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I have panels made with foam like the one in the picture, but compressed and glued together to be quite stable. foam chippings

The idea is to glue it to the lower side of the hood of my car to reduce the noise coming out of it. Some more will be applied between engine bay and feet area, to reduce the transmission on that path too.

How to glue that kind of panels to the painted metal? Is silicon ok? (the one applied with a gun) I know there are various types. I have a tube barely used for my bathroom, and it will dry out if not used, it would a good way to use it.

Or is Bostik glue (yellow, sticky, elastic also when dry) suitable? I would not use single points but spread a sort of spiral of glue on the panel and then keep it pressed for a while on the hood.

Should I add also some additional structure (thin wooden frame) to retain the panel for increased safety? The panel is quite light per unit of area, so the vertical force is not relevant, but to be sure...

What if I apply to the hood some bitumen/aluminium foil strips to dampen vibrations too? How to stick the dense foam panel to aluminium (or whatever it is) foil?

I also thought about using pins sticking through the panel (the part sticking out bent appropriately) and having on the other side a small plate that I can glue to the hood. https://www.gamma.nl/assortiment/isolatie-bevestigingspennen-zelfklevend-zink-10-stuks/p/B194520

Concerning safety and heat resistance: in the appendices of this study (on 3x as powerful cars as mine) it is shown that the hood never reaches 100°C unless idling for several minutes and at higher RPM than minimum. I will test the foam in the oven to be sure. Also, my mechanic suggested me a similar solution. In any case, if this foam appears not to be suitable, I can use sound absorbing foam, with a better fire safety rating. The question above still applies.

Concerning space: this is a 20+ old car, there is enough space below the hood and also the panel is not thicker than the metal bars already attached to the hood to strengthen it against flexing and torsion. cofano

migrated from diy.stackexchange.com Nov 13 '15 at 17:03

This question came from our site for contractors and serious DIYers.

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    That material doesn't look like it would hold up to temperatures under the hood. Is it OK with temperatures up to 300F, and is it oil resistant? Is it flammable? Sounds like a bad idea unless it has really been tested. Also, how thick is it? Many modern cars don't have any room to glue panels to the hood without hitting the engine. – JPhi1618 Nov 13 '15 at 15:57
  • I will test all of these aspects, but such temperatures are not reached in my car except close to the exhaust and given how clean my current hood is after so many years, I think nothing except road dust will touch it. And not even a lot of it. – FarO Nov 13 '15 at 17:45
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    Sounds like you're willing to make sure its a safe product, so that's great. Also keep in mind that the material safety needs to be tested based on what could happen rather than what normally happens. Look at the mats that some cars come with to get better ideas on how it's made and attached. – JPhi1618 Nov 13 '15 at 19:24
  • I also have sound absorbing foam for studio (the weavy one) that is also tested for automotive environment like engine compartment. It is quite less dense than the one I initially meant, but the question stays the same... – FarO Nov 13 '15 at 23:00
  • Are you really trying to reduce the sound reaching you in the cabin? If so, maybe we should address that problem instead. It's related but the options are much safer. – Bob Cross Nov 14 '15 at 4:23
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It appears the material you are describing would be considered and usable for carpet padding (the stuff which goes under carpeting). There is a plethora of reasons for not using something like this in the manner you've described.

  • Heat, of course is one of the biggest issues. The stuff you've shown will not handle the underhood temps which occur. I don't know where you got the information that shows underhood temps will not exceed 100°C, but can tell you engines normally run in excess of this (normally up to 116°C). The area under the bonnet will quite easily exceed 100°C on a regular basis.
  • If the glue should fail (more than likely, a thin strip which is stuck to the glue will hold, but the rest will separate fall off), this material will fall down onto the exhaust manifolds and catch fire ... poof! There goes the vehicle and whatever investment you have in it.
  • Does the term "Ugly as sin" mean anything? Enough said ...

