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How do I vent this 1916 house roof?!!

doitall | Posted in Construction Techniques on March 15, 2007 02:26am

I have a 2-story 1916 home with a large attic space that I am starting to finish off.  The roof has exposed rafter tails, and the only ventilation presently is 4 rectangular vents(which will now be blocked by the new ceiling/insulation/baffles).  It gets hot as hey in the summer up there now and it’s unfinished, and I don’t want to cause ice dams later, either.  

I am planning to install baffles between the roof insulation and the roof decking, starting at the back of the new knee walls and contiunuing to the ridge (to provide an air channel), and will have a continuous vent cut in along the ridge.  This will provide air flow along the underside of the roof – if I get some source of air into the attic.

But since I do not have eave venting, should I cut in new rectangular vents behind the new knee walls (in the unfinished space) so that I get proper air flow up the underside of the roof?  I was thinking about cutting in vents on the back and sides of the roof, approximately 4ft above the eave.  Is that a good height? Lower? Higher? Also, what is the recommended ratio of venting sf to attic square footage.

Any and all advice is appreciated.

 

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  1. User avater
    RichBeckman | Mar 15, 2007 03:39am | #1

    Is there a reason you cannot add soffit (under eave) venting? That would be the preferred solution I believe.

    Rich Beckman

    This signature line intentionally left blank.

  2. yammy | Mar 15, 2007 03:55am | #2

    could you drill out the blocking on the top of the plate,between the rafters,and insert (i call them button vents)small circular vents to fit the drilled holes?

  3. User avater
    McDesign | Mar 15, 2007 04:04am | #3

    Where are you located, climate-wise?  Venting attics is so . . . 20th century.  Use spray foam, or dense pack with cellulose or chopped fiberglass (I'm doing that now on a job - Attic Recovery thread is in the Photo section)

    Forrest

    1. Caulk | Mar 15, 2007 04:23am | #5

      Here here!!  In about 3 weeks I'm gutting my finished attic space due to recurring ice dam problems (central NY).  Previous owners did a nice job with the sheetrock but unfortunately only put in R-19 fiberglass in the rafter bays.  Plus the house has a hip roof with 3 dormers so even if they tried to vent it (which they didn't), it would have been nearly impossible.  So I get to pay a demo crew to destroy the space, an insulation contractor to spray foam the entire roof area (no venting- a "hot roof"), and then rebuild the room. 

      To the original poster- seriously consider this advice especially since you're saying it's hot up there already.  It's not likely to improve after finishing it off.  Plus, unless you're furring out the rafters considerably, you probably won't be able to get nearly enough R value using fiberglass or cellulose.  Take it from me, you don't want to do what I'm about to have done!

      1. doitall | Mar 15, 2007 05:42am | #6

        I'm confused here.  I thought that it was critical to properly vent an attic space.

        My intention is to fill between all 2x8 joists with blown-in cellulose (roughly 7-1/2" of fill - I pulled up the existing T&G flooring boards which will be replaced by T&G plywood (only under the finished space footprint - not back to the eaves again), and then blow-in cellulose to a 16" depth (I believe that's R-44, if I remember correctly) behind all knee walls right up to the underside of the deck.

        So, if I have Icynene or and equivalent sprayed to the bottom of the roof sheathing, I don't need venting?  Do I still need to fill the space behing the knee walls to R-44 with cellulose?  Is the entire underside of the roof sprayed (ridge to eave), or only the area behind the finished drywall ceiling?

        Is OK to insulate the walls with fiberglass (I was going to sheath the back of the new stud walls with a layer of rigid board insulation, taping/caulking all seams), and then use R-13 bewteen the stud bays.  Or should the walls be sprayed instead?

        Questions, questions.....

        Thanks a million.

         

        DIA.

