Welp, No Going Back Now. I Demolished Our Ductwork.
I own a 70-foot single-wide in north Florida. Our ductwork was shot - rust, holes, barely hanging on, full of rat droppings and deteriorating insulation. Heating bills were insane because at least 20-30% of our conditioned air was just leaking into the crawlspace.
So I’m replacing it. All of it. And I’m currently in the middle of this project, covered in fiberglass and questioning my life choices.
Here’s what I’ve learned so far - including the $300 worth of wrong parts I almost bought.
The PPE I Actually Needed (Because This Is Disgusting)
Before I crawled under there, I stocked up:
- Tyvek suits - The full-body kind. You’re gonna be covered in nasty insulation and who knows what else
- Respirator - Not a dust mask. An actual respirator. Trust me.
- Goggles - Fiberglass in your eyes is not a good time
- Heavy duty scissors and shears - For cutting through old ductwork and belly wrap
- Utility knives - Multiple kinds. They get dull fast
- Anabec mold cleaner - For cleaning as I go (spoiler: there was mold)
- Trash bags - Industrial strength, and lots of them
- Shop vac - For the stuff that doesn’t make it into the bags
Total PPE cost: ~$120. Worth every penny.
My Demo Strategy: The Tarp Burrito Method
This is taking way longer than I thought. I’m on day 5 and I’ve only got about 80% of the underbelly and old insulation out.
Here’s what worked:
- Drag a couple tarps under the mobile home
- Cut open a section of belly wrap
- Pull out old insulation and let it drop onto the tarp
- Vacuum up anything that missed
- Drag the tarp out burrito-style
- Dump into trash bags
- Repeat dozens of times
What I found: The old ductwork was even worse than I expected. Barely attached, full of holes, rusted out and beat up. Critters were definitely getting in. Some sections were just hanging by the thinnest metal straps.
I just started gutting the actual ductwork today. I’m snipping the hangers and undoing the ducts section by section, cleaning with Anabec as I go. Hope to get half of it done tomorrow so I can access the 14” main flex duct that I’ll need to replace.
My Timeline (The Real One, Not the Optimistic One)
- Days 1-5: Cut away old underbelly and insulation (tarp burrito method)
- Today (Day 6): Started gutting ductwork
- Tomorrow & Saturday: Finish gutting ductwork
- Sunday/Monday: Spray foam the floor
- Early next week: New duct materials arrive (fingers crossed)
- Mid-week: Install new ductwork
Ask me in two weeks if this timeline held up. I’m betting it didn’t.
Day-by-Day Log (I’m Updating This As I Go)
Days 1-5: Demo Hell
Cut away old underbelly and insulation using the tarp burrito method. Filled dozens of trash bags. Found mold, rat droppings, and deteriorating fiberglass everywhere.
Day 6 - Friday, November 27
Started gutting the actual ductwork today.
The game-changer: Milwaukee M12 cutoff tool. Cuts through tin ductwork like butter. I’m cutting the ducts into small pieces so I can wiggle them out from under the house without destroying my back.
What I found inside: I’m so glad I did this. The ducts were FULL of nasty old insulation, broken glass (no idea how that got in there), and an old receipt from 15 years ago. These things were so clogged I’m surprised we got any airflow at all. And they were barely attached - just hanging by a few screws in some spots.
Got about halfway done today. The main 14” supply line connects to the old tin trunk at the middle of the house. I’m up to that connection point now.
I’m taking photos of the growing pile of destroyed tin ductwork. It’s gonna be so satisfying when this is done.
Tomorrow (Saturday): Cut away the rest of the old ductwork, detach and cap the 14” main line temporarily, keep cleaning with Anabec as I go. Should be able to finish gutting now that I have a system down.
Sunday/Monday: Spray foam the floor
Early next week: Parts arrive, install new system
(I’ll keep updating this section as I make progress)
The Big Decision: Plenum Box vs Replacing the Entire Trunk Line
When I started planning this, I thought I’d just replace the old metal trunk line with flex duct. Run a 14” flex duct down the center, slap some saddle boots on it, call it a day.
