As I hope to be able to use the greenhouse for all but the coldest months, insulation is important. No time like “Construction Time” to insulate the floor. I picked up two 1.5” thick sheets of pink extruded polystyrene foam – pink board insulation — with an R-value of about 7.5 . It is a “closed cell” styrofoam and does not absorb water. The supplier kindly cut the 4’x8’ sheets into 14.5” wide lengths for me, easing transport and on site handling.
I needed to trim some long edges as I placed the styrofoam lengths between the joists as lumber is not completely straight . I scouted around online and saw references for styrofoam cutting devices such as a “hot knife,” fashioned from a soldering iron or wood burning tool. I possess neither so I used a jigsaw for trimming and had little chipping of the closed cell styrofoam at low speed. I trimmed the ends as the lengths were slightly more than 8’.
To hold the insulation in place beneath the floor, I used strapping. If you have access to recycled/damaged boards that you can cut into smaller dimensions for the purpose, there is an opportunity to save some money and some trees; otherwise, it’s a purchase of some low-cost 1” x 3” spruce strapping.
A standard square has a 2” and a 1.5” arm, so it is easy to scribe a horizontal line 1.5” down and along each joist.
Dry fit the strapping, 1/16” below the scribed line to avoid having the 1.5” thick insulation ride high when placed between the joists. To save time, I double checked the depth with a scrap piece of the styrofoam to ensure that it did not ride above the joist.
To vapor barrier or not to vapor barrier?
If I was to include a vapor barrier, it would go on the cold side of the insulation — on the exterior and the underside of the joists. As this structure is a greenhouse and I’m expecting moisture because of that use, I decided not to vapor barrier at all so that spilled water and condensation seep away and/or are controlled through window venting.