On 2008-02-06, Edwin Pawlowski wrote:

Insulation depends on air to do the work and compressing thicker
batts will reduce the amount of insulating value.

For a given thickness cavity, denser insulation will have a higher
R-value. For a 3.5" cavity, you can get low density (R-11), medium
density (R-13) and high density (R-15) fiberglass batts. So if you
took a medium density 5.5" batt (R-21) and stuffed it in a 3.5"
cavity, you would lose R-value relative to the R-21, but you'd still
be higher than R-13.

Cheers, Wayne