R30 Attic Walls

R15 or R21 as allowed in a knee wall with 2x4 or 2x6 framing, is not enough where a thicker wall is permitted, against an unconditioned space (roof not insulated, perhaps vented). The goal should be R30 or more, leaving rule makers behind. This >R30 goal is developed in my Insulation Math.

Here is an approach for R30:

Using a spacer set, I have preset 2x3 KDDF rails outside a 2x4 knee wall. I have a 3" screw through two or three crossings for each 2x3 stick. R15 is placed first in the outer wall. It's too bad I didn't set this up before the wall outlets were run, The horizontal 2x3's are inviting as cable runs, not demanding drilling of joists and complication of drywall, by wire protection plates.

Here is the finished appearance from the back side. Note absence of flooring. The homeowner doesn't need storage space, but does need service access including that of a gas line. Sacrifice of a 4" slice of closet volume is no big deal. The space vents to an upper attic, but is not served well by soffit vents. I think safety would be better served by fire blocks in the room sloped ceilings. Based on Southface.org advice below, I will return to apply 1/2" OSB covering the back side, after drywall is installed, to better press all voids out of the insulation.

Here is the finished room-side appearance, with a MidMade wall portal ready to install after drywall. There is still a missing step. An air tight PVC vapor barrier should be installed before drywall. I am awaiting homeowner instruction on slope ceiling fire blocks, and direction to install Swedish Tenoarm vapor barrier The wall portal and the Tenoarm are supplied by Resource Conservation Technology, Inc., in Baltimore. I follow their advice in using unfaced insulation, and the Tenoarm.

Here is the R30 knee wall, with applied Tenoarm vapor barrier, just before application of drywall. The second photo is looking to the left, through the MidMade access door. I remain in conflict with this rule, in not covering the outside of the knee wall with a vapor-permeable air barrier, "vpab." If I were to comply, I could apply drywall, or least-costly wood, perhaps 3/8" plywood. I would NOT use house wrap. But, why a covering at all? The knee wall is better and more-stably insulated than the adjacent exposed wall of a bathroom, not a knee wall, and not commanded to have vpab. For that bathroom wall I placed an over-fill of 2x4 framing, with quite-well-retained unfaced R19 batt insulation. Since this closet is unused, and insulation is well retained, the only reason for hard-facing the walls will be to increase R-values by blockage of wind washing. I may ask to do that some day, and would then double the insulation on the R19 wall, too.

Here are before and after photos in a floor insulation upgrade from R18 to R38, encouraged by generous rebate. A floor at under R19 is rebated as if found uninsulated. The return on spent public purpose found would be about double if wall insulation were upgraded from sometimes-R10, to R20, with the same per-square foot incentive.

Hard-Facing Advice from Southface.org:

This graphic is found at southface.org., in a seven-page pdf document, credited to the 2006 IECC. It better informs my thinking for both jobs in the photos above. The attic-side air barrier indicated here is not yet applied in my work. In one other job, I attempted compliance by least effort, and at least cost, by applying house wrap. House wrap might meet the intent of my rule-makers, who require "a durable, vapor permeable air barrier material to prevent air penetration of the insulation, and to ensure that the insulation is held in full contact with the wall cavity." House wrap was very unsatisfactory, first, because no suitable tape to close and secure overlap was available. The wrap becomes unsealed, and is prone to be caught and pulled down. I think most "compliant" contractors slash sheet plastic, failing to accomplish anything. I am informed by this graphic, to know that the only purpose of a covering is to hold it in place and protect it against traffic. The material must be a durable shell, OSB, plywood or drywall, of as least 3/8" thickness. Calling it an air barrier, or thinking it is for health, blocking dust, is misleading. If air penetration or health were issues, the need is then overlooked with floor insulation. The overlooking for floor insulation is of great political significance, for one must not impugn low-density loose-fill with wind-washing concerns. I am happy to so impugn, but insulation manufacturers offer strong denial.

An R30 double-wall improves upon the Southface recommendation. My Insulation Math defends virtue of adding R30, rather than just R15, aiming for maximum insulation that has return on investment of more than, say 15%. R30 for this wall is still not enough, consistent with a floor R38 requirement.

Southface is wrong to think R19 amply fills 2x6 joists. The illustration should call for at least R21. Where do you buy R3 or R5? Do they appreciate the need to overfill to preclude free convection in voids between drywall and insulation, fed around edges? The illustration says no, with disconnect from a message that an outer shell helps to maintain insulation contact with wall material. ALWAYS OVER-FILL. NEVER UNDER-FILL.

I note more Southface advice for the first customer on bottom-blocking slope-ceiling insulation, but find that inconsistent with illustrated expanded polystyrene roof-cooling insulation baffles. Such baffles are 1 1/2" thickness with egg-crate stiffening, and significantly occupy 2x6 bays that are supposed to be R21 or more. I'm wanting to give up using such baffles, letting a roof run a bit hotter. I doubt the long-term survival of EPS passing hot air. I have seen fully-eroded EPS placed under failed exterior siding. I intend hereafter to use only plywood soffit-path insulation baffles.

Applying advice from Southface.org:

Both of the crossing insulation layers here are Johns Manville R15. The second layer is compressed slightly by applied 3/8" CDX select plywood. 3/8" OSB at $8 per 4x8 sheet does not measure up in function and value, to CDX at $12 per sheet.