Flooring insulation and Subfloor

[Tango]

Just make sure it is "closed cell". Otherwise, it is a sponge for moisture.

[bus-bro]

That pink polystyrene board, Foamular, can support 15 pounds per square inch (and you can get it in up to 60 psi). That would be a ton per square foot.

[jjhwick119]

I went with blue foam from lowes, and plywood on top, no frame. I did put a few screws in each panel though to keep everything flat. Wood bowed slightly in areas

[Sevier]

Originally Posted by Robin97396

You're going to want to use plywood heavier than 3/8ths for flooring. You won't find 3/8ths with a tung and groove anyway.

You think about this. Insulation isn't made for strength. If you glue down the relatively soft insulation and then glue the plywood to the top of the insulation, where exactly are you getting enough strength from to hold it all in place?

If it was a vertical, wall instead of a floor, you'd never consider gluing plywood over the top of insulation that's glued to the wall. You'd screw it to the wall.

You're over thinking this. There are many methods for accomplishing the same goal. Use your basic building skills to decide your best method of attaching the floor.

Hey folks -

Have read every thread I can find on here about floor insulation, so grateful for everyone's perspectives! even when they conflict, up to us all to make up our own minds!

Two questions I would love feedback on:

1) Plan is to build a wood floor grid and insert foam board inside the grid (put glue down of course), then top with plywood. Sounds like there is no need to screw into the bus floor (we really dont want to do that anyway), rather weigh it down w blocks while it cures. We were going to line the cross beams of the floor grid up with where the future room divider walls will be (and the bathroom framing) so that there is something to screw them into (again, not wanting to screw into the metal bus floor).

I know many feel a grid isnt necessary, but being inexperienced, I just cant see how the walls will be sturdily attached otherwise.

Thoughts?

2) What is MOST important to us, which I think it pretty important to everyone, is that the floor remains as dry as possible. Secondly, non-toxic. I get its going to be necessary to use toxins, but something as large as insulation needs to be zero VOC. There are plenty of non toxic products out there - non toxic spray foam, and non toxic plywood etc. But the only non toxic foam board I can find is EPS. Again, concerned about water damage (we are pulling out all our windows and resealing them) dont want to use wool etc.

So - what is the difference between EPS and XPS board foam? We wanted to go with EPS but dont see anyone referencing it and cant figure out why.


Thanks. all!

[ECU-Pirates]

Originally Posted by jazty

I put down 2" of ship-lap XPS foam and 5/8" tongue-and-groove plywood several years back and it has held up amazingly. Not a squeak, not a shift. Rock solid.

Here's what I did:
First clean, prep and paint the floor. Let the paint completely cure. If you're using Rustoleum then give it several days beyond what they recommend on the tin. That stuff is slooooowwwww to dry.

Start at the rear of the bus. Cut your first piece of rigid XPS foam (not EPS. PolyIso is ok) to fit the floor, but leave a gap at the walls. Put a zig-zag bead of original (slow dry) PL Premium adhesive on the downward side of the foam and fit it in place.
There is also a PL foam adhesive, but it's a weird, thick consistency and doesn't hold nearly as well. I tested PL Premium on XPS foam and it cured flawlessly without burning through and held much better. I've used both Liquid Nails products and PL products and PL Premium has always come out on top.

Put a light bead on the ship-lap edge and fit another. That should give enough of a foam surface to put down a piece of plywood. Doing it this way gives you two firm surfaces to stand on. Just like the foam, cut the plywood so there's a wall gap. Note that you want the plywood edges to be centered on the foam board. You don't want a foam edge and a plywood edge meeting up.

Now, at this point I went ahead with driving screws through the plywood, the foam and into the metal floor. If I were to do the job again I would skip this step and use weight to sandwich the layers (bricks perhaps?) while the adhesive cures. Screws turn out to be unnecessary and moisture condenses on the screw heads in the cold. Not a big loss of heat, but I commonly have little wet spots on the floor in the winter. I determined that the screws were unnecessary when removing a chunk of the flooring+foam to put in a shower. I had to break and scrape the foam out even after removing the screws.

Continue the process until the floor is covered. Once everything is laid and dry go ahead and put some canned spray foam into the wall gap. Use the window and door foam so the floor can expand and contract without cracking the spray foam.

That should do it! It has worked gangbusters for me. Once the foam is sandwiched between steel and plywood it is incredibly solid. That is, as long as the plywood is thick enough. I would say that 3/8" is a bit flimsy. I wouldn't go with anything less than 5/8".

EDIT: Oh, one more note. PL Premium cures in the presence of humidity. The tube recommends spraying a light mist of water onto non-permeable surfaces after laying the bead. XPS foam isn't 100% air tight so the remaining humidity between the surfaces will wick out. Don't be concerned about water being trapped between layers. Again, it all worked out very well for me.

Hi Jazty,

I saw your post about glueing the XPS straight to the floor, and then the subfloor on top, without using any framing.

How is it holding up? And I saw you like using PL Premium instead of liquid nail. Does that still hold true?

Im thinking of doing the same on my 2010 Freightliner C2, but with 1 inch of XPS (Height reasons)

Thank you for your post! Its been very helpful.

Thanks,
Mike

[jazty]

Hi Mike
Many years and many miles have past since that post. The floor is still as solid as it was in the beginning. Everything I said in that old quote still holds true.
PL Premium is da bomb. Liquid nails dries really brittle (cracks easy) and its holding strength is not even close to comparable.

