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Under Bonnet Insulation: Why Every Car Needs It for Performance and Protection
Engine noise gets into the cabin two ways. It comes through the firewall, and it comes up and over the bonnet. Most people treat the firewall and forget that the bonnet is a large, flat, unlined steel panel sitting directly above the loudest thing in the car.
Under bonnet insulation fixes both halves of that at once. It cuts the noise reflecting off the underside of the bonnet, and it stops engine bay heat baking the paint above it.
What under bonnet insulation does
Two jobs in one product.
Noise. A bare bonnet reflects engine noise back down and out. A lined bonnet absorbs part of it instead of bouncing it around the engine bay. Diesel engines benefit most, because the clatter they produce is exactly the kind of noise a bare panel reflects well.
Heat. Engine bay heat rises and sits against the underside of the bonnet. Over years that heat works on the paint from the inside. A lined bonnet keeps the worst of it off the panel.
Most factory cars left the showroom with some form of lining. Plenty have had it removed, or it has crumbled and been thrown out. If you can see bare painted metal when you open the bonnet, there is nothing there.
Under bonnet lining, pads and felt
Kool Wrap supplies three under bonnet options.
| Product | What it is | Best for |
|---|---|---|
| 10mm adhesive foam pad | Self-adhesive foam, trimmed to fit | A standard replacement for missing factory under bonnet lining |
| 12mm adhesive foam pad | The thicker version of the same thing | Noisier engines, and bonnets with more clearance |
| Premium compressed felt | A dense felt, self-adhesive, trimmed to fit | The quietest option, and the closest to a factory lining |
All three are supplied as sheet and trimmed to your bonnet. Browse the range on the under bonnet insulation page.
Where under bonnet insulation goes
On the underside of the bonnet, between the structural ribs.
- Open the bonnet and look at the underside. There is a stamped frame with flat bays between the ribs.
- Measure the bays. Make a paper or cardboard template if the shapes are awkward.
- Clean each bay with a degreaser. Old lining leaves adhesive residue and that has to come off.
- Cut the sheet to your templates.
- Peel the backing and press each piece into its bay, from the centre outward.
- Keep clear of the bonnet latch, the strut mounts, the washer lines and anything that moves.
Do not cover the whole underside in one piece. It will not follow the ribs and it will lift at the edges.
Under bonnet sound deadening is only half the engine noise job
The bonnet handles the noise going up. The firewall handles the noise coming at you.
The firewall is the panel between the engine bay and your feet, and it is the shortest path from the engine to your ears. Treat the cabin side of it with sound deadening sheets, on clean metal, under the carpet. Our guide to car sound deadener explained covers how the material works and where else it goes.
Do both and the change is bigger than either one alone. Do one and you have halved the problem.
Diesel engine soundproofing
Diesels are the strongest case for this work, in a light commercial, a 4WD or a ute. The noise is sharper and more percussive than a petrol engine’s, it sits in a frequency range that carries well through panels, and it is present at idle rather than only under load.
Under bonnet insulation, firewall deadening and floor deadening in combination are what makes a difference in a diesel. There is no single product that does it, and anybody selling one answer for diesel clatter is overselling.
If your 4WD or ute is also a touring vehicle, the 4WD, ute and canopy guide covers the rest of the build.
Under bonnet heat is a separate question
If your problem is heat rather than noise, be specific about where it is coming from.
Heat radiating off a turbo or a manifold is a shielding job, and it is answered by a turbo blanket or exhaust wrap at the source rather than by lining the bonnet. Heat baking a component in the engine bay wants a heat shield between the two. Under bonnet insulation protects the bonnet itself, and that is what it is for.
Never stick an adhesive foam or felt product onto an exhaust pipe, a manifold or a turbo housing.
Under bonnet insulation questions
Will under bonnet insulation make my car quieter inside?
It reduces engine noise reflecting off the bonnet, which is part of what you hear. The firewall and floor carry the rest. Treat all three for a real change.
Does under bonnet insulation cause overheating?
No. It sits against the bonnet, not against the engine, and it does not restrict airflow through the bay. Keep it clear of the latch, the struts and any vents in the panel.
What thickness under bonnet insulation should I use?
10mm suits most cars and matches what the factory usually fitted. 12mm or the compressed felt suits a louder engine, and both need enough clearance that the bonnet closes without pressing on anything below.
Can I fit under bonnet insulation myself?
Yes. It is self-adhesive, it trims with scissors or a sharp blade, and it is one of the few jobs on a car you can do standing up with the bonnet open.
Get the right under bonnet option
Tell us your vehicle and whether you are chasing noise, heat or both. Call 1300 44 33 34 or email sales@koolwrap.com.au. Kool Wrap has supplied heat and sound protection in Australia since 1995 and ships Australia-wide with tracking sent after dispatch.
