Duct Joining and Sealing with Mastic

Duct sealing can be done at lowest material cost, and with assured complete air tightness, using a two-part tape method, or a simple brush-on or spray-on of mastic. This photo shows one duct element solidly installed with materials I judge to be Hardcast Two-Part: RTA adhesive, and DT 5300 mineral-gypsum-compound-impregnated fiber tape. A two-part joint has alignment strength that is comparable to that with Foilmastic tape, but stable alignment must be achieved by other means, before tape application. The method was easily applied where a small-diameter duct was at rest on blocks. Brush-on mastic has its place where duct joints are not under bending stress. Mastic covering is not suitable where a duct must be lifted for subsequent insulation wrapping.

Hardcast materials are conveniently available at GENSCO.

Duct Joining and Sealing With Foilmastic Tape

What is an insulation installer to do, if a customer has not found and hired a competent HVAC mechanic, to fix defective ducts, in advance of his work? I gamely did what had to be done, in this first trial of methods. A 16" diameter return-air duct has been re-routed, with some new, stronger ducts and elbows. Several detached joints were made stable, and were locked and sealed using Nashua Polyken 367-17 Foilmastic UL 181B-FX listed tape. GENSCO will be a good supplier of all elements of the job. No one offfered me a wrap and goop method, and I would have found that unsuitable here. I needed to progressively, experimentally and securely assemble the ducts. Wrap and goop is both too permanent and too insecure.

Foilmastic is useful for all joining and sealing of solid ducts. It is also useful as a means of flaring and stabilizing a flexible duct where it joins a solid duct. This flexible duct was not large enough to engage the steel duct. I cut 120 degree “flaps” that were drawn over the steel ducts, with good control of engagement. I taped the flaps down with a couple of full turns of 3” foil tape. A full round or two of poly-skrim-kraft tape completes the mating with insulating wrap. Several dollars of tape are involved. It is worth it.

A crawl space might incude a large number of supply-side ducts of variable diameter, with perhaps a need of detachability for access. I note the following in a 1996 report:

• Duct mastic works, although cold and wet weather make it difficult to apply.
• Workers don't like mastic. It ruins clothes and tools, especially when users are novices.
• Tape-applied mastic seals well. These are the aluminum tapes with 15-mil butyl backing.
• Air handlers are leaky.

These duct elements had been bound with duct tape, which of course was dried out and detached. Joints were stable but not secure. There were no leaks more significant than that occurring at the elbow joints. Foilmastic is used to assure joining strength, and sealing was not the first objective. If simple sealing were the objective, brush-on mastic would do, and all joints should be painted. Here, near the crawl apace entrance and with relatively good head room, Foilmastic was not the only choice. Please see the next posting.

At 4/12/2015, upon seeing this as a prominent find in search of Nashua Polyken 367-17, add referral to my newer views that most steel ducts should be replaced with well-set flex ducts.
http://energyconservationhowto.blogspot.com/search/label/Better%20HVAC%20Flexible%20Ducts 

The process employs 367-17 tape only at steel connector-fittings. All joining of flex ducts is with very excellent Nashua 557 tape, having just-right adhesiveness. Why aren't these excellent Nashua tapes in most stores, displacing and eliminating most sales of awful "duct tape." Choose black 557 Pro-Grade UL Listed Duct Tape. I doubt that where sold as silver 557 "duct tape" at Home Depot and a few other stores, that it measures up. Why do most stores insist on selling non-professional junk?

Conditioned Crawl Space, First Experiment

In a very three-dimensional crawl space, with a trench and a gas furnace, Visqueen, 6-mil poly, whatever the cheap covering commonly available, does not suffice.

Here is a trial of 16-mil reinforced polyethylene called Goldentouch, from Americover. The material is similar to that offered by Basement Systems.

All seams are strongly attached and sealed with tough 1 1/2" width double-back butyl tape. Topside overlap is dressed with 4" width "vapor bond" sealing tape, matching the tough Goldentouch white vinyl.

The material cost here was about $500. 

Note added 8/21/2023:

Please read on to learn of the full successful and inexpensive  passive control of Radon in this crawl space. Follow the Conditioned Crawl Spaces Labels-link below.

Find many details in a job photo album:

My Conditioned Crawl Space 

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.

Coping With Attic Vermiculite

In December, 2006, I added batt insulation in this attic, grading vermiculite to a uniform distribution, while wearing a respirator. I would do better now, with advice from a competent testing laboratory, and mindful of US Government advice at:

http://www.epa.gov/asbestos/pubs/insulation.html

Without negative confirmation, it should be presumed that vermiculite and many other construction materials have small asbestos content. Testing to a negative for vermiculite is expensive in the current method, with three gallons of material, taken in one gallon complete cross sections.  If an installation is positive for asbestos, mitigation is expensive, perhaps $10,000 in this very large attic. After mitigation, by a licensed abatement contractor, I would still not bet on absence of asbestos. Who knows where particles might have settled, to be stirred up by intrusive action such as drilling for wiring placement. I would rather be warned by visible vermiculite, to take protective measures.

