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Vaulted Ceiling Insulation

Noc Nomore | Posted in General Questions on

I am building a ranch house in RI. The house is 2400sqft, has SIP walls and is a slab on grade with radiant. My question concerns insulating my vented, vaulted ceiling. My living room is made with scissor trusses and has a 12″ energy heal (inside the 9′ SIP walls). I have had a few insulation companies come and give me an estimate on insulating the ceiling and I have as many recommendations on how to do the vaults. One company wants to do 12″ of fiberglasswool batts; another wants to do cellulose and yet another insists on rock wool. My truss engineer suggests that I go with cellulose but use scrap plywood or OSB to make little dams every 4 feet or so running across the cathedral slopes to prevent the cellulose from falling down to the walls and possibly obstructing the soffit vents. Oh yes…the rock wool guy insists that the rock wool will not fall down the slopes due to it’s weight, does not need a vapor barrier and guaranties everything for life…I am skeptical to say the least. What do I do?

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Replies

  1. Torsten Hansen | | #1

    Regardless of your choice of insulation material, you need a sturdy vertical barrier between your trusses and flush with the exterior face of your wall. This barrier mates with the baffles that you have installed to allow air to flow in from the soffit vents and will keep the insulation out of the soffits. Use OSB or similar.

    Blown materials are preferable because you get a continuous insulation blanket without the gaps that batts will have. Cellulose and rock wool will both work. Whatever you choose, do not let your contractor blow the material without properly air sealing the floor of the attic, a topic that has been written about extensively on this website.

  2. Jeff Newman | | #2

    What is the slope of your ceiling? You will definitely need a vertical stop at the bottom (basically a continuation of the exterior wall sheathing) leaving room for ventilation below the roof sheathing. My opinion would be that if the ceiling slope is 6/12 or less cellulose or rock wool should not slide down. Batt insulation of any kind is a bad idea unless you can be guaranteed of near perfect installation. That might require doing it yourself because few insulation contractors figure in the time it takes to do perfect installations of fiberglass batts, thus, it just doesn't get done properly.
    One thing I would add to the above post by Mr. Hansen is that, yes, your attic floor should be airsealed. But, reading between the lines, since you have used sip panels on the walls I am guessing that your goal is a high efficiency home, meaning that if you are building this yourself, or even have a well intentioned but uninformed building contractor, you are probably sealing everything up pretty tightly. Great idea, IF it is done correctly. DON'T forget about ventilation. Contact someone to do a blower door test. Any BPI energy auditor or Resnet HERS rater can do this for a couple hundred bucks. This is money well spent when considering the damage that will occur to your home from mold and poor indoor air quality due to being too airtight. ASHRAE recommends .35 airchanges per hr. (That is 1/3 of the air in your home should be exchanged for fresh air every hour) If you are much tighter than this you will need a ventilation sytem, ideally an hrv or erv. Well intentioned homeowners as well as building contractors that are not familiar with building science and the dangers associated with tight, well insulated homes are doing far more damage than they realize and will eventually incur repair costs that will exponentially exceed the savings they recieved from the high efficiency they sought.

  3. Eric Novotny | | #3

    Jeff Newman,

    The BPI standard allow for a tightening to 0.25 ACH and I seem to recall that the newer ASHRAE standard allows for 0.15 ACH.

    Why are you vented your vaulted ceiling?

    https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

  4. GBA Editor
    Martin Holladay | | #4

    Noc,
    If you are installing fluffy insulation on a slope, it's always best to have an air barrier above the insulation. This is a tricky slope to insulate, because the scissors trusses have members that interfere with the installation of an above-the-insulation air barrier.

    These dilemmas drive me nuts -- because the architect who chose the roof trusses didn't have a good insulation plan at the time that the plans were drawn up.

  5. James Morgan | | #5

    Martin:
    "These dilemmas drive me nuts -- because the architect who chose the roof trusses didn't have a good insulation plan at the time that the plans were drawn up."
    I see no mention of an architect in NOC's post. Most US homes are built without benefit of a design professional of any stripe other than for the few engineering elements which are required by code.

  6. GBA Editor
    Martin Holladay | | #6

    James,
    Good point.

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