Side Wall Construction

[GeorgeT]

Just weighed two halves of a 4' x 8' piece of 3/4 inch sanded plywood from Home Depot to use for my floor. The 4x8 weighs 74.4 lbs. I'll be using 1/2 inch walls with a sandwich construction door to deal with the warping. Am using 3/4 inch flooring as it is 5 feet wide and extends 6 inches beyond my HF frame. I'm in Texas too and cold weather is not much of an issue. I camped in a pop up tent trailer for a while. I figure 1/2 inch walls all around me will be better than the pop up tent walls. I guess it matters most as to what you're used to as to what upgrades you are moving up to and are essential.

[R Keller]

I don't consider what you are doing as an insulated floor as much as it is a lightweight composite floor. If all voids are filled and it is well sealed I think you will have something really cool there.Cary

Cary: I was going to respond to your earlier post until I saw you 2nd post! I was going to ask what the difference was between an insulated floor and insulated sidewalls/roof in terms of moisture infiltration potential, and why you seemed to be saying that an insulated floor was a big problem but that insulated sidewalls/roof weren't. Seems like you addressed that.

My new modified question is while I understand that a "belly pan" would be a bad idea for the floor, how is this different from the aluminum applied to the roof and sidewalls. Doesn't the air gap between the plywood and aluminum on the sidewalls and roof allow for condensation to form and premature rotting to take place? This question has been bothering me for a while, and I haven't seen anybody address it in any forum. Does anybody have any thoughts on the matter?

I agree with you 100% about not using fiberglass insulation.

Finally, I have to respectfully disagree with you regarding your statement that "if you are building a wall 1" or thinner you will not see a significant enough difference in R value over 3/4" plywood to justify insulating sidewalls."

As far as relative R-values, plywood has an R-value of about 1.25 per inch thickness. 3/4" plywood thus has an R-value of about 0.94. In comparison, with Styrofoam having an R-value of about 5 per inch thickness, the composite sidewalls on my trailer (5/32" ply/3/4' Styrofoam/5/32" ply) have an R-value of about 4.14 (not counting reduced insulation because of thermal bridging where there is wood framing). That's about 4 1/2 times the insulating value of 3/4 plywood. That's a big difference. But does that translate into a real-life difference in terms?

The answer is YES! Since my front storage box (top half is accessible from the inside) has uninsulated sidewalls (only 1/4" thick, but still comparable to 3/4" ply in terms of R-value), but my main body has insulated sidewalls, I have been able to conduct real-life experiments with a direct comparison.

The result: in cold weather (I have been in the trailer down to mid-20s F), the insulated sidewalls (even just 3/4" insulation) make a HUGE difference. The storage box sidewalls become very cold to ther touch and because of this, LOTS of condensation forms on them (so much so that I will probably retrofit some insulation to them). The main body side walls stay warm and get no condensation.

Also, in hot weather when the trailer is in the sun, you get similar benefits from the insulated sidewalls. The thermal gain on the inside is reduced tremendously.

Of course, it is a lot more labor intensive and expensive to construct composite insulated sidewalls. For a lot of people who aren't camping in extreme conditions, the benefits probably aren't going to be worth it.

Keep up the great discussion!

Rik

[Cary Winch]

Rik,

All your points are right on the money. The biggest reason for the floor being the most problematic area is the fact that teardrops don't stay on dry concrete their whole life. There is a conciderable amount of moisture that comes out of the ground in a day. Add in some wet grass and dew and you see where I am headed here. The condensation cycle will cause moisture to form under the trailer and hang on the floor. If you have an aluminum belly pan and the temperature does get low enough you will have condensation forming in the pan.

Now you are correct about the walls and roof too. This is part of what I was trying to say before about filling the roof voids in with low pressure foam. You want absolutely no air pockets in the roof where condensation can form. We tend to be pretty fussy about this on our units. We use hard foam sheet to insulate and then fill the voids with low pressure foam and shave it off flush with a long sharpened piece of aluminum.

As for the walls this is part of the reason for using plywood walls, it eliminates the possiblity of voids in the wall. But, on a aluminum clad unit you do have a air gap between the wood and the plywood. This can be minimized by good construction technique and thick aluminum but is present none the less. The best we have found for this problem is to completely seal the wood with a couple of coats of spar urethane before applying the aluminum. We go through about 2 1/2" gallons of this stuff on every trailer.

