Camper Van Conversion Subfloor: How To DIY
The subfloor makes up the foundation for the floor of your van.
It helps to insulate your mobile home from the outside temperature, as well as provides structure to screw wall framing in down the road. Quality insulation is necessary to keep the interior comfortable enough to live full-time, through all four seasons.
Besides wanting to create space for thick insulation, having wooden framing on the floor, walls and ceiling makes it easier to sheet the walls, secure furniture or appliances, erect new walls (for a closet or bathroom), build cabinetry, and many other things we’ll get into further along in the build process. For now, let’s talk about the four components of a subfloor: framing, insulation, plywood, and underlayment.
Materials:
2” Construction screws
Sikaflex (construction adhesive)
2 ½” kreg screws
2x3’s
3 sheets of ~ ¾” plywood
4 sheets polyiso insulation (depth may vary, I used 2 sheets 1” and 2 sheets ½”)
Underlayment (optional*)
3 cans spray foam insulation
1 can gorilla glue spray adhesive
1 roll gorilla tape
1 roll HVAC tape
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Framing
Thoroughly clean the van floor (I used a scrub brush with heavy duty degreaser and a pressure washer).
Cut 2x3s to surround the perimeter of the van (excluding wheel wells for the time being).
Use a generous amount of construction adhesive (rated for wood and metal) to secure 2x3’s to the original van floor.
Use pocket screws to join 2x3 members.
Add horizontal cross members before and after the wheel wells. I broke my subfloor into 3 sections: before wheel wells, between wheel wells, and after wheel wells.
Add vertical cross members in each section to support the plywood and prevent the floor from bowing.
Frame the wheel wells in a similar manner, starting with the perimeter and adding cross members as necessary.
Optional: Consider where you might want walls for a bathroom or closet. Add 2x3 members here to ensure a more solid support for wall framing.
Using construction adhesive to secure 2x3’s to the original van floor.
Insulation
Better insulation means less energy spent trying to heat or cool your home with diesel heaters or battery powered fans.
The thickness of insulation should correspond with the thickness of your subfloor framing. I used 2x3s for my framing, which are 1.5” thick. I wanted to buy 1.5” polyiso foam insulation, but I couldn’t find it at the big box hardware stores, so I ended up stacking 1” and ½” insulation. ½” Rmax has an R-value of 3.2, and 1” has an R-value of 6. With polyiso, the R-value is additive, so combining these sheets of insulation yields an R-value of 9.2!
That said, if you can special order 1.5” Rmax from Lowes or Home Depot without buying a crazy large quantity, you’ll save time dealing with combining two sheets and you’ll get an R-value of 9.6. I was working with limited time so I just bought what was available and made it work.
If using a single sheet, skip to step 2. If using multiple sheets to acquire appropriate depth to match subfloor framing, spray adhesive sheets to be combined. Lay some heavy things on top of combined sheets (tool bags, dumbbells, bags of dog food, etc.) Tape corners of insulation to further prevent sheets from separating. Let dry for ~45 mins.
Sketch subfloor framing on a notepad, label each section that will need insulation.
Measure the openings between the vertical and horizontal cross members in the subfloor framing. Fill in the measurements on notepad (Tip: You’ll save a lot of time by measuring everything at once and then making all cuts).
Mark and cut insulation with flexible putty knife or razor blade. (Tip: I found the putty knife worked best. I used a grinding wheel to sharpen the edges of the knife and it cut through the polyiso like warm butter).
Press insulation into place, then use HVAC tape around the edges to get an airtight seal.
After the plywood is in position, we’ll spray foam all gaps around the perimeter (step 5 below).
Combining sheets of polyiso to get the right thickness.
The flexible putty knife, post-sharpening.
HVAC tape seal on wheel well insulation.
Plywood
Plywood makes for a very strong subfloor surface. I used 23/32” plywood overtop my framing.
Measure between walls to determine length and width of plywood (As previously stated, I broke my subfloor into 3 sections; before wheel wells, between wheel wells, and after wheel wells).
Cut plywood with table saw or circular saw (I used 3 pieces of plywood, one for each section).
Set plywood in position overtop subfloor framing. Use impact drill to screw plywood down into framing with 2” construction screws. (Be sure to inset screw head into plywood just enough so that it’s not protruding out of plywood).
Repeat steps for each section of subfloor and wheel wells.
Note: My floor had a service panel that could be opened to access ports in the top of the gas tank. I recommend leaving these uncovered so you can access whatever your service panel leads to down the road if needed. I framed around the panel, laid plywood overtop and then used a router to create an opening in the plywood above the panel. I’ll insert polyiso insulation and the plywood I routed out to fill the opening, but I’ll always be able to easily remove these.
Spray foam all gaps around perimeter.
Subfloor fin, sans underlayment.
Leaving the service panel accessible.
Close-up: the panel fully routed out.
Underlayment
If you intend on laying your finished flooring (vinyl, linoleum, etc.) immediately following the subfloor installation, be sure to use underlayment. I chose to hold off on final flooring because I didn’t want to damage a nice floor while doing all the work that’s to come. The final floor will be one of the last projects, and I’ll include a writeup on underlayment at that point.