[ECU-Pirates]

Originally Posted by jazty

Hi Mike
Many years and many miles have past since that post. The floor is still as solid as it was in the beginning. Everything I said in that old quote still holds true.
PL Premium is da bomb. Liquid nails dries really brittle (cracks easy) and its holding strength is not even close to comparable.

Thank you for this and your other contributions to the forum!! They've been helping us for years!

[SamT]

Originally Posted by superdave

you should use a t&g flooring product and glue the t&g together. insulation is a must, use fan fold at least. I got some self drilling screws from ebay to screw down the ply. just a few screws will do, mainly in the center where you walk to keep it from flexing.

I'm about to lay my flooring in my skoolie today, I have my insulation laid down and my ply cut to size and laid down as well but I am unsure of how to secure it. Is it okay to use self drilling screws to go through the ply, insulation, and the thin metal floor or should you not puncture that metal? I did not frame my floors, just went right into laying insulation and ply so I don't have wood to secure it to.

[SamT]

I'm about to lay my flooring in my skoolie today, I have my insulation laid down and my ply cut to size and laid down as well but I am unsure of how to secure it. Is it okay to use self drilling screws to go through the ply, insulation, and the thin metal floor or should you not puncture that metal? I did not frame my floors, just went right into laying insulation and ply so I don't have wood to secure it to.

[o1marc]

Originally Posted by SamT

I'm about to lay my flooring in my skoolie today, I have my insulation laid down and my ply cut to size and laid down as well but I am unsure of how to secure it. Is it okay to use self drilling screws to go through the ply, insulation, and the thin metal floor or should you not puncture that metal? I did not frame my floors, just went right into laying insulation and ply so I don't have wood to secure it to.

Anywhere a screw goes through the floor is a spot rust can start. My whole floor is floating, not secured at all. Everything on it is attached to the walls, not the floor.

[T-Bolt]

I used a glue approved for use on foam and glued the insulation down. The foam is rated at 25# psi so floor framing was not needed. Then glued the subfloor over the foam. Everything built in the bus helps to hold it all together. We have put thousands of miles on our bus and it has all worked perfect.

[cntryby]

Clean and paint / treat the floor. Foam board with adhesive on the edges of the seams, seam tape over the seams. Bonding the edges helps prevent squeaks and helps prevent water from having a place to sit.

Ply board thickness of choice, all surfaces painted prior to laying down, seal edges of seams with adhesive whether tongue and groove or not. Painting the ply before installation helps prevent the boards from wicking any moisture introduced.

Screw the boards to the metal floor and use undercoating spray to heavily cover the screws under the bus to prevent / slow thermal bridging.

Sealant of choice over the ply board. I like DIY bed liner. Try to get flooring in after the sealant is cured to the touch but still slightly pliable. This probably means cutting and laying the floor and then staging it for quick installation after putting down the sealant. (Bed liner seals good even after curing) All this is just trying to have the sealant seal to the screws. (For wood flooring I prefer screws to nails because of the flex and movement of a bus.) For tongue & groove, a pliable adhesive in the grooves, helps keep water out of the floor.

If using natural wood, use a sealer on all the edges and bottom prior to installation. The floor surface can't be seal or it will need a hell of a sanding to be able to stain it.

Stain and finish to preference.

[JamiFids]

Originally Posted by CadaSan

This thread had some great information.

I do have one question along the same lines of this thread.
I am ready to do my subfloor. I noticed no one mentioned framing the subfloor with 2x4, etc.

What are the pros and cons of framing the subfloor?
I am trying to keep as much head space as possible and if I can forgo framing the sub floor then I would like to just lay down insulation and and plywood then weight it down until it cures and boom, I got a sub floor.

Does this sound doable?


(I need to start my build thread, we removed all the seats, and flooring and rust converted and painted with rustoleum safety red.)

I recommend doing a floor cover. In any case, it won't be any worse. But you need to spend extra time and money on this.

[Delos356]

I finished my subfloor a couple days ago, and while I am no expert, I read a ton TON of the threads really over analyzed every aspect. Because that's just who I am. I have pictures if anyone thinks they could be useful. I went with what seems to be more or less the universally agreed upon method.

1- I treated the rust with 2 coats of oshpo then swept really well and painted with 2 good coats of Rustoleum.
2- I patched EVERY single hole, and sealed the edge of every patch so it was both water and air tight. For big holes I used steal and rivets, smaller holes I used water and air tight butyl tape with a metal backing. (like duct tape but the butyl is WAY thicker).
3- I painted everything again with Rustoleum.
4-I framed out only the perimeter and wheel wells with 2x4s layed flat, that were secured with liquid nails. This way I have something good to secure the future wall framing to.
5-Then I did 2 layers of rigid foam. This is because I was able to score 25 sheets of the really expensive stuff for only $50 bucks but it was only half inch. So I did one layer of regular 1 inch rigid foam, then a layer of my fancy foam. Each layer was secured with the Loctite foam glue. I found i liked it a lot better than the Liquid nails version, just because it was softer so easier for me to squirt it with my caulk gun. Every seam between each layer was sealed with the same tape mentioned above.
6- Then a layer of plywood on top. The plywood was screwed down at the perimeter into the 2x4s, but I used the Loctite glue everywhere else to adhere it to the foam insulation.

Reasoning:
I really did not want to punch hundreds of holes into a floor that I just worked so hard to seal up. As someone said, any screw hole is a place for moisture or cold air to wick up.
And I figured its not like the floor is going to jump up and run away, especially one it starts to get secured to everything else.
I chose minimal framing to reduce thermal bridging.

I will say, the floor is very quiet and nice to walk on. So I feel okay about my choices there.

[cntryby]

Nicely done!