         

        1. Caulk | Mar 15, 2007 06:47am | #10

          There are two main schools of thought on roofs- vented and unvented.  Vented is the most popular.  From what I understand, an unvented roof should only be done with spray foam insulation and, even then, only certain types that qualify as a vapor retarder.  Probably partly because the unvented approach requires a much more expensive insulation (my insulation contractor is charging about $7k to insulate about 1100 sq. ft. of the roof to about an R-40) and because builders tend to stick with tried and true methods, the unvented approach is not nearly as common.  From what I've read (thanks to links provided in past discussions on this forum) however, unvented or "hot roofs" can be successfully constructed.  Some pretty reputable building institutes have analyzed the two approaches and have concluded that there is not a significant performance benefit of vented roofs compared to unvented.  They also point out that vented roofs can be tricky to build with sufficient ventilation to function as intended.  Plus, fiberglass tends to lose its R value when exposed to moving air, which is precisely what happens in a vented roof.  I'm going the unvented route with my place because I can't afford to lose the additional headroom it would take to get to a comparable R value with cellulose or fiberglass plus the complex roof of the house makes a vented design practically impossible.  

          As to where to apply the insulation, there are different approaches on that as well.  Some prefer to insulate the knee walls and only the portion of the roof over the conditioned space.  Others prefer to insulate the entire roof and not the knee walls.  (I'm going the latter route primarily so I can utilize the space behind the knee walls for future storage and because I think that will offer the best success at fighting the ice dam problem.  It sounds like you're planning on a hybrid because you're insulating the knee walls and completely filling the triangular space behind the knee walls?  I'm not sure on that approach.  If you opt for a spray foam product, you'll likely be contracting that out so that contractor could provide guidance for you.  (Spray foam is available to the average joe but the material cost for my project ended up more expensive than the contractor's installed price.)

          One other thing- I'm by no means an expert on this topic- just a homeowner (and part-time civil engineer when I'm not working on the house) who has had to researching the issue to death.  Hopefully this helps you out. 

        2. Ragnar17 | Mar 15, 2007 08:24am | #11

          Here are a couple of links:

          Icynene (FAQ): http://prairiefoam.com/faq.htm#faq24

          Corbond website: http://www.corbond.com/corbondproduct.htm

    2. doitall | Mar 15, 2007 05:44am | #7

      Location is St. Paul, MN.

  4. mccarty12 | Mar 15, 2007 04:16am | #4

    If you have the money I would use some closed cell foam insulation such as Corbond which negates the need for ventilation. I've only used it for one remodeling project but it gave me a good feeling.



    Edited 3/14/2007 10:01 pm ET by McCarty12

    1. doitall | Mar 15, 2007 05:44am | #8

      Is Corbond the same as Icynene?

      Thanks!

      1. Ragnar17 | Mar 15, 2007 06:01am | #9

        doitall,

        Corbond is heavier than icynene, and also has a higher R-value per inch.  Also costs more, of course.  The only drawback I'm aware of is that it is more difficult to cut into if you need to undergo remodel work -- their website says it "cuts easier than wood" -- that's saying a lot!  ;)

        I've used icynene several times and have been happy with the results.  Once, I had to run some more wire, and I just used a hand saw to easily cut a big slit, and slip the wire in.  There's a bit of a hard "shell" on the exterior, but once you get past that, it's soft enough to stick your fingers into.  You could pull it out with your hands if you wanted to.

        I used Corbond last winter on another remodel project and am also happy with the results.  The only reason we didn't use icynene is that we needed to get a code-compliant R-value in a 2x4 wall cavity; icynene did not have a high enough R-value per inch, and would have required a thicker wall.   The Corbond feels very hard on the outside, but I've never tried to cut it yet.

  5. reinvent | Mar 15, 2007 02:22pm | #12

    I agree with what others have said about doing an unvented roof using corbond. Will be the best money you have spent. Also agree with 'caulk' on the knee wall. Just spray the roof, that way you simplify the knee wall detail.
    Venting is a waste of time IMO. The air exchanges needed for it to be effective are near impossible to achieve.
    Read this:

    http://www.jlconline.com/cgi-bin/jlconline.storefront/45f9297f000f974927177f00000105a0/UserTemplate/82?s=45eaceff0018964427177f00000105d2&c=99bed6c24948e67882b7559f41b7f06d&p=1

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