Cutting holes in a sandwich wall?

[mreidsma]

I've been adding 3/4" foam to the inside of my build this week, but I've been leaving certain areas bare since I still need to cut some holes for things, like the shore power connector and some soffit vents. But tonight I started wondering: would it be easier to foam everything, then add my internal wall, and then cut out my holes (assuming I'm using a hole saw for round holes)? I started thinking about lining up cuts between the inside and outside walls and thought it might be easier to do it all at once. But, I don't want to destroy the foam inside the wall once it's all sealed up.

Outside is currently 1/4" OSB, inside will be 1/8" hardboard. Foam is 3/4" XPS (white pebble kind). Plan is to later add wood siding or canvas/glue/paint to the outside, but I feel more comfortable working around existing holes that way for some reason.

-M

[Woodbutcher]

I would put the walls together completely first. Most hole saws have a centering bit in them. Drill from the outside partway in. The centering bit will have poked through the other side. Then go inside and using the centering hole drill back toward the outside and you are done. Nice clean hole.

[KCStudly]

What Woodbutcher said +1.

I will add, the technique I used was to have blocking where ever a hole would be, since there would invariably need to be fasteners around what ever would be going in the hole any way. Even for a window that uses a clamp ring, you don't want the foam around the opening to crush.

If you have a router, another good technique is to run the hole saw thru the blocking before gluing it to the skin, foam around the block, and run the router around the inside with a flush trim bit to match the hole. That's how I did the small vent fan opening in my bulkhead.

[mreidsma]

Thanks, Woodbutcher and KC! For the shore power connector I'll have blocking, but the soffit vents are slip in, sealed most likely with silicone or butyl tape, so I think Woodbutcher's technique will work perfectly. For the holes near blocking, I'll make sure to mark where the centering hole needs to be on one side before I cover up the inside.

Thanks again!

-Matthew

[RAYVILLIAN]

Just be sure there ain't nothing on the inside. I didn't and chewed a hole in the top of a brand new battery. Lessons learned, take the battery out next time and hot glue is the only think I found that will stick to a battery. After patching the battery still stood up for 5 years.

Gary

PS If you can't laugh at yourself with others don't do stupid stuff.

[mreidsma]

Ray,

Good advice! Most of the areas I'll be adding holes don't have much in the walls except insulation, but your story made me draw the locations of all the wiring and other hidden goodies on the outside just to be safe!

-Matthew

[martymcfly]

I used JB weld to glue a optima battery back together after I dropped it and broke a corner off.