How do you determine how much insulation is needed in the house?

Get to know your local home improvement managers and workers. They love to help. They will know what you need for your area. Since you did not say where you live (state) it is hard to give you an answer here. For our mountainous northwest area, we have to have very high rating insulation even under the floors to protect pipes and floors from losing heat. R26 is not uncommon for us with 6 inch walls. However when we lived in TN we had 4 inch walls an R15 insulation. Be sure the attic and walls have insulation as well.

Check your local building codes, can easily be found on your city or county websites. An easy Google search. If you give us the city, town or county, I can find them for you.

Agree with Lenora! And don't forget inside walls. It really helps with sound proofing and is not that expensive.

Attic or walls?

Insulation is labeled and sold by square footage. Measure walls height and width. multiply the two figures and you get the square footage. Round off any inches to the foot. Attic spaces are done the same way. If you choose you can double the insulation in the attic. I always add about ten percent extra when I buy any building materials in case there's an oops or unforeseeable miss.

Many formulas are on line, but you must also consider local building codes, the summer and winter extremes in temperature, the composition of the outside walls, the type of roof and attic, as well as the type of foundation and flooring.

And there are several kinds of insulation. In my opinion, the best comes rolls of different "values", or ability to keep out extreme temps. Get the best you can afford, at least 5", up to 8". But get advice from at least one local, reputable home builder.

Best wishes 😇