Wall construction and insulation

[pappawheely]

I need some advice on proper wall construction, and insulation. The walls on my box truck are constructed from an aluminum outer skin, and vertical aluminum extrusions that are "Z" shaped in profile. The distance between the outer skin, and where the extrusions return is 1-3/4". I was going to slip rigid insulation under the extrusion return, but I don't have any local options that include 1-3/4" thick material. I was thinking instead that I would put a vapor barrier up, then 2" thick rigid panel insulation over it. Then plywood panels. That would leave an air gap of 1-3/4" between the insulation and the outside panel. I have some questions. Will this air gap control condensation? Will the walls have enough support screwing through the insulation, or do I need to add firing strips to attach the wood paneling? Ay advice would be appreciated. I wanted to go with spray foam, but I found R-13 rigid panel insulation that will be less than half the cost.

 

[Joe917]

Your vapour barrier should be inside the insulation. That air gap to the exterior aluminum is the perfect place for condensation and ice formation, followed by water damage . Bite the bullet and go with the spray foam, unless this is just a cheap short term build.

 

[Alloy]

Most of the time (if air conditioning isn't used) water vapor is moving from the inside (hot to cold) to outside so a vapor barrier goes on the inside of the wall.

Like Joe917 said spray foam is best ..it is a vapor barrier and fills voids 100%. Best to use 1/2lb density in the roof in case something leaks then water will come through the 1/2lb foam. To keep the interior paneling away from the alum and prevent thermal bridging I'd cap the alum with firing strips of something that will hold a screw and bring the spray foam out to that level.

The weight of spray foam might be a factor to consider.

Next best is 2 1/2" XPS foam notched to fit tight into the return leg of the Z extrusion. It is important that the seams be caulked to prevent air movement. The exposed aluminum between the XPS can be covered with strips of XPS, Reflexcite, EPDM foam or Polyethylene foam This will keep the interior paneling away from the alum and prevent thermal bridging.

Plan out routing for electrical and plumbing before insulation



A vapor barrier inside the XPS isn't a bad idea but it needs to be sealed around windows, doors and electrical enclosures.

 

[pappawheely]

Thank you both for your input. I know spray foam is the best, but I took a vow of poverty to become an automotive journalist. The 2 inch rigid I found is R13, and it's a lot cheaper than foam. At least the box came with it's own supply of lumber that I can re-purpose.

 

[luthj]

Here is my thoughts. I would just use 2 different foam thicknesses stacked. Such as 1" and 1/2" or 1" and 3/4" if available. Use 3m spray adhesive to bond the foam to the metal. Do the same for the second layer of foam to the first. Then take great stuff expanding foam, and seal all the edges of the panels to the cavity. This eliminates the condensation issue by preventing active air ingress, and removing the chances moisture laden air will contact the skin.

You will want some type of thermal break over the ribs. 1/8" of closed cell foam will work, then just put your wall panels over top of that. It goes without saying but avoid excessive weigh (such as thick plywood wall panels).

You might price out spray foam, it could be cost competitive if you have a local operator who will do you as a side job. Some folks report 500$ for a larger van, if they show up to the job site with the rig pre-masked and ready to go. You may get a bit of panel distortion in very cold or very hot weather. Do not go with anyone that does high temp sprayed products, they tend to warp a lot more.

 

[luthj]

Another option would be to use furring strips to raise the wall ribs enough to use 2" insulation.

 

[vintageracer]

You might want to go to the Airstream Forum for other less expensive solutions to your insulation needs. The aluminum skin and spars in your box are similar to Airstream trailer insulation issues that can create a unique situation with regard to condensation and temperature change that Airstreamer's have had to deal with for years.

You might find a less expensive and workable solution there.

www.airforums.com

 

[pappawheely]

I figured out a way to slot the insulation and fit it tight in the struts. I can also rip down some furring strips to attach the wall panels. Thanks everyone for the input. Great ideas shared.

 

[FireAce]

Following your progress, great info here. I have an aluminum truck body that I’m converting to a camper and have been debating how to insulate. I have been leaning toward no vapor barrier at all and just letting moisture condense/evaporate as it likes. Such a tough call.

I have been avoiding spray foam because if you want to go back and run wiring, plumbing, etc. it’s a real hassle to try and remove the stuff.

Great tip by the person who posted about the airstream forums! Will check those out for sure!

 

[Alloy]

Here is my thoughts. I would just use 2 different foam thicknesses stacked. Such as 1" and 1/2" or 1" and 3/4" if available. Use 3m spray adhesive to bond the foam to the metal. Do the same for the second layer of foam to the first. Then take great stuff expanding foam, and seal all the edges of the panels to the cavity. This eliminates the condensation issue by preventing active air ingress, and removing the chances moisture laden air will contact the skin.

You will want some type of thermal break over the ribs. 1/8" of closed cell foam will work, then just put your wall panels over top of that. It goes without saying but avoid excessive weigh (such as thick plywood wall panels).

You might price out spray foam, it could be cost competitive if you have a local operator who will do you as a side job. Some folks report 500$ for a larger van, if they show up to the job site with the rig pre-masked and ready to go. You may get a bit of panel distortion in very cold or very hot weather. Do not go with anyone that does high temp sprayed products, they tend to warp a lot more.

My thought for the 2 1/2" was to prevent the thermal break from being compressed.

Great stuff won't bond to aluminum so air/moisture will leak into the wall. Depending on temperature there will be condensation between the 2 layer of foam. A vapor barrier is the best way to prevent moisture from entering the wall. Even the caulking I suggested would be 80% at best and it would add weight.

Found this....except I limit the number of staples because I put tape or acoustic seal on all of them