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Community and Q&A

Somewhat vented scissor truss roof

Josh Beck, Wisconsin-6B | Posted in General Questions on

I have lurked here for the past couple of months and have not found a definitive answer to my question. I’ll get to the question further down the post, right now I’ll give you the details on the house. Forgive my ignorance that will soon become obvious. We live in central Wisconsin zone 5.

My wifes father built this house in ’68-69. The roof is the aforementioned scissor-truss (2×4) style with a 4/12 roof and 2/12 sloped ceiling. There is no ridge vent, but only continuous soffit vents and a small venting fan. The house is 25×45 ranch with no hips, valleys, gables, etc. There is, however, a flat ceiling above the bedrooms on the south end of the building. The ceiling drops about 24” and that area of the house has the conventional roof truss design.

I’m trying to figure out how to insulate the scissor truss portion of the attic. I’ve downloaded some of the CAD files (4-00412) on this type of design. I dont have very much space to insulate, and the omission of a ridge vent has me as confused as a fawn-caught-in-the-headlights.

I’m sure you are going to fire off a boatload of questions, I will answer to the best of my limited knowledge and insufficient experience. I can also take pics to better illustrate what I’m trying to say. I appreciate any/all advise. Thank you.

Josh

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Replies

  1. Josh Beck, Wisconsin-6B | | #1

    This pic is looking at the transition from the conventional truss over the flat ceiling into the scissor truss section. Whada mess eh?

  2. Josh Beck, Wisconsin-6B | | #2

    The only ventilation besides the soffit vent and the airleaks which I'm sure are prevalent throughout the house.

  3. Josh Beck, Wisconsin-6B | | #3

    Yes, that is a hole in the roof, but hey, it's covered by shingles!????

  4. Josh Beck, Wisconsin-6B | | #4

    Another look

  5. Josh Beck, Wisconsin-6B | | #5

    This pic is the transition from the original house into the "porch" area. The porch was added in 1975, I know this not by my wifes father who has passed, but by the wallet I found in the ceiling insulation. I tracked down the owner of the wallet, he had lost it when he insulated this house. Cool huh!

    I'm not sure if I should knock the plywood out or make this a separate attic space, thoughts?

  6. Josh Beck, Wisconsin-6B | | #6

    Sorry, wrong pic. This is the correct one.

  7. Riversong | | #7

    Are you planning to leave the fiberglass in the flat ceiling area? How much height do you have in the scissors truss - at the eaves and at the peak? It looks like there was cellulose there before.

    I would remove the vent fan and cut in a ridge vent, installing either AirVent ShingleVent II or Lomanco OR-4 (the only two with external wind baffles), make sure the soffit vents are clear, install ventilation baffles far enough up the roof to clear whatever insulation depth you'll install, and fill with cellulose. You can cover the existing fiberglass with cellulose as well to both add R-value and decrease convective losses.

    Seal the ceilings below as well as possible, including at any penetrations. If the porch is conditioned space, then include that in the insulated attic. If not, then leave it separate.

    You also have the option of dropping a flat ceiling under the scissors truss section and increasing the depth of the insulation cavity.

  8. Josh Beck, Wisconsin-6B | | #8

    Thank you for responding Robert,

    As for your first question, I will attempt to super insulate the whole house-this is just the beginning of the very large hole I've dug myself into. I am going to replace all of the fiberglass with blown in cellulose, my goal is R38 or better in the flat section of the attic.

    Measurements are as follows-30'' at the peak and 5'' at the junction of the rafter and the wall top plate.

    The soffits need to be cleaned out, some cellulose has washed in from the attic.

    I thought about your idea of making a flat ceiling under the trusses, I received a "no freaking way" from my wife.

    I also should tell you that there is a wood stove being installed where the old fireplace was.

    I like the ridge vent suggestions, I will look into them further.

    Happy New Year

  9. Riversong | | #9

    Josh,

    The ceiling strategy should be integrated with the wall insulation strategy for continuity of air barrier.

    If, for instance, you're planning to build a secondary exterior wall inside the existing ones to create a deep cavity for dense-pack celllulose, you can chamfer the outer perimeter of the cathedral ceiling by angling a sub-rafter from near the top of outer walls to existing bottom chord in order to create a 12" deep cavity to the eaves. This would allow room for a vent channel and consistent R-38 ceiling insulation, while using interior drywall as the air barrier.

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Josh,
    The fact that you have only 5 inches of space for ceiling insulation at the perimeter of you house is a problem. If you install ventilation chutes, cellulose will only provide you with about R-14 or R-15 at this location. That's not enough.

    There are two possible solutions: either fur down your framing to create a deeper cavity for insulation, or switch to closed-cell spray polyurethane foam., which might provide about R-30 (or a little more) if you don't install ventilation chutes. There are several disadvantages with this last option, including:
    1. the possibility that foam insulation will disguise roof leaks and sheathing rot,
    2. the fact that R-30 still isn't much, and
    3. the environmental disadvantages of spray foam.

  11. Josh Beck, Wisconsin-6B | | #11

    Excellent suggestions.

    Robert, I'm trying to get my brain wrapped around your idea. Even if I attached a subrafter to the bottom cord (like furring it out, creating depth?), wouldn't I still have an issue with the 5'' at the top wall plate junction? I'm thinking even if I have a deep cavity (because the ceiling is now x-amount of inches lower down the wall), wouldn't I have bridging issues through the top plate section because the lateral distance from the top plate to the ceiling will be nowhere near adequate (the shortest distance between the top plate and the ceiling)?

    Perhaps I'm overthinking the idea, or, I don't have a clue (I'm thinking the latter).

