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Roof insulation – 1/2″ EPS worth it?

wzrdmatt | Posted in General Questions on

Hi Everyone,

I posted earlier about this roof assembly, but a different question this time around. 
California Zone 3 climate zone.

We’re doing a metal roof, with R-38 spray foam on the underside of the roof deck.   Our roofer is recommending 1/2 EPS foam on TOP of the roof.  Why is unclear.  He says it might help smooth out the sheeting…or help with insulation…or perform some magic 🙂

From an energy perspective, is there any material value in laying down 1/2″ of EPS, at an R-Value of ~2?   I guess I could say I get an R-40 roof now?!!  

I asked my manual-j rater to update and see what happens, and he just changed the roof insulation from R-38 to R-40, and it lowered our BTU requirements by ~500BTU.   

$6,800 dollars for the upgrade. Feels like I should spend my money elsewhere. 

Am I missing any other energy savings value? Would the 1/2″ help create any materially beneficial thermal break between the metal roof and the spray foam? 

Thanks for your ideas and advice!


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  1. walta100 | | #1

    Is it to late to change your plans?

    Spray foam is a mistake in new construction! It is the most expensive and least green way to buy r value.

    Consider taking the time to plan an HVAC system that can be located below the ceiling inside the conditioned space and covering the attic floor with cheap fluffy insulation.


  2. Expert Member
    Deleted | | #2


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