Kneewall Closet Air Barrier

This kneewall closet was to be insulated in compliance with new Energy Trust of Oregon requirements as follows:

Kneewall insulation, whether new or pre-existing, shall be covered with 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. Air barrier material shall be tested and labeled to meet Oregon fire protection standards. The air barrier material shall be permanently fastened so that it supports the kneewall insulation.

Insulation and drywall on the kneewall would suffice, but this closet is large enough to offer useful storage. With a complete envelope of insulation and house wrap, the space is conditioned, and the door is no longer challenged by cold and drafts. The floor is stuffed with a maximum of insulation.

House wrap is tried here as an economical barrier method. If durable and cheerful enough, it avoids a larger cost for drywall. This installation enclosing the outer wall and sloped ceiling (roof joists) has tight overlap of wrap with stapling at end walls, and only one other seam, sealed with Tyvek tape.

I have been confounded by the covering requirement, added by Energy Trust of Oregon without explanation or prior discussion with affected contractors. Technical support at Johns Manville denies that house wrap will have measurable effect on insulation R-value. I accept that there are two objectives. First Objective: blocking air infiltration if the kneewall lacks another air barrier, as without complete interior drywall and gasket sealing of access doors. Second Objective: as stated at the first page of the ETO Specifications, there is concern for occupant exposure to insulation fibers:

Insulation installed and receiving an incentive from the Home Energy Solutions (HES) program in attics, basements, garages, storage areas, or other areas where occupants go for routine maintenance or storage (Human Contact Areas), shall be covered with a vapor permeable air barrier to limit occupant exposure to insulation fibers. Unless a barrier (such as a wall) exists, fibrous insulation shall be covered. Fibrous insulation used as a dam around storage areas in attics shall be covered and extend at least sixteen inches from storage area. Attic hatches and knee wall-access doors insulated with fibrous insulation shall also be covered. All covering shall meet applicable codes.

The first objective might not dictate wrap on the outer wall or the sloped ceiling, and would apply whether or not the closet would ever be occupied. With this objective, there is concern that wrap is complete, with tape that securely seals all overlap. The Tyvek tape failed in this service, under blower coor conditions. See added comment below. It's adhesion is WIMPY. Surely all Tyvek tape should be recalled, as demands of adhesion are even greater for exterior use under siding. Johns Manville makes a tape for compatibility with its Gorilla Wrap, called JM SealIt, but my supplier does not stock it, and I could not buy it on my own. My supplier stocks Gorilla Wrap only in response to the new kneewall rules, and this suggests other installers are not taping seams or overlap. I don't yet know how I will respond to liability to my customer, for failed tape.

The second objective would dictate wrap only in a utilized closet, but on all insulation surfaces.

With either argument, air should be blocked in all kneewalls, even where access outside the kneewall is difficult. Where access is difficult, I think it would be better to just ensure the kneewall is air-tight on inside wall surfaces, as with drywall replacement of leaky paneling.

I have another concern related to the kneewall ruling. Often there are wall areas accessible on the outside, within an upper attic used for storage. It is painful to not find incentive to insulate those walls to R21 or more, with two layers, one crossing and covering wall joists. With encouragement to do this, would there be demand of outside covering with air barrier material?

If exposure to insulation fibers is the principal concern, what of floor insulation in an upper attic? I brought covering of attic-floor insulation into my discussion of house wrap, with Johns Manville technical support. I thought I might learn that floor R-value is diminished, especially with low-density loose-fill insulation, without a covering air barrier. A lid on that discussion is closed. I still entertain thought of laying manageable pieces of house wrap on an attic floor, especially as a block of loose-fill, whereby covering fiberglass batts, during handling, would not dirty their surroundings. I have much concern about fibers with cellulose and rock-wool insulation, and almost none with dense Johns Manville batts. I think the concern might serve to ban cellulose and rock wool as insulation in an attic floor.

By my trial (photo) I have illustrated confusion in the ETO Specifications. With this report, I ask for clarification. In the absence of clarity, I have foregone several jobs involving kneewalls.

At March 27, 2009, I add these comments:

Draft 2009 specifications from Energy Trust do not clarify intent of air barrier material. I think the ruling is still misdirected at covering insulation fibers so they won't be breathed. My observation of the failed tape seam in the closet described here, under minus 50 pascal blower door conditions, is that draft IS the issue. Tape releases under any pressure differential if an overlap is not tightly pinched. Often in unusable spaces of bungalow homes, the wall of the "bonus room" is gapped paneling. A tight air barrier is hard to achieve, especially where there is not a lower, outer kneewall, in tiny closets. Still, I will try, with use of more-agressive tapes. I do now have a roll of Johns Manville Gorilla Wrap tape. It is just 1.89" wide, hard to aim onto an overlap seam, and looks like the Tyvek tape. I doubt I will even test it. Certainly in the closet described here, added drywall will be the best solution, and housewrap was not needed. In tiny never-accessed closets, air sealing attempts outside kneewalls will fail. There, the correct solution is replacement or overlay of paneling, with good drywall.

At 1/24/2010 I add discovery of more history in the unexplained ETO rule. The Building Performance Institute, BPI, weighed in on this, in 2003. They say:

Insulation installed in kneewalls or other exposed vertical areas must be covered on the cold side with an air barrier such as plywood or housewrap to protect the insulation from wind-washing and free convection within the insulation. This measure is not necessary if rigid foam insulation is used.

To BPI, the issue is insulation effectiveness. They believe air stilling in insulation improves its effectiveness. I'm inclined to agree, but am overruled by experts at Johns Manville. They claim (wrongly I accuse) that even loose-fill insulation on an attic floor would have exactly the same thermal performance whether or not there is a covering that might still the contained air.


At 3/21/2010, I direct the reader to Southface.org interpretation, and my related experiments:
http://sites.google.com/site/phillipnormanatticaccess/Home/r30-knee-wall.


At 11/16/2010, I report condition of the house wrap in this little-used closet.  All Tyvek housewrap tape has released. The customer still does not want to apply drywall, and I promised to just repair the one fallen-down panel. I will apply 3" width Vapor-Bond tape, by Americover.