insulating bare aluminum skin and tube frame

[Padilen]

Hey check your thread, we went talking about the same kind of framing.

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[fourbtgait]

If you note back where I talked of it, I said I had channels.
True, he was talking tubing, but the same applies.
Though if I was to do tubing on an aluminum frame I welded up, I would use Corpour

[low277]

I have heard of doing this, drilling holes and squirting low expanding door and window jam foam inside, but I never actually seen it done or personally know anyone that has done it. I too would be curious if someone has details about doing this?

[ssuuki19]

Interesting stuff, I appreciate the dialogue, and I wasn't even thinking about the inside of the tubes which is something to take note. I was more thinking just how that whole 1" framed wall is a giant heat-sink to the interior.. I was going to insulate 'over the tubes' on the inside with 1/2" of that foam insulation with shiny backing, but in some spots there will be 1" x 1" point thermal bridges. I'll see if I can get an r-value for the insulation I bought its open cell foam so not as good as closed.

[fourbtgait]

3m VHB tape to attach the outer skin to the tubing instead of screws/rivets.
No insulation factor but will help cut down direct transfer.

[tylerjd]

I'll throw in my 2 cents here. Caveat: I haven't built my trailer yet! So I speak not from any trailer specific experience, but from having studied heat transfer in school, designed wall assemblies at work with thermal bridging in mind (I'm an HVAC engineer), and having thought about my trailer a lot. Insulation and overall thermal performance is something I enjoy nerding out about in general.

Here's a diagram of how I plan on doing my wall for my CTC:


In English: I will install rigid foam insulation in the cavities in between the 1" frame tubing.
Then the original 1/2" plywood will go back on.
And then I will fasten another 1" of *continuous* rigid foam insulation over that plywood. Continuous is a term used in the building industry to imply that the insulation is not thermally broken by studs or anything else.

To my mind, continuous insulation is the only effective way of combating the significant thermal bridging that you have with aluminum framing.

Also, a note about foam insulation on the inside of the tubing: it is my respectful belief, based not on experience but on, well, math, that adding foam insulation to the inside of metal tubing would have almost zero mitigating effect on thermal bridging.

The red lines in my image show the path that heat is most going to want to take. It is going to "want" to go from the Al skin, to the tubing, to the plywood, and then on to the inside. The heat will not want to go through the dead air space in the middle of the tubing, it is going to want to flow from metal surface to metal surface. Air, when it isn't moving, is actually a pretty good insulator. And inside a 1" tube, it doesn't have enough room to move and set up convective currents, so a 1" Al tube is effectively already insulated inside. (Sort of like a really skinny double-pane window...)

3m VHB tape to attach the outer skin to the tubing instead of screws/rivets.
No insulation factor but will help cut down direct transfer.

I would think this would help, have been thinking about doing something similar with my floor assembly. (Maybe some sort of thick thermal tape as well as screws though... I'd be worried about the lack of mechanical fastening?)

[fourbtgait]

I agree that insulating the inside of the tube does not negate thermal bridgeing. It simply helps to reduce it.
A 1/2" to 4" dead air space as shown on commonly accepted tables only provides an R1.

[tylerjd]

I agree that insulating the inside of the tube does not negate thermal bridgeing. It simply helps to reduce it.
A 1/2" to 4" dead air space as shown on commonly accepted tables only provides an R1.

I certainly agree, you are correct, but if you run the numbers, the overall benefit you get from filling that tube with foam is almost nothing. Not worth the effort, I would argue.

No one actually cares what the "R-Value" of any material is, that's just a made-up number some engineers came up with that makes it easy to compare materials. The units of R-value are Btu/(hr F ft)... I studied engineering for 5 years and I've no real notion of what that means, intuitively.

What people actually care about is *how much heat is getting in to (or leaking out of) my space?"

And if we run the numbers on that (otherwise known as heat transfer), we see that insulating the inside of your tube isn't doing much:

(I wasn't sure if the tubes are steel or Al, so I did both)

If you put those numbers on a graph, the heat transfer through air vs. foam is barely even visible:


So by making the path through the center of the tube about 6x as insulated as just air, you are improving the *overall thermal performance of your wall* by some unimaginably tiny fraction of a percent!

This is why - to my mind - the only way to meaningfully mitigate thermal bridging (and I assume condensation issues) is through the use of a continuous insulation layer. Now, that means you lose a few inches of interior space, which might not be worth it. If you only camp in reasonable weather, continuous insulation might not make sense.

[tylerjd]

^^My analysis above only holds for square tube framing... for those roof bows that you posted pictures of, now that I go back and look at them, I would expect the insulating foam to have a more significant effect.

