Thoughts on not stripping off the wall plywood

Converting Cargo Trailers into TTTs

Thoughts on not stripping off the wall plywood

Postby jimreo » Fri Jan 12, 2018 11:57 am

I am looking to buy a cargo trailer for conversion to a camper and was thinking of getting the 8.5x20 and then not tear off the plywood inside and just build 1.5” thick walls inside of the plywood and insulate just the new interior walls. My questions are:

1) Does this give me an advantage (besides saving effort and time), as far as insulating value and why?
2) Does this put me at a disadvantage (besides losing a few inches) and why?
3) If I do this, will the trailer sweat on the aluminum sides, cause mold and now I can’t get to it to clean the walls?

Thanks for any thoughts, tips or advise.
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Re: Thoughts on not stripping off the wall plywood

Postby NotJammer » Fri Jan 12, 2018 1:52 pm

Read this. I found it last month. I am not heavily re-insulating my CT.

Mold to me is a big concern.

http://www.mychemicalfreehouse.net/2017 ... rsion.html
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Re: Thoughts on not stripping off the wall plywood

Postby Woodbutcher » Fri Jan 12, 2018 1:55 pm

Can you order the trailer without insulation and interior plywood? Then build out how you want. I would not want to lose the space,or the added weight.
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Re: Thoughts on not stripping off the wall plywood

Postby jimreo » Fri Jan 12, 2018 3:23 pm

I will be looking at a trailer or two this weekend and will ask them about leaving the plywood off. I'm in Oregon and I know at least one of the brands is made here, so maybe they would do it since the trailer isn't traveling cross country. At 8.5 wide I'm not too concerned about losing 3 to 4 inches. I was mostly curious if leaving the air space would add R-Value or deplete it? Keep it cooler in the summer or make it hotter? I think the mold factor would be my biggest factor in whether to take the plywood off or not.

Before I joined, I had read on this forum, someone stating that if they had to do it all over again they would have built to the inside and not taken off the inside plywood.
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Re: Thoughts on not stripping off the wall plywood

Postby NotJammer » Fri Jan 12, 2018 6:56 pm

Correction. Per the website below air space has an R1 value.


Air space has almost no R-value. Plywood has a low R rating like R-0.75 for 3/4 in.

I was on this site last night. https://www.archtoolbox.com/materials-s ... alues.html
Last edited by NotJammer on Fri Jan 12, 2018 7:31 pm, edited 1 time in total.
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Re: Thoughts on not stripping off the wall plywood

Postby tylerjd » Fri Jan 12, 2018 7:23 pm

Comparing these two options:

1. You take the plywood off, put in 1" of insulation in between the ribs, and put the plywood back on, and that's it.
2. You leave the plywood on, and put *continuous* 1.5" insulation on the inside of the plywood, and then sandwich it between paneling that you affix by sinking screws all the way to the existing plywood.

Option 2 will have a *significantly* higher R-value. It will, in fact, be about R-7.5, since there is nothing effecting the R-value of the insulation.
Option 1 will be significantly lower - the R value of 1" XPS is about 5, but with the ribs, the overall R-value of the wall will be something like R-2 or R-3 at best, I would guess.

The air gap probably won't help you. It might help if you could ensure it is totally sealed up and there is no air leakage/movement in there, but that's probably not going to happen.

If you don't care about using up a few interior inches, interior continuous insulation is the best way to achieve halfway decent thermal ratings.

I have no idea about the mold issue... my mold mitigation strategy is to always be in dry places.
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Re: Thoughts on not stripping off the wall plywood

Postby featherliteCT1 » Fri Jan 12, 2018 7:27 pm

NotJammer wrote:Air space has almost no R-value. Plywood has a low R rating like R-0.75 for 3/4 in.



I agree with NotJammer ++
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Re: Thoughts on not stripping off the wall plywood

Postby Iconfabul8 » Fri Jan 12, 2018 8:48 pm

If I were thinking along the lines of an air space, I would take off the wood and put a reflective foil on it and put it back up. then insulate the inside. This would help immensely with the radiant summer heat. Not sure you can assign an R value to this but there is a lot of info out there about reflective foil and air space.
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Re: Thoughts on not stripping off the wall plywood

Postby CoventryKid » Fri Jan 12, 2018 11:24 pm

Does this put me at a disadvantage (besides losing a few inches) and why?

I had to be conscious of the finished weight of my CT because of my tow vehicle. One thing that comes to mind is that you will have twice the weight in plywood (walls). Lose space, add weight - wouldn't be my first choice. JMHO.

Hope this helps.
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Re: Thoughts on not stripping off the wall plywood

Postby featherliteCT1 » Sat Jan 13, 2018 8:26 am

Iconfabul8 wrote:If I were thinking along the lines of an air space, I would take off the wood and put a reflective foil on it and put it back up. then insulate the inside. This would help immensely with the radiant summer heat. Not sure you can assign an R value to this but there is a lot of info out there about reflective foil and air space.


