DanTwoLakes comments bring up an important question. Are you trying to reduce sound, or provide heat insulation, or both?
- If you are trying to reduce heat you need good complete coverage of the panel. For example you need complete coverage of the firewall to prevent engine heat from transferring to the passenger compartment.
- If you are trying to only reduce noise, you don't need full coverage to reduce the panel resonance. A few well-placed sections of insulation will significantly reduce the panel's ability to amplify or transmit noise. That is why many products are sold in relatively small sheets. These may work very well to reduce noise for door panels or trunks.
Closed cell foam does not absorb water, and that is an advantage for some applications. However, I'm not sure that affects its noise absorption or insulation capability.