Originally Posted by cechardy
I started on this project today, and it turns out, there are no support "rafters" in this unit? The ceiling consists of only styrofoam glued to the fibreglass and panels glued to that. This should make this job a lot easier than anticipated!
Sent from my iPad using Fiberglass RV
Those layers of fiberglass with foam adhered all across the surface followed by another layer of panels adhered all across the surface form what is known in the industry as a "SIP" which stands for structural insulated panel. SIPs are self supporting panels (no roof beams needed) when engineered correctly for the loads. But they can be delaminated when overloaded. That may happen if for instance a person put most of their body weight
up there to work on an AC unit. Or the AC used was too heavy, very heavy snow loads, combination of such things, etc.
Rebuilding a SIP can be tricky but can be done. You need to have as close to 100% percent coverage of glue over the surface as possible to adhere materials together. Putting down rows of adhesive from a caulk gun is not sufficient for rebuilding a SIP unless those rows are spaced such that when compressed the glue spreads into that 100% coverage. Then you need to put well distributed, even compression all across the surface, done from from underneath, against the materials until the adhesive has been well cured. That means lots of spring poles that apply firm pressure all across the panels. In the factory the SIP was created while the roof was upside down and they could put weight
on the SIP layers while the adhesive cured. The 100% coverage of adhesive is what gives SIPs and also stress skin panels their ability to support loads because it turns the light weight
, lower strength materials into a single, thick structural unit of material.
You need to use the right adhesive for the job. You do not have absorbent materials against each other when you are dealing with foam and fiberglass. So there is no way for VOCs or water to evaporate or in the case of moisture curing glues (poyurethanes)no way for moisture to arrive from the environment. Although you can use polyurethane when gluing ridged foam to wood as wood provides its own supply of moisture if you very slightly dampen the wood surface that will kick of the polyurethane curing. But there is still some risk that the adhesive did not get enough moisture to cure and will remain soft and then the panels will delaminate.
Contact cements might be the answer or a slow curing epoxy (does not generate a lot of heat which would melt the foam). But a water based contact cement won't work unless one of the surfaces you are adhering is absorbent.