I was stoked to tackle the skoolie subfloor!
It marked a significant milestone in our school bus conversion project. It looms as the first major step in construction rather deconstruction! It felt nice to move from patching rust on the metal floor to adding some new material!
It seems, too, that so much depends on banging out this step. “Finalizing” the floorplan, framing wall, and running plumbing/wiring really need to have the subfloor in first.
So we were pumped to get the school bus floor project underway.
It turned out to be a paradoxically frustrating and exhilarating two-day adventure.
The first part of this post describes the process. The second half explains our material choices. Read on, enjoy, and feel free to cajole, criticize or question in the comments!
Work Flow
So I finally had a full day with temperatures above 40! I knew I wanted an uninterrupted work day, and I also wanted it to be warm enough for adhesive effectively glue the insulation foam board to the metal floor.
Now I had it.
Of course, the warm spring-like weather brought challenges.
Mud.
I didn’t want to track mud all over the place, make a mess and compromise the integrity of the glue bond, so I did my best to set up a workstation to limit mud trafficking.
I pulled all the Advantek onto a couple of sawhorses just outside the back door of the bus off to the side. In the center, I set up my table saw. Behind my saw, I parked my truck bed facing the saw to use as an outfeed table.
I stacked the insulation on sawhorses to the right of the saw.
Then, in another attempt to reduce mud, I pulled the rubber truck bed lining out to set on the ground as a mat.
Of course, in wrestling with the liner, thinking it was the tailgate, I stepped onto the overhanging edge of the plywood.
And went ass over teakettle.
Table saw: flung.
Truck bed liner: flung.
Pride and dignity: flung.
I should have taken this as a bad omen and played fortnite for the day.
But I’m stubborn.
And don’t play fortnite.
So, I reset up the saw and plowed ahead.
Gluing Down the Insulation
The bus measured 90″ across the floor. So I threw in a piece of foam, measured the distance remaining, and, you guessed it, got 42″.
Sometimes things like that work out.
I set my table saw to 6″ and used it to trim the 2nd piece of 48″ wide foam down to 42″.
It was a tight, (carpenters sometimes call it a “dry”) fit, but it went in. For future cuts I trimmed it down to 41 â…ž” to make it a bit easier.
Now came the trials with the glue.
DAP Smartbond Heavy Duty
I had kept it inside to keep the temperature in the 70’s which I thought the glue would appreciate.
It didn’t.
I shook it up as the directions directed.
I held the can upside down as directed.
I squeezed the trigger.
There was a sudden WHOOSH sound, then nothing.
No glue.
Nothing.
Any male would have been decidedly disheartened. Humiliated, perhaps.
I was flat out angry.
After spending $86 for adhesive that I was gloriously looking forward to applying with ease…and it didn’t even come out of the can.
Two other cans provided the same results.
Rats.
And other expletives.
Loctite 3X
So off to the hardware store for plan B: Loctite 3X.
I returned to the bus project, fitted the tube of loctite into my large caulking gun, (I needed to reassert my masculinity after the embarrassing spray can incident) and started pump, pump, pumping glue onto the foam board.
I was already frustrated that this was taking as long as I knew it would when I had ordered the spray foam when the glue tube split down the side oozing adhesive all over the world.
Rats.
And other expletives.
So I donned some rubber gloves and began finger painting with glue.
Messy, messy, messy.
And one tube of Loctite 3x gone.
Anyway, I got the glue on, flipped the foam and put in place.
One.
So I put a new tube of glue into my big caulking gun and repeated the process.
Yup, the tube split spewing glue all over the universe.
Rats.
And other, more violent, expletives.
After another finger painting activity, I got back in the truck and headed back to the store for another big caulking gun. Evidently mine was defective. (The plunger was bent causing it to push up against the side of the tube.)
By this time it was 2:00 in the afternoon. I had cartwheeled off the tailgate, suffered from aerosol failure and blown up two $8 cartridges of glue.
I should start playing fortnite.
But, as a tribute to my bullheadedness, I continued on.
Advantech Subfloor
I had laid the foam the long way in the bus, and as a general rule I avoid overlapping seams, so I decided to put the OSB across the bus.
I measured a whisker shy of 90″, snapped a chalk line, and used my skill saw for the rip cut.
Now I’m a fairly accomplished plywood porter. But lifting 70 pound sheets up into the bus was tougher than expected. Not necessarily the weight, but the pivot points are all off for my 5’7″ frame.
Moreover, once I finally got it up and in, I couldn’t navigate it past the chair rails onto the floor.
I had worried about that but had hoped angling would circumvent them.
It didn’t.
Wrestling didn’t help.
I ended up bleeding.
Advantek 1, Ted 0.
Rats and !@#!#!#$.
