Friday, March 24, 2023


My RV’s furnace ductwork is exposed to the elements. How do I insulate it?

Dear Dave,
My furnace heat is distributed through cut-out channels in the RV’s flooring. It’s all surrounded by the floor Styrofoam and is quite efficient. However, the duct over to the floor inlet is just plain painted steel, under the coach, and exposed to frigid air. What would you suggest to insulate the “naked” duct under the floor? It’s about 20″ wide, 8″ deep, and 40″ long. Insulating even the belly of it (20″ x 40″) would, I think, have benefits. —Gregory, 2003 Winnebago/Itasca Sunova

Dear Gregory,
When I first read your question I was puzzled as to why the duct would be exposed to the outside. The Sunova is a basement model and the ductwork is typically fastened to the underside of the floor. The pass-through compartments cover it and also provide some heat. Then I went to the website to look at the 3D spec’s but they do not have them for this old of a model. However, looking at the brochure, it all made sense! For a few years, Winnebago engineers designed storage compartments underneath the slide room that were attached or “hung” underneath and came out with the room. The concept was called StoreMore.

Access to the compartments

The idea was that you could open the compartments without needing to crawl under the slide room. You could also open the door without worrying about crushing the door if the room was retracted or extended.

The problem with this design is the compartments had to be sealed on the back side and so there was no pass-through storage and heat like their competitors had. Therefore, they got killed in the market and went back to the popular pass-through design after a few years. A classic example of engineers and designers that are not actual RVers. With the closed compartments being hung under the slide room floor, this left the space between the chassis rails open, and that is why your ductwork is exposed.

According to the brochure on the Winnebago site, the 27C was only available on the Workhorse 19,000 lb. chassis, while the 30B and 33L were standard on the Ford 26,000 lb. chassis, and had the Workhorse 21,000 lb. chassis optional. I don’t think any of the options would be any more difficult than the others. It is just a matter of accessing the ductwork between the frame rails.

You most likely will not be able to access the entire ductwork as there are chassis cross members and other obstructions. However, any additional insulation you can apply will help. Here you can see some of the challenges on the underside of this 2000 Winnebago Adventurer: the fuel tank, cross members, and wiring harnesses.

Spray foam

You could use a spray foam. However, since it’s a 2003 model, I would assume that the ductwork and other components are not very clean and the foam may not adhere to the oil, rust, and other corrosion. You would need to clean it thoroughly and let it dry thoroughly before applying the foam. There are several over-the-counter spray foams such as Great Stuff. However, I would contact an autobody shop about getting a commercial application. Plus, you don’t want to get a product that expands too much as it could work its way into areas that you don’t want to cover or push with the expansion. You could also contact a commercial building insulation company that sprays foam on pole buildings and metal roofs inside.

Reflective insulation

Since the underside of the floor is a wood material, you could wrap a reflective insulation such as Reflectix around the ductwork and fasten it with a batten strip screwed to the underside of the floor on each side.

There are other products with a similar corrugated design available at home improvement stores and some even have an adhesive backing. However, I would not trust the adhesive to stick in driving conditions, so maybe a combination of the adhesive with the bracing on the side would be best.

False underbelly

You could cover the entire underside between the frame rails similar to what the travel trailers and 5th wheels do. Use a corrugated plastic material to create an air space and limit the contact to cold air. This would also need to be fastened to a support underneath. You can find it on Amazon here.

Block foam insulation

Probably the best insulation is the same block foam used in the floor and the sidewall. However, it is not flexible and will be more of a challenge to install. You can get various sizes of thickness and density which will determine the R-Factor. This product can be found at home improvement stores as well as most lumberyards. The white bead board style would not be strong enough, in my opinion. I would go with the blue or pink hardboard material like this on Amazon.

You could build a U-shaped box using HVAC tape on the edges and secure it with banding or cross braces. Again, I would not try to use an adhesive to apply as it will probably not stay on going down the road.

RV AirSkirt

Another option for times you would be stationary at a campground would be to use an inflation product such as the AirSkirt. It could be placed underneath and inflated to rise up and cover the entire ductwork area front to back


 You might also enjoy this from Dave 

How should I winterize the RV to use it part-time in the winter?

Dear Dave,
We live in Long Island and want to use our motorhome part-time during the winter months. Is there an alternative to going through the winterizing process? —Walt

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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Thomas D
10 days ago

Using Reflectix might give you a warm and fuzzy feeling but it’s only about R1.2. Not much. I removed all the 4″ round single layer aluminum ducts and replaced it with round, insulated ductwork with a plastic covering. R 4. Still not great but better. And then there’s hard core fiberglass duct with aluminum covering that’s around R11 but hard to work with and then you have airborne fiberglass. I’d stay away from that.

Neal Davis
11 days ago

No question, but I do enjoy reading your column. I learn a lot and appreciate you sharing your knowledge; thank you!

Jim Johnson
11 days ago

Not the same, but the same issue… The RV manufacturer designed our 21′ TT with the entire fresh water system inside the cabin EXCEPT for the run between the water pump and the rest of the system. You know, the part of the system you need when the park forbids connection to their water system?
I used rigid foam insulation leftover from a foundation project to fill space below the line between the outer wall and the frame. It was attached to the poly film under the floor via adhesive intended for the foam and all joints and edges sealed with canned foam – it has to be water-tight or water trapped between the foam and floor could rot the floor. Where I couldn’t use the block foam, I carefully cleaned the TT underside and taped plastic shopping bags, then filled the void with spray foam – carefully removing the bags once the sprayed foam had expanded and substantially dried. I also shortened the low-point drain lines, installed 1/4 turn ball valves and put pipe insulation the drains.

10 days ago
Reply to  Jim Johnson

I like your choice of foam panels for the bulk of the work load but instead of canned foam which will likely absorb water at the joints, or oxidize or deteriorate, I would simply use a tube of urethane caulk to seal the joints of the rigid iso foam panels. I recommend a Sherwin Williams product called LOXON, or Stampede if LOXON is not available in your area. (No, I don’t work for Sherwin).

Jim Johnson has it right tho, use riidgid iso 8 sulation panels, and seal the joints well.

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