Problems with sandwich construction - need advice

[okedoketear]

Ok, so I decided to go with a sandwich construction for this build and now I have some potential issues that I need opinions/advice on.

1. I sandwiched a 1/4" lauan, 3/4" Dow Styrofoam Rigid, 1/4" lauan. I used PL POLYURETHANE construction adhesive as my "glue" between the plywood and foam board. After a day I read the fine print on the adhesive tube and it stated that you should NEVER use POLYURETHANE adhesive with any POLYSTYRENE products. I called both DOW and Henkel, the maker of PL and they told me that the adhesive can burn through and destroy the foam. I however have not seen any breaking down of the foam and it appears to be strong. Does anyone have any thoughts about this?

2. When I cut out my profile I noticed a few areas that did not bond well during gluing/clamping. I reglued those areas and clamped then. My question is this: How critical in the sandwich construction method is it to have 100% glued coverage? Do small areas or gaps in the adhesion present MAJOR problems structurally? Will any gap cause an overall failure in the sidewall? Anyone have any thoughts on this?

Please help. I hate to throw away 2 sidewalls but I also do not want a potentially dangerous platform. Thanks.

[starleen2]

The answer to your question depends on two scenarios
1. The two luan panels are joined to a submember that completely surrounds the perimeter of the foam. The submember can be either wood or metal as long as it is the same thickness of the foam. A mechanical bond to this submember will prevent the two pieces of Luan from separating from the foam
2. Trying to get a perfect bond between foam and luan is possible – However if nothing else exist to strengthen the bond, then a possibility exist that the three layers separate and thus the application of adhesive is VERY critical. When done in a factory setting, the foam is injected and makes a chemical bond to the ply – but since you are trying to duplicate the process with foam that is in its solid (post injection state), the application of adhesive is vital – unless some other way of joining the outer halves together is used (as per example 1)

opinions may vary
it would be helpful if you could post a picture

[Steve_Cox]

Did you leave the clear film on the styrofoam?

Oh yeah, welcome to the forum and all that stuff.

[okedoketear]

No, I completely removed the protective clear film.

[asianflava]

I used epoxy thickened with micro fibers. I started to do 100% coverage but found that it was using more epoxy than I had. After the initial tries, I just concentrated on the stringers and where there was wood-wood contact.

[Gage]

No, I completely removed the protective clear film.

Oh well. Now rebuild your sides with framing. Wedge the foam in between the framing and glue the ply to the framing. Besides, how were you going to finish off your doors and windows with just a sandwich wall and no framing.

Stick a Styrofoam cup in a pan of gas and see what happens. It's the same principle. That's what the experts were trying to tell you when you called them. Sometimes you have to accept a different answer than what your looking for.

I'll probably catch hell with this post, oh well.

[asianflava]

Besides, how were you going to finish off your doors and windows with just a sandwich wall and no framing.

I didn't see anything about not having any framing. Then again, I didn't see anything about having framing either, so you could be right.

[okedoketear]

There is framing within the sandwiched wall. I DO understand the concept of foam and solvent. My question was did anyone else have any negative experiences using a polyurethane based adhesive with polystrene foam? I have seen no evidence of burn through but wanted to be on the safe side.

[starleen2]

There is framing within the sandwiched wall. I DO understand the concept of foam and solvent. My question was did anyone else have any negative experiences using a polyurethane based adhesive with polystrene foam? I have seen no evidence of burn through but wanted to be on the safe side.

If you have internal framing - then a few voids where the adhesive did not attach the foam well will not make for much difference. I just cut the foam and wedged it in between the framing strips - sealed the joints with alum. duct tape (not the silver fabric type but the actual foil type) If the adhesive didn’t eat through it right away then you’re good to go

[S. Heisley]

I have a small piece of uncovered Styrofoam insulation that I've been testing products on. I happened to test PL construction glue on it about a week ago. If you're worried about degradation, it has not degraded the Styrofoam yet and doesn't look like it's going to. However, the glue doesn't stick very well. These good fellows are right. You need the wood framing.

Regards,
Sharon

[Gage]

There is framing within the sandwiched wall. I DO understand the concept of foam and solvent. My question was did anyone else have any negative experiences using a polyurethane based adhesive with polystrene foam? I have seen no evidence of burn through but wanted to be on the safe side.

Well, when I built my teardrop, I did a test piece. No, it didn't burn through but it did deform the foam enough that it didn't make good contact with the ply. Therefore I decided to just wedge the foam between the framming and apply glue only on wood to wood. Oh and by the way. My roof is 1/8 birch ply, 3/4 foam and another 1/8 birch ply with 1"x2"s laid flat and spaced at about 12". It's strong enough for a 200 lb+ person to stand on. The teardrop is also 5' wide.

[Mike Angeles]

[Gage]

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[brian_bp]

...When done in a factory setting, the foam is injected and makes a chemical bond to the ply ...

Yes, structural insulated panels are built this way, with foams such as polyurethane or polyisocyanurate; Bigfoot does this for their Fibrecore Wallsystem. On the other hand, I believe that this is rarely done in recreational vehicles with structural sandwich walls, which normally use polystyrene boards (and varying types and quanties of framing) glued to inner and outer wood panels. Those built-up walls are either clamped (in enormous presses) or run through pinch rollers to make contact, and I believe that the emphasis for bonding is on the framing members.

[cuyeda]

I will have to check when I get home, but isn't there adhesive for foam insulation. I believe at HD and Lowes, it is normally stocked near the foam insulation panels. I have a picture of the tube foam adhesive at home.