The completed 2x4 framing for the Tiny House floor was divided effectively into 30 rectangular spaces by joists and blocking pieces installed to stabilize them. These rectangles were of roughly similar lengths and widths, and all were 3.5” deep - the width of a 2x4. Mikey’s plan called for each space to be filled with rigid foam insulation, to be provided in three layers of 2”, 1” and 1/2” thicknesses respectively. A grand total of 90 rectangles of rigid foam had to be cut from available 4x8 sheets of the three thicknesses.
The bottom layer in each space, 2” thick, was cut allowing for a 1/4” gap on all sides between the rigid foam and the surrounding 2x4 framing. The 1/4” gap was filled with Great Stuff Pro, dispensed with a handy foam gun attachment from several 20-oz. aerosol cans. A nice foam gun can be had for ~$30-$50 and is highly recommended if you’re going to be doing a significant amount of foaming. It will allow you to reuse the same container of Great Stuff until it’s fully used up. We used the “Doors & Windows” variety of Great Stuff, since it doesn’t expand as much and cures with a bit of flexibility, to hopefully allow for some shrinking/expanding of the framing.
Before installing 2” foam in all spaces, a trial application of the next layer - 1” thick - was made in 3 spaces to validate the technique and check if there would be enough room left above each 1” piece to allow a 1/2” piece to fit flush with the top surfaces of surrounding wood framing. Two of the first spaces we had done to be re-done entirely (not easy) because the bottom piece of foam was slightly bowed and the top 1/2” piece would consequently protrude above the framing. This was before we developed a good system for installing the 2” pieces, using clamps at each end of the piece to hold it down while the Great Stuff cured.
After the trial, the thirty 2” pieces were installed -foamed in place - throughout. Next, excess foam had to be cut away so that the next, 1” layer would lie flat on the first layer. Once the 1” layer was complete, Michael considered a key decision: whether to go ahead with the 1/2” layer as initially planned or leave it out to ensure that the plywood subfloor would lie completely flat on the 2x4 framing. If the 1/2” layer did not fit perfectly, some of the subfloor weight (and weight of structure above) would be supported by rigid foam rather than the framing itself! (The rigid foam was effectively incompressible).
Michael decided to proceed with the planned 1/2” layer, and there was an immediate need for me (Jon) -who had been cutting almost all the foam rectangles to that point - to get busy and cut thirty more pieces, this time 1/2” thick. A race was on, with Michael trimming excess foam, vacuuming up trimmings, and placing 1/2” pieces about as rapidly as Jon could cut them. The 1/2” pieces were not foamed in place; they were cut to the full dimensions of the cavities and simply pressed into place after trimming excess foam above the 1” layer.
The cutting process itself merits some attention. Among all the spaces to be filled, there were two different widths (wide and narrow) and two different lengths (long and short). For each rigid foam layer, there were 16 long & wide pieces, 8 short & wide pieces, 4 long & narrow pieces, and 2 short & narrow pieces. Width markings for cutting the 4x8 sheets were simplified and expedited somewhat by use a of a homemade marking gauge of length 17 3/16” (for wide dimension) and another gauge a half-inch longer for marking out of full-dimensional 1/2” layer pieces. Michael’s excellent cordless Ryobi jig saw (fairly light) was used for all cuts.
When the final 1/2” piece was put in place near nightfall on 6/19/16, there was nothing but satisfaction with the decision to install all three layers. With only a few tiny exceptions, the entire expanse to be covered by plywood subfloor was so totally flush that it looked like the cavities were filled with something like reflective liquid mercury (foil backing on rigid foam was shiny).
On to the plywood!
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