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Rick Alston 01-26-2004 12:14 PM

Under floor insulation
When I removed the belly pan and fiberglass insulation, I thought I would replace the fiberglass with "styrofoam" insulation.

I've got two questions:

1. Is using this type of insulation a good idea for this application? and

2. I plan to screw it in place by driving a screw (complete with wide washer) through the insulation and into the flooring about one quarter of an inch. Is this a good idea assuming number one is acceptable?


Chas 01-26-2004 12:22 PM

Other than it being HIGHLY FLAMMABLE I don't see a real problem, at least it won't soak up the bellypan water.

Ahh yes, those were the days, poking around into the bowels of an Airstream floor, rotted steel & wood, snakeskins and other assorted varmints.

Kinda enjoying my time off from Airstreams for a while but I feel the bug still within me, damn it, just can't seem to shake it!!


TomW 01-26-2004 12:22 PM

Someone in a previous post, indicated the floor needs to breathe. How much, I don't know. I'm in a similar situation as you right now, and hope someone posts a good answer.

Was cost a consideration to you? While it won't break the bank, it seems styrofoam would be more expensive.

Rick Alston 01-26-2004 12:37 PM

Cost is not yet a consideration.


Perhaps my use of the term "styrofoam" is too loose. When I say "styrofoam", I mean housing grade insulation which is approved for installation in residential and commercial buildings. Every time I pass a constructions site I see either pink or blue insulation boards in 4X8 sheets (about 3 inches thick) and think how nice that might work under the floor.


Pahaska 01-26-2004 12:40 PM

Another suggestion
On a previous SOB, I used the foil-bubble-foil stuff stapled in place. I understand Airstream now uses this material on Limited models with slideouts. The stuff is light and can't soak up water. The fact that Airstream is using it says it should be safe for the floor.

I can vouch for the efficiency of the foam-bubble-foam stuff. My whole Scamp was lined with the stuff and it was a super snug trailer.

Ken J 01-26-2004 12:47 PM

This may be unpopular - but what I do is take out the insulation and I don't put anything back - I figure all it does is hold moisture.

Ken J.

Rick Alston 01-26-2004 01:11 PM


I think we are discussing the same type of generic foam insulation. I like the idea that it won't hold water, too.


I've got to admit that I, too, thought of not replacing the insulation. I have never camped below 31 degrees, so I don't think I need all that added insulation. However, the next owner may want the added protection. Since re-insulating the unit is pretty simple, I will go ahead and do it.

I am intrigued by the issue of the flooring needing to breathe. That sounds reasonable, but does it breathe adequately from the other side? What about laying down floor tiles with glue? I have a feeling that the breathing issue may not be as strong a concern as some people think.


Janets Husband 01-26-2004 02:33 PM

I'm sort of thinking like Rick on this subject, but I'm not really sure.
I have a few questions I can't seem to clear up in my mind.
Conventional thinking on insulation is
a.] The vapor barrier goes toward the heated side.
b.] Insulation should have adequate ventalation on one side to keep it dry
The question is if the insulation is between the floor and the belly pan where is the "vapor barrier" and where is the "air space" for ventalation?
This brings up another question: If your insulation gets wet for what ever reason does it not dry out by the water vapor going through the wood floor? Does this cause rot?
Is there a better way, maybe styrofoam?
Nothing clear in this head.:confused: :confused: :confused:

wahoonc 01-26-2004 02:45 PM

My take on this is that the belly pan is not sealed air tight, also with the tank connections and vent stacks and what have you going thru the floor, there are plenty of opportunities for vapor to escape. I like the idea of a vapor barrier, but wonder if it is necessary. I like the idea of the batt insulation only because it is easy to stuff around things. The styrofoam (FWIW know as extruded polystyrene) has the advantage of not absorbing water, doesn't lend itself well to nesting critters and won't absorb and hold odors. Downfalls; it is quite noxious if burned, and being ridgid is hard to fit in and around things. That being said...want to guess what I insulated the roof of my house and barn with? :D


Ken J 01-26-2004 02:55 PM

I've been in my trailer down to about 10 degrees and did not seem to miss the insulation...

As far as breathing goes what that refers to is you don't want your belly 100% sealed - it should breathe - otherwise any mositure in there would stay there + you would get condensation.

