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Thread: Attic Insulation

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  1. #1
    DIY Member Semon's Avatar
    Join Date
    Oct 2011

    Default Attic Insulation

    I just had my attic insulation I need to put soffit vents in it yet. I looked in it from the soffit end the blown insulation doesn't meet the roof at any place on the exterior wall.
    Shouldn't it go up to the roof sheeting?

  2. #2
    Nuclear Engineer nukeman's Avatar
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    Nov 2009


    Typically not, if I understand your question correctly. There should be some baffles installed in the attic at each rafter bay. These keep the insulation from covering up the soffit vents and blocking the flow. So, there is usually a space between the exterior of the wall out to the distance of the overhang (where the soffit vents are located) where no insulation will exist. This doesn't matter in terms of heat loss as this area is not above the living space.

    If you are saying that the blown insulation is no where near the baffles in the attic (or no baffles are installed), then that should be fixed in the attic.

  3. #3
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    You need at least 1.5" (some say 2") of air space between the insulation and the roof decking so that the roof decking can always dry quickly, and wintertime moist air leakage that may migrate up from the conditioned space during winter is always diluted by vented attic air (that tracks the outdoor dew point) before it can condense on the roof decking. This is true in any vented attic, whether gable-vented or soffit/ridge vented. There are commercially made chutes that get tacked to the roof decking specifically to guarantee a minimum clearance for that purpose.

    Blown insulation needs to be the same depth every where to get the full benefit, and this is an issue when going for higher-R when there isn't sufficient clearance between the top of the studwall and roof decking. Blown insulation tends to run between R3/inch and R3.5/inch, and if foot (R36-R42) needs to taper down to 6" (R18-R21) at the top of the stud plate it's a significant performance hit. This is because every square foot at the edge has 2x the heat loss/gain as in the deeper center, drastically reducing the average performance.

    To mitigate that you can stack rigid iso insulation (R6/inch, or R36 @ 6" thickness) at the edges and use rigid iso as the clearance chute to bring the R-value at the tapered edges more in line with the rest of the attic.

  4. #4


    +1 to both comment.

    Minimum ventilation gap between the sheathing and the insulation should be 1" on a steep roof and 2"+ on a lower pitch.

    Hopefully they proper installed baffles and you are not blocking off the soffits against proper air flow.

    Air sealing should always be done prior to insulation but 20/20 hindsight is always and exact science.

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