The other really serious area to worry about is the bottom edges (that pesky underside again) of the walls. This is where condensation hangs and gets into stuff. There is one solution only. Lots and lots of silicone. All edge trim and such needs to packed heavy with silicone so that the water has no way to work into the edge of the walls.

Now, when I say the difference in R value is negligable what I am getting at is the final result from a heating and cooling impact is not an issue. First off the the amount of wall surface area on a 8' teardrop that is actually in the cabin seriously reduces its impact. In that same area you will have areas with studs and wooden structure. Most of the time that will be more of a heat loss than solid wall of equal thickness due to air infiltration around the stud. To put it simply there is no way you can get R 4-5 on a insulated wall unless all voids are removed and wall structure. About the only way would be to make a composite with no structure in the walls at all just a foam core.

There is a huge difference in 1/4" and 3/4" as far as insulation value. R factors do not translate into a linear curve for heat transmission so these first steps in R value have the most impact. Even the difference in thermal conductivity of a 1/2" wall and a 3/4" wall is very noticable. You most certainly will see considerable condensation on a 1/4" wall at that temperature. Of course I will say that at a under 30F temp a 3/4" wall will start to see condensation also. But, like you said most people are not out in these temps.

Keep in mind the simple great points of all teardrops. They are easy to keep warm and cooling is not that big a problem becuase you are only in them at night (sorry, that is the teardrop salesman speaking). A teardrop with a mattress warmer or ceramic space heater will be toasty inside with either the plywood wall or stick built insulated wall, it won't care. For most, two people inside provides alot of heat. I believe that an average human being puts out heat comparable with a 100w bulb, correct me if I am wrong on that one because I am not sure. Now throw in a dog with you and you will be sleeping with a window wide open on a cold night.

My biggest reason for liking a plywood wall has nothing to do with insulation value anyways. It really comes down to structural design. Most teardrops have way more frame under them then they should. A stick built wall is one main reason for this. Typically they are built like a house which requires a good foundation under it. Your frame ends up being the foundation in those cases. I prefer a design where the frame is not structural. This can be done in several methods. One is to employee a plywood box structure like we do that is self supporting. The other is to do a geodetic structure like an aircraft fusalage. I mess around with homebuilt airplanes (I have one under construction right now) and can't help but think that a strong lightweight teardrop could be built using similiar construction techniques. My last airplane had a wooden fuselage, this one has a welded tube fuselage. Material is different but the structure is pretty much the same. Anybody out there thinking the same?

Cary

[Woody]

Cary,
Yes, I was thinking the same thing you were about a light weight frame using aircraft construction techniques as a model for constructing a teardrop.It would be interesting to explore the concept if nothing else.

[Cary Winch]

Woody,

Heh heh, there are all kinds of ideas here. From a simpler geodetic wood frame to a all aluminum riveted construction and even a true composite glass unit. Lets not forget a welded tube frame. Here is one to get people thinking. Aircraft fabric on the outside. I having been trying to get people to try this one for a long time. The neat thing about a fabric coverd one is it wouldn't need to be a flat sided tear. Someone could form some stringers to shape it out into a more curvy, swoopy shape.

Cary

[Larry Messaros]

Woody,
Here is one to get people thinking. Aircraft fabric on the outside. I having been trying to get people to try this one for a long time. The neat thing about a fabric coverd one is it wouldn't need to be a flat sided tear. Someone could form some stringers to shape it out into a more curvy, swoopy shape.

Cary

Cary,

That's an interesting thought. What kind of fabric is used? How is it waterproofed? What kind of maintenance is involved?

[R Keller]

Cary,

Thanks for the detailed reply! And good insight into your construction techniques too.

This statement of yours was particularly interesting: "There is a huge difference in 1/4" and 3/4" as far as insulation value. R factors do not translate into a linear curve for heat transmission so these first steps in R value have the most impact. Even the difference in thermal conductivity of a 1/2" wall and a 3/4" wall is very noticable."

Since the R-value of wood is so low, I didn't know that there would be such a difference between 1/2" and 3/4" ply. I'll have to do a little more research...

