I found this video (no affiliation) and thought that it provided a fair overview of what is involved in making your own sandwich panels using XPS foam and fiberglass. The builder speaks well, seems to be very thorough and organised, and was able to produce what appears to be a quality product. That said, there are several errors that must be corrected.
First, the builder states numerous times that he used 15 and 25-pound Density XPS foam. There is simply no such thing. Density is often given in Pounds per Cubic Foot (PCF) in the US or kg/m3 in the ROTW. A 15 PCF density XPS foam would have the same mass as Coosa's Nautical 15 boards, a MUCH stronger product than XPS foam. The latter can be broken with your fingers. More on this, later.
Product names can be misleading: Foamular 150 does not have a compressive strength of 150 PSI, nor does it have a mass of 15 PCF. The compressive strength is only 15 PSI. The same naming convention holds true for Foamular 250, 400, 600 and 1000. Dow, Kingspan and other OEMs uses similar designations. Check the specs to ensure that you are getting what you want. Technical spec sheets are readily-available on line.
Second correction: R-value does not increase with density. If anything it would decrease, as more "product" means less trapped air. That said, majour manufacturers rate commonly-available XPS foam sheets at R-5 per inch. The builder states, "If I had to do it again, I would use 2-inches of 25-pound density (sic) for the additional strength and insulation." (see notes on Coosa, below)
Third correction: XPS is foam insulation. it is not a structural foam core like Diab, Gurit or Coosa. I realise that many people use XPS, but many people smoke cigarettes and eat at McDonald's too. ;-).
In a three-part sandwich panel, skins are bonded to a core (Oreo cookie). The core is generally exposed to shearing forces (top of the Oreo cookie goes left and the bottom of the Oreo cookie goes right), while the skins are exposed to tensile (pulling) and compressive (pushing or crunching) type loads. The shear strength of XPS foam insulation is orders of magnitude less than that of Diab's H80 or Coosa's Nautical 15 (15 PCF) or their Bluewater 26 (26 PCF), which are sort of sandwich panels on their own and pretty darn tough.
If these DIY XPS + fiberglass panels were used to 'build a box', and all seams/joints were filleted with epoxy, so that the panels could be tabbed to each other using fiberglass 'tapes', you would have created a monocoque, two seamless, stressed skins, separated by 2" of core material. If this is done properly, it is unlikely that there will ever be a significant load placed on one portion of the core material in a manner that would cause it to shear or otherwise fail catastrophically. Loads are spread throughout the structure, unless there is a high point load (mounting bracket or puncture).
Notes: XPS is a closed cell foam but it will absorb water over time if continually exposed (pooled water from a leak). The property that makes it resistant to absorbing water also makes it resistant to absorbing epoxy resin, which is required. You cannot use vinly-ester or poly-ester resin on XPS foam.
This builder bought scored sheets/boards and filled each score line with caulking. While only a very small portion of the panel, this adds time and materials and is completely unnecessary. Buy smooth sheets from your favourite box store.
Amines are water soluble and can easily be removed using soap, water and a soft brush or cloth. Peel ply isn't necessary and if the builder had bought a quality peel ply, he would have doubled his 'fabric' budget. IIRC, the e=glass cost slightly less than the quality peel ply he suggested. Non-blushing epoxy resins are available, but are quite pricey.
Multiple thin layers, i.e. 6 ounce (per square yard - I think this is ~200 GSM), will 'wet out' nicer than a thick/heavy, single layer cloth, i.e. 18 ounce.
Neither XPS foam nor a few layers of fiberglass will hold fasteners of any kind. So, over-sized holes must be drilled and 'potted', then tapped, structural inserts must be used or something like a French cleat or aircraft track must be 'screwed and glued' to the skin, allowing loads to be spread out over a larger area.
Please do not put aluminum, steel tubing or wood in the walls to 'reinforce' the structure, mount cabinets or for any other purpose. They will expand and contract at a different rate than the plastics and wood holds moisture. Use Coosa or a structural foam (H200 or H250) if you need something "beefy" in the wall.
