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Would Tyvek on top of rafters and under roof sheeeting meet R806.4 for non-vented attics?

elmo1943 | Posted in Building Code Questions on

Wish to have a non-vented attic but desire blown insulation. It appears R806.4 does not allow this. Would Tyvek remove requirement for spray insulation against roof sheeting?

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  1. Expert Member
    Dana Dorsett | | #1

    Short answer: Nope!

    Did you actually mean R804.5 / Table R804.5? (Doesn't change the answer though.):

  2. elmo1943 | | #2

    My understanding was that R806.4 required sprayed insulation against the roof sheeting in non-vented attics. This came from a Green Building Advisor Q&A on non-vented attics.

  3. Expert Member
    Dana Dorsett | | #3

    R804.4 in the IRC 2012 (linked to above) reads:

    R806.4 Installation and weather protection.

    Ventilators shall be installed in accordance with manufacturer’s installation instructions. Installation of ventilators in roof systems shall be in accordance with the requirements of Section R903. Installation of ventilators in wall systems shall be in accordance with the requirements of Section R703.1."

    R804.5 begins:


    "R806.5 Unvented attic and unvented enclosed rafter assemblies.
    Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted if all the following conditions are met:

    1. The unvented attic space is completely contained within the building thermal envelope.

    2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed rafter assembly.

    3. Where wood shingles or shakes are used, a minimum 1/4-inch (6 mm) vented air space separates the shingles or shakes and the roofing underlayment above the structural sheathing.

    4. In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class III vapor retarder coating or covering in direct contact with the underside of the insulation.

    5. Either Items 5.1, 5.2 or 5.3 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.

    5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing.

    5.2. Air-permeable insulation only. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control.

    5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

    5.4. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.


    Table 806.5 (I'm not including that here for formating reason) indicates how much air-impermeable insulation is required between the roof deck & fiber insulation by US climate zone, assuming the total-R is at the IRC 2012 chapter 11 minimums are being met:

    The ratio of air-impermeable R to total R is important, since that's what determines the average temperature at the foam/fiber boundary, which in turn determines the risk of excessive wintertime moisture accumulation.

    Tyvek, while a good air barrier, is EXTREMELY permeable to water vapor (30 perms or more), as is fiber insulation. Putting an air-barrier there is necessary, but not sufficient. Vapor diffusion alone through a 30 perm layer would move enough moisture to rot your roof, deck, even if it was perfectly air-tight.

    If you are going to cheat on R806.5, you need to use something with a vapor permeance lower than 1 US perm on the interior (as well as build it air-tight), but sufficiently vapor permeable that the roof deck can dry toward the interior in the summer. The optimal permeance of the interior will vary by climate and even by roof-color. (The amount of solar heating==drying of the roof deck will vary by the reflectivity of the roofing materials to the solar spectrum- white roofs dry more slowly than dark roofs.)

    A common solution that works for most US climates would be to use a "smart vapor retarder" between the interior ceiling gypsum and the fiber insulation, such as Certainteed MemBrain (tm) or Intello Plus (tm). These membranes need to be air tigth, but the vapor permeance is variable, sensitive to the relative humidity of the proximate air mass (on either side of the membrane.) When the RH is below 35% or so, the way it likely will be during winter when the outdoor dew points are below 30F, the vapor permeance is less than 1 perm. In the spring when the outdoor air temps and solar heating of the roof deck drives the moisture in the roof deck into the entrained air in the fiber layer the membrane the RH of that air rises to where the membrane becomes vapor open,allowing the moisture to escape at a much higher rate than when it was accumulating.

    On an early spring night when the temps drop, the remaining moisture in the cavity is re-adsorbed into the wood, and the vapor permeance drops again, which slows the rate of moisture transfer from the room air into the cavity. During these shoulder-seasons conditions the assembly dries as quickly as if it had no vapor retarder other than the paint (3-5 perms) during the day, but at night it stems the flow of moisture almost as if it had a layer of 2-3mil polyethylene sheeting.

    The code is silent regarding variable permeance vapor retarders, but they mostly work. The colder the climate, the riskier that approach is, but it's not super-risky even in climate zone 7.

  4. elmo1943 | | #4

    Thank you Dana. I will have to re-think sprayed insulation as the first layer.

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