The Weather Barrier Under Your New Siding: How an Ames Re-Side Seals Out Water, Layer by Layer
The crew pulls the last stretch of brittle vinyl off the north wall and steps back. Underneath, the sheathing is streaked dark below a window, soft to a thumb press, and there is not a scrap of housewrap in sight. Somebody in the 1990s nailed panels straight onto bare OSB and called it done. This is the moment a good siding replacement earns its keep, and it happens hours before a single new plank goes up.
Homeowners tend to picture new siding as the pretty part: color, texture, clean lines. But the layers hiding behind those planks are what keep an Iowa wall dry through 30 winters and every August downpour. Here is how that hidden system goes on, stage by stage.
Step 1: Strip the old siding to bare sheathing
Everything comes off. On a 1,800-square-foot two-story, that is roughly 1,900 to 2,100 square feet of wall surface once you count gables and the second floor. The crew pulls panels, trim, and old flashing, then sweeps the wall clean so nothing telegraphs through the new work.
Why full tear-off matters: you cannot inspect what you cannot see. Going over old siding hides rot, hides pest damage, and buries any chance to add a proper weather barrier. If your quote skips the tear-off, ask why. Our siding replacement service starts here for a reason.
Step 2: Inspect and repair the sheathing
With the wall bare, we read it. Dark staining below windows, spongy spots at the base of walls, and daylight around penetrations all tell a story. On that streaked north wall, expect to replace two or three 4-by-8 sheets of OSB, which typically runs $40 to $70 per sheet in materials plus the labor to cut them in.
This is also where the whole repair-versus-replace question gets settled for good. If only one wall is soft, we can sometimes patch it, but widespread punk means the sheathing is done. Our guide on when a repair is enough and when replacement pays off walks through that call in more detail.
Step 3: Flash the openings first
Before any housewrap, the crew handles the windows and doors, because water finds openings first. Each one gets flashing tape at the sill, then up the jambs, layered so gravity carries water out and down rather than in and behind. On a home with 14 windows and two exterior doors, that is 16 openings each getting sill and jamb treatment.
The order here is everything. Sill flashing goes down, then the sides overlap it, then the head flashing overlaps the sides. Water always sheds onto the layer below it, never behind it. Get the sequence backwards and you have built a funnel into the wall.
Step 4: Roll on the weather-resistive barrier
Now the housewrap. This is the sheet that wraps the whole house and drains any water that sneaks past the siding. A standard roll covers about 9 feet by 100 feet, so 900 square feet, meaning our 2,000-square-foot wall needs a little over two rolls plus overlap.
We start at the bottom and work up, so each higher course laps over the one below like shingles. Horizontal overlaps run 6 inches, vertical seams a foot. The barrier tucks over the sill flashing at windows and laps under the head flashing, tying every opening into the drainage plane. Done right, a drop of water landing anywhere on that wall has a continuous path down and out.
Step 5: Tape the seams and integrate the details
Loose overlaps are not enough. Every seam gets taped, every fastener penetration in the barrier gets sealed, and the housewrap gets cut and folded into window openings with precision. This is the fussy, slow stage, and it is exactly where a siding-only crew shows its edge. When siding is all you do, you have seen every way a wall leaks and you close each one.
Air sealing rides along here too. Tightening these seams cuts the drafts that make an Ames living room feel cold in January even with the furnace running. Weather-tightness and comfort are the same job.
Step 6: Add the drainage gap where it belongs
Behind fiber cement and behind quality insulated vinyl, a rainscreen gap or a bonded drainage layer gives trapped moisture room to dry. On many Story County re-sides they install furring strips or a drainage mat so the back of the siding is not sitting flat against the housewrap. That small air space is what lets a wall shed a wind-driven August rain instead of holding it.
If you are weighing materials, our breakdown of insulated versus standard vinyl grades covers how the backing on premium panels changes this step.
Step 7: Hang the new siding
Only now do the planks go on. Every course laps the one below, butt joints land over solid framing, and the trim caps the whole assembly. Because the layers underneath were built to drain, the siding gets to be what it should be: the tough, good-looking outer shell, not the last and only line of defense.
If you want to picture the full jobsite flow around this, our post on getting your home ready for a siding replacement sets the stage, and the line-by-line cost breakdown shows where these hidden layers land on the invoice.
Frequently Asked Questions
Do I really need housewrap if my siding looked fine for years?
Fine on the surface and dry underneath are different things. Plenty of Ames homes went decades with no weather barrier and quietly rotted sheathing behind intact-looking panels. A re-side is your one clean shot to add that drainage layer, so it is worth doing while the wall is open.
Can you tell the barrier is bad before tearing off the siding?
Not always, and that is the honest answer. Soft spots, interior staining, and musty smells are clues, but the wall usually tells its full story only once the old siding comes off. That is why a good contractor builds inspection into the tear-off stage rather than guessing from the outside.
Does a better weather barrier cost a lot more?
The materials themselves, housewrap, flashing tape, and drainage strips, are a modest slice of the total. The value is in the labor and the sequencing, which any specialist crew should include as standard. If a bid is unusually cheap, check whether these layers are in it at all.
Wondering what is behind your own walls? Call Ames Siding for a free estimate and we will take a look before the first plank ever comes off.