Plywood/Foam Wall material

[redbicycle]

Hey all,

I have been trying to plan my construction plans and I am running into a bit of a roadblock. I am struggling with finding materials that align with correct thickness. Specifically I am trying to align plywood with foam board.

For my floor I am planning a layer from top to bottom:

1/2" Hickory engineered hardwood
5mm Underlayment
1x2 furring strips(common .75) with .75 Kingspan foam board (common .75)
7/32 plywood

I think this will work successfully for my floor. I am struggling with the walls. I would prefer to carve my wall interior structure from plywood vice assembling it with sticks and a Kregg jig. The problem I am running into is finding a combination of plywood and foam that are the same thickness. The best combination I have found so far is 19/32 plywood. The 19/32 sheeting has a thickness of .59. If I use green Kingspan foam the panel has a thickness of common .56.

If I look at thicker plywood the best I can find is 23/32 sheeting with a thickness of .72. This appears to be far enough away from the 3/4 foam board thickness that the foam board will not work in combination with that plywood. Anything I am finding in 3/4 ply thickness is either closer to .62, a more expensive hardwood, or pressure treated. None of these are good selections for what I am trying to do.

Am I just failing in my search for a good 3/4 plywood that aligns with a 3/4 foam panel or is this a strange thickness combination?

[tony.latham]

Am I just failing in my search for a good 3/4 plywood that aligns with a 3/4 foam panel or is this a strange thickness combination?

I've never understood the fact that wood building materials are "nominal" but foam sheets ––that are going in the same structure-- aren't.

You can take the foam down to the level of the skeleton with a random orbital sander. It's quick and dirty.





Tony

[Tom&Shelly]

We solved the problem by making a hot-wire foam cutter:



We use 1 inch foam and cut a quarter inch off. The quarter inch bends so well, we may try and use those (stacked) between our ceiling and roof. It's not perfect because our floor wasn't perfectly flat, and I hand sanded about a 32nd inch off of what was left of the foam.

There are several sites on the internet that discuss foam cutters. Here is where we bought our wire and transformer: http://www.jacobs-online.biz/nichrome/NichromeCalc.html

I got the idea from one my Dad made for building model airplanes in the 60's using a World War II surplus variac. A modern (safer) version uses a transformer and light dimmer:



The baseline design uses Nichrome wire, but that site also sells Rene wire (whatever that is) claiming it's stronger. It is, but also springy. I wear safety glasses when working with it. We hold ours in a home-made bow with rubber bands to keep tension (the wire expands a little when heated:



That's a plastic slippery washer I cut out of a coffee creamer jar. We built our bow to be five feet long so we can trim the roof foam. Smaller would be easier to handle and would require a less expensive transformer.

The fumes are likely carcinogenic and at our age may kill us by the time we are 135; probably best to do the cutting with a window open.



Tom

[Aguyfromohio]

Very nice! Hot wire slicing off a thin layer from a full sheet.

We did ours the other way, much harder.
We used common fir 2 x 4 framing, and used a Dewalt thickness planer to cut it all down to match the foam.
Hours of time sending wood through the planer and bags and bags of shavings.

You method make much more sense.

[Philip]

Hot wire setup is the fastest and cleanest way to deal with foam size differences. On my build I had to shave almost all the foam I used. If you look threw my build it shows some of the cutters I made and the power supplies I made the cutters from.

[tony.latham]

I've designed my ceilings and hatches* so I don't have to cut the foam. It takes about ten minutes with the orbital sander to take the wall foam down using 80 grit paper. It's one of those sanding tasks that encourage hooking up the vacuum.

Tony

*I add the foam to the hatch to support the 1/8" ply. It may be overkill.

[Tom&Shelly]

I've designed my ceilings and hatches* so I don't have to cut the foam. It takes about ten minutes with the orbital sander to take the wall foam down using 80 grit paper. It's one of those sanding tasks that encourage hooking up the vacuum.

Tony

*I add the foam to the hatch to support the 1/8" ply. It may be overkill.

Took me an afternoon calculating and ordering what I needed for the foam cutter, 3 days until the parts arrived, and half a day to build the contraption. Then only about 10 minutes to cut the foam. Umm, then another ten minutes or so to hand sand the parts that didn't come out perfectly flush, and another minute to vacuum that dust. Which may be a hint as to why our tear isn't even close to being finished after ten months! But it's all part of the fun!

I'll think about foam in the hatch. Does it also help keep the galley (and cooler there in) temperate?

Tom

[tony.latham]

I've designed my ceilings and hatches* so I don't have to cut the foam. It takes about ten minutes with the orbital sander to take the wall foam down using 80 grit paper. It's one of those sanding tasks that encourage hooking up the vacuum.

Tony

*I add the foam to the hatch to support the 1/8" ply. It may be overkill.

Took me an afternoon calculating and ordering what I needed for the foam cutter, 3 days until the parts arrived, and half a day to build the contraption. Then only about 10 minutes to cut the foam. Umm, then another ten minutes or so to hand sand the parts that didn't come out perfectly flush, and another minute to vacuum that dust. Which may be a hint as to why our tear isn't even close to being finished after ten months! But it's all part of the fun!

I'll think about foam in the hatch. Does it also help keep the galley (and cooler there in) temperate?

Tom

It must on smoking hot days. But how much I don’t know. I probably need to stick s thermometer in there.

T

[GPW]

Guys why not cut out those Nice “lightened” profiles out of some really Good thin plywood and then just glue that to a piece of 2” thick foam … done … Insulated wall , with lots of places to attach things , and really Light too … CNC helps , or if you’re really patient with a jig saw… just an idea … ( that’s been done before by our Fearless Leader… )