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Insulation at Attic Floor vs. Roof Line

GBCif | Posted in Green Building Techniques on

My 15×30 cabin has R15 (3 inches of foam) on the exterior. This continues up to the attic. I put in raised heels to allow for venting without constricting the airflow around the top plates.

I’m deciding now whether I want to condition the space (rafters) or insulate the attic floor (cj’s).

I don’t plan on storing anything in this attic ever. The only mechanical or HVAC equipment that I plan on ever having will not be in this area. I do have the plumbing vent passing though.

If I insulated the floor, I wondered if I could block off the undersides of the rafters to allow the venting to go from one end of the roof to another without creating a breeze in the main attic space. Of course, there would be a vapor barrier from the house.

I’m planning on using salvage iso rigid foam.The floor is likely a much easier job and I won’t have to heat the attic.

Is there anything I’m missing?

Thanks

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Replies

  1. Walter Ahlgrim | | #1

    I say make a new plan without ductwork and HVAC in the attic.

    Ductwork an HVAC in the attic is shoot yourself in the foot silly choice.

    If you choose to conditioned the attic it will likely be a slightly than HVAC in a vented attic but only slightly. One will cost 50% more to operate and the other will cost 40% more to operate.

    The conditioned attic will have twice the surface area to insulate and loose heat and require the use of expensive spray foam insulation. You are likely to end up with an R 25 or 30 spray foam almost no one get the code required 38 most likely the cost will be 5X the cost of R60 cellulose on the attic floor would have been.

    Walta

    1. Andrew C | | #3

      Walter, reading the original post it doesn't sound like there is HVAC currently or planned in the attic, unless I'm misinterpreting. Your warning about HVAC is valid but perhaps not applicable here?

      I agree that cellulose on the attic floor is almost always the first choice.

  2. Steve Knapp CZ 3A Georgia | | #2

    I think Walter missed that you don't plan on putting any HVAC or ducts in the attic. From a cost standpoint, you can buy a lot of fiberglass or cellulose for not much money. Just be sure to air seal the ceiling while it's still easy to access that area.

  3. Expert Member
    Zephyr7 | | #4

    I'm assuming you have the usual soffit and ridge vents to ventilate the attic. I would put baffles in at the eaves to both keep insulation clear of the soffit vents, and to prevent wind washing effects. Exactly what kind of baffles you use depends on what fits. I would air seal the attic floor in the usual way with caulk and canned foam. Get all penetrations (wires, pipes, etc.), any/all electrical boxes, and the usual leaky transition points around top plates. There are lots of good articles on GBA about how to do all of this air sealing work.

    I would skip the reclaimed polyiso. I would use blown cellulose instead -- it's cheaper, and it will do a better job. R per inch doesn't really matter here since you're not space constrained, just blow in enough cellulose to hit whatever your target R value is.

    I would not bother with a vapor retarder on the attic floor, it's not really needed. Any moisture that sneaks through will get carried away by the attic vents.

    Bill

    1. Eric Habegger | | #5

      I agree completely here with Zephyr. I did exactly this on my own home and it was cheap and very effective.

  4. Walter Ahlgrim | | #6

    Sorry I missed the word "not" in your post about having HVAC in the attic.

    As you can tell I don’t like conditioned attics. A vented attic filled with lots of cheep fluffy insulation is a clear winner in my book.

    Near the eave you will need something to keep the insulation out of the soffits and direct the air flow above the insulation. Often this is cardboard attached with staples. I am not happy with the cardboard and staples. The wind has knock lose the cardboard from the joists and the wind can move the insulation around. If I did it again I would not use the per cut cardboard alone. After the precut was installed I would put long pieces of cardboard across 6 joists 3 feet above the insulation.

    Walta

  5. GBA Editor
    Kiley Jacques | | #7

    In my three years working at FHB and GBA, I have heard again and again that insulating at the floor is the way to go—it certainly makes sense in your case, given you don’t plan on equipment up there. Here’s a helpful article that addresses the ventilation baffles that Walta mentioned: How to Insulate an Attic Floor.

  6. GBCif | | #8

    Looks like blown cellulose is the best option. I have the raised heels, so no need really for baffles, though I might encapsulate the top plate with foam.

    I'll have (foam gasketed) led light mounted through the ceiling. I'm considering making little drywall boxes to encapsulate the boxes and to reduce the mess if I ever need to replace them.

    I'm also not putting in a huge entrance. Any suggestions on how to insulate and seal an entryway that is the width of studs in 16" center?

    1. Andrew C | | #9

      By "entrance", you mean you're putting in an attic hatch?
      If there's an option, I'd avoid putting a hole in your ceiling; put an access hatch on the "end" of your attic. Whether or not this is possible or easy depends on your roof design.

      If you put in a traditional hatch/hole in your ceiling, include gaskets and some cam-lock latches (think double-hung windows) to compress the gasket. You'll probably want to attach some rigid foam to the back of the hatch door so that you don't have to deal with loose insulation as much when the hatch gets used.

      BTW, there may be some code requirements for hatch openings. Think of the people that have to fit thru that hole...might have to be wider than 16"...more like 22x30, with 30" minimum clearance overhead.

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