That was almost a huge mistake.
Here’s the problem with that approach:
Just replacing trunk with flex:
- Flex duct compresses and kinks when you run it 60+ feet
- Air velocity drops off at the far registers
- Hard to support properly without it sagging
- Saddle boots can leak over time
Plenum box with individual branches (what I’m doing):
- Rigid fiberglass plenum sits under the center of the home
- Individual insulated flex branches run to each register (mine are 6”)
- Way better airflow distribution
- Each branch can be properly supported
- Professional-grade setup
I went with the plenum box. It cost about $150 more, but I’d rather do this once and do it right.
The $300 Mistake I Almost Made: Ductwork Sizing
This is where I almost screwed up badly. I was about to order 8-inch flex duct for my branches because “bigger is better,” right?
Wrong.
Here’s the problem: I have 6” register boots in my floor. If I bought 8” flex duct, I’d need reducers to connect to the boots. Not only would that cost an extra ~$100, but reducers create turbulence and reduce airflow. Plus, 8” flex is way harder to maneuver in a cramped crawlspace.
Here’s how to size it correctly:
Main Supply Line (from your heat pump)
- For most single-wide mobile homes with 2-3 ton units: 14 inches
- Mine is a Goodman GPHH33641AA (3-ton unit) = 14” supply
- Check your unit’s manual or look for the supply collar size
Plenum Box
- Match it to your supply: 24” x 24” x 24” for 14” supply
- The plenum needs a 14” starting collar to connect to your main line
- Then seven 6” collars for the branch takeoffs (one per register)
Branch Ducts (from plenum to each register)
- Match your register boot size - mine are 6”, so I’m using 6” flex duct
- Don’t go bigger thinking it’s better
- Don’t go smaller thinking you’ll save money
My almost-mistake: I was about to buy 8” flex duct with 6” boots. They don’t connect without reducers. Would have wasted $300 and created airflow problems.
The Parts List (What I Actually Ordered)
The Parts List (What I Actually Ordered)
These parts are arriving early next week. I’ll update this once I actually install them and know if I screwed anything up.
1. Fiberglass Plenum Box
IMPERIAL 24x24x24 Fiberglass Duct Board Plenum - $85 at Lowe’s
This is the central box that sits under your home. The 14” main supply connects to it, then seven 6” branches come out to each register.
Why 24x24x24? That’s the right size for a 14” supply and gives you room for 7 takeoff collars without them being too close together.
2. Starting Collars (Metal Rings)
These are what you attach to the plenum box where the flex duct connects.
What I ordered:
- One 14” collar (IMPERIAL GV2477) - $15 - for the main supply
- Seven 6” collars (IMPERIAL GVL0068-A) - $10 each ($70 total) - for the branches
Critical: Match collar size to your flex duct. I’m using 6” flex, so I need 6” collars. Don’t mix sizes.
3. Insulated Flex Duct
Six rolls of 6” x 25ft R-6 Insulated Flex Duct - Dundas Jafine from Amazon - $44/roll
I’m replacing seven runs. Most are under 20 feet, but I’ve got one long run that’ll need about 30 feet. So six 25-foot rolls gives me ~150 feet total, with some buffer for mistakes.
Why Amazon instead of Lowe’s? Lowe’s had R-8 IMPERIAL brand for $55/roll, but it was out of stock. R-6 vs R-8 doesn’t matter much when you’re going from bare tin (basically R-0). Either one is a massive upgrade.
Important: Get the insulated kind with the UV-resistant jacket. The cheap non-insulated stuff will sweat and drip condensation everywhere.
4. Register Boots (Floor to Duct Connectors)
Seven 4x10 Register Boots with 6” round connection (IMPERIAL GVL0074-A) - $12 each at Lowe’s
These connect your flex duct to the register openings in your floor.