Rik

[Woody]

Cary,
It does bring on some major possibilities for some wild shapes that come to mind, of course at the moment my mind went blank, Oh Yea the oscar mayer wiener mobile teardrop variant in the woods.

[Cary Winch]

Larry,

The common type of fabric today is the Ceconite brand. There are several methods out there for doing the shrinking/sealing/UV protection/finish. I can't believe you go wrong with any of them.

The process is kind of like a big model airplane project. The fabric is a synthetic blend that is pretty heavyweight (cotton for those old school types). You glue it on to the structure and glue a fabric tape over seams and such. Once done you shrink it in place with a little heat from a heat gun first and then by applying nitrate type dope. This will get the fabric drum tight. Then there are layers of butyrate dope mixed with aluminum powder. This is what is called your silver coat. This provides the necessary UV protection the fabric needs. Plus it is a fairly thick coating so this is what you sand and sand and sand to get a good finish. You could leave it silver if you chose (ala the spirit of St. Louis) or you can paint it.

It is very weather and rain proof. It also has a nice impact resistance due to its flexibility. Of course your paint job would have to be flexible.

Since the final finish is sort of like your automobiles finish it will be very low maintainance. You can get many many years out of a fabric job. The sun is the big enemy. But of course I have seen many fabric covered airplanes sitting outside 24/7 and the fabric hold up Ok for many years.

Aircraft spruce and specialty is a great source for this type of material. Here is their website: http://www.aircraft-spruce.com/ .

Go down to your local airport and grab the first pilot you see. Ask him to point you to a fabric covered airplane and its owner. He or she won't bite and I think you will find a huge amount info in a short amount of time.

I hope this helps.

Cary

[Cary Winch]

Rik,

I thought about this today. It really occured to me that that statement would make no sense to anyone but me probably. I tend to not make sense alot.

The R value is a value of conductive resistance. This would plot out linear. But, when factored into heat dissipation calculations it affects heat transfer differently. Then when you figure in temperature differentials it gets real weird. I used to do heatsink design for the power electronics industry and this was a very frustrating thing to deal with. Since R value is sort of the reverse of heat conduction heatsink design kind of lends itself to it.

Basically my thought is the temperature differential creates a time constraint dictated by the R value. If you have a heat source inside that is equal to 200watts (two people) the R value will slow down the heat transfer enough that 200w will still raise the temperature inside. As your ambient outside temperature lowers outside it requires a higher R value to keep above that 200w input. This is why you have a noticible effect with your insulated walls in a 20 degree temp. It would be possible with a plywood wall to cross over than heat generation balance at higher point on the curve. This is also why in our gathering pictures on our website you will see a picture of a space heater in my trailer. My wife was at home. So, I only had the 100watts of heat. I would have been cold without the space heater. But, Craig and Betsey were in one next to me without a heater and had to have a window open because it was so warm inside. All a matter of balance. It was in the low thirties that night.

So, like you said. Insulated walls do improve. But do they need to? It all depends on your heat generation and outside temp. Being from rural Wisconsin I have been in way more than my share of old farm houses in the dead of winter that had no insulation in the walls and single pane windows that were hot inside. Why, because they had a big wood furnace and had more heat than needed.

Cary

[Cary Winch]

Woody,

Have you ever seen the movie "The egg and I" ? It was about a very weird rounded teardrop. I haven't seen it but have been looking for it. I have seen pictures of the "egg" and makes for an intriquing teardrop shape.


Cary

[Woody]

Cary,
Yes, I have seen it. This does bring a whole mutitude of possibilities of shapes and designs. I know you have seen that teardrop the guy built that matches the old model "T" type car. It sleeps one, very retro looking with the old style wire spoke wheels, Looks like it is partially made with canvas and wood and very narrow. I will have to look for it again, very cool concept to adapt into something larger and alittle more modern maybe. I am sure that Mike will find pretty quick once he reads this thread.

[mikeschn]

That would the the egg in this thread, I believe... scroll down to the third message...

http://tnttt.com/viewto ... t=egg#2781

Mike...

[Woody]

Mike, How do you keep all that stuff straight in your head

[mikeschn]

LOL... Woody... it's easy... this forum has a really good search function... learn how to really use it, and it'll yield you treasure beyond your wildest dreams!!!

Mike...