Should You Replace Windows and Siding at the Same Time?
Why the Two Jobs Are Connected
Siding is not what keeps a wall dry on its own. Behind it is a water-resistive barrier (house wrap or building paper), and every window opening is a break in that barrier. The building code ties them together: the 2024 International Residential Code, which the City of Austin enforces, requires flashing at window openings to extend to the wall finish or to the water-resistive barrier "for subsequent drainage," and says "overlapped flashing shall be applied in shingle fashion." Shingle fashion means each upper layer laps over the one below it, so water running down the wall always stays on the outside.
James Hardie's installation instructions say the same from the siding side: a water-resistive barrier is required and "must be appropriately installed with penetration and junction flashing." The siding manufacturer also states it "will assume no responsibility for water infiltration," which leaves the flashing details to the installer.
When you replace a window without removing siding, the installer has to work those laps in from the outside around the existing cladding. When the siding is off, the whole barrier and every opening are exposed, and the layers can be put in the right order.
The Right Order When You Do Both
With siding removed, a typical sequence for each wall looks like this:
- Remove siding and trim, inspect the sheathing, and replace any wet or rotted wood, especially below window corners.
- Install the water-resistive barrier below the window openings first.
- Install the sill pan so it laps over that barrier. A Building Science Corporation guide for the U.S. Department of Energy notes that the barrier must be installed at the bottom of the window before the pan, to prevent reverse lapping.
- Set and fasten the window per the window manufacturer's instructions, leaving the sill free to drain.
- Flash the sides and head, with the barrier above the window lapping over the head flashing.
- Install siding and trim, keeping the manufacturer's clearances and gaps at trim and flashing.
The code lets window flashing follow the window manufacturer's instructions, and where those give no flashing details, it requires a pan at the sill. Either way, the goal is the same: every layer sheds water onto the layer below it.
When Doing Both Together Makes Sense
- Both are worn out. Fogged or failing windows and siding that is swollen, cracked or rotting.
- You have signs of water at the windows. Soft trim, stained drywall below window corners, or siding rot under the openings. Fixing the windows without opening the wall can leave the damage in place.
- You are changing window sizes or styles. New openings mean new framing and new flashing, which is cleaner with the siding off.
- You are switching siding materials. Going from wood or old composite siding to fiber cement changes the trim details around every window anyway.
Doing both also gives you one set of trim details designed together, rather than new windows fitted to old trim or new trim fitted around old windows.
When It Is Fine to Split Them
Not every home needs both at once. If the siding is sound and only a few windows have failed seals, replacing those windows on their own is reasonable, as long as the installer flashes each opening properly and repairs the siding and trim around it. If the siding is failing but the windows are fairly new and dry, siding can be replaced around them, with each window's existing flashing inspected and integrated with the new barrier.
The rule when splitting: whoever does the second job has to tie into the first job's flashing correctly, not just caulk around it. If you know you will do the other job within a few years, tell the first contractor so they can plan the trim and flashing details for it.
Permits in Austin
The City of Austin's permit-exempt list includes siding "that does not exceed 128 square feet," replacing exterior trim, fascia and soffits, and "replacing doors of the same size and operation." A whole-house siding replacement goes well past 128 square feet, and window replacement does not appear on the exempt list, so ask your contractor which permits your project needs and who pulls them. Outside the city limits, check with your own city.
Questions to Ask Your Contractor
- Will the water-resistive barrier be replaced, and how will each window be flashed into it?
- Will you use sill pans, and how will the sill drain?
- Whose installation instructions are you following for the windows and for the siding?
- How will you handle rotted sheathing found under the windows?
- Who is responsible if a window leaks into the wall: the window installer or the siding installer?
That last question is the strongest argument for one contractor handling both. Driftwood installs James Hardie siding and replacement windows, so both can be planned as one job, with the flashing at each opening worked out before the new siding goes on.
Sources: 2024 International Residential Code sections R703.2, R703.4 and R703.4.1; James Hardie HardiePlank Lap Siding installation instructions (HZ10, effective December 2019); Building Science Corporation Information Sheet 302, prepared for the U.S. Department of Energy Building America program; City of Austin, Work Exempt from Building Permits.
Frequently Asked Questions
If I replace both, which is installed first?
When both are being replaced, windows are installed and flashed into the water-resistive barrier before the siding and trim go on. That way each layer laps over the one below it and water stays on the outside.
Can I replace windows without removing siding?
Yes. Many window replacements are done without removing siding. The installer still has to flash each opening correctly and repair the siding and trim around it.
Can I replace siding and keep my existing windows?
Yes, if the windows are sound. The installer should inspect each window’s existing flashing and integrate it with the new water-resistive barrier rather than caulking around it.
Why does flashing matter so much?
Windows are breaks in the wall’s water barrier. The 2024 IRC requires overlapped flashing to be applied in shingle fashion so water drains outward. Poor flashing lets water into the wall, where it rots sheathing and framing out of sight.
Do I need a permit for windows and siding in Austin?
Siding over 128 square feet is not on the City of Austin’s permit-exempt list, and window replacement is not listed either. Ask your contractor which permits apply and who pulls them.