Sizing is critical:
- Measure your floor register openings - mine are 4x10 (that’s 4 inches by 10 inches)
- The round connection HAS to match your flex duct - 6” boots for 6” flex
- Don’t buy 8” boots for 6” flex thinking you’ll “make it work” - you won’t
5. Worm Gear Clamps
14x 6-7” Stainless Steel Adjustable Clamps - $3 each at Lowe’s
These clamp the flex duct onto the collars and boots. You need:
- 7 clamps for the plenum connections
- 7 clamps for the boot connections
- Total: 14 clamps
Size note: 6-7” clamps fit over 6” insulated flex duct. The insulation adds thickness, so the outer diameter is actually around 9 inches.
6. Mastic Sealing Tape (NOT Regular Duct Tape)
Three rolls of 3M Professional Grade HVAC Sealing Tape - $18/roll at Lowe’s
This is foil-faced mastic tape. It seals all your connections permanently.
Do NOT use regular duct tape. It’ll dry out and fall off in a year. Trust me, you don’t want to crawl back under there to redo this.
7. Support Straps
Two rolls of plastic duct support webbing - $15/roll at Lowe’s
Flex duct needs support every 4-6 feet or it’ll sag and restrict airflow. This webbing wraps around the duct and attaches to your floor joists.
8. Odds and Ends
- #8 x 1/2” sheet metal screws (for attaching collars to plenum): ~$8
- Utility knife with extra blades (for cutting duct board): ~$15
What This Actually Costs
| Item | Quantity | Unit Price | Total |
|---|---|---|---|
| Fiberglass Plenum 24x24x24 | 1 | $85 | $85 |
| 14” Starting Collar | 1 | $15 | $15 |
| 6” Starting Collars | 7 | $10 | $70 |
| 6” x 25ft R-6 Flex Duct | 6 rolls | $44 | $264 |
| 4x10 Register Boots | 7 | $12 | $84 |
| 6-7” Worm Gear Clamps | 14 | $3 | $42 |
| Mastic Sealing Tape | 3 | $18 | $54 |
| Support Webbing | 2 | $15 | $30 |
| Sheet Metal Screws | 1 box | $8 | $8 |
| Utility Knife | 1 | $15 | $15 |
| TOTAL | $667 |
Compare to professional installation: $2,500-4,000 for the same work.
The Mistakes I Almost Made (And You Should Avoid)
Almost-Mistake #1: Buying 8” Flex Duct for 6” Boots
I was about to order 8” flex duct because “bigger is better,” right? Then I actually looked at my register boots. They’re 6”.
The problem: 8” flex doesn’t connect to 6” boots without reducers. I would’ve spent $300 on the wrong flex duct, then another $100 on reducers that create airflow problems.
The lesson: Match your flex duct size to your boot size. Period.
Almost-Mistake #2: Trying to Save Money on the Plenum Box
My first plan was to run one long 14” flex duct as the main trunk with saddle boots branching off. It seemed cheaper.
The problem: Flex duct sags, kinks, and compresses over long runs. Airflow at the far end would’ve sucked. And supporting 60+ feet of heavy flex duct is a nightmare.
The lesson: Spend the extra $150 on a plenum box. It’s the right way to do it.
Almost-Mistake #3: Using Regular Duct Tape
I figured mastic tape was just overpriced duct tape. Almost bought the cheap stuff.
The problem: Regular duct tape dries out and falls off in 6-12 months. You do NOT want to crawl back under there to reseal everything.
The lesson: Buy the foil-faced mastic tape rated for HVAC. It’s like $18/roll instead of $8, but it actually lasts.
Almost-Mistake #4: Not Planning on Paper First
I almost just ordered parts from three different places without checking if they’d work together.
The problem: Different brands sometimes have slightly different collar sizes. And if you mix up one measurement, you’re stuck with parts that don’t fit.
The lesson: Plan your entire system on paper. Make sure every connection point makes sense before you order anything.
Where to Buy (And Where NOT to Waste Your Time)
Best Option: Ferguson HVAC Supply
If you’re near a major city, Ferguson is a one-stop shop for HVAC wholesalers. They have everything and it’s in stock.
Pros:
- Professional-grade parts
- Everything in stock
- Knowledgeable staff
Cons:
- Some locations require contractor license
- Not in rural areas
- Call ahead to confirm they sell to homeowners
Good Option: Home Depot
Decent selection of flex duct and basic fittings.
