How to Install Under Hood Insulation

There is a lot of heat generated from the engine and exhaust which can cause concern of potentially blistering the hood’s paint. Additionally, there should be a benefit of sound reduction in the interior of the car. Also, having a good appearance under the hood was a top priority with the choice of which product to use.

First I’ll give you the great results followed by the installation process. Prior to the install, the engine was ran to operating temperature. I then checked temperatures with one of Titan Tools infrared temp gun with the hood closed. The surface of the hood measured 160 degrees. After the insulation was installed and the engine was operating for the same amount time, the hood’s surface temp dropped to 140 degrees. A nice 20 degree drop.

There was also a noticeable change to interior sound resonance. In addition to that, it helped with closing the hood, which gives a lot less tinny sound.

When driving at 2,500 RPM, at 68 MPH, I was surprised at the lower measured sound level. At highway speeds, there is a lot of undesirable road and wind sound. A one-decibel change, as achieved here, can be a noticeable amount.

My choice was the DEI/Boom Mat Under Hood Sound Deadening Layer, Black, 32 x 59 Inch. Speedway Motors Part # 356050129. This one seems to fit the outcome I was looking for. The insulation is ½” thick, high temp adhesive-backed and the face is a tough black fabric type of material. DEI describes it as a carbon fiber look, I’ll describe it as a fine waffle pattern.

Removal of hood made the installation process much easier than trying to work upside down. The hinge location was marked with masking tape to make it easier to relocate the hood when reinstalling. I like to use studs instead of bolts for ease of aligning the hood to the hinges. A fine point Sharpie was also used to draw around the washers for the nuts on the hinges. Remember to put padding down between the hood and whatever you are laying the hood on. The 4 triangular sections in the hood structure are where the insulation would be applied to.

The inner structure varied slightly but measured ¼” away from the hood’s outer skin. The plan is to cut the insulation a little larger than the triangular opening then tuck it under the inner structure. Tucking it under the edges would hide the cut edges and give the insulation a bit of pillowed and clean-edged look.

To make a pattern to cut the insulation, a piece of paper was held in place on the hood as I pushed my fingernail around the edge to form a crease. Be careful to not cut your finger on the edge of the sheet metal.

The paper pattern was cut about 3/8” larger than the crease. This would give extra insulation material to tuck between the inner structure and the outer hood. Transfer the pattern to the insulation and cut out.

When ready to install remove the back side adhesive backing. I then used the backing and cut about ½” off all the way around and then put back on the adhesive side of the insulation. Since the insulation will get pushed between the hood’s inner structure I want it to slide underneath. That smooth backing will cover up the adhesive and allow it to slide under. The adhesive is very sticky and does not want to reposition easily, if at all. If you are not going to tuck or slide it under any structure, you don’t want to leave an edge of the adhesive covered up.

The piece of insulation then adheres to the bottom side of the hood. The edge will not readily slip between the structure and the hood. The easiest way I found was to fold the insulation back over on itself and push under using the thin edge of a plastic pot scraper from the kitchen. You might find something else that will work as good. Work the insulation under all the way around. That ½” strip of adhesive back stays on and there would be no way to remove anyway.

Here is another use of the DECO Magnetic Angle Finder Gauge tool from Speedway Motors Part # 91089409. I have not installed a hood this way before, but boy did it work great. Hang the hood on the hoist. Match the angle of the hood to the angle of the hood hinge, then push the car underneath and lower the hood.