Where Failed Flex Ducts in a Crawl Space Were a Magnet to Mice and Rats

This is what happens when R19 kraft-faced batts are stapled to floor sheathing, in a crawl space with framing 4x8@48, so hard to insulate. Twine everywhere just densified the jungle. It was much worse before a couple of rounds, carting away fallen-down batts; really ugly.

Many generations of mice thrived atop the kraft facing and in falling-down caverns.

Half of the batts did not share this decrepitude, sadder then to pull them down for disposal.

About 1200 gallons of compacted batts were sent off to an Eastern Oregon landfill, via this transfer station and rail cars. This is one carload of 250 gallons, a crime, but nothing compared to a mountain of plastic foam that should have had an alternative use.

HVAC ducting came into view, this an awfully-leaky 14" return line. All other ducts were Thermaflex flex, far more to be hated, as probably the major cause of the vermin infestation, the magnets of the post title.

The flex ducts were to be hated for their thoughtless construction. Here a duct is partially detached where it stressfully ducked under a water pipe.

In imagining how to craft vermin-proof new crawl space insulation, consider a romex lead to a living room floor outlet. A competitor suggested  spray foam, nicely ruled out by the wiring obstruction. At about this time I would confront a ruined very-expensive home in a  sad bid visit, uninhabitable due to stink of foam, white hardness coating even the ground.

Horrible decrepitude of the flex ducts was not easily seen. A Northwest corner of the crawl space, least accessible, was littered with rat traps. Why?

Where my discoveries began in Mid-July, 2015, my mandate was just to fix the ruined insulation. I spent part of a day pulling down batts. Then I brought lumber and insulation, to try out a method that had been on-my-mind for years. Here it is:

And with adjusted attitude and method, it works. But, what of interfering ducts? I'm told I must cope with them. I have impending family travel, and a Summer of air conditioning is wanted. I will return, and will find workable paths, in Early-October.

It is late-September, and I will attack the ducts, not really authorized. This is the near duct I had crushed, thought to be attached OK. The liner has a leak, but there is much worse. See the detached spring coils? All Thermaflex ducts in this home are of an early, brittle polyethylene formula. Wires are detached. There are associated liner rips, everywhere.

I expected decrepitude at the thoughtless runaround of laundry water pipes. Curse that damned "duct tape!"

See lots of leakage in the steel piping. See a large separation in the plastic liner, that had been hidden but not blocked, by insulation lining.

See more detached wire, and holes everywhere, inside the flex duct.

Everywhere, is not much exaggeration.

A D-box is a hydraulic monster, itself with usually-leaky attachments.

Incredible.

And, Thermaflex will poo poo, chemistry from mice somehow caused this.

This was the main entry to ducting by rats and mice, then having access to the house interior. The foulness of the liner is evident. Yet a pest abatement contractor spliced in an oversize aluminum flexible duct as the sufficient cure years ago.

Within the insulation lining, not before explored, I found this. The stick was within the air path. With the report I found immense home owner gratitude that I had insisted on demolition and re-imagined ducts.

Crude D-Box tab-in collars can not be sealed.

An insulation-lined turning box replaces the D-box. Use springy lumber to gently find alignment.

A found strap is useful, the rest of the demolished D-box is recycling.

A 16-12-12 wye was an obvious need in this alternative to ridiculous job-hacked D-boxes. Aligning it for clearance of a nearby post is challenging. Begin to see thick flexible grout coating inside all unlined steel fittings, to isolate them somewhat from temperature cycles of carried air. If the steel does not fully swing with air temperature, it has important reduction of thermal mass. Most of the volume in flexible grout is insulative ceramic microballoons.

In a chosen wye orientation, bring on insulating wrap, and secure fittings with screws and UL181 Foilmastic tape.

Some of the weight of steel components is borne by cantilever from the turning box and furnace. Most weight is carried by taut wires at joining pieces. The wires attach to eye bolts in the sreel fittings and to eye screws in the beams above. The ample reserves of insulation jackets are rolled into place as duct assemblies are completed.

In advance of each joining, pull on ample lengths of insulation jacket. Jacket should fully cover all steel and insulation.

Work now with the left-side 12" branch of the 16-12-12 wye. There is a quick 6" wye take-off to a register.

Note the excess liner lengths needed to contain all steel, insulated, out of sight.

Please rely on this diagram of the as-built ducts, to locate the work in following photos.

This attachment to a 12-12-6 wye is with necessary change of direction. Pull quite a bit of tension to limit flow resistance. Call the axial taping strain relief.

Here is finished appearance at the 12-12-6 wye, in a high-traffic area. Interleaving insulation and wrap from two directions can be bumpy.

This is the previously-hated low Northwest corner of the crawl space, close up. Leaving at end of an October day, the furnace is back on and all but two registers are served.

Back in the Northwest corner, for some clear-headed, intense duct construction.

Ducts all done, but to rework a branch warming the kitchen. I will leave some of this now-dirty OSB, though I don't expect ever to return.

Here is the Northwest corner at a distance, now not foreign or hated. I see the rocky dirt as dry. I will not bother to pull up ill-fitting black visqueen, over the rocks and concrete piers.

Survey the completed ducting, starting at the furnace. Yes, there is a rat trap down there, never to be needed again, soon to be gone and forgotten. Everywhere there is smoothness overhead, and I have been under there in the duct-running.

North wall alley. Just duck under, to get past the furnace.

The 16-12-12 wye at the furnace nicely split about a post. All weight is carried by duct steel fittings with topside eye bolts, tied by wires to the beams above.

The 12" branch toward the kitchen is pulled over vent piping, off the ground, without much flow resistance at a formation to oval.

The house had a single 7" register for the kitchen, laundry and a little bathroom. Sadly resigned that can't be improved, minimize resistance to the 7" register, new, firmed-up. 8" flex duct transitions through a 7" reducer.

Kitchen HVAC duct finally done. In the distance, I will now finish an obstructing 6" duct from the kitchen grill and fan. Overhead see another approach to hard-covered vermin-proof crawl space insulation using rips of OSB.

A determined recycler, I took apart ducts belatedly hauled from the job, too dirty and bulky for hauling with bagged insulation trash. Recycle all metal, including spring coils chopped to about 12" lengths.  Take a pause. Send that rat ridden end above for examination by Thermaflex in South Carolina.

Thermaflex was nice, accepting a length of the brittle liner for examination at my cost, against accusation failure is general; a cause of mouse and rat access, not a symptom, surely this not the only instance. To my shock and dismay, they would disagree with me, blaming mice as cause of embrittlement. And, they offered no comment whether current production is with better material.

Sharing this blog post then with Thermaflex, I studied their product claims. Theirs are guaranteed for ten years, said to be better than with competitors .These had "served" perhaps 25 years. What are we to expect past-warranty? Have materials improved in current production? We need to know these things. 

Let's have some discussion here. What have others found? Will a manufacturer speak up about materials evolution?

I suggest to Thermaflex that I am worthy as a commenter, for teaching good flex duct installation practices. May we talk about this, too?

More HVAC Circuitry, In A Crawl Space

This is a hydraulic design study considering before and after conditions of HVAC ducts in 

the crawl space overhaul reported here:

A Deep Energy Retrofit, Fall of 2013

The home is a well-built farm house, 1590 sf, completed in 1973.

It was necessary to remove and dispose all crawl space HVAC ducts, including some of the elbows approaching floor boots. This permitted cinching heavy R25 cotton batts into needed contact with the floor sheathing.  

Believe that the ducts demolition was needed for access to tie up the batts. 

Know that once again I have done a ducts rebuild by my own commonsense rules, without first characterizing the performance of the found ducts. And, what does performance  characterization mean? Characterization includes Diagnostic Static Pressure Measurements and a measurable sense of the equality of bulk air velocity at each floor register. I'm thinking the register equality is sensed by comparison of noise level at each register.

Removal of all found HVAC ducts was a needed first step. I found then that all supply distribution was with stupid leaky hacks of a single D-box directly under the furnace.

Five flow paths out of this D-box are visible in this photo. On the RHS (front), two 9" ducts. On the side, two 6" ducts and on the back, one 6" duct. 

Find this Medusa distribution of conditioned air to be stupid and offensive, never to be taught to anyone. Air in motion must constantly be guided in least-resistance paths; else it must generate the net motion through gradients in wasteful and noisy, unstable turbulence.

The three 6" duct outlets each served one nearby 6" register. One 9" outlet served four 6" registers of southern rooms. One 9" outlet served an 8" register and three 6" registers in northern rooms. Assuning that the D-box outlets delivered equal bulk velocities, calculate register flows thus: 

Assume equal pressure drops at the five paths in the supply D-box. This happens with equal path bulk velocity in the paths if they have identical geometric loss, say k = 1.8  in each path, an imperfect assumption.  A direction having a majority of flow, to the East at 9" starters. will have higher drive pressure.

The sum of D-box exit areas a bit less than .7854 * ( ( 3 * 36 ) + (2 * 81) ) = 212 sq in

The three 6" registers each have total area 84.8 sq in.

The South-serving four 6" registers have area 113.1 sq in

The North-serving three 6" registers and one 8" register have total area 135.1 sq in.

The total area of pipes leading to registers is 333 sq in. Where pressure drops in various paths are in proportion to square of velocity, Q/A, the majority of fluid energy is dissipated at the D-box walls, mostly-regulating the flow distribution. The square of (212/333) is 0.4. The North-serving registers were favored by almost  135.1/113.1, 20%.

Accurate math here is impossible and futile.

Study further the sizing of the improved HVAC supply ducts:

The trunk area is 14 in x 19 in , 266 sq in.

The flow constriction through electric resistance heaters is to area area about 120 sq in, with extreme high velocity and unfortunate high pressure drop equally reducing all path flows.We should wish some day to have a ground-source heat pump, eliminating the supplemental heaters.

