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We're planning to put R60 cellulose insulation and a radiant barrier foil. We will staple those barriers into the roof framing. Will the radiant barrier foil withhold an R60 insulation to prevent it from going to the soffits or we will still need to install PVC baffles?

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  • A drawing of the proposed design, as well as specifying which radiant foil you're using would probably be helpful. Whether the foil will work as a baffle may depend on how thick it is (will it tear when you push the insulation up against it). Of course, installing the baffles (which aren't all that expensive) seems like cheap insurance.
    – FreeMan
    Sep 14, 2023 at 15:32
  • Fair point, unfortunately, we already ordered a thin radiant foil. However, we may order a thicker one or a foam base if it's enough to hold the insulation or if others already tried it and can share how it goes. I also believe that installing the baffles will be safer but wanted to hear others thought first.
    – Josh
    Sep 14, 2023 at 16:19
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    PVC baffles? Most I've seen are Expanded PolyStyrene, or eXtruded PolyStyrene (EPS or XPS) and the (mostly older) cardboard ones. Is this a stright-triangle attic, or do you have a high-heel truss or equivalent framing to provide space for full insulation at the edges? That makes a difference as to whether you're holding back insulation for a couple of feet at the edges, or much more than that.
    – Ecnerwal
    Sep 14, 2023 at 16:21
  • I've seen Styrofoam (XPS) baffles, but would not recommend them. the plastic ones I got (not sure exactly what kind of plastic) were very flexible and needed to be. The XPS ones were more rigid and I can only imagine that they'd have broken many times based on the handling it took to get them into the roof. The thin plastic ones I used just flexed right into place.
    – FreeMan
    Sep 14, 2023 at 16:24
  • We have different attics for different STR properties that we have and planning to improve insulation so I'm not sure yet but I think it is mostly straight-triangle. We only saw PVC baffles from HD but I think we can order cardboard from Amazon.
    – Josh
    Sep 14, 2023 at 16:38

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As was implied by the clarifying questions posed in comments, this depends entirely on what the product you mentioned actually is. Mylar and similar films tear very easily, so if it doesn't have reinforcing strands or a more robust backing it's likely to fail as an insulation support where small tears exist from fastening. I would consider installing the baffles upside down so that the flat panel supports the foil.

That said, once insulation is blown it doesn't move much, and all weight is away from the baffles. Gusty winds can shift the top layer a bit, but eventually it settles in and stays put due to additives and the binding nature of insulation fibers. So you don't need a lot of strength to do the job.

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  • Mylar is so flimsy that it tears from staples under its own flapping about. My main insulation is glass fibre in the attic floor, but I've lined the roof with aluminised mylar (to reflect light when working up there, reduce summer solar gain, and keep a tiny bit of heat in - in roughly that order, with the latter two mainly about making conditions nicer for the contents of the loft). Where it's stapled, I had to reinforce with clear tape to prevent tearing
    – Chris H
    Oct 19, 2023 at 9:22

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