This was something I had concerns with several years ago because the belly on my 59 was not 100% sealed - I was told not to over vulcum........

Ken J.

markdoane 01-26-2004 02:57 PM

Aaron: I think it's the other way, expanded polystyrene (3.8) has a higher R-value than fibreglas (3.2). Extruded polystyrene R-value is 4.8, and sprayed polyurethane R-value is 5.9.

Chas 01-26-2004 02:59 PM

The problem is along the lower seam, behind the rubrail, water comes in quite easily.

From the factory they are somewhat sealed with double edge tape which holds for maybe a few months, perhaps a couple of years. If any of the flooring is exposed anywhere near this seam the water wicks into the plywood and you have rot. Then insulation gets wet also and further contributes to the problem. Aluminum starts corroding at the rivets and then the belly starts falling off while the steel framing turns to rust.

The more I think about it the recent AS models had a lot going for them with the abscence of a bellypan. If I had anothe Airstream I would pull all the insulation out and put a bunch of "soffit-like vents" in the bellyskin, keeping it well aired out.


wahoonc 01-26-2004 03:06 PM

Re: R-value

Originally posted by markdoane
Aaron: I think it's the other way, expanded polystyrene (3.8) has a higher R-value than fibreglas (3.2). Extruded polystyrene R-value is 4.8, and sprayed polyurethane R-value is 5.9.
OOPS:o I was thinking of Expanded Polystyrene. :rolleyes:

Aaron :cool:

Cruiser 01-26-2004 04:34 PM

I used 2" polyisocyanurate rigid insulation with foil backing under our newly installed floor. I used panel adhesive with a few screws as you propose to hold it in place overnight. The stuff has less noxious off-gassing than polystyrene, it doesn't absorb water, and it has high R-values (7-10). I used spray foam in cracks around edges of panels with advice from Geof to increase R-values further. It was tedious cutting and fitting the polyisocyanurate insulation into so many odd sized panels. I used 1-1/2" in the walls and managed to get it to bend okay. The trailer didn't heat up nearly as much in the hot sun last summer (no A/C). Might make a difference in the floor, too, if you're on hot pavement?

markdoane 01-26-2004 06:49 PM

Shame on you! Tell us how neat it is (isocyan' w/foil) and don't tell us where to get it or what it cost.:D

Davydd 01-26-2004 07:35 PM

Don't use expanded polystyrene

Expanded polystyrene (compressed polystyrene beads) will hold water between the beads. Extruded will not absorb water. Also, I know for a fact that critters will attack polystyrene in any form. I have extruded closed cell polystyrene enveloping my entire house protected on the interior side with 2" T&G pine boards. At night I can hear the mice in the insulation. Luckily they've never penetrated the pine and gotten into the house. BTW, if the boards are white they are expanded polystyrene. If the boards are blue (Dow), pink (Owens Corning) or yellow (Certifoam) they are extruded closed cell polystyrene.

Doug's idea of using polyisocyanurate boards with an aluminum foil vapor barrier is commonly used in commercial roof insulation. They can, by code, be applied directly over a metal roof deck without a fire barrier. Polystyrenes require a gypsum board fire barrier. The foil faced type of boards can be used inside a building with the foil on the warm side. Polyisocyanurates are very similar to polyurethanes, are closed cell non water absorbing--and safer than polystyrene.

Cruiser 01-26-2004 07:47 PM

Did I mention I'm a distributor? :rolleyes: (just kidding) I found polyisocyanurate insulation in several sizes at our local small town home center/ lumber yard. I've seen it at the big box stores, too. Some of the sales folks have never heard of it, even though they carry it. I think the problem is that none of us know how to pronounce it. :o You may have to do some explaining for folks to know what you're talking about. I can't remember what I paid - didn't think it was too expensive, maybe $10 or $15/ 4'x8' sheet.

Davydd 01-26-2004 07:58 PM

Polly I so sigh an your ate
I think that is phonetically close. :)

74Argosy24MH 01-26-2004 08:02 PM

One of the trade names is Thermax, made by Dow; I have seen it at HD and Lowes. I also used under my floor and really like the way it went in and it doesn't hold water.


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