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Community and Q&A

How thick can you go on roof deck insulation?

GBA Editor | Posted in Green Building Techniques on

Hi again. You may recall that I live in Baltimore, Maryland, and am planning an addition to my home. I live in a brick masonry 2 wythe load bearing house built in 1938. I see this also as an opportunity to retrofit my house, that would keep the house as an integrated system. In have been thinking about doing a “chainsaw retrofit”/PERSIST, except there is no chainsaw because my existing roof has no overhang or soffit, with only gable end window ventilation. After removing the existing roof (which needs replacement) I would wrap the entire house (existing and addition) (i.e., all my roof sheathing, 2-wythe brick walls and masonry – perhaps AAC – addition) with a class I (yes, perm<0.1) vapor retarder (in keeping with Joe Lstiburek’s “Perfect Wall” and recommendations for vapor control in the humid south), to convert the small attic space into a conditioned space, and put exclusively external insulation (I’m thinking mineral wool because of its good water repellence and termite-free and off-gassing free properties) over top, then a rainscreen gap by furring, then the cladding (shingle for existing roof, perhaps metal cool roof for south-facing addition, lath and stucco for walls – with brick slips “lumpy stucco” over the existing house to maintain current appearance). For external roof insulation, how many inches of insulation can you logistically put up there? I see Joe Lstiburek and Betsy Pettit point out that 6″ of polyisocyanurate (faced R 6.5) can get you to R-40, which passes the R-38 for code. I also see that the REMOTE manual uses 6″ of foam insulation. But I’m not thrilled by its gradual loss of R-value and that the pentane that is off-gassed might contribute to smog in my city. But I see that Martin in his blog writes that the PERSIST system has been used up to 8″ of external roof deck foam with a further 2″ thick sleepers. This would mean 10″ below the shingle nailing bed. And I can use R-4.2/inch mineral wool or rigid board fiberglass for 9″ and have the 1″ drainage gap and so pass code with the same 10″. I like to be able to see the underside of my roof sheathing from the attic space, and feel for moisture, as well as to keep mineral fibers as much out of the conditioned space as possible. Mineral wool would also give better vapor permeability compared to foam, to allow drying to the outside. If I found appropriate structural screws to go through the plywood/OSB and then 10” of drainage space and rigid board insulation, could this work? Are there such screws? Is there a limit to the above deck thickness for any other reasons? I would of course pass this by my structural engineer, but I am interested to know the experience in the green building community.

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  1. GBA Editor
    Martin Holladay | | #1

    As far as I know, the only limit to foam thickness is the length of the screws -- and the installers' ability to hit the rafters with the long screws without cursing so much that the neighbors complain.

    If you are dead-set on mineral wool, you might consider adding deep framing on top of your roof, and laying mineral wool batts between the new framing members. If you cross-hatch your framing members -- first rafters, then purlins, then rafters again -- you can go as deep as you want while addressing thermal bridging.

  2. Daniel Ernst | | #2

    You'll find some screws here that could be used for this type of application:

    Check out the compressive strength of mineral wool or rigid fiberglass board insulation before you go much further on your design. Roxul's RockBoard 80 (one of their higher density products) is rated at 2.5 psi under 10% load deformation.®

    Polyisocyanurate and XPS have a much higher compressive strength. Dow's TUFF-R Commercial polyiso is rated at 25 psi. Their XPS boards range from 15 - 100 psi (also rated at 10% deformation):

  3. Richard | | #3

    Martin and Daniel,

    Thanks for your responses. This gives me the lay of the land of what is possible and things to look into. It will prepare me for my discussions with the future builder and structural engineer.


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