Ask This Old House Season 23 Episode 24: Duct Noise, Overseeding, and Mark McCullough's Tappy-Tap Stucco Fix

Season 23, episode 24 puts three unrelated problems on the same bill: a return register howling at roughly 80 dBA, a lawn gone brown through a drought, and a cracked stucco skirt sitting on a poured concrete foundation in Austin. The whistle, the brown patch, and the crack are the symptoms. The work in each segment goes into the movement and the moisture underneath them.

Ross Trethewey handles sound, Lee Gilliam handles turf recovery, and Mark McCullough handles the masonry. Only one of those three jobs is anywhere near as quick as the edit makes it look.

Ross Trethewey on duct noise: velocity, vibration, and where you choke the air

Ross built a prop, and Kevin noticed immediately. The prop is a return register feeding an air handler, shown as a black blower wheel, pushing air out through a supply trunk to supply registers. Kevin gets in the line about prop making being genetic, which is the kind of aside this show does better than most.

Two things make residential HVAC noise, per Ross. The first is the fan itself: a blower wheel spinning faster generates more vibration, and vibration becomes the sound waves your ear picks up. The second is air velocity. Move air too fast and it whistles through the duct or at the register.

He measures it with a sound pressure meter on the A-weighted dBA scale and reads about 80 dBA near the open return, where sound travels by line of sight straight through the grille. Add return ducting with a couple of bends and some distance, he says, and that number can fall from 80 to 70. On that scale, the drop represents half the perceived loudness. That is the whole argument for ducted returns in one measurement.

The counterintuitive part is what happens when a homeowner tries to quiet a room by closing down the register. Choking it increases velocity at that grille and pushes more air out of the other registers on the branch. The fix that works is upstream: a balancing damper back near the main branch, so the turbulence and the noise happen further from your ears. Running flex duct fully extended rather than compressed matters too, because a squashed length of flex stops muffling anything.

Room-level fixes are genuinely approachable. Ross compares two 4x10 grilles, one metal and one wood. The wooden face is far more restrictive, which means more velocity and more chance of a whistle through the same boot and the same duct. A slightly larger 6x10 with a new boot, fed by the same duct size, is another option. Kevin presses him on the balancing damper being behind drywall and gets an honest answer: that is a design decision or a lucky retrofit, not a weekend project. Air handler speed adjustments are usually a technician's setting.

If you read the Miami airflow segment in episode 23, this is the same discipline with different units. There, Ross measured a house depressurizing to -0.9 pascals. Here he measures decibels. Same lesson: get the number first, then decide what to change.

Lee Gilliam shows Valerie how to aerate and overseed a lawn that went brown

Valerie's lawn was fine until brown spots showed up over the summer. Lee reads the site before recommending anything: drought, humidity, and what looks like crabgrass. The lawn is level and the property is in good shape, which is why he goes after the soil instead of the grass.

The core aerator comes first. It drives hollow tines into the ground, pulls up plugs in seven rows, and opens the thatch layer so air, water, fertilizer, and seed can get down where roots live. Lee points out that you rent these from a box store or a rental center rather than buying one, which is the right call for a machine you use a few times a year. He hands Valerie ear protection before she starts, walks her through the throttle, and compares feathering it to a clutch in a car. The cores stay on the lawn and break down.

His target is aeration three times a year: early spring, midsummer, and fall. That is worth saying out loud, because most of us do it once and wonder why the lawn shrugs.

The seed is a blend, tall fescue plus something in the Kentucky blue family. Valerie asks why two, and Lee gives two reasons: a single species invites disease, and a blend looks better as it fills in. The starter fertilizer is chosen for the middle number, phosphate, which supports root growth. Nitrogen is the springtime number for color and quick top growth, not what you want under fresh seed.

Then the part Lee calls the black magic: compost, spread by shovel and raked even, dropping into the aeration holes. Seed and fertilizer go down with a spreader set to open about 20 inches, and Lee warns Valerie to keep it off the driveway and out of the beds, since fertilizer does not check your intentions. Peat moss goes on last, by hand, in a light coverage.

