Best Three Season Bottom Floor Insulation

One thing many don't know about fiberglass batt insulation is that if you compress it, it substantially reduces the R rating. It's very soft an easy to compress. I realize it's very commonly used by the manufacturers though. It's going to get compressed under the tanks for sure.

If I were to do it, my first choice would be Roxul mineral wool insulation. It's denser and doesn't compress anywhere near as easily as fiberglass. It's relatively easy to cut and work with and if it ever got wet, it wouldn't turn to mush.

Closed cell rigid insulation would also be a good choice. You can buy it in various thicknesses. The problem I would see though is working around all the obstructions, tanks, pipes, etc. Do-able, but awfully time consuming.

The insulating would depend on what you are trying to achieve. If you just want to make the floor warmer to walk on, you could get away with an inch or two of rigid foam glued to the underside of the subfloor. If you want to prevent plumbing and tanks from freezing, I'd look at Roxul. If you have a heated underbelly, you don't want to block movement of air.

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For my RV/TT's underbelly, I didn't over analyze the situation.

For my 2006 Jayco TT, I simply:
- Removed the underside coroplast (hard plastic) layer.
- Removed its soft fabric layer.
- Removed its water holding fiberglass batt insulation.
- Replaced its underside fabric heat duct pipes with real metal flex pipes.
- Installed its wiring within plastic wire loop and created smooth round corners - before securing wire loom to underside floor.
- Removed its thin 12 gauge linear brake wiring.
- Installed new thick 10 gauge STAR design brake wiring.
- Taped shinny plastic on areas that don't need over spray.
- Using wood, created access door under tub's gooseneck pipe.
- Using landscaping plastic fabric (from home hardware store), used staple gun and duct tape to apply between tank's gap. Thus, stopping spray foam getting between floor and tank gap.
- Called Spray Foam company and they sent over their mobile truck.
- The guy crawled under my TT, applied spray foam CC @ 2.5" - 3" thick
- I did after inspection - for their completion Sign-Off.
- I then removed over spray plastic areas myself.

Since I did before and after prep work myself, the spray foam company gave me a huge discount. And with their mobile truck, I didn't have to tow my RV/TT to their shop location either.

For water protection, I can then re-install the previously removed soft fabric sheet. And for even more water protection, I could re-install previously remove coroplast plastic sheets as well. Same water layer protection as factory build.

If (and if) a wire needs to be replaced or a tank needs to be replaced, I simply crawl under the RV/TT, find each end of the plastic loom piping and fish new wires. Or, cut around the plastic barrier around each tank, then remove the tank. Or, access the tub's gooseneck piping via trap door.

If you feel risk of replacing underbelly wiring is too high, simply install PVC electrical piping and where needed, use smooth 45 - 90 degree elbows and secure to underside floor. Then, fish wiring into these new 1/2" or 3/4" support pipes. Then, spray foam over the PVC piping. If wires need to be replaced, simply fish new wires down the much smoother PVC pipes (instead of corrugated wire loom plastic wire covers).

For my 2006 RV/TT, I never regretted spray foaming its underbelly. re: - click here - Today, they now make Spray Foam that is fire rated. And, its Closed Cell formula is much better (compared to formula I used 4 years ago). And when my RV's inner roof cavity and/or its inner walls need repair, I'll be using Spray Foam in these locations as well.

For me, replacing factory fiberglass batt with Spray Foam Closed Cell was very low risk / very easy solution. No brainer in my books.

Hope this helps..

I'm assuming your trailer has a black woven cover on the underbody with the tanks hanging under that. When the RV manufacturers insulate from this point they usually use fiberglass insulation (think pink spun fiberglass insulation from home depot)They hold this up with a large sheet of coroplast. This is like cardboard made out of plastic. They put the insulation up then they put the coroplast up, and run a bunch of self tapping screws through the coroplast and into the bottom of the frame rails. Add a bit of self expanding spray foam where the drain pipes exit the coroplast and you're done.

coroplast

roxul mineral wool, mold amd water proof, just need to keep it in place. can get it in dual density panels for easiest install.

I would have it sprayed with urethane insulation. Make sure to prep all area's that need to be accessed and spray away. Is you best bet. Look it up.I use it all the time at work.

Fiberglass insulation "breathes" but it will probably be a real chore getting it installed and to stay in place.
Do you have a coroplast under belly? This material is a vinyl extrusion, usually pleated. If you want to maintain access to the plumbing, wiring, and subfloor, you may be able to replace the original liner with foam insulation board.

The tank area is something I'm working on. I think I've pretty much relegated myself to building a frame around my tanks and drains. Inside the welded steel frame, I'll install 1" (or so) polyisocyanurate foam board and also a skin of light gauge aluminum sheet. I'll have to engineer access to the area to work on drain pipes and valves. I won't worry about making this accessible for tank removal. Tanks last forever.
I figured if I framed in the whole area, I could heat this area as a whole rather than using multiple electrical cable heaters and tank pads. Since this would cover just about the whole under-floor area of the bathroom, it should keep the bathroom nice and toasty. We'll see. Fortunately, work on this won't start until Spring.