Mobile Greenhouse - Observations

It's been about a week since the mobile greenhouse was pulled into its place behind the vegetable beds.  An earthbox seeded with lettuce and a limestone block was set inside.  I have been monitoring the temperature using a wireless temperature monitor (base station located inside my house) and two remote sensors, one inside the greenhouse in a little box to block any direct sunlight from falling on the sensor, and another sensor on the back side of the greenhouse (also in shade) to monitor the outside temperature.  The base unit can monitor up to three channels and will record minimum and maximum temperatures since the last reset.   You can buy these units at many big box stores for about 20 dollars, although additional sensors will cost more.

Since the greenhouse was set up outdoors the weather has gone from one extreme to another.  One day there was a high temperature of 55 degrees F.  Last night the low temp was 2 degrees.  I haven’t kept detailed temperature records, but have come up with a few general observations.  Initially I had one limestone block in the greenhouse for thermal mass, but after a few days of warm weather some blocks which were frozen together came unstuck and were put inside the greenhouse.  There’s probably about 200 pounds of stone inside the greenhouse. 

A few observations:

 

The greenhouse will be about 20 degrees warmer than the outside temperature on a cloudy day, and as much as 35 degrees warmer on a sunny day.  At noon today it was 15 degrees, sunny and very windy outside. 

 

The temperature inside the greenhouse at noon was 45 degrees, a 30 degree difference.  By 1 PM it was 53 degrees (outside temp 18 degrees), for a 35 degree difference.  It is 100% sunshine today so that temperature difference is not surprising. 

Once the sun goes down the temperature difference slowly diminishes.  Last night was the coldest night this winter.  By midnight the outside temperature was 12 degrees, while the greenhouse temperature was about 25 degrees.  By this morning the outside temperature had dropped to 2 degrees and the greenhouse reached a minimum of 11 degrees.  After sunrise the inside temperature rose quickly.

 

11 degrees is pretty cold for most plants, but there are not many winter nights that get to near zero degrees.  Another consideration is the actual soil temperature inside the Earthbox.  The temperature of the soil will not reach the temperature extremes of the air around the Earthbox.

Last year I compared the temperatures inside the plastic greenhouse that goes over one of the raised beds each year where spinach is overwintered.  As with the mobile greenhouse the temperature inside rose quickly whenever the sun was shining.  What surprised me was just how quickly the temperature fell once the sun went down.  In less than an hour there was no difference between the inside and outside temperature.   I thought that the soil in the bed would retain some heat and keep the space inside a little warmer but that was not the case.  A single layer of plastic film just does not insulate well enough to hold in any heat.   This is a photo of the plastic greenhouse with the new greenhouse in the background.

 

So how does the greenhouse perform on a more typical winter day for this area?  Actually quite well in my opinion.  A few days ago we had an "average" January day – a high of about 35 degrees, partly cloudy with occasional peeks of sun, then a low the next morning of 20.  The temperature inside the greenhouse stabilized at just under 60 degrees and held at that until sunset, when it slowly fell until it was 35 degrees at midnight.  By the next morning the greenhouse temperature reached a low of 28 degrees (outside temp 20 degrees) then climbed quickly after the sun came up. 

The climate inside the greenhouse that day was similar to a March day with a nightime frost.  That’s enough to give me hope that lettuce and spinach can be grown in this thing much of the winter.  Now if that lettuce would hurry up and germinate. 

Mobile Greenhouse - Ready for Plants

Tuesday evening the greenhouse was deemed ready to go into the real world, actually my backyard.  I wheeled it from the pole barn to it’s location behind the beds.  It was not hard to move, although a little awkard.  It was set north of the raised beds and faced toward the south.  I leveled it some with scrap wood.  It looks like a solar panel teetering at the edge of the pond, although the shoreline is about 40 feet away. 

 

The pond doubled in size over the weekend.  Before last weekend it was at it’s lowest level ever, about 5 feet below the overflow, thanks to the epic drought we had last summer.   When it got warm for a few days the 15 inches of snow on the ground quickly melted, and then we got 4 inches of rain to add to the runoff.  In a few days the pond rose to the overflow and probably doubled its surface area.  Sure looks better. 

This morning I filled up the one earthbox I have with potting mix and planted it with lettuce.  I was going to leave the earthbox in the house until the lettuce germinated but decided to just go ahead and put it in the greenhouse.  That’s a limestone block at the front of the bench left over from last year's landscaping.  It's there for thermal mass and weighs about 60 pounds.  I’ll put some more blocks in there later but they are still frozen together right now.  (For those of you who don’t live in the midwest it’s not uncommon to get huge temperature swings from day to day as weather systems from every direction push through this part of the country).  The little wood box in front of the planter holds a remote sensor so I can monitor the temperature inside.  I wanted to make sure it was not exposed to direct sun so I can get accurate readings.