[fourbtgait]

The question one has to ask, whether it is trailer skins or building an energy efficient home is, at what point does initial cost over ride long term payback? And if a person sells before payback of the initial cost from energy savings, is it at all possible to recoup that difference from the buyer, if they even care? When I sold my Benroy, the people couldnt care less about the insulation or led lighting.
So when is goos enough, good enough?
Even with the insulation upgrades I have done to this trailer, because the walls, roof are so thin, after so many hours, the temperature will basically equalize. By evening, outside air temp is 90+ degrees, trailer is 85+ degrees and holds it. By 6 am, outside air is 55 degrees and trailer is 55-60 degrees. 1" of R6 is basically nothing. Now yes, if I had continuous instead of cavity insulation, and thicker, I could acheive much better passiveness. If I do not mind the loss of floor area.
I toyed with the idea of using SIPS panels to build my own "cargo" trailer.

[tylerjd]

Agreed, all this stuff is so personal and depends on intended use, climate, does one actually care about the $$$ payback or is one doing it mostly for kicks to see just how well insulated it is possible to get a CTC, etc.

One of the reasons I'm into this is that one of my goals with my ctc is to be able to button it up on a cold night, let's say down in the 20's or even 10's F, and with just two people's body heat (and maybe a laptop or two) keep it no lower than 45 or 50 inside. I do understand that's a tall ask with these trailers.

A SIPS trailer would be awesome to see!

My bad if I came on a little strong with my above posts... I got a little excited to do math again.

[aggie79]

A couple of items for thought:

1. I agree that "spray foaming" the square tubing will negligible if any impact on heating and cooling. There may however be the following possible benefits of insulating square tubing:
a. Displacing the air with closed cell insulation should reduce or eliminate condensation within the square tubes
b. If you outer trailer skin is attached by screws or rivets into the tubing, that are subject to loosening over time due to vibration and flexing, the closed cell insulation should act as a seal to prevent moisture intrusion.

2. Continuous insulation works from a thermal standpoint, but:
a. The typical variability in insulation panel thicknesses will make it difficult to get a smooth plane for interior finish skins.
b. Attaching upper cabinets, if being used, will be a little more tricky. Because the insulation can compress, cabinets may requiring anchoring to a substrate (plywood) on behind the insulation - not the interior finish skin - or to the directly to the CTC's square tubing.

[tylerjd]

A couple of items for thought:

1. I agree that "spray foaming" the square tubing will negligible if any impact on heating and cooling. There may however be the following possible benefits of insulating square tubing:
a. Displacing the air with closed cell insulation should reduce or eliminate condensation within the square tubes
b. If you outer trailer skin is attached by screws or rivets into the tubing, that are subject to loosening over time due to vibration and flexing, the closed cell insulation should act as a seal to prevent moisture intrusion.

2. Continuous insulation works from a thermal standpoint, but:
a. The typical variability in insulation panel thicknesses will make it difficult to get a smooth plane for interior finish skins.
b. Attaching upper cabinets, if being used, will be a little more tricky. Because the insulation can compress, cabinets may requiring anchoring to a substrate (plywood) on behind the insulation - not the interior finish skin - or to the directly to the CTC's square tubing.

Good points - I wonder if insulating the tubing would cut down on noise transmission as well?

I just wanted to follow up having just installed my inner continuous insulation + wall paneling.

The inner wall paneling is certainly not as smooth/flat as one expects in travel trailers, as you say. My paneling sort of has a chesterfield couch contour to it (but way more subtle than a chesterfield). You can feel it if you run your hands over the wall that it 'dips' in a little at the screws, but it's not easy to see unless you put your eye flush with the wall and have light shining just right. I'm using 1" XPS insulation. I actually kind of like it, it gives the walls a touch of character haha.

I haven't gotten to attaching cabinets, walls, shelves, etc. I thought about putting in furring strips for better connection points, but decided against it because I didn't want to have to figure out precisely where I wanted all that stuff at this time and then be committed. So, I'm committed to figuring out an alternative. For most things I need to tie in to the wall, I'm planning on just using extra long screws (2-1/2" to 3", I imagine) to ensure my walls/cabinets/etc are anchored into the 1/2" plywood. For overhead cabinets, I'd add some bracing in the ceiling off the ribs... I'll update if/when I run into issues with all this.

Cheers.

[OverTheTopCargoTrailer]

I gotta say ten stars for the man named tylerjd. where were you when OTTCT was born ? You really payed attention in school.....

I'm cooling to 72F with solar and mini split in 7 x18 CarMate wood frame trailer using under 300 watts per hour parked in full sun 100F or Texas 90F with 85% humidity. 3" dow blue board 6" denim in the roof. no heat required until outside drops below 32F

cheers OTTCT

[dogpound101]

I gotta say ten stars for the man named tylerjd. where were you when OTTCT was born ? You really payed attention in school.....

I'm cooling to 72F with solar and mini split in 7 x18 CarMate wood frame trailer using under 300 watts per hour parked in full sun 100F or Texas 90F with 85% humidity. 3" dow blue board 6" denim in the roof. no heat required until outside drops below 32F

cheers OTTCT

Did you say mini split How about some info on how you did this. Pleaseeeeee


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