Although I personally would not use an air space, if I did, I would agree with Iconfabul8 ++
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Re: Thoughts on not stripping off the wall plywood

Postby hankaye » Sat Jan 13, 2018 10:18 am

Howdy All;

Call me crazy, lots of others have ... "GoneFishin" has only the bubble-wrap
type of insulation inside his walls and yet he is able to camp (he states),
comfortably with his heating system in some fairly coolish to cold weather.

Most of these trailers have no sealant where the sidewalls (exterior), over-
lap allowing for air to somewhat circulate. Those same walls are not sealed
at the bottoms allowing for some drainage of condensation.

When I first bought mine I was on my way to Montana to do some fishin' and
meet some friends. The weather that week was cold with scattered snow and
rain/sleet that was near the end of JUNE. Well, I had the winter that I'd missed
in N.M. back in Dec.-March. Yet I was comfortable with just a space heater set
on low (900watt), to keep me warm and comfortable. The part that needed the
insulation was the ceiling, the sound of rain dripping on it was almost deafening.

Just some thoughts about my observations and from what I've been able to surmise
from my readings here.

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Re: Thoughts on not stripping off the wall plywood

Postby NotJammer » Sat Jan 13, 2018 12:39 pm

My CT has only Reflectix, with taped aluminum outside panels and 7\16" Advantech inside. I checked it was there yesterday. Nicely factory installed. 1" air gap.

I am running tests today and some last week. The white CT is facing south into a bright sun today. Lots of snow reflection. I have a remote thermometer in it. All windows and roof vent are covered in Reflectix also. Floor is 3/4" Advantech. No Sun getting inside.

Reflectix has no R value at all as far as I can find. So maybe R2 walls and R1 floor. Today the CT inside temp is exactly following the outside air temp. Sun is doing nothing, it's radiant heat is being reflected. Night time tests of heat rise inside with 1350 watt electric heater has been getting the inside up as far as 80 with outside temps in the teens. Heat is being reflected back in.

At noon I am turning on the electric heater from inside my warm home and will see how long until I can go play house. Meaning above 60 F.

I had wanted the factory to spray foam the underneath, They refused. I have noticed some high-end RV's use Coroplast (corrugated plastic) to cover the bottom of the trailer. Seals everything in and adds an airspace...I am considering doing that. Coroplast is cheap, tough and easy to work. I might stick it up with Scotch 3M . https://www.scotchbrand.com/3M/en_US/sc ... 497&rt=rud
CoroPlast https://www.homedepot.com/p/Coroplast-4 ... AQodtqUBdg

I have used the 3M for lots of things. Hangs Coroplast signage anywhere for years in the North. I think it would be too strong to remove if it was used in a continuous strip on a CT bottom.

It would slow air infiltration underneath thus keeping the floor warmer if heated. Maybe add Reflectix between the floor and the Coroplast.

Have you ever slept in only a sleeping bag on the metal floor of a pickup? I have in 25 F. It was very cold. The cold seemed to intensify. I soon built a platform bed and it was much better.

Now its noon and 26 f inside the CT with 1350 watt just turned on. I went outside for a minute and damn near fell. My cement pad has thin snow covered in ice and very slippery...I won't use salt.
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Re: Thoughts on not stripping off the wall plywood

Postby tony.latham » Sat Jan 13, 2018 1:57 pm

Me? I think it would depend on how hard it is to remove the plywood. I've read where the screw-looking fasteners aren't screws and are difficult to remove and reinstall. (I think it is part of the structural integrity.)

I'd be tempted to staple Visqueen to the existing plywood to prevent interior moisture from condensing on the metal sheathing. I'd then add 3/4" foam insulation and furring strips. I'd cover that with prefinished 1/4" subfloor plywood (It's cheaper and better looking than ACX). But you could cover it with wood paneling and that would save you a ton of finish work.

But I'm a teardrop guy, so what do I know?

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Re: Thoughts on not stripping off the wall plywood

Postby NotJammer » Sat Jan 13, 2018 5:31 pm

Results of today's electric heater test in a 7X17 CT.

I wrote about it above.

Outside air temp was 27F interior was also 27F

At noon turned on 1350 watt electric Honeywell heater

At 3 pm the outside temp had fallen to 20F and was still dropping. Inside the CT temp rose to 50F and stopped rising.

At 4 pm outside was 18F and inside was 49F.

End of test.
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Re: Thoughts on not stripping off the wall plywood

Postby featherliteCT1 » Sat Jan 13, 2018 6:50 pm

NotJammer,

Thanks for taking the effort to post that data (at 27 to 18 degrees, 4 hours to get to 50 to 49 degrees). Good to know!

In your previous post, you noted that “Night time tests of heat rise inside with 1350 watt electric heater has been getting the inside up as far as 80 with outside temps in the teens. Heat is being reflected back in”.

How do the two tests compare? Did the 80 degrees result because you ran the heater for a lot longer than 4 hours?
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