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Douggy 07-17-2018 12:24 AM

Floor insulation thickness - quick opinion needed
Hi guys,

looking to get a quick opinion on floor insulation and sub-floor thickness. Spent many, many, many hours digging through the site, but seem to see varying opinions.

Background - standard bus height (6.2?) with metal floor. I removed the rubber and ply original floor (absolutely rotten, with some metal pitting underneath - removing the original floor is great advice to take!!). Metal floor was ground, phosphate-treated, primed, and painted. Looking to insulate, then add plywood sub-floor, and vinyl planking top layer. Bus is mainly for weekend/week vacations, intermittently - not fully-timing in it. Located in the Louisiana, so it’s basically hot as hell most of the year. Mainly looking to stop too much heat getting in and too much a/c cooling getting out. BUT, looking to loose as little interior height as possible.


1..... 1/2” XPS, with 1/2” ply, then vinyl plank top
2...... 1/2” XPS, with 3/4” ply, then vinyl plank top
3..... 1” XPS, with 1/2” ply, then vinyl plank top
4...... 1” XPS, with 3/4” ply, then vinyl plank top

I know option 4 is going to be the best of them all, but is it that much better than option 1? ......thought I would throw it out to the community to get some real-life experience from y’all!


Mark_In_MA 07-17-2018 12:30 AM

I wouldn't bother with anything over 1/2 plywood, especially if it's just going to be a sub-floor.

I'd do the 1/2 XPS and 1/2 Plywood.

I think you'll loose more cooling through the windows, etc. - keep the headroom.

Douggy 07-17-2018 12:37 AM

Thank for replying Mark - I was thinking with the 1/2” ply and vinyl planking, things would be fairly rigid up top, but its good to ask and get opions - Cheers!

CoryBodiker 07-17-2018 05:52 AM

I did 1” foam with 1/4” 3ply plywood and just laid my flooring on it it worked grate.and There hasn’t been any warping Of any kind.

Johnny Mullet 07-17-2018 07:15 AM

I used 1/4" foam and 5/8" plywood to save roof height. Still made a HUGE difference in road noise.

brokedown 07-17-2018 07:49 AM

I went with option #4. So far so good.

djdalfaro 07-17-2018 09:39 AM

Man, I have gone over this exact question so many times (my floor is still not ripped out). Here's what I'm thinking.

The floor is going to be the surface that sees the least radiation, and anywhere you park it is going to be cool as the shade of the bus is over it. As such, my thoughts are that if you are looking for the most bang for your buck the additional thickness of insulation will do the most good on the ceiling and walls.

Of course this is only taking into consideration the hot outside/cool inside situation. The cold outside/warm inside is different and you will probably notice the lack of adequate insulation in the floor if you spend time in cold climates.

All that being said, I ran some rough heat transfer calcs and realized that no matter how much insulation I put in the floor/walls/ceiling, if I didn't skin over the windows (I'm not going to), the difference between the insulation thicknesses didn't matter much when you consider how much heat transfer is going through the glass windows.

*Rough heat transfer calculations ahead*

The difference between an uninsulated floor and an R3 floor at a temperature difference of 20 F is about 8000 BTU/(hr*sqft), apply that over the entire floor, say 200 sqft and it's a difference of about 1,600,000 BTU/hr.

The difference between an R3 and R5 is only 2.66 BTU/(hr*sqft), apply that over the entire floor, and it's only a difference of about 500 BTU/hr.

Consequently EACH un-insulated window flows about 650 BTU/hr.

In short, unless you are dealing with extreme temps, full timing, have skinned over all your windows... IT DOESN'T MATTER.

Of course, that won't stop me from going with option 4. Why, because I like to make my life difficult and expensive.

somewhereinusa 07-17-2018 09:44 AM

I have 2" insulation over top ot original wood floor with 3/4" wood on top of that. Heat is in grooves cut into the insulation. My floors are warm when it's below 0°.

thrifty1 07-17-2018 06:24 PM

Option 4 for me as well. Tongue and groove advantec ply

Douggy 07-17-2018 10:49 PM

Thanks for all the input guys.

Thinking I’ll play with Option #4 - buy a board of 1” insulation and board of 3/4” ply, stick it in the bus, and wander around on it seeing how many times I bash my head on things - there is little to get damaged inside of it.

Ninjakitty 07-18-2018 06:52 AM

I think it depends on how tall you are. I ended up framing my floor with 2x3 because of how crazy uneven it was. So inlayed down 1" and 3/4" foam board and compressed it by screwing down 1/2 inch ply. However, I'm 5'5" and my wife is 5'2" (maybe) so losing an inch of headroom wasn't a huge deal for us. I think you can go with 1/2" foam and 1/2" ply if you're looking to save headroom.

EastCoastCB 07-18-2018 01:37 PM


Originally Posted by Douggy (Post 281069)
Thanks for all the input guys.

Thinking I’ll play with Option #4 - buy a board of 1” insulation and board of 3/4” ply, stick it in the bus, and wander around on it seeing how many times I bash my head on things - there is little to get damaged inside of it.

I'm not gonna live in my bus, but I'm going with 3/4" insulation and 5/8 or 3/4" marine grade ply.

thrifty1 07-18-2018 01:53 PM

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Totally posted this in wrong thread

atthebeginning 07-25-2018 05:59 PM

Literally the foam people just left an hour ago. With all the windows open and no fans the bus is comfortable. Yesterday the metal roof was hot enough that you didn't want to leave your hand on it. The ceiling closed cell foam is almost two inches. The walls are almost three inches. The floor is metal + 1/2" dense blue foam + 3/4" plywood flooring. 5 windows were eliminated on each side of an 11 window bus. The foaming was $1250.00. I'm impressed, better than I thought it would be.

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