 

It’s a partly cloudy day with the sun peaking through now and then.   At noon the temperature was about 30 degrees F while inside the greenhouse the temperature was 55 degrees F.   By 1 PM it looks like the temperature inside the greenhouse has leveled off at 60 degrees, with a little more cloud cover now.  It looks like it is working, in fact I’ll have to watch out for overheating on warmer, sunnier days.  

 

I noticed that the earthbox walls have gotten quite warm and that will heat up the soil inside it.  It’s going to take a few days to warm up all the stone once it is inside the greenhouse.  I’m hoping this thermal mass will release enough heat at night to keep the water in the earthbox reservoir from freezing.  This spring I plan to put several flats of onion seedlings in the greenhouse so they get the natural day length.  And of course it will be used as a cold frame to harden off plants. 

There’s still a few changes I plan to make when I build the next greenhouse.  It’s a little bit tippy, although the weight of the stone inside makes it more stable.  I’ll probably angle the back wall a little on the next one to give it a little wider stance.  And there’s still some issues to work out with the door props and some other things.  But it’s finished enough to give it a real world test and I’m looking forward to getting an early crop of lettuce.      

Mobile Greenhouse - nearly complete

Just a note before the real blog.  I tried numerous times yesterday to upload photos from my computer to blogger with no success.  After doing some research on Google (kind of ironic) I found that many bloggers are having this problem.  Apparently Google wants people who use Blogger to use Google Chrome as their default browser, and pictures will no longer upload in other browsers.  I had to download Picasa web album (a Google product) and learn to use it (took me over an hour because I'm not so good on the computer) so I could make this post.  This is the kind of petty stuff that companies engage in all too often, especially media companies.  I'm kind of stubborn, and since Google is trying to pressure me into using their browser I will make sure that I never use their browser.  On to the post. 
  
The greenhouse is about ready for the real world test – growing lettuce in winter.  For those who are new to this project here is what I’m trying to do.  I don’t have room (or money) to put up a real walk-in greenhouse.  I wanted a mini-greenhouse with a planting bench at a comfortable level that could be moved by one person.  The greenhouse should perform better than a coldframe or a hoophouse covered in plastic.  It needs to maximize sun exposure, have a high insulation value and use thermal mass to hold warmth in overnight.  I’ll find out over the next few weeks how well it meets those goals. 

From the last post, the next step was the installation of the twinwall plastic glazing.  This is really quite amazing stuff.  It’s incredibly light, in fact similar in weight and feel to strong cardboard.  It’s also impact resistant, has an R-value of 1.6, and a transmissivity of 80%.  I can see why it is widely used for greenhouses.  This is not the only glazing option of course.  Polyethylene greenhouse film could be stapled on the inside and outside of the lid frame to give a comparable R-value.  It wouldn’t be as durable but is a little cheaper. 

The twinwall sheets must be installed with the channels running vertical so water can escape. One side is treated for UV exposure and must be the outside face.  I used vinyl siding saw blade in the circular saw to make the cuts.  This worked perfectly with no chipping of the sheets.  Here a guide has been set up to crosscut the glazing for the lid.

Once the front pieces were cut I started on the sides.  To make sure the dimensions were right I first made a template for the glazing with a piece of plywood, then cut the glazing from the template.  I wasn’t going to take a chance on getting a cut wrong.  It’s 30 miles to the lumberyard and this glazing is 42 bucks a sheet. 

Cutting the glazing left some fine dust in the channels.  That’s were an air compressor comes in handy to blow the dust out.  Once the pieces were cut the edges with exposed channels have to be covered to keep out insects and dust.   I cut duct tape into two equal strips and applied the tape to the ends.  Once the tape was on I punctured the tape with a thumbtack over each channel.  Very tedious work but the channels need some airflow to remove condensation, and they have to be covered to prevent insects from getting into them.  You can buy a special tape to do this but it is a dollar a foot.  Too much money!

First I installed glazing in the sides.  I wanted glazing to go all the way up the side, but since the glazing has one outer face that changed the plan I had for cutting the triangles. The piece that was left after cutting the front pieces was not large enough to cut both triangle pieces.  I had to leave out the upper corner of the triangle then fill in that section on the greenhouse with wood.  I like to keep the design clean and consider this an unwelcome afterthought.  This is something I’ll have to work on when I make the next one.  The side pieces were attached with screws with rubber washers.  (I took this picture after all the glazing was installed). 

