Here is a link to what you need.
These are fast easy and inexpensive.

Here is a link that might be useful: Insulation clips

Or you can do like many of like many of us here and throw a clear, solar pool cover over your greenhouse in the winter. I put one on my 10x16 glass house this winter and it worked very well as extra insulation. I took it off a couple weeks ago.
The Ultra Therm covers are clear, from Americas Best Pool Supply.

Here is a link that might be useful: Ultra Therm Pool Covers

I'm in the process of insulating my new GH now. I put 3/4"
polystyrene on the north wall and half way up the east and west sides. The rest of the walls and roof will be bubble warp. Do a search on this forum and read how many say they would not buy the pool cover again. Not only is it hard to put on and remove but high wind will remove it for you. No such issues with the insulation on the inside. The two foot wide is easy enough that a woman can put it up
A pool cover would be a bear to put on my GH with the 10 foot roof.

foil backed PS is considerably better than plain, for a number of reasons: a) aluminium is a good reflector of IR and thus a poor emitter b) the foil stops water moving through the foam, which reduces the insulation values. Painting or cement-acrylic rendering the foam will have the same effect as b).

3/4" has a US R-value of about 4.2 compared to glass/PC/PE with R of 1. I would double that thickness to 1.5" if I could afford it. Bubble wrap has an R-value of about 2, but you get some light through it.

: nathanhurst The Polystyrene that I installed has foil on both sides I considered thicker but cost was double and r value only increased a small amount didn't seem like it was worth it. I will only be heating a couple months a year start plants from seed in the spring about 9 months a year the GH will sit empty

I'm using reflectex insulation on my north wall...I like it.

I attached it to the panels with double sided tape as I was installing them. Seems to work well. But I was rushed last fall so I didn't insulate as well as I should have which I'll fix this summer.

Dave
Milwaukee