I've been doing some reading on this site and others and the consensus is that doing a Manual J load calc is the best thing to do to properly size the equipment.

What information do I need to give to a contractor so he can do a load calc?

I'm just in the initial stages of talking to contractors, and had the first one out last night. I asked how he figured out how big of a system I need and he said he went off of existing and without me asking (he probably knew where I Was going with it) he mentioned that an engineered load analysis could be done, but we would need to give him R numbers of what's in the attic, our windows, our roof, etc. Are these things that can be measured, or would I need to find specs? Is this something I need to do or the contractors should be able to do?

Any help would greatly be appreciated.