My suggestion to you is to purchase the materials which are designed to do exactly what you want. One such product is called Dynamat (there are others available, you just need to look). Dynamat (or the like) will provide the heat shielding and sound deadening you are looking for without the other worries (ugly/fire). When installed correctly, it will not lose adhesion and fall off. Plus, there is a bunch of different designs to do it exactly like you want it to.

  • The material I described is meant for walls. Quite close to what you expected. And about the temperature, I put the link to a study where the temperature was actually measured. – FarO Nov 13 '15 at 23:04
  • I also have sound absorbing foam meant for recording studios AND engine compartments (certified for it). I may use that one (monolithic) if the other one is not suitable. Especially thanks to your second point, I didn't think about the foam separating in pieces (as it was before compacting and glueing). That is quite possible. Provided that the foam is suitable for higher temperatures (and I will check), I could hold it along the whole surface using spray glue. Or switch to the other monolithic foam, that is however lighter (less dense) and therefore less able to dampen sound. – FarO Nov 13 '15 at 23:06
  • Dynamat (well, the similar bitumen band with alu foil you can buy for roofs) was already planned to be installed on the hood before applying the foam, as I wrote in my question. Bitumen band/Dynamat = vibration dampening due to increase in mass, foam = sound absorption. Different things. – FarO Nov 13 '15 at 23:08
  • Wait a moment: the hood easily getting to 100°C? I never heard of a hood so hot that a drop of water on it boils. Not in Europe. And not with 45 HP under the hood. – FarO Nov 13 '15 at 23:22
  • @OlafM - Just because the hood doesn't get to that temp doesn't mean you cannot get to that temp under neath of it. Also, I'm not saying it would happen all the time, I'm saying it would happen readily. Also, you need to recheck as one of Dynamat's properties is sound deadening. – Pᴀᴜʟsᴛᴇʀ2 Nov 14 '15 at 1:49
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If you are sure it is a suitable material for the job, then an impact adhesive should do a good job, for example - http://www.bostik.co.uk/diy/product/evo-stik/Impact/6

However, the Datasheet for this glue says it's serviceable temperature range is up to 55 degrees C. While the hood shouldn't get above this temperature from the engine, especially when protected by the insulation, you may live somewhere where the direct sunlight may raise the panel's temperature quite high.

  • Heh, In Texas, I can't imaging any part of my car being under 55C, but I guess things are a lot better in Europe. – JPhi1618 Nov 13 '15 at 19:30
  • @JPhi1618 I'm jealous :) – HandyHowie Nov 13 '15 at 21:27
  • I will check in the local store which kind of spray glue they have. I never considered them, but in fact they would allow me to cover the whole surface of the foam (whatever foam I decide to use) and avoid the issue with the point n. 2 of the answer by Paulster2. I can always add a frame to retain the foam... – FarO Nov 13 '15 at 23:10
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Given the recommendations from other answers and the need for a heat resistant glue, capable of holding at least 100°C, spray glues are not suitable, at least the ones I found in the biggest store around here. In general they hold 60-80°C.

Liquid glues (that hold foam and paper extremely well) also are not certified for temperatures above 80°C, usually. At least not the ones available. Furthermore, not all of them are suitable for metallic surfaces.

Silicon glues can hold to 80°C and some even up to 120°C. Among the ones suitable for metals and resistant to both water and UV (and rated for outdoor use) I found the glue "100%" (http://www.pattex.nl/dhz/producten/100-procent.html) suitable up to 110°C. It is not officially suggested for PE, PP and PTFE but since I don't know the composition of the sound absorbing foam, I tried it.

As a matter of fact, it penetrates enough in the foam and bonds so strongly to the bitumen tape (http://www.bostikdhz.nl/uploads/Products/product_9/TI_BitBand_Zelfklevend.pdf) that trying to separate the two materials, after about 18 hours, produces strnog tearing of the foam.

All the considerations about usefulness of the foam to reduce noise from the engine compartment (see comments) and about safety are still valid, but if a glue is needed to fix foam to metal, the silicone one could be a solution.

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