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  1. #1
    Join Date
    Jun 2004
    Location
    South Norwalk CT
    Posts
    438

    Default Venting rafter bays with Benjamin Obdykes Home Slicker?

    We have been asked to insulate the roof of a house built in 1787. The existing rafters are approximately 5” round with one side flattened and on 38” to 42” centers. The roof assembly is old skip sheathing covered with plywood and asphalt shingle roof. There is a ridge vent.

    We are planning to sister 2x6s to the existing rafters and add another one in between so we’ll have something like 19-21” centers. We then plan to press fit R-23 Roxul into the bays and then sheath across the rafters with 2” R-13 foil faced fire rated TSX-8500 foam board for a total of R-36.

    We want to vent the roof and I’m leery that Styrofoam baffles would collapse when we press in the Roxul. We could put 1x furring strips along the rafters and span that with Luan plywood but that seems very labor intensive so I was consider stapling Benjamin Obdykes 10mm (.44) thick Home Slicker in to the bays before we press in the Roxul. It would be fast to install and could be cut to fit the odd sized rafter bays.
    Anyone ever done something like this or if not any opinions on whether it would provide enough ventilation?

    Thanks.-Jud

  2. #2
    Join Date
    Jun 2004
    Location
    Martinez, California
    Posts
    14,684

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    Jud:

    Here is a picture of plastic sim cath 2.JPG baffles in a home our member Sim built, note how he is conducting air under the steel, the insulation is rock wool.
    “‎A general State education is a mere contrivance for moulding people to be exactly like one another; and as the mould in which it casts them is that which pleases the dominant power in the government, whether this be a monarch, an aristocracy, or a majority of the existing generation; in proportion as it is efficient and successful, it establishes a despotism over the mind, leading by a natural tendency to one over the body.” ― John Stuart Mill

  3. #3
    Join Date
    Nov 2007
    Location
    Dallas,PA
    Posts
    1,014

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    Jud, I am assuming the house is in CT. Why not add the additional framing as described and spray the back of the deck with closed cell foam?
    "ALS IK KAN" - Stickley

  4. #4
    Join Date
    Nov 2007
    Location
    Dallas,PA
    Posts
    1,014

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    Quote Originally Posted by Dick Seibert View Post
    Jud:

    Here is a picture of plastic Attachment 29487 baffles in a home our member Sim built, note how he is conducting air under the steel, the insulation is rock wool.
    Dick, the insulation in this picture is Knauf fiberglass not rock wool. The baffles used there seem to be placed in a manor that will minimize the insulation that can be applied. May be ok in CA but not too good in the NE. These baffles always intrigued me since they do provide an air channel between the insulation and the roof deck but they are not permeable and in a cold climate would constitute a condensing surface for moisture escaping from the interior of the structure.
    "ALS IK KAN" - Stickley

  5. #5
    Join Date
    Nov 2004
    Location
    Boston, MA
    Posts
    4,612

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    I would use Accuvent by Brentwood.

    http://www.brentwoodindustries.com/p...f-ventilation/

    There are others with a center ridge, we used to denspack against them.
    I will never buy a Nissan Cargo Van.

  6. #6
    Join Date
    Nov 2009
    Posts
    537

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    I use the same baffles as Sim. At the plate I install ¼" fanfold to seal off the bay. The baffle is installed about 1-½" past the fanfold.

    This is an existing home, the HAP is 5", the baffle drops 1-½" so I get 3.5" above the top plate.

    I'll have to see if I can get the Accuvent locally (or online if I plan ahead), it would save some time. Thanks for the link Ted.

    Tom
    Attached Images Attached Images

  7. #7
    Join Date
    Dec 2005
    Location
    Michigan
    Posts
    7,261

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    What is the purpose of the additional rafters?

    If it's just to facilitate the addition of insulation, it seems like a lot of work for very little/no gain. Just spray closed cell between/over the existing rafters. Or insulate at the attic floor.

  8. #8
    Join Date
    Jul 2008
    Location
    Suburbia (Washington, DC area)
    Posts
    1,932

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    Joe Lstiburek has written a couple of times that 1" vent space is either too small or possibly just barely enough, and 2" is much better for effective ventilation.
    I would think .44", partially occluded by the mesh, would not perform that well unless the rafter bays are quite short.
    Additionally it seems to me that's not very much insulation.
    Would it make sense to put the 2" foam board between the rafters with 1.5" space above, seal well to the rafters, install new framing 11" below (at any spacing you want, you could probably even just use 2x4s), then net and blow with a good layer of cellulose? You'd be at code R-49, the vent space would be substantial, and the foam would be on the "correct" side of the assembly. Actually if you have good venting either side is OK I suppose.
    Or possibly stitch netting on the existing rafters leaving a 2"ish space (which will be less in the middle after dense-packing) then install rafters, netting, blow cellulose in there?

  9. #9
    Join Date
    Jun 2004
    Location
    South Norwalk CT
    Posts
    438

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    Ted, the Accuvent looks promising, if you have dense packed against those then I’m sure we could press fit the Roxul against them without them collapsing.

    Concerning spray foam. Owner doesn’t want it because this is a historic home so any modifications or improvements need to be “Reversible” or easily taken apart in case someone ever decides to put the home back to its “original condition”.

    Concerning the attic Floor. Going to be putting HVAC duct work in the attic so we don’t want to insulate the floor since that would put the mechanicals outside the envelope.

    Doug, personally I like dense pack, most of my house is done with it, but if we take more than 6” away from either side it will be too small for the equipment we need to get in there. And dense packing a 2x6 would only give a R-17.5 at best.

  10. #10
    Join Date
    Jun 2004
    Location
    Portland, ME
    Posts
    6,297

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    Jud - can you build out the exterior? Seems like that would be a better solution, but perhaps you can't change the exterior profile.

  11. #11
    Join Date
    Jun 2004
    Location
    South Norwalk CT
    Posts
    438

    Default Re: Venting rafter bays with Benjamin Obdykes Home Slicker?

    Dan-
    Your correct, we can't alter the exterior either. The owner would never allow it.

    Having done more research on this we will us Accuvent Cathedral vents, seems like a great product.

    -Jud

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