Should You Replace Windows or Siding First?

If both are going, replace the windows first, then the water-resistive barrier, then the siding. Window flashing has to lap into that barrier, and once new cladding covers it the next installer is cutting back into finished work. Doing only one is often defensible. Remodel Now installs replