Attic and Crawlspace Duct Insulation Repair

Purpose

This standing instruction restores duct insulation outside conditioned space so it delivers both of the things it is there for: the R-value, and a continuous vapor retarder.

Only one of the two ever gets inspected by eye, and it is the wrong one. A crew can put back the right thickness of the right product and still leave a duct that sweats, because humid attic or crawlspace air reaches cold metal through an unsealed seam and condenses inside the jacket where nobody can see it. The insulation wets, loses most of its R, and the first evidence anyone gets is a stain on a ceiling. The other half of the failure is compression: a wrap strapped to two thirds of its thickness is not the R-value on its label.

Safety actions that gate this work

• Duct wrap, duct tape and pipe covering in pre-1980s buildings may contain asbestos, and this procedure does not disturb it. Corrugated paper wrap, canvas-and-paste jackets and old fibrous tape are the common forms. Do not cut, tear, scrape or pull suspect material. Where the age or the material is unknown, it is presumed asbestos-containing and sampled by a qualified party, and the work stops until that comes back.
• Rodent droppings in attics and crawlspaces are not swept or vacuumed. Dry disturbance aerosolizes them. Wet them down with a disinfectant, let them soak, and remove them damp, with respiratory protection selected under a written program meeting 29 CFR 1910.134.
• Fiberglass duct wrap sheds fibers every time it is cut, unrolled or trimmed overhead. Cut with a sharp blade rather than tearing, sleeves down and eye protection on, and keep the respiratory protection on for the whole install rather than the first cut.
• Attic heat and crawlspace footing are the injury risks. Walk on structure only, carry two lights, and never go under a house alone.

Scope

Covers repair and replacement of insulation and vapor retarder on ducts in attics, crawlspaces, garages and other unconditioned space on residential and small light-commercial systems, including the condensation assessment that decides whether the repair was adequate.

Does not cover asbestos abatement, which is not this trade's work. Does not cover duct sealing workmanship or flexible duct geometry, owned by their own SOPs, although both must be complete before insulation goes on. Does not cover attic air sealing or envelope insulation.

Two requirements, not one

The R-value comes from the duct insulation table in the energy code edition your jurisdiction has adopted, and it binds new and replacement work through the permit. Recent editions commonly require a higher value for supply ducts in attics than for other ducts outside conditioned space, and both the values and the qualifying conditions move between editions, so pull the table from your adopted edition rather than repeating a number from the last job.

The vapor retarder is the facing, and it stops humid outdoor-grade air reaching cold metal. It works only if it is continuous: every seam lapped and sealed, every end closed at the boot and the plenum, every hanger penetration addressed. A jacket that is 95 percent continuous is not 95 percent effective, because the air finds the other 5 percent.

The field test that ties them together is a dew point comparison. Read the space's air temperature and relative humidity, take the dew point from the instrument, and measure the outer jacket surface temperature at the same time. Shop gate: the jacket surface stays at least 5 F above the space's dew point at every point measured. Take that surface temperature with a contact probe, or with an infrared instrument only after correcting for the facing's emissivity, because a bright foil facing has low emissivity and an uncorrected infrared reading on it is not a temperature.

One relationship needs its condition attached: for a fibrous blanket, R is roughly proportional to thickness at a given density. Compressing it raises density, which raises R per inch slightly, so a compressed wrap keeps a little more than the straight proportion and nowhere near enough to matter. Treat a wrap strapped to two thirds of its thickness as delivering on the order of two thirds of its rated R.

Roles and handoffs

Procedure

1. Identify the material before a blade touches anything. Acceptance: system age, wrap type and facing type recorded, and any suspect material photographed rather than handled. Stop rule: thermal system insulation in a building constructed no later than 1980 is presumed asbestos-containing under the asbestos standard unless testing says otherwise, so it stops the job on the spot, gets referred for sampling, and nobody on this crew cuts, scrapes or pulls it. Hazard: this is the inhalation hazard of the whole job, and the control is not disturbing it, not wearing something while you do.

2. Establish the required R-value from your adopted code edition and the duct's location. Acceptance: the value, the edition it came from, and the location category it applies to, written on the ticket. Wrong looks like the value somebody remembers from a job in another county. Stop rule: no confirmed value means no material order. Hazard: none physical; a desk step.

3. Measure the condition before deciding what to do about it. Acceptance: space air temperature and relative humidity with the dew point from the instrument, surface temperature at every defect and at an intact section taken with a contact probe, installed insulation thickness at three points, and the jacket continuity mapped. Wrong looks like an infrared reading taken straight off a foil facing. Stop rule: a surface below the space dew point is a condensing condition and the scope becomes re-wrap plus vapor retarder rather than a patch. Hazard: attic footing and heat; walk on structure and carry two lights.

4. Repair and seal the metal before anything is wrapped over it. Acceptance: every joint fastened and sealed per the duct sealing SOP, and any disconnected or corroded section replaced, with photographs taken before covering. Wrong looks like new wrap over a leaking joint, which blows conditioned air into the blanket and wets it from the inside. Stop rule: a leak found under the old wrap is sealed today or the wrap does not go back on. Hazard: sealant vapor in a crawlspace needs the ventilation and respiratory controls that SOP states.

5. Install the wrap to its stated thickness, without compression. Acceptance: installed thickness measured under the strapping at three points and matching the thickness the product needs to make its rated R, laps of at least the manufacturer's stated overlap, and support spacing that carries the wrap rather than cinching it. Stop rule: measured thickness materially below the product's rated thickness means re-strapping with wider or more frequent supports, because the delivered R falls with it. Hazard: cutting and unrolling overhead sheds fibers, so protection stays on for the whole install.

6. Make the vapor retarder continuous, not merely present. Acceptance: every lap and butt joint sealed with a facing tape or mastic listed for that facing, ends closed at the boot and the plenum, hanger penetrations sealed, and no exposed insulation face on the run. Wrong looks like a neat wrap with an open butt joint behind a truss where nobody looks. Stop rule: an unsealed seam is not left for the next visit; it is the defect the job exists to prevent. Hazard: overhead work with tape and knife, so cut away from your body and keep the blade retracted between cuts.

7. Verify against dew point, restore and record. Acceptance: after at least 20 minutes of run time, jacket surface temperature at least 5 F above the space's dew point at every point measured, installed thickness recorded, filters in, panels on, and the blower-door interlock proven by opening the door on a running blower. Stop rule: a surface still inside the margin means the R-value or the sealing is inadequate and the job is not complete. Hazard: this step puts the system back with the family present, so restore at the disconnect with the panel closed and clear the attic of tools and offcuts first.

The record this produces

• Material identification, system age, and any suspect material with its photograph and referral
• Required R-value, the code edition it came from, and the location category
• Space air temperature, relative humidity and dew point, with the time taken
• Surface temperatures before and after, the instrument and method used, and the margin against dew point
• Installed thickness at three points against the product's rated thickness
• Metal repairs and sealing done under the wrap, with photographs
• Seam and end treatment, and the facing closure product used

The next tech reads the suspect-material line before opening anything, and the office reads the dew point margin if a ceiling stain reappears. The photographs are the only evidence of the sealed metal that will exist once the wrap is closed.

Worked pass: attic supply trunk over a stained ceiling

Step 1: system dates from the mid-1990s and the wrap is a modern foil-faced fiberglass blanket, not a suspect material, so the job proceeds. That determination is recorded rather than assumed.

Step 2: the adopted energy code edition's duct insulation table is pulled, and the required value for a supply duct in a vented attic is recorded with the edition on the ticket.

Step 3: attic air reads 88 F at 65 percent relative humidity and the instrument returns a dew point of 75 F. A 3 ft section of jacket is torn open and the bare metal reads 58 F on a contact probe, which is 75 minus 58, or 17 degrees F below the space dew point. That section has been condensing every cooling hour and it sits directly over the stain. Intact jacket elsewhere reads 83 F.

Step 4: with the wrap opened, a longitudinal seam on the trunk is found unsealed, and it is fastened and sealed before anything goes back over it, photographed first.

Step 5 FAILS. The replacement blanket needs 2 in of installed thickness to make its rated R, and measured under the new strapping it reads 1 1/4 in at two of three points. That is 1.25 divided by 2, about 63 percent of rated thickness, so the delivered R is on the order of two thirds of the label value, a little better than that because compression raises density, and still well short of the required figure. Stop rule taken: the section is re-strapped on wider supports at closer spacing and re-measured at 2 in, 2 in and 2 in.

Step 6: laps sealed to the manufacturer's stated overlap with a facing tape listed for that facing, both ends closed at the boot and the plenum, and the two hanger penetrations sealed.

Step 7: after 25 minutes of run time, jacket surface reads 84 F at the repaired section against the same 75 F space dew point, a margin of 9 degrees F, which clears the 5 F gate. The previously intact section reads 83 F, an 8 F margin. Interlock proven by opening the door on a running blower, tools and offcuts cleared from the attic.

Checking the pass against the sections above: the dew point margin is the gate stated in the requirements section, reported at every point the step named and in the same units; surface temperatures were taken with a contact probe rather than an infrared reading off foil, which is the method that section specifies; the compression relationship is applied with its density caveat rather than as a clean proportion; and the failing step is step 5, whose stop rule re-strapped the run rather than accepting a thickness that would quietly have cost a third of the R.

References

• The energy code your jurisdiction has adopted, in its adopted edition, for the duct insulation R-value table, which binds new and replacement work through the permit
• The insulation manufacturer's data for rated thickness, lap dimension and the facing closure products listed for that facing
• 29 CFR 1910.134 for the written respirator program governing protection used with fiberglass and with rodent contamination
• The asbestos standard that covers your activity, 29 CFR 1926.1101 for construction and renovation work or 29 CFR 1910.1001 for general industry, for the presumption that governs step 1
• See related: the duct sealing and mastic application standard SOP and the flex duct inspection and correction SOP