The above diagram of this crawl space warm air duct system is a 1:120 scale drawing, with overlay of structure details upon a Google satellite photo. Please note similarity to planning graphics and involved ducting elements for similar achievement in an attic.

The really curious may examine too, a full photo album A Deep Energy Retrofit, Fall of 2013

 including found conditions of the failed UltraTouch batt installation by "twining" method, and of impassable and leaking conditions in the found system of steel warm air ducts. A separate blog post will address the cotton batt insulation repair. The involved home in Hillsboro, Oregon is also the site of a publicly-shared full photo album documenting attic floor weatherization with UltraTouch cotton batts: Real Sealing Fixes HPwES Sealing In An Oregon Attic. Full pdf photo albums always tell a story best, but please know they are poorly rendered when viewed in GoogleDocs. If you want to examine such albums, please bother to download them.



Elements of a new warm air duct system are in two nearly-equal branches, called North and South. The branches include a well-considered set of plenum components below the furnace. A 14x19 turning plenum replaces a crude D-box.

The furnace plena have a 1" insulation liner and end in a triangular head with two 12" starter collars serving a system of flexible ducts.

Two 6" registers near the furnace are served by the first of four lateral take-offs, circuit label T12x6. These and all steel couplings of flexible ducts, are hung from the overhead via eye bolts. These two, hanging low, are pulled up by wire loops. Never support steel couplings by strapping upon the flexible ducts. Don't rely on screws and tape for suspension via metal couplings.

Here note the 1" insulation lining, and turning vanes. Getting prepared with such well-conceived and well-built elements is a small step of installer professionalism. Expect you may need a couple of weeks lead time for fabrication by a superior sheet metal fabricator. My fabricator in Portland, Oregon is Vinje & Son.

Don't accept dissing of flexible ducts, that they have extraordinary friction. Where each duct section is fully supported by straps and by steel couplings, draw small tension in the liner, taking up coils as needed, to align the duct and coupling. High-resistance problems are always the result of careless alignment, with tripping over steel edges. Plan excess of insulation jackets, to draw them fully-overlapping, over couplings.

This is coupling component R8x6, used two places at branch ends, where 6" ducts to registers are of length not more than six feet. I wish for simpler one-piece reducers, but want ample starter lengths for liner take-up and secure taping. Here and in the previous photo, see that the eye bolt hanger of a steel fitting passes through a hole in plywood overhead. A wood dowel cut from nimble tree branch passes through the bolt eye to releasably hold everything up. The release can allow better fit of insulation jacket overlap.

Here is the completed end of the North branch at a 6" register. The hanger of the R8x6 is out if sight above the jacketed water pipe. See only tough black liner, strapping and some Nashua 557 tape. Leave no opening for mice. Wherever possible, draw ducting tight against plywood guides, as barrier to mice.

This is the new heart of the duct system under the furnace, with a turning plenum.

This is the well-strapped head of the North branch 12" duct, tightly in contact with overhead 1/2"  plywood strips. One can readily crawl anywhere in this crawl space, sliding under the smooth plastic covering if necessary.








Please see that residential HVAC should be naturally balanced with good flow dynamics in orientation and more, of plena, take-offs and reducer or wye fittings. Air flow in thoughtless ducting can be extremely chaotic, and not amenable to tweaking. Do accounting math as in this example for the construction planning. With intelligent design, achieve equal register discharge velocity and noise level at the  eleven floor registers.

The practical concerns in duct design are to allow overall free flow. Never think that a damper hidden and forgotten somewhere would allow residents to fix unbalanced, bad duct design. If someone finds too little heat in a favored room, just tackle the duct path to that room. Find an obstruction, perhaps a crushed or pinched duct, or misalignment in a coupling. Failing that, increase the size of ducting for that room, perhaps just in a more-efficient register. Look for missing insulation as the cause of discomfort.

Know that I am unaware of any other person sharing my practice of novel insights. I observe that taught common practice, persisting in new homes, is absurd and wasteful. Google HVAC Duct Design ACCA Manual D and easily find instruction in the wrong practice I have undone in this home. I engage in this practice as a rare person experienced in the federally-subsidized design of commercial nuclear power reactors, foolishly thinking to find value in my service in the engineering operation of nuclear submarines as a Regular Line Officer, then obtaining my first Masters Degree, in Niclear Engineering, at UC Berkeley in 1972. At now-all-gone Combustion Engineering in Windsor, CT, I had a senior PhD-level assignment as a sole Principal Investigator engaged in controlling the even distribution of water cooling nuclear power reactors, to enable higher power output. Not really a very helpful career and short-lived.

Here are example illustrations found in  a wasted hour of looking at results of that Google Manual D search:

Do silly Manual D Worksheet entries upon duct  friction lossesof energy, when geometric effects dominate. Pooh pooh the virtue of fitting chamfers; accept needless abrupt change of velocity.

Medusa foolishness!