Step 7 : Covering the Greenhouse

Once the roof line was completed it was time to cover.   We decided to use a twin wall polycarbonate called Polygal.  The twinwall material has internal air channels, providing for good thermal insulation and light transmission qualities.  Like most coverings sold for greenhouse use, it is available in four foot widths and fit the arch spacing of our greenhouse with minimal trimming.    

We had to trim about 3/4 inch off the sheet widths to compensate for the aluminum pieces used to attach them to our frame.  The plastic sheets were easily cut using a jigsaw with Bosch U234X3 "Progressor" saw blades.  It is important to cut the polycarbonate sheets without damage and we found this particular saw blade cut the sheets with excellent results.

We started with the end walls, carefully framing around the "windows" and door frame with the aluminum supports.  In order to cut the plastic sheets and aluminum extrusions to the proper shape, we first held them in place using the wood frame as a guide to draw a line on each piece where trimming was needed.  We started on one end, working across piece by piece until finished.  The double splice aluminum pieces must be screwed down with the flat surface on the outside.  We did not realize this until the end walls were already completed.   It took quite a bit of effort to box in around the fan cutouts, but as shown in these photos it gave a nice finished look to the end walls.

Before each plastic sheet was fastened, we applied a metal tape along the top edge and a mesh tape along the bottom.  The aluminum tape at the top of each piece is used to prevent water from getting into the channels inside the plastic walls.  The mesh tape at the bottom allows moisture to escape out the bottom and  stops spiders or insects from crawling up into the plastic walls.  Self drilling wood screws were used to hold the plastic to the wooden frame.  The plastic was pre-drilled at these locations, and a special self sealing gasket washer was used under the screw head to seal the hole from rain and snow. 

The toughest part of the plastic construction for us was getting the sheets up on to the arches.  Because they are 12 feet long, it took quite a bit of work to push each sheet up into the top extrusion, while also pushing it sideways into the extrusion along the arch.  To ensure the plastic sheets were slid up into the aluminum at the top, we drew a line across them about 3/4 inch down from the top edge.  Without this line it was difficult to be sure we had them pushed up into the aluminum bracket at the top of the greenhouse.  With the line drawn across the top, however, we were then able to make sure that the plastic was pushed up into the top as far as possible.  We also found that it easier to get the plastic up if we only screwed down the aluminum on the top half of each arch.  If they were bent down beforehand, it would be too difficult to push the plastic into place.  It was much easier to get the plastic into the aluminum tracks before we bent them down on to the arches.

Attaching the plastic to our frame was not a difficult task, but was much more challenging than we expected.  There were other problems encountered as we worked, but eventually we became much better at the job.  The biggest mistake we made, however, was not realizing that spacers must be used on the inside to hold the plastic sheets away from the horizontal purlins.  Because we attached the plastic directly onto the wood frame, any condensation inside the greenhouse was not able to run down the walls.  We eventually discovered that the condensation inside our greenhouse would run down the wall until it hit the purlins, and then drip down onto our orchids.  We needed to unscrew the plastic sheets in order to slide a special foam spacer between the plastic and our wood purlins.  These spacers now keep the sheets away from the wood framing, allowing the moisture to run down the plastic without dripping onto the plants below.