Sidewall weight comparison.

[57Dan]

I am wondering if anyone has any thoughts about whether a sandwiched constructed sidewall is much if any lighter than just 3/4 inch plywood. Considering you have the inside and outside skins at 1/4 inch each, that's 1/2 inch, just a 1/4 inch less than the 3/4 plywood. Add in your framing and the insulation are you coming out ahead much if any in weight savings? If not wouldn't just using the 3/4 save a lot of time?

[Pmullen503]

Sandwich can be lighter. Use 1/8 skins instead of 1/4. Sandwich will be stiffer and you can add insulation. You'll be glad you insulated, a 3/4" ply box is like an oven on a sunny day.

[lrrowe]

Interesting you should have this thought now. I also have it and just used intuitive logic to,say, yes it is lighter. In my case, I would substitue the 3/4" plywood wall for a 1" foam wall, using two pieces of 1/4 in plywood and 1/2" foam. Why 1/2"? Because I have never seen 1/4" XPS foam. For the heck of it, later today I will just go ahead and weight the materials just to put an actual weight to this. BTW, I will be making walls inside my CT and want to go as light as I can.

[bobhenry]

I have been playing with my left over insulated 7/16 zip wall



A fiberglass faced 7/16 osb base with 1" of dense fiberglass foam insulation giving a R 6.6. I have used it in the house in a house to
insulate the cold concrete floor and line the sidewall of the closets.



The foam sandwich has a heavy build paper attached to it that accepts glue well so it would be easy to line the interior
with a light paneling. As to electrical just dig a trench where needed



We are looking at phase two of another building that should net 125 or so pieces 3' x 4' as scrap.

I am thinking about squirling it away for another day for what ever project gets in the way.

[noseoil]

Bob, I'd have to look at my notes, but I think my "skeletonized" panel for the 5 X 9 benroy with just one 1/8" skin was about 27#. If it was 3/4" ply, it would have been closer to 75# after trimming. With the 2nd skin in place and insulation, it would be closer to 40 pounds, but this is still much lighter than a solid panel. Considering insulation properties & weight it's much better.

If you look at cost, the 3/4" panel wins every time as it's about half as expensive and much faster to build with a solid panel than a layered one. And so it goes......

[booyah]

Going off the weights here
http://www.comptonlbr.com/weight-sheet.html

In my ongoing build I used 3/8" ACX for the external skin, 1/4" luanan for the internal, 1x2 lining around the door, and profile, and then 3/4" Rmax panels
external skin 36lbs
internal skin 25lbs
Rmax-R3 (3/4") panel 7lbs (home depot claim) seems heavy to me, these panels are LIGHT

total 68lbs

the same site claims 3/4" plywod is 71lbs

I'm pretty sure I didnt use more than a few lbs of 1x2s to line, but say I did, I draw nearly even on the weight, but I score MUCH higher on improved sound and thermal isolation.

Yeah the effort is a lot higher too, but I'll take it for the payoff

[lrrowe]

Bob, I'd have to look at my notes, but I think my "skeletonized" panel for the 5 X 9 benroy with just one 1/8" skin was about 27#. If it was 3/4" ply, it would have been closer to 75# after trimming. With the 2nd skin in place and insulation, it would be closer to 40 pounds, but this is still much lighter than a solid panel. Considering insulation properties & weight it's much better.

If you look at cost, the 3/4" panel wins every time as it's about half as expensive and much faster to build with a solid panel than a layered one. And so it goes......

Noseoil, I will use your numbers as they both sound reasonable and are backed up by extrapolation using booyah's listed weights below (for 1/8 in panels and 1/2 in foam).

For me the question is whether these walls using 1/8 in luan and 1/2 in foam is strong enough for supporting shelf type weights on interior CT walls (like around a shower enclosure). Maybe I will build one and test it for strenght.

[dales133]

I've used 6 mm ply internal and 3/4" Dow XPS foam and 1/8 external ply with minimal framing.
While I don't have an exact weight without the 1/8 external skin the walls were extremely light.
If i had to guess I'd say somewhere in the 20 pound range and that's with using hardwood framimg

[tony.latham]

There are many advantages of a sandwiched wall (in my opinion). Building inside out, mortising your bulkheads, and insulation are three that I can come up with real fast.
Weight isn't one of them. The weight of course depends on your skins and how "skinny" your skeleton is.

The only advantage that I can think of for a solid wall is that it makes for a quicker build.

I prefer 1/4" subfloor ply in and out, 3/4" skeletonized AC ply and foam for the internal space. The weight has got to be close to a teardrop shaped piece of 3/4" AC ply. Higher or lower, I don't know.








