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I am having a horrible time trying to glue the polyiso onto the metal. I have wiped it down with alcohol, used M77 &m and m90. Used 1" side walls, really hard to find 1/2", same on roof. then did find 1/2" over ribs. will not glue. In CA not cold. With 1" some stuck but some areas had 1/3" gap. Tried spraying into gap and used wood slats to keep it tight. Removed slats and it came back down right away. I decided not to worry about gaps and tried 1/2"over it. Roof doesn't stick much. Enough that I pulled it right off. Cleaned glue off of ribs, 1" leaves gap so no contact. wiped down insulation and ribs to remove old glue supported over night and part of day, didn't stick at all. Okay, try screwing down with self pierciing sheet metal screws. They don't go through metal. Tried pre-drilling with titanium bit, several  and types of screw heads. Hex heads the best so far but don't want to lay back against. No prob with pan head once drilled so will use them. Finally got a couple through with light pressure or a tremendous amount of pressure. Extremely difficult. I have cut all insulation, wood for walls, used 1/8th" plywood for walls but can't put it on without glue. To prevent mold have sealed panels with spray foam insulation and metal tape to prevent change of temp between metal and insulation and am using Kills to prime wood to kill mold. I am supposed to be at my sons wedding on 10/19 and don't think I will make it. Have been working on this for weeks now. Any suggestion????? OMG! Help!!!!
Pictures please!!! Is the glue sticking to the insulation? Are you spraying both surfaces with the glue? are they flat surfaces? foamies section has a section on how to glue foam to other surfaces that might help. Sometimes glue will not stick well to a slick surfaces unless both surfaces are very lightly sanded, wiped clean and dried.
3M 77 and 3M90 are contact adhesives. They don't work if the two surfaces are not in direct contact with each other. So if you have a gap you need to switch to a different method of installation that does not involve contact adhesives.

For instance: you can apply tape to the surface of the foam and have that tape extend onto the metal surfaces to hold it in place. This will require the least amount of labor of the solutions I am presenting to you.

You can do a tight pressure fit by making the insulating panel very slightly oversized so that when installed they press against the sides of the cavity. This is not overly labor intensive but does require careful measuring. If you cut a piece too small you can glue or tape a scrap of foam to the edge to rebuild the width of the panel.

You can use battens to keep the sheets retained in the cavity. Fastening the battens to the ribs of the structure. Then when you apply your interior wall sheathing it can be attached to those battens. Labor intensive but it is a solution many people prefer.

There are some types of thick adhesive caulks that will do a quick grab. Just remember that caulk needs a way to dry and the metal of a van is not absorbent of moisture and the foam is also not absorbent. So you can't put a wide area of caulking on those surfaces and expect it to dry all the way through. On a wide area of caulk the edges dry and that prevents the inside area from curing which means not a lot of strength to the bond. So your lines of caulking should be an inch or so apart and the gap the caulk has to span should not be any larger than what is specified on the tube of caulking. One way around this is to apply wood battens to the surface of the metal. The thickness of the battens can be adjusted to provide a more level area of contact for the foam sheet. It is OK if there is a gap between the battens, you can level that up with some of the thin foam used for placing on sill plates when framing up a house or you can just leave that space to have a void. Then when you apply your sheet of foam you only apply adhesive to the surface of the battens and to the foam where it touches the battens.
Thank you so much for the options. I think maybe the tape might be a good option. I understand they have to have contact for the glue to stick and some does and in other areas there is a gap so the only thing it can stick to is the ribs. I did try the caulking type of glue but maybe didn't use enough. The polyiso does have an aluminium moisture barrier on both sides so maybe it is too slick but I can't scuff it up. I do have the adhesive on both sides and waited for it to get "tacky" before sticking it. I have spent so much money trying to find something to work. Maybe the tape would be affordable and easy to apply. I am not sure exactly what type of tape to use but I can probably find out. thank you so much!
How large are the sections you are trying to glue? if they are too big, the weight of the foam might not let the glue set unless you clamp it and let it set.

If you cut into smaller sections it will be lighter and it should hold without clamping.

If you just want to do it quickly get a hotglue gun and cut in small sections and install. Its all I use in my van. Hotglue drys in minutes and its a permanent job. On the roof I have 6 layers of 1/2 inch foam, its been installed going on 6 years, never had any come loose no matter how hot it gets. Hotglue sticks extremely well to foam and metal. You can get a hotglue gun for 20 dollars and use the generic gluesticks.

Picture of my roof, notice how the sections are small, reason for that is hotglue drys quickly, if the sections are too big, some areas might dry before you apply it all.
Yes with foil on both sides do not try to scuff it up! You are basically trying to glue metal to metal.
I  have no idea why its being done that way.  We used cork rolls for flooring.
Cheap, sound proof, water proof and mold proof.
You would need the 1/2" thick to get any substantial Rvalue out of that material.
"Maybe the tape would be affordable and easy to apply. I am not sure exactly what type of tape to use"??
I used adhesive that did not work but the tape held in place and I did use a few battens to put up the plywood sheets. Be careful with your screw length so you don't go all the way through.  The tape I used was the aluminum tape used with reflectix about 3 dollars a roll at Home Depot.    -crofter
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