Pace

    I loved reading about your hoop house. Did the computer fan reduce the 100 degree temperatures in the air as well as warm the soil or did you also ventilate? Did you make holes in the drainage pipe or was it just the warm air going through it that warmed the soil? Very interesting article. I did so wish for photos though. Also wondering what is your plan for the urine – is it going on the compost heap in the Spring? And why did you dye it rather than paint the bottles black?

  • Norm G

    I, too, wondered why you didn’t paint the jugs black. I have a question about the jugs, though. I read an article suggesting people don’t use water filled jugs for heat in tunnels. A scientist said thermodynamics will result in the jugs cooling or getting the environment in the tunnel colder than with no jugs. It had something to do with the transfer of heat being faster (cooling) than warming…sort of like a refridge or air conditioner. Also, because it takes more energy to warm the water than cool it, so energy is displaced, somehow. Anyway, wondered if as a scientist you have compared two tunnels end to end; one with jugs and one without to see if it makes a difference in tunnel/soil temperatures.

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