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  1. #1
    Join Date
    Dec 2011
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    3

    Default extending width of rafters for insulation

    My son purchased a 1940's home and we plan on making the attic space a bedroom. The roof rafters are 2x6 on 24" centers, we would like to add as much R value insulation as possible. Our thought is to extend the depth of the 2x6 by adding or attaching a 2x4 to the rafter, making it now a 2x10. Our thought was to glue and screw the 2x4, then add 9.5" batts for an R value of 30. Then, attached to the added 2x4, we would cover it with 1/2" drywall. There would be a ceiling about 8' tall and a 4' knee wall. Is this doable?

    thanks

    tom

  2. #2
    Join Date
    Dec 2010
    Location
    Houston Texas
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    Default Re: extending width of rafters for insulation

    Certainly doable but would you want to?

    Normally insulation should not be in direct contact with the plywood of the roof system. You'll collect moisture there and cook the shingles off the roof.

    You may find spray foam insulation a better option when combined with a venting system for the roof sheathing.

  3. #3
    Join Date
    Dec 2011
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    3

    Default Re: extending width of rafters for insulation

    Quote Originally Posted by HoustonRemodeler View Post
    Normally insulation should not be in direct contact with the plywood of the roof system. You'll collect moisture there and cook the shingles off the roof.

    You may find spray foam insulation a better option when combined with a venting system for the roof sheathing.
    Hi HustonRemodeler

    My plan was to use those foam panels that allow air to move from the sofit to the ridge vent. The problem with high density foam is it must be done by a professional and the CO$T. hoping to keep the cost down and the R value high.

    thanks

    tom

  4. #4
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    Dec 2010
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    Houston Texas
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    Default Re: extending width of rafters for insulation

    sounds good to me Tom

  5. #5
    Join Date
    Mar 2008
    Location
    nova scotia, canada
    Posts
    1,522

    Default Re: extending width of rafters for insulation

    regarding spray foam insulation, yes it does HAVE to be done by a professional. they have the gear both to spray and to protect themselves from breathing it in. its also 3x the cost of batt insulation.. locally 2lb foam is $3.75 a sq/ft and 6 lb foam is around $5
    fire up the saw and make some dust

  6. #6
    Join Date
    Jan 2010
    Location
    Northern Virginia
    Posts
    975

    Default Re: extending width of rafters for insulation

    Hi,
    I would put in 6" r-19 batts, then cover the whole thing with 2" rigid foam for another r14, use 3" drywall screws to hang the rock. You end up with better than r33, and save an awful lot of lumber. And the foam spanning over the rafters gives a full thermal break. So you can also decide whether to put in baffles and ridge vent it.
    Casey
    Remove not the ancient landmark, which your fathers have set.

  7. #7
    Join Date
    May 2011
    Posts
    251

    Default Re: extending width of rafters for insulation

    Use open cell spray foam and completely seal the attic space. Any air movement through fiberglass and it's R value drops fast. Trying to vent effectively in a narrow space.

    The measured shingle surface temp in the field of sealed attics vs. vented attics is 5-10F depending on conditions. That might matter in AZ, but not so much anywhere else.

    Sealed attics stop air movement. You'll find that the downstairs is less drafty as well.

    I've read of one person venting an attic that was sealed and finding that it made his home drafty and cold because it accelerated the natural stack effect in the house.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  8. #8
    Join Date
    Jul 2010
    Location
    SoCal
    Posts
    5,084

    Default Re: extending width of rafters for insulation

    All good suggestions. I'd choose sombreuil's, and here's why:

    1. you can do it yourself.
    2. not so expensive.
    3. very effective.
    4. no fumes, no mess.

    make sure you don't forget baffels. Ridge vent may or may not be necessary depending on your current vents.

  9. #9
    Join Date
    Dec 2010
    Location
    Houston Texas
    Posts
    2,360

    Default Re: extending width of rafters for insulation

    I've done the layered insulation board method before with great success. 2 layers with staggered joints works very well.

  10. #10
    Join Date
    Dec 2011
    Posts
    3

    Default Re: extending width of rafters for insulation

    Thanks everyone, great information. never thought about what sombreuil_mongrel suggested. Have to think/plan that one out. Just to make sure, the 3" screws going through the 2" of rigid foam should hold both with no issues? Should the foam board be glued to the rafters? What type of foam board should I look for and should it be faced?
    thanks

    tom

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