It is important to know and understand what type of insulation is used in the various areas of your home and what the needed insulation R value is. For example when insulating a normal attic flat the best and most cost effective product is blown in cellulose insulation. You can use this on top of the existing attic insulation and it fills in all of the nooks and crannies nicely and effectively.
You can achieve the desired goal of an R-50 with blown in cellulose insulation without spending too mush money as cellulose blown in insulation is relatively inexpensive. Blown in cellulose insulation achieves an R-50 value at approximately 16” in depth which is possible in most attics.
As you get closer to the eaves of the home where the roof line ends, the cavity for insulation gets smaller. Sometimes it makes sense to use an insulation with a higher R-value per inch like 2 part spray foam insulation or a Polyisocyanurate ridged foam commonly referred to as Polyiso. Cellulose yields an R-3.6 per inch 2 part spray foam and poly-iso ridged foam yield an R-7 per inch. This is very important when space is at a premium and you need the most insulation per inch.
Make sure whatever insulation company you hire is well versed in different kinds of spray foam insulation, blown in cellulose insulation and roll insulation and knows the proper application needed throughout your home.