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How much air space to roof deck with only R10 above the top plate?

mikeolder | Posted in General Questions on

The ends of my 2×2 trusses are 3″ tall, so I have that small space to work with.  

How much of a air gap should I leave between the foam and the roof decking?   Any suggestions on how to cut the foam to that shape would be appreciated as well. 


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  1. Expert Member
    Michael Maines | | #1

    The IRC specifies a minimum 1" vent space; though famously based on flimsy evidence, it seems to work and it's the least I would do. If possible, I would add insulation on the interior by furring down the ceiling at the eaves.

    1. mikeolder | | #2

      But that last 1" would be another R6 and I wonder with only a 3" space if utilizing the air space for insulation is a better approach because R14 wont stop melting or ice damns anyway. What insulation board would you recommend given the small space that opens up to 14"? Polyiso?

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