Plywood back out of the bus, trimmed down to 89″, wrestle back into the bus and fit.
Made it past the chair rail, but now there’s a 1″ gap.
Oh, well, I guess that’s what’s going to be. I can fill it in later. (Although I’ll still leave an 1/2″ gap to accommodate heat expansion.)
So, I essentially repeated this process going down the bus, through the wheel wells until I got to the front.
Julie came home from work and helped.
This improved morale. She’s cute.
I did stagger the foam board joints, meaning I alternated putting the trimmed piece on either side of the bus at each course. I also taped all the joints with aluminum tape to help seal any gaps.
By this time, it was late, so we called it a day.
Templates and Angle Tools
I returned, apprehensively, the next day to finish the project.
I contemplated sacrificing a goat to the subfloor gods to secure their benevolence. But I couldn’t find a goat.
I briefly considered the dog, but then Julie wouldn’t be as cute with me anymore.
So I floundered ahead without any divine appeasement.
Angle Tool
To navigate the straightforward angles of the driver’s heater, I used my handy-dandy angle measuring tool to trace the angles onto the insulation and plywood. (At this time, too, I was out of full sheets so was using the cutoffs, which wasn’t bad because I they were smaller, lighter, and more easily heaved around the irregular faces of the front of the bus.)
I traced, cut, fit, adjusted, recut, and refit.
Then glued.
Templates
To get the area around the motor cover with multiple funky angles and curves, I laid down sheets of paper in the area, taped them all together, and used a box cutter knife to trip the paper along the irregular surfaces.
Then, I lifted the template, set it on top of the foam and Advantec, traced and cut. I had to make a few minor adjustments, but for the most part, the pieces fit.
Hallelujah.
So now we have a subfloor.
The foundation for greatness.
I lost a little blood, more hair, and skin cells from glue and caulking, but we have a floor.
I’m not sure if I’m wiser, though.
Just older.
But, we are stoked to have a floor!
I’m starting to think about wall framing now.
I’ve got to find a goat.
Skoolie Subfloor Material Rationale
If you’ve ever built a house, you know that making 1.7 million decisions can overwhelm the gray matter. On several occasions working for clients I’ve asked them, what color wall plates do you want on your outlets and they’ve responded, “Oh, jeez, I don’t know. Just pick something that matches!”
Indeed, there’s a lot to think about.
So here is the rationale for our material choices in our skoolie subfloor.
Skoolie Subfloor Insulation
XPS
I plan on writing a much more detailed post on insulation coming up, so I’ll cover that in extensive detail down the road. For now, though, know that we chose to insulate the floor with 1inch Rigid Foam Extruded Polystyrene. Or XPS for short.
We don’t plan on living in our bus full time all year round. We will live in it full time in the summer and for a bit in the spring and fall in temperate climes. In the winter we plan to flee frozen temps by migrating to South America where the paddling is good and the beer is cheap.
In addition, though not towering giants by any means at 5’7″, we elected not to invest the time, effort, sweat and most likely tears into a roof-raise. So keeping the floor low with 1″ foam instead of 2″ made more sense.
One inch of XPS provides an R-Value of 5. Not great, but certainly adequate for our needs. However, XPS has better RV values of EPS, and it holds up much better under compression.
Polyiso
Some people opt for a foil-backed foam to serve as a vapor barrier. While this certainly makes sense, we opted for a less expensive option. Here’s why. According to the Polyisocyanurate Insulation Manufacturers Association, “Water vapor enters walls through permeable materials and penetrations.” However, we glued 3/4″ Advantek as a subfloor. I doubt that water vapor can penetrate Advantek and, since we glued it down, there will be limited penetrations in our skoolie subfloor for vapor to invade .
Floating Skoolie Subfloor
Framing
We elected not to frame. Instead, we opted for a floating floor.
I know some folks want framing to screw stuff into. We just didn’t see the need.
These furring strips, kind of like floor joists, in my opinion, add more thermal bridging than support. In other words, the wood conducts cold from outside the metal floor, up through the wood subfloor and into the bus. And who wants cold feet?
Adhesive
The loctite PL premium 400 adhesive we used has a shear strength of 221 psi. Covering (or nearly covering) a sheet of Advantek gives you thousands of pounds per square inch to work with. And we ended up using Loctite 3X because it’s even stronger.
Loctite PL Premium FAR exceeds Liquid Nails in multiple tests…DOUBLES Liquid Nails in fact!
Not only do we have the strength of the glue to work with, we will also have walls extending from floor to ceiling keeping everything in place as well as furniture secured to the walls.
In addition, framing needs a way to be secured to the floor. If we decided to bolt it to the floor, we would have created more holes in the galvanized floor that we had patched. Remember that part about water and water vapor finding its way through penetrations? We don’t want that.
