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Best way to insulate a roof without attic access…

Jesse Lizer | Posted in Energy Efficiency and Durability on

On my new house design, it is a prairie style house with wide over hangs and low (3:12) pitches. I have a clearstory area that bumps up with a 4 way hip roof capping it off. Being the low pitch, and getting 14-16″ of insulation up there, it fills up a lot of the roof.

So my question is, what is the best method of insulating a roof like this where you have no access from above (blowing in insulation) and would prefer to not use spray foams (green and cost factor)? Obviously I could staple thick batts up there which isnt as ideal either. I have considered going with thick batts and then adding 2″ of rigid to the bottom of the trusses, installing furring strips, and then installing gyp to those. However with this I run into additional issues such as lighting boxes and things. But I think I could attach them to the furring strips, maybe, and was planning on boxing and sealing them out from above anyway. But after the cost and time to do all of this, its probably best to just shoot the thing with a few inches closed cell foam to seal it up, then install thick batts.

Any other thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    Is there an architect or a designer involved in this project? It always amazes me that anyone can choose a design -- and in some cases even begin construction -- without have determined (a) whether it is possible to install insulation, and (b) if it is possible, which type of insulation will be used.

    So, if there are any GBA readers who may be planning to build a house, here's one take-away from Jesse's question: before settling on a design, know how you will be installing the home's air barrier and insulation.

    We need to know more information to answer your question:
    1. Do you have flat ceilings or sloped ceilings?

    2. Do you intend to leave a gap above the insulation or do you intend to fill the rafter bays?

    3. How deep are your ceiling joists? What type of rafters or roof trusses do you have?

    4. What is your climate zone or location?

  2. Jesse Lizer | | #2

    I am the architect (really, that is my profession...), designed from the ground up with the help of this great forum and others similar as maybe you gathered by my other questions on here. I, and especially my firm, is new to super efficient design so I am spearheading a change in our office, starting with my own house.
    Working through my details made the question surface as I was debating the best way to insulate that space. I also have a construction background, so it comes into play as I figure out how to build it as well.

    The ceiling is flat, the roof will be a 2x4 truss system with a 1' energy heal on the double stud wall. I was planning on using a vented attic unless I spray foam it of course.
    Zone is 6, my HDD is 7200 and design temp is -20, although that is fairly extreme and probably in the 98th percentile.

  3. GBA Editor
    Martin Holladay | | #3

    The best way to insulate a flat ceiling with a vented unconditioned attic above is with cellulose. But it's hard to know if there is enough room to make that possible.

    Can you include an attic access hatch at the location with the greatest head room? Is there enough room up there for a contractor to get up there and manipulate a hose?

  4. Jesse Lizer | | #4

    That is the issue. Even if I change the 3:12 to a 4:12 (which I probably will) I only have about 4.5' to the peak, and it all slopes/hips down from that point. The room is a large open space, so no place for an access. The room is 29x17. One thought I had was to leave a hole in the gyp in the center that someone could get up there and just spray from that center point, but I think the room is too big for that. Then we would simply install the gyp over the hole and tape it up.

  5. GBA Editor
    Martin Holladay | | #5

    If there is no room to insulate it from above, you have to insulate it from below before the drywall goes up. I think that means you are restricted to insulating the sloped roof.

    If you want to insulate a sloped roof with cellulose, you need to install rafters, not trusses. Since you have chosen trusses, that limits you to spray foam.

    Even if you find a way to insulate the flat ceiling, you have all of the disadvantages of an attic with none of the advantages. The good thing about an attic is you can inspect roof leaks and correct insulation flaws. With no access, there's no way to gain these advantages.

  6. Michael Chandler | | #6

    I would talk with an insulator about using a Blown-in-Batt system where you staple up a scrim and poke a hole through from below to fill the area above with insulation. You then would only need to provide access for quality control inspection and for the building inspector to confirm fill. We've done this with cellulose as well as fiberglass. you would have to provide baffles on the underside of the roof for adequate ventilation. My neighbors house has an "ocean wave" asymmetrical curved roof built with 20" floor trusses on saddles on curved walls done this way with a full 20" (less baffle depth) of fiberglass packed in there. He strapped over the scrim w/ 1x4s 24" OC before blowing the insulation so he didn't have to fight the bulging scrim when hanging the curved ceiling.

  7. Jesse Lizer | | #7

    I have contacted a very experienced insulator who does a lot of our projects to get his opinion on the job....he will most likely me the one doing it anyway. I will report back with his thoughts.
    thanks for your input.

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