I've owned teardrops built both ways and much prefer sandwiched walls for condensation reasons (and yes, I crack the windows.)

Tony

[KCStudly]

My walls are 5 mm inside ply and 1-1/2 blue foam with minimal cedar framing; just 2x2 along the base and around the door. The doors are the same except just 1x2 framing around the perimeter and around the window with some radius blocks at the corners. Before skinning the inside I let in 1x pine blocking, mostly 3-1/2 squares, except I used a full standing 1x4 let in flat at the bulkhead. These blocks and some more let into the outside were all strategically placed to accept screws for everything from my cabinet face frames (part of the 'unit construction' design) to my lights and coat hooks, etc. Plan is to use canvas as an outer skin, so that was not included in the weight. I had the door frame in place and skinned it with the inside wall all at once, so the door (but not the window unit or hinges) is included in the weight.

Dimension of the wall is about 49-1/2 inches tall (48 inches of foam on top of the 2x sill) at the crest of the profile and 9ft-8in at the base. Because it is similar to a Benroy profile there isn't much area cut off of the front and rear, in fact the rear has a "bustle" that sticks out a bit. Before cutting the door free, and I think before adding the inside skin to the galley area, one of my walls weighed within one lb. of 45 lbs (it was either 44 or 46, I'd have to look back).

I could easily handle my walls by myself throughout construction.

So yeah, lots of insulation for heat/cool/sound, and much lighter than the same size of good 3/4 ply; but lots of work routing pockets and gluing all of those blocks in (and you have to know where to put them!!!), and the premium select cedar (which is very light and rot resistant) wasn't cheap.

You pays your money and you takes your choice.

[S. Heisley]

As a general rule,

A 1/8" thick, 4x8' plywood sheet weighs in the vicinity of 9 pounds. (I found that all my 1/8" plywood sheets weighed exactly 9 lbs each, regardless of type.)
Double that for 1/4" thick plywood (18 pounds each).
Multiply that 9 pounds by 6 for 3/4" thick plywood. (54 pounds each).

If you use minimal 1x2 framing and use Styrofoam in between,

a 3/4" thick, 4x8' Styrofoam sheet weights 1.5 pounds.

So, the framing that you use can make the biggest difference, both the type of wood and the amount and size of it. However, I would assume that you would use some framing wood, even when building with 3/4" plywood. I used 1x2 poplar boards and even those would differ in weight, based on, believe it or not, the color! The darker the wood, the more it weighed. Also, all pine boards tended to weigh more than poplar boards. (... possibly because of the wood grain?...dunno.)

Sandwiched walls provide the most benefit by insulating against heat and cold as well as blocking noise. Sandwiched walls are also helpful in minimizing condensation. Weight can be less only if the builder is mindful while he is building. (Using 2x4's is not being mindful. HaHa!) Everything depends on the builder.

...Hope this helps.

[pchast]

Several lumber dealers posted that the typical plywood is 25 lbs per 1/4 inch per 4' x 8' sheet.
This is the data I used in figuring my choice to finally go foam.

[57Dan]

Going with 3/4 is tempting for easier and quicker construction. I know about insulating against the cold but I'm forgetting about insulating against the hot summers days also. So you all talked me into it. Sandwich walls it is. Thanks for the input.

[noseoil]

Dan, take a look at my walls on the build I'm doing for more details of how I set mine up. I didn't leave a lot of meat in the 3/4" ply panel, except where it was necessary for fastening from the outside & joining the walls to the deck & headliner. If you have access to a router & a 1/2" (bearing pilot) flush-cut bit, it's pretty easy to make one side & then use the router to duplicate a second panel. It takes a bit of planning, so do a full scale layout with everything you can think of first (shelves, bulkheads, galley, footboard, headboard, etc.). I missed the hatch supports at the galley on the first panel, but was able to use the repair to do the second one without any trouble once the first one was fixed.

The nice thing about the skeleton wall build is the ability to add things if you forget something along the way. I had to add blocks for my drink holders (an afterthought) and one of the legs for my outside galley shelves. I still need to add for the reading lights, but I'm waiting until I get to that point in the build. I've found that I'm better off going slowly as this is my first build and once the walls are buttoned up, it's a PITA to have to add later.

[Tomterrific]

I built an extremely light and fast 4x8 build with 3/16" (approximate) underpayment from Lowes. I sandwiched the top but did a board and batten thing on the sides. I painted it white to reflect the sun. The flimsy sides were stiffened dramatically with the 1x2 battens. The battens were added after the roof was completed. I can lift it easily and drag it around my yard.

Tom