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Creating more space for attic insulation

Pascalli2 | Posted in Energy Efficiency and Durability on

Ok, I’m sure this is a stupid question, but I really do appreciate all of the patient answers I have received in the past.

Because I will be building my home with blocks that are 1′ tall, my top floor will be 8’8″ from floor to the top of the tallest course of blocks, where the roof trusses normally rest. I really only need 8′ ceilings on that floor. I got to wondering what to do with that extra 8″, and came up with this idea to add some extra insulation since I would like to have a floor for at least part of the attic.

I was thinking that it might be possible to attach some 2x8s or something like that perpendicularly to the roof trusses along the bottom, and then attach drywall and what not to that. The extra space could then be filled with more insulation, offering about an extra R20 (minus bridging from the wood) if using cellulose. I attached a picture to try and illustrate what I mean. The yellow rectangles are the ends of the lumber that is attached to the trusses.

Does this make sense? Is it something that could be done? Is it something that should be done? Would I be better off just making the ceiling taller or cutting the blocks to size?


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  1. charlie_sullivan | | #1

    I see no reason this wouldn't work--it makes sense to me. It does seem a little overly material intensive for what you get. An more common way to get more insulation space is a "raised heel truss." ( I could imagine that a clever structural engineer could design a truss for you that would include a lowered ceiling as in your design, but using less material, but I doubt it would use any less material than a raised heel truss, and it would require more engineering.

    If I were you, I think I'd simply make the ceilings higher than needed and use a raised heel truss. Or cut the blocks down and use a raised heel truss. Even though I see no reason your idea wouldn't work.

    What kind of blocks? ICF?

  2. Pascalli2 | | #2

    Thanks Charlie. It looks like raised heel trusses are more about increasing the insulation around the perimeter of the roof? If I want to get near the R60 that is often recommended for my zone (6) without foam, I am thinking I will need all the depth I can get - would it make sense to do a raised heel truss as well as dropping the ceiling further down into the envelope? Another benefit to building down is that it would let me have more depth of insulation where I am putting an attic floor. Otherwise I am limited to the space between the attic floor and the bottom of the trusses.

    You make a good point about the materials - I would have to do some sort of cost-benefit analysis to determine if it is efficient or not. I wonder also if there might be some issues in trying to get drywall to install nice and smooth and even.

    I plan on using Durisol ICF blocks (the pros and cons of which are for a different discussion).

  3. iLikeDirt | | #3

    I'd love to see that discussion. Those blocks look interesting.

  4. dankolbert | | #4

    How deep is the bottom chord of the truss? 2x6? 2x8?

    What kind of insulation are you using? With dense pack cellulose you'd about 16" to get to R60.

    Cross-framing with 2x8 seems like a huge waste of lumber. I'd just do it with 2x4 and hang it off the studs on the exterior walls and the bottom chords of the trusses. I guess you'd want to talk to the truss manufacturer about load, and think a little about how to attach interior partitions to allow for truss lift.

  5. Pascalli2 | | #5

    We have not ordered our trusses yet, so the bottom chord dimensions are not yet determined. I think 8" would allow more depth for insulation. Thinking about material use and performance gave me another idea of a way to do this. I am attaching another drawing because I find it difficult to describe accurately.

    Basically, what about sticking a 1x2 board all the way around the rim of the building at the level we want our ceiling to be, and using it like a rim joist to hang what is, essentially, a false ceiling? We could use 2x4s between the exterior walls and various interior partitions, etc, and attach the drywall to the underside of those. If we cut a notch on the end, it could hang off of the board on the rim so that the ceiling can go all the way to the wall. By using 2x4s, the quantity of wood used is cut almost in half.

    If we did that, and filled all the way from the top of the roof trusses bottom chord to the drywall with dense-pack cellulose, it would give almost 16" of insulation in addition to an almost complete thermal break in the space between the false ceiling and the bottom of the roof trusses. It would also eliminate any problems associated with truss lift and load on the trusses, since the ceiling would not be attached to the trusses at all. I am not sure how to tell if the insulation and drywall would be too much weight for the 2x4's over the space, though.

    Also, I'm not sure how much extra labor this would take.

    Any thoughts on my updated idea?

  6. dankolbert | | #6


  7. Pascalli2 | | #7

    Uh-oh, sounds like I'm way off? I thought it was somewhat similar to your suggestion, but could easily be confused.

  8. dankolbert | | #8

    No, my point is that's exactly what I was suggesting. I would just do a 2x4 rim as well. What will the spans for the 2x4's be?

  9. Pascalli2 | | #9

    Ahh gotcha. Yeah, I was revising my plan based on your suggestions, but didn't realize it was exactly as you were thinking. Although, I think I missed an important detail based on your question about the spans. I would have to span almost 15 feet unsupported if I want to avoid any attachment to the roof trusses. Connecting to the trusses brings the issue of truss lift back into play. That could be the biggest problem, as I am planning a nice smoothly painted drywall ceiling.

  10. wjrobinson | | #10

    Frame with 2x6 and you will be fine. We have 2x6 sheds with that span that the snow loads around here have not crushed.... yet. You could attach to the trusses near the exterior as only the middle of the truss moves up and down mostly. That could cut your span back plenty, attach at 5' and you are left with 10' to the center up lift point. 10' span with 2x6 is a no brainer. If you are really concerned drop your spacing to 12" on center. Go to 2x8 but at some point you are way into overkill.

    Span tables are everywhere as is free span software. Run the numbers.

  11. Pascalli2 | | #11

    Digging around in some span tables, it looks like you're right AJ. Good point about the edges of the trusses, as well. If I could keep everything under 10' unsupported spans, then it might even be possible with 2x4, depending on the wood and final load. There would be pretty much zero live load.

    Would a framer charge a lot extra over a normal ceiling to do something like this? I have no idea how much time or effort it would be...

  12. dankolbert | | #12

    It's a pretty simple job. Even I could handle it.

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