I have an older 1920's house. Short of tearing down all the plaster and starting over again, I'm not sure that there is too much you can do to stop air infiltration. I've done a lot, and it has helped a lot, but the house still "breathes" plenty well as far as I can tell - no lingering cooking odors, no excess humidity, etc. McBride - great list, it's giving me some things to think about. I've done the following on my older home: 1. Installed foam covers on all outside wall outlet and switch covers 2. Insulated outside walls with blown-in fiberglass (all interior walls are plaster with wallpaper over them, so there is not much moisture migration, I believe) 3. Sealed attic penetrations (there weren't many) 4. Insulated the attic hatch with foam board and sealed the edges with EPDM weatherstrip 5. Insulated the bottom edge of the double hung window with EPDM weatherstrip 6. Weatherstripped the sides of the double hung windows with sprung bronze 7. Caulked all of the floor penetrations with fire stop caulk 8. Insulated the above ground basement walls with 2" of spray on foam 9. Caulked the joint between the sill and the top plate with silicone caulk I've probably done a few other things that don't come to mind right now. So, to the point of the original poster, I wouldn't be too concerned about weatherizing an older house - there is not enough you can do to ever tighten it up too much without extreme renovations. For a newer house, I wouldn't be too concerned either unless you knew the builder and all contractors were trained in building a tight house. Otherwise, they will leave lots of opportunities for infiltration either through ignorance or neglect. For commercial buildings - perhaps a different story. In that case, often there are inoperable windows and a tighter building envelope.