 

I put the 1 x 8 bandboard back on the side.  It’s mainly for esthetics but it also makes for a good handle to pick up the end since it stands away from the glazing. 

Then I installed the glazing in the lid.  The glazing is not fastened to the lid but sets in grooves in the lid frame.  The bottom piece of the lid frame was removed and the glazing slid into the grooves.  Then the bottom piece was reattached.  The two pieces of glazing in the lid are held together by  a plastic H-channel made for that purpose.  

Next the Reflectix insulation went on the back side.  Here’s what it looks like, kind of like silvered bubble wrap. 

This material not only helps hold the heat in, but with its high reflectivity should boost the solar irradiation during the weak sun of winter.  I considered putting this on top of the bench but decided it would take too much abuse to hold up there.  I may still tack on a layer underneath the bench.   Here’s the front view with the Reflectix installed. 

And here’s what it looks like with the lid propped open.

 
There’s a few details left.  I have to develop an easy-to-use prop to hold the lid open and a good latching system to hold the lid shut.  Those last details are never straightforward, in fact I’ve experimented with different designs for several hours and still haven’t worked out anything I’m satisfied with.  I also have to think about a temperature actuated lid opener.  I think it’s possible that the interior could overheat very quickly in the springtime. 

I’ll make another post on this after I wheel it out into the yard and set in an Earthbox with lettuce planted in it.  Until then cheers.         
  

Mobile Greenhouse Post 5

It’s been awhile since the last post on the greenhouse project.  There was Christmas, travel, then a winter storm with howling winds that dumped a foot of snow.  The weather people said that the storm met the criteria and could be called a blizzard.  I believe them.  Snowed in for two days, I paid a neighbor with a tractor to plow the driveway.

Once I got back to working on the greenhouse the trigger switch on the miter saw gave up.  The switch would not shut the saw off.  Well the saw had served me well for 10 years.  It should be easy enough to find a replacement on the Internet, I thought.  Not so, I found.  The switch was classified obsolete and in spite of the fact that it was used on a number of Ryobi models it could not be found on parts websites or Ryobi’s site.  But it was on eBay.  It looks like an entrepreneur bought up the world’s supply of these switches and was now selling them for $24 dollars a piece. Given the price of a new compound miter saw I was happy to pay it.  My guess is the failed switches tendency to keep the saw running had thoughts of litigation dancing in suppliers heads.  I really don’t like making angle cuts with a circular saw, they are just not clean enough to make quality work, but there weren’t many cuts left to make, so the project went on.

I had to find a way to build a lid, which will contain the glazing, that will shed water and operate on hinges.  Finding the design wasn’t easy.  Finally it came to me.  If the lid has a lip around the perimeter the hinge can attach to the lip and the assembly will swing free of the base when it is swung up.  Every other option I considered either required too much additional structure or the lid would interfere with the base when it was lifted.

With the plan in mind I notched some scraps of wood on the table saw, constructed a mockup corner that I attached with hinges to the ridgetop.  The mockup swung away from the ridge without interfering and will prevent water from entering at the top.  Problem is the notch removed too much wood from the frame pieces and did not leave enough thickness to make strong groove for the glazing. 

 
I went with plan B: instead of making a groove in the lid a strip of wood will be glued and fastened around the outside of the frame to make the lip - addition instead of subtraction.  It won’t look as clean as a single piece of wood but the frame will not be weakened by removing a large groove.  Using this design I made mockups of the top and bottom corners.  With the corners set in place I could get accurate measurements of the lid frame.   (At this point I had also attached some wheels to the legs). 

Once the lid frame pieces were cut to size I screwed the frame together to make sure it was the proper size then disassembled the frame and cut the grooves for the glazing on the table saw.  Constructing then deconstructing may seem like an excess of caution, but there are a million things that can go wrong when building a new design and I wanted to make sure it would work before proceeding.

It's getting close.  I planed off the top edge off the 1 x 8 (the white strip you see in the picture) to get the lid to seat on all four sides.  Once I purchase the glazing I’ll unscrew the bottom 2 x 2 from the lid and slide the glazing in then put the bottom piece back on.  But first I’ll drill a series of weep holes in the groove to channel away water.  It’s been through a lot of changes but it’s finally taking shape.  When it’s finished I’ll wheel it into the back yard and plant some lettuce in containers and see how it does.