What are those lumpy under bonnet carpets for?


The insulation mat under the bonnet can be removed without affecting the vehicle’s operation.

Photo credit: Shutterstock

What you need to know:

  • These mats are soft and bulky because their primary role is sound-proofing and heat-proofing. 
  • Removing these mats would make the car noisier in the street and hotter in the passenger compartment.

Why do modern cars have “carpets” of sponge and felt matting under their bonnet lids and over the engine compartment bulkhead?

They trap dirt and water and often get very tatty. Are they not a fire hazard? Would it be a mistake to remove them altogether, leaving just painted metal?

Ruben T.

These mats are soft and bulky because their primary role is sound-proofing and heat-proofing. Insulation. They can be removed without affecting the vehicle’s operation, but the car would then be noisier in the street and both noisier and hotter in the passenger compartment.

It is surprising that manufacturers do not use more robust materials; presumably it is a cost thing. And in markets with less severe motoring conditions than Kenya, the traditional matting doesn’t have so much dirt to absorb, and not so much vibration to make the mats peel away and tear and fray prematurely.

The insulating qualities of the mats are positive – you would almost certainly notice the difference if they were not there.

While hoping for smarter technology/economics to arrive, users should be gentle when cleaning the mats (no high-pressure washers; thorough but gentle rinsing) and promptly attend to peeling or fraying with suitable tapes and adhesives.

Insulation mats

If you use a vacuum cleaner to get the dust-out, use it on low power. Many have clips to help keep them in place, but these are often not repositioned correctly when mechanics need to get behind the mats for service work. It is worth regularly checking that those are in place and gripping correctly. Beading, held on by self-tapping screws (not pop rivets) for easy removal, can also help.

Insulation mats under the bonnet lid need glue that is flexible but much stronger than for sticking objects to floors and walls in stationary places (like a building). The underside of the bonnet is a “ceiling” and the straight pull of gravity (over time) is unrelenting.

I once owned an old Series II Land-Rover (don’t tell anyone) with a body that rattled like the inside of a kettle drum. So I carpeted (with carpet!) the entire inside of the passenger compartment. Easy enough to do in a shape like a biscuit tin, and it was the roof lining, not the walls, that occasionally needed re-sticking and additional holder-uppers.

But then, it wouldn’t be an old Land-Rover if things didn’t keep falling off. I love ‘em dearly, but eccentric affection is not the same thing as admiration, which the car does not always deserve but its owners often do for their patience and courage.