In the Pink

ImageAs 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.


ImageA 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.









ImageDry 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.





ImageThe strapping was then screwed into place.








ImageI dry fit the styrofoam insulation, trimming the width and length as necessary to fit.
















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.


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