The peat is also a moisture gauge. Light and maroon when dry, dark when wet. Water daily, sneak in a noon watering if you can, and expect germination in roughly eight to ten days. This is where the episode compresses the most: shoveling and raking compost and peat across a yard is hours of real labor, and the watering schedule runs longer than the planting did. Overwatering and underwatering both kill new seed, which is the same water-management problem that showed up in the drainage segment earlier in this season: you are not watering a plant so much as managing where the water sits and how long it stays.

Mark McCullough in Austin: cracked stucco over a concrete foundation

Adam's foundation looked odd enough to write in about. Mark starts outside the crack.

There is dirt built up in one spot from runoff, which means something is carrying soil across the yard. The condenser pad has settled, and the grade around it should be near the top of that pad rather than two inches below the valve cover box. The grade has dropped enough to expose raw concrete. Stucco exists to dress that rough foundation, and once the soil pulls away, the dressing has nothing to protect.

The crack at the corner is a minor structural concern, not an emergency. Soft stucco sitting on hard concrete expands and contracts at different rates, so the corner wants to fall off. What would worry Mark is a vertical crack working up through the foundation and transferring into the softer brick above. That one gets investigated. Adam asks the exact right question here, and Mark answers it without drama.

The diagnosis tool is Mark's ears. He taps the wall and listens for hollow. Solid stucco sounds solid; unbonded stucco rings hollow, and whatever wants to come off comes off. What he finds is that the original work skipped the bonding agent, which he describes as glue applied before the stucco to give it stickability.

The sequence from there is mostly preparation. Adam rubs down the form marks with a rubbing tool, knocking down the high spots; the pock marks left by the forms stay, because they help the new coat bind. Mark hoses the foundation down so dry concrete does not pull water out of fresh stucco, and the sprayer also kills the dust. Bonding agent goes on from the bottom up with Adam brushing behind him. The damaged corner gets rebuilt and left to set.

On the mix, Mark starts with a full 90-pound bag of Portland cement, four shovelfuls of lime, and 20 to 24 shovelfuls of sand, matched to the fine sand in the existing finish. He is fair about pre-mixed bags: a mason contractor on big walls mixes from scratch, but a homeowner patching a small area is usually better off with a bag of pre-mixed stucco. Then the trowels. Small work area means a small steel trowel and a margin trowel, while Adam loads a brick trowel and works material up from the bottom in the scratch coat, notched so the finish coat has something to bite into.

The finish coat gets worked with a sponge float in a circular motion, with the last sweep riding up over the patch edge to erase the line. If you do not own a sponge float, a household sponge can work if its texture matches closely enough. Mark's homework is specific: hydrate the wall lightly after about an hour, then again the next morning, at lunchtime, and in the evening.

This is the same approach Mark took on the adobe floor and brick in the New Mexico project, where matching the material and controlling moisture through curing mattered more than the visible patch. His chimney crown work runs on the same logic: find the crack, remove what has failed, mix a patch that matches.

DIY Confidence Scale

• Duct noise diagnosis and register swap: 4/10. Measuring is easy to understand, and swapping a restrictive wooden grille for an open metal one is a screwdriver job. Air handler speed settings and balancing dampers behind drywall are not. Know which half you are in before you start.
• Aerate and overseed: 8/10. The technique is learnable and the machine is rentable, with ear protection and a gentle hand on the throttle. The grading is for physical effort and follow-through, since the watering schedule decides whether any of it worked.
• Stucco patch over a foundation: 5/10. Prepping, soaking, and sponging are within reach for a careful homeowner. Chasing hollow stucco by ear, rebuilding a corner, and matching a finish sand take a mason's judgment.

Additional Resources

• Watch on PBS
• Official This Old House episode page

Three fixes, three different senses in play: Ross reading a meter, Mark reading a wall with his knuckles, and Lee reading a lawn by what is growing in it. Next week the show steps away from house calls to remember Roger Cook, which is its own kind of episode. Which of these three would you actually attempt, and which one would you be on the phone about by Tuesday?