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So I was able to insulate the mid part of my roof because I could screw into it but the front and back parts are straight up fiberglass and apparently you can't screw into that.  So I've tried reflectix with liquid nails and caulk but nothing will stick.  I'm thinking of resorting to something more heavy duty but idk what.  Any advice?
Is that a vertical surface you are referring to? Using polyiso?
When I was trying to adhere to fiberglass, I had to use alcohol to thoroughly strip away any wax or whatever it was not letting my industrial strength velcro stick. Mine was on the outside though. Reflectix needs an air gap to work properly although direct adhesion will help reflect heat back inside. Reflectix is not really insulation but a radiant barrier.
Try hotglue, that sticks to everything. I been using it for years to install my rtech foam insulation in my van. It also dries quickly, less then 5 minutes. In 5 years never had any foam insulation come loose. I also use it in my swampcoolers, usually nothing sticks to plastic but hotglue sticks real well. When I take them apart I have to scrape the hotglue off.

This 20 dollar hotglue gun "surebonder dt-360f" cost less then 20 dollars works very well on fragile materials like foam, it has 2 settings low and high, I use high for the foam. Its one of my favorite tools for building things. Is only 60 watts so a small inverter will run it.

This ryobi cordless hotglue gun (30 dollars for tool only) is for more rugged materials, this one melts glue quickly. Too hot for foam. Also you can use the more specialized gluesticks that need a hotter temperature. Very convenient for outside work.
I should add, i think technically you can screw into fiberglass but the nail would go right through the roof. It's also a curved roof on both ends, which makes it more complicated. Thanks for the suggestions so far Smile
3M Spray adhesive #90 works well also.

It's a green can on the shelves at the big box home improvement stores.

Make sure to clean or wipe the fiberglass surface first. 

Spray both surfaces, wait about 3-5 minutes, then press insulation in place. Hold, brace, or clamp items together for a few minutes.

Ambient temps should be warm, above about 65 degrees if possible, and be sure to have ventilation.

Very easy to work with, but will not hold a stiff foam board against a curved surface.

It's not a permanent bond, so the items can be peeled apart later if needed.
Possible to fiberglass in some 1x1 or 1x2 wood in designated spots for anchor points.

This link has some really good info. RV fiberglass work is pretty basic, nothing like boats.
x2 on the 3M adhesive. what ever you use you must properly prep the fiberglass or you are right nothing will stick. I have done Ensolite and 3M adhesive on fiberglass many times. Ensolite will conform to almost any shape. highdesertranger
Use the right solvent and rough up the surface.

Sikaflex and 3M are tops, read the data sheets or call tech support. Especially temperature and humidity conditions.

VHB tape is amazing.

Do **not** use 3M 5200 unless you want it to be permanent, as in centuries

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(11-18-2018, 06:37 PM)John61CT Wrote: [ -> ]...and rough up the surface.

Unless the fiberglass you want to stick to is already rough because it has no gel coat, or it's so old the gel coat is no longer glossy.
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