    Martin, thanks for chiming in. I know there are some great qualities to ccsf, but, I plan to use it in limited quantities (think rim joists) because of the reasons you stated and the cost.

    I like the idea of furring down and also adding 1.5-2.0'' ridgid (w/1x3 wood furring strips). Install insulation baffles and pack in as much cellulose to get my R38 or better. Very similar to CAD 4-0019.

    Thoughts?

  12. Riversong | | #12

    Josh,

    Since you say you want to superinsulate the house and minimize the use of foam, what I was suggesting is building a double wall system by adding an additional inner wall to the current exterior walls to create a space sufficient for lots of dense-pack cellulose. Then you can drop an angled subrafter from below the top of the inner wall to the bottom chord (see attached illustration) to create enough truss depth for ventilation and R-38 cellulose.

    This eliminates thermal bridging at the top plates as well as through the wall framing without the need for foam.

  13. Working Roofer | | #13

    With all due respect to Robert Riversong, the spouse is probably not going to agree to give up interior space to build a new house inside the existing one. It's technically a great idea, but a hard sell emotionally and from a value point of view. I think settling for some high-R rigid foam in the attic along the perimeter where the depth only allows 5 inches of insulation makes the most sense. It will also work well to help hold back the cellulose added on top of the fiberglass. The attic sheathing in framing looks to be in excellent condition, so whatever venting has been in place there seems to be working.

  14. Riversong | | #14

    Working Roofer,

    Let's let the wife make her own decisions. Josh said he wanted to superinsulate the house and wants to avoid foam, so this is clearly an excellent solution. And the value (real as well as "emotional") of a super efficient and warm and cozy home that does not contribute to fossil fuel depletion, global warming or petrochemical waste is literally priceless.

  15. Josh Beck, Wisconsin-6B | | #15

    I see said the blind man. My spatial analysis isn't as strong as I would like. The diagram helps considerably.

    As WR put it, my wife isn't keen on the idea of shrinking the space, I am open to the idea of furring out the studs to create a deeper cavity, however.

    Perhaps superinsulate isn't the word that should be used, I think damn good or the best-I-can-do-given-with-what-I-have-to-work-with would be a better term. I plan on bringing this house "up to par" for the future owners and for my own education. I'd rather learn from my mistakes now.

    WR,

    Could you elaborate on your idea?

    Robert,

    Do I have other options?

  16. Riversong | | #16

    If you're open to building outward, you can fabricate Larsen Trusses (parallel-chord trusses) to create an additional insulation cavity outside the existing walls. This will probably require extending the eaves (and rakes) to overhang the new walls and that would provide an opportunity to build up the roof depth on the outside with a change in pitch (something like below).

    [edited to change illustration]

  17. Josh Beck, Wisconsin-6B | | #17

    Robert,

    I don't want to build out, I think that would be beyond my budget.

    What would be your thoughts on making insulation baffles out of 2'' XPS, installing them 1-2'' below the roof deck, and foam out that area around the top plate/rafter junction (kinda like Martin said?). I'll install ridgid foam under the furred trusses and blow in as much dense-pack as required.

  18. Josh Beck, Wisconsin-6B | | #18

    Bump

  19. ROY HARMON | | #19

    Robert,
    I like detail #16, what would be happening at the bottom?
    I actually have a client interested in upgrading an early 60's rancher. Full 12" block foundation.
    Sorry for the use of your post on this one Josh ~ I owe you one.

  20. Riversong | | #20

    Josh,

    Rigid foam baffles and foaming the top plate area while using cellulose in the deeper cavity is a good compromise in this situation. I would use foil-faced polyiso, though, instead of XPS. It's a bit more climate change friendly and the foil will create a radiant barrier to summer heat gain.

    An easy way to improve wall performance without a major build-out is to install foil-faced polyiso on the interior of the studwall, strap horizontally, blow dense-pack behind and leave the 1½" radiant air gap for additional "free" R-3 and increasing the mean radiant wall temperature for comfort. This will require moving electric boxes inward (and foaming around them) as well as window jamb extensions, but it puts the vapor barrier on the right side for a cold climate. As long as the siding allows drying to the outside, this strategy will work well.

  21. Riversong | | #21

    Roy,

    Some kind of transition, ideally semi-structural, is required at the bottom of the trusses. That could be angle iron bolted to the foundation (which would serve as a termite shield) or a PT ledger with angled brackets to the outer truss chord, or some other creative solution. Ideally, the trusses and exterior insulation should extend down beyond the sill to stop both the thermal bridging and the typical air leakage (if there's sufficient ground clearance).

  22. Josh Beck, Wisconsin-6B | | #22

    Good stuff Robert, this is what I was looking for. Thank you so much, I'm sure you'll be hearing from me in the near future.

    Roy,
    No problem. Impart your knowledge and experience when I ask questions and we'll call it even.

  23. John Brooks | | #23

    I think that the new Photo & Drawing posting Feature has made GBA Better
    By an Order of Magnitude

    this thread is a great example
    thank you Josh & All

  24. Josh Beck, Wisconsin-6B | | #24

    John,
    It helps tremendously, I had not a clue what Robert was talking about until I saw his drawing. Many less experienced/knowledgeable homeowners (like myself) will benefit when you can see a visual to go along with what is said. It also benefits us aforementioned homeowners when we can post a photo to help explain the point we're trying to get across.

    This site is worth every penny!

  25. John Brooks | | #25

    Josh,
    I am an Architect and I have great difficulty "following" many of the verbal descriptions.
    The person describing the assembly usually assumes we understand what they are talking about.

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