Adding CI to old house in MD – GreenBuildingAdvisor

“Old” fiberglass insulation in a 2×4 wall is probably R11, which was the standard R value for that type of insulation in that depth wall cavity for a very long time. Maryland looks to be entirely CZ4 from the IBC map I have, which would need R2.5 as a minimum based on Martin’s classic article, or R5 to meet code. You’d need to go with R5 here for code reasons, which is also plenty to satisfy the rules in Martin’s article about insulation ratios too. 1″ Foamular (XPS) is rated R5, so you should be good to go here.

Note that polyiso is a greener option than XPS, and can often be found cheaper if you have an insulation reclaimer in your area. 1″ polyiso will get you R6.

Bill