The protective measures include careful wearing of a HEPA-filtered respirator, with appropriate control of respirator elements and clothing. 

In an attic floor, vermiculite adds to step-through safety concerns, adding justification of lighting and safety decking. 

R30 Crawl Space Plan, 48" Joist Spacing

Please double-click, for a somewhat larger image. This is an installation I will be trying in the next several weeks. I post this for comment, especially from Energy Trust. Is there a simpler way of doing this? Don't we agree insulation and support means should not hang down, with danger to the installation, and obstruction of passage? Certainly, infrequent supports must not permit insulation to fall out of contact with floor sheathing. Most crawl spaces do not have an excess of headroom.

R30 Crawl Space Insulation, Materials Choices

In Oregon at 2008, weatherization sponsor Energy Trust  requires R30 for crawl space insulation. For floor framing of 2x8 at 16" on center, the available Johns Manville batts are 16" by 48." This photo shows placement of a kraft-faced batt within 2x10 joists. Although free-standing batts are 10.25" thickness, batts stably compress to less than the 7.25" joist depth. Side compression and friction give sustantial retentive force. The batt is forced against floor sheathing, and is fluffed down to just fill the cavity. R30 Cathedral (high density) batts are inferior in this application. Contractors who place twine to hold up batts are misguided and do a disservice. I blame irresponsible advice of Bonneville Power Administration for much wasteful placement of floor insulation. BPA calls for R19 batts resting on twine or wire, making tempting habitat for critters, until the insulation falls down, and having no insulating effect. Twine will do nothing to force floor contact of a batt R30 or less in 2x8 framing, even at 16" oc. An R38 batt might be retained in floor contact, but not with joist spacing more than 16" oc.

Requirements do not specify whether batts should be kraft-faced or unfaced. I will choose kraft-faced, with belief that flooring often contributes to house infiltration. Foundation vents, even with typically-crummy plugs, leave the crawl space as the entry point for the chimney effect of air circulation. The kraft facing will be an imperfect barrier, but may help. Wouldn't we do more good where the crawl space is substantially open to outside air, to run house wrap under the insulation? For full value, insulation must be confined air-tight all around. No excuse for a crawl space. No  excuse for an attic floor.


I have tried both R30 and R38 Cathedral, high density batts. For appearance and security of fit, I prefer the R38c, to ordinary-density R30, but find the added cost not justified. The R38c better fit is in part from manufacture at 15.5" width. I wish there were more choices of width for R30 batts.


This illustration for floor framing of 4x8 on 48" centers expresses a dilemma. Whether kraft faced or unfaced, R30 batts are available to me at only 16" and 24" width. I want batts to be retained without billowing down below the joists, because space is often limited, and slack batts are at risk of destruction by a challenged, snagging worker. Insulation must somehow be kept in intimate contact with floor sheathing, everywhere; billowing batts with slack wire or twine, must not be allowed. But, what of the batt interference illustrated, where batts are tried without any custom sawing?

At 9/13/2012, note that better information can be maintained on a web page. Here are pages for several joist configurations:
4x6 At 32" On Center 
4x8 At 48" On Center 
2x8 At 16" On Center 

At 5/2/2016, consider this proposal submitted for 2018 revision of the International Energy Conservation Code:

Revise Table R402.1.2 Footnote g, and apply it for all climate zones.

g. Floor insulation shall be in full contact with overhead sheathing and shall be retained by an underside air barrier. If the air barrier is set against joist bottom faces, bays must be over-filled, generally compressed to much less than batt free thickness. An excess of insulation, more than label R30, shall be required for common 7.25" joist depths if batts and air barrier material are not raised to make contact with the subfloor, above joist bottom faces. Where a crawl space or unconditioned basement does not follow outdoor temperature extremes, with outdoor vents safely, easily and assuredly closed during colder weather, air barrier-faced batts shall achieve floor R-value 19 in any climate zone . 

My code suggestion derives in part from success with hard-covering of insulation in a crawl space overhead for immunity to mice and rats:
http://energyconservationhowto.blogspot.com/2016/02/where-failed-flex-ducts-in-crawl-space.html 

The proposal needs substantial revision to look like "code."


The outcome of passage might be be manufacturer offering of fibrous, compliant insulation material bonded to OSB in useful sizes including 43" width for the difficult 4x8@48 situation. Perhaps a standard R19 or so, for any climate zone with CS not vented in extreme Winter.

In the matters of climate zone and practices of winter closure of vents of an unconditioned crawl space, note that venting for radon is a seasonal concern. My home in Climate Zone 4 with vents closed experiences radon levels above 4 picocuries per liter only in wet spring weather with ground water-saturated and is well below action levels in other seasons. I am getting to know how my crawl space behaves while devising the means to condition it as unvented year-round.