Or, you could glue it to the skoolie subfloor as many people do, but then you’re still relying on glue to hold the whole kit and kaboodle into place. Which is what we planned to do anyway.
Adding framing then, wouldn’t help secure it to the bus floor, it would just provide less insulation and less thermal breaks for cold to radiate up and nip at our tootsie.
Compression
Some people elect to use framing to protect the insulation from compressing and lowering it’s R-Value. XPS, however, has a compression strength of 30 psi. So if I spread out my rugged friend Todd’s 260 pounds over 4000 odd square inches of Advantec, if averages out to about 18 pounds a square inch. This gives him plenty of latitude to safely drop some 80’s dance moves without compressing the rigid foam.
Plywood or OSB
It seems people debate this as much as Red Sox / Yankees or Bud / Bud Lite. Traditionally, OSB has been thought of as the cheap knock off on plywood. But technology has changed.
Plywood
Plywood stood as a champion sheather for years. Manufacturers shaved veneers from tree trunks, then glued layers together with the grains at perpendicular angles. This created a strong material resistant to shearing. (Ever tried to split a piece of plywood with an axe? Good-luck.)
OSB (Oriented Strand Board)
Shaving veneers, however, requires bigger trees. Thus, using “chips” allows manufacturers to use less perfect, and smaller, trees but still build a quality product.
A Quick Delve into Particle Board
Don’t mistake OSB for particle board. Or “Fall Aparticle Board” as we like to call it. Particle board, as it’s named, is made from particles…essentially sawdust and glue. It comes in different densities. And it can serve a purpose. If you need some cheap shelves or panels for furniture where it won’t get wet or be abused too much, it can be an economical way to go. I wouldn’t count on it, however, for other uses!
Back to OSB
The larger “strands,” or chips, in OSB get “oriented” much like the larger veneers in plywood, then combined with waxes and resins, then pressed together with thousands of pounds of force. Thus, when the resins dry, you have a pretty rugged material!
Plywood vs OSB
When OSB first came out, the waxes and resins weren’t as good, so OSB, like particle board, had more of a tendency to fall apart.
Also, the weaker resins didn’t hold nails and screws as well, so they would tear out of OSB more easily than plywood.
Because of the physics involved in the longer vs shorter strands of material, plywood wouldn’t sag as much.
If water found its way into the edges of plywood, the wood would swell, but dry quickly and return to its original shape. OSB dried more slowly, and would often stay swollen, forcing builders to sand down ridges.
Plywood is also lighter.
OSB is significantly cheaper, however.
And technology has improved, however. Better waxes and resins have made OSB more resistant to water. In fact, Adventec OSB boasts a 500 day “no sand” guarantee. And with these technological advances, shear strength, fastener holding, water resistance and consistent thickness, OSB matches or exceeds traditional plywood.
So, for our skoolie subfloor, we opted for Advantek OSB.
It was tempting to choose a less expensive OSB. Advantec goes for around $35 a sheet. We could have gotten a bargain brand for around $20. I mean, how much water is really going to get inside the bus, through the eventual vinyl plank flooring and into the subfloor.
But it might.
So, for around $100 more, we have a floor that will “endure wind and weather.” (Little Shakespeare quote there to balance all the science and math with some art.)
More on Adhesive
I agonized over adhesive. There are just so many choices.
And, with a floating floor, there won’t be many mechanical fasteners like screw, nails, bolts, or rivets. So we needed to make the adhesive count.
For years I have always used Loctite’s PL400 when building houses. Primarily we have used it on floor joists and stairs to prevent squeaking. You could argue that it’s practically an industry standard.
But, in researching my options I found a few instances that gave me pause.
First, there seem to be more cases where the adhesive is not sticking to the newer Advantek as well. Second, those tubes of adhesive require SO MUCH pumping! It’s slow and gets tiring to cover entire sheets of insulation and OSB on the skoolie subfloor. And I just didn’t feel like shelling out another $30-60 for a pneumatic caulking gun.
In examining strength studies for various adhesives, I found DAP Smartbond. Tests showed that it was stronger than PL400. In addition, DAP Smartbond comes in spray cans. Rather than pump, pump, pump the glue out, you just press a button and it comes out as a foam, then turns into a gel adhesive.
My forearms were practically giddy at the thought.
We ended up choosing DAP Smartbond Heavy Duty in order to better bond, we hope, the insulation to the steel floor and the Advantec to the insulation.
This DID NOT work out for us, however, as I explained above!
We ended up using Loctite 3x
Conclusion
So now we are all set up for the final laminate floor that will go on later!
I hope that this was helpful for any of you aspiring skoolie subfloor installers!
Share your experiences, questions, commendations or criticisms in the comments below!