What they have:
- Master Flow R-8 insulated flex duct
- Register boots
- Clamps
- Mastic tape
What they DON’T have:
- Plenum boxes (hit or miss)
- Starting collars (rarely in stock)
Hit or Miss: Lowe’s
My local Lowe’s had almost nothing in stock for mobile home HVAC.
Your mileage may vary - call ahead before driving.
Online: Amazon/Specialty Suppliers
Where I got my flex duct because local stores were out.
Pros:
- Ships fast (2-3 days)
- Often cheaper than big box stores
Cons:
- Can’t see/touch before buying
- Returns are harder
What’s Next: Installation Plan
I haven’t installed any of this yet. Parts arrive early next week. Here’s my plan:
Saturday (tomorrow):
- Finish gutting old ductwork
- Get access to the 14” main supply line
- Final vacuum and Anabec cleaning
Sunday/Monday:
- Spray foam the floor (separate project, but doing it while I’m under there)
- Let foam cure
Tuesday/Wednesday (when parts arrive):
- Install the plenum box under center of home
- Connect 14” supply to plenum
- Run the seven 6” flex branches to each register
- Install boots and connect everything
- Seal with mastic tape
- Add support straps
What could go wrong:
- Parts arrive late
- Spray foam takes longer to cure
- I discover more problems while installing
- My measurements were off
I’ll update this article once I actually complete the install and know if my plan worked or if I had to change anything.
Should You DIY This?
DIY if you:
- Can crawl around under your home (it sucks, but it’s doable)
- Have basic tool skills
- Can follow instructions
- Want to save $2,000+
- Don’t mind spending a week under your house
Hire a pro if:
- You’re not comfortable in tight, dirty spaces
- Your existing system is complicated
- You value your time more than $2,000
- You want warranty coverage
- You have back/knee problems
My take: I’m doing this myself because I’m young, handy, and $2,500 is a lot of money. But I won’t lie - crawling around in fiberglass insulation and rat droppings is miserable. If you can afford to hire it out, that’s not a bad call.
What I’ll Know After This Is Done
Ask me in two weeks and I’ll tell you:
- If the plenum box approach actually worked
- If I sized everything correctly
- What my heating bill looks like
- What mistakes I made that I didn’t anticipate
- If it was worth the effort
Bottom Line
Total cost: $667 for materials + $120 for PPE = $787
Professional install quote: $2,500-4,000
Savings: ~$2,000
Time investment: ~10 days (5 days demo, 2 days spray foam, 3 days install)
Worth it? Ask me when I’m done.
What I’ve Learned So Far
The big stuff:
- Plenum box > trunk line replacement - Costs $150 more but way better
- Match ALL your sizes - 6” flex → 6” collars → 6” boots (no mixing)
- Don’t overbuy duct size - Bigger is NOT better, it just won’t fit
- Good mastic tape matters - Not regular duct tape, get the real stuff
- PPE is not optional - Tyvek suit, respirator, goggles minimum
- It takes longer than you think - I’m on day 6 and still haven’t started installing
If You’re Doing This, Here’s What to Do First
Before you order anything:
-
Crawl under there and look - How bad is it really? Take pictures.
- Measure everything:
- Main supply line diameter (look where it comes off your air handler)
- Count your floor registers
- Measure your register openings (probably 4x10)
- Estimate duct run lengths
-
Check what boots you have - What’s the round connection size? That determines your flex duct size.
-
Draw a rough diagram - Where’s your air handler? Where will the plenum go? How will branches run?
-
Build your shopping list from this guide - Triple-check that sizes match
-
Call stores to check stock - Don’t assume they have plenum boxes or collars
- Order everything at once - You don’t want to be halfway done and realize you’re missing clamps
I’m writing this article as I do the project. It’s November 27, 2024, and I’m currently in the middle of gutting my old ductwork. I’ll update this once I actually complete the installation and know if my plan worked. If you’re reading this later and the article still says “in progress,” drop a comment and yell at me to finish it.
This is for a 70-foot single-wide mobile home in north Florida. Your setup might be different, but the sizing principles are the same.