Re: Insulating from inside - flat roof dilemma
In my understanding the reason that a Poly based foam has a superior insulating property is the poly has very good resistance to thermal transfer. In the case of closed cell foam ..... air pockets are encased with the poly.
Air in a stagnant environment has good insulating properties and combined with the poly provides a superior insulation material.
Unlike fiberglass batts where moving air diminishes the insulation performance as much as 40%. However .... in a static air environment fiberglass can be adequate.
In this case if there is a budget allowed .... you might have closed cell foam sprayed along the joist ends and about 2 inches thick along the underside of the roof deck to seal air infiltration and provide a very good insulation barrier. Then conventional fiberglass or rock wool batts can be installed for added R value.
If the budget doesn't allow spray foam .... then using 2 inch thick rigid foam can be used instead along the same areas. With this it's important to provide a tight air seal. One thing that can be done is use spray foam in a can to seal along all the edges of the foam sheets or a good caulking.
2 cents worth.
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