Attic insulation – floor vs underside sheathing
We’re building a new home in zone-5, Colorado Springs area. We have radiant heat and an HRV (distributed with an air handler). Our original plan for attic insulation was on the ‘floor’ of the attic using 2in. closed cell with additional blown fiberglass to bring it to R63. However, on the walk through with the insulation contractor yesterday, he expressed concerns about accessing the hip corners, mainly due to the 5/12 roof pitch. As an alternative, he recommended R38 open cell installed under the roof decking/sheathing. He said he’s been doing a lot of houses that way with good results. He’s had personal experience with poor cellulose installations and settling in his own house so strongly recommends against that approach.
I’ve been frantically re-reading the articles here about using spray foam on the in this location, especially the potential for damp sheathing. (https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing)
Is the additional thickness of spray foam enough to compensate for the decreased R-value? Is an easier installation a better approach than a more difficult, and therefore potentially inadequate, but better option?
It sounds like I should also be asking about closed cell and/or a vapor retarder.
What other options or questions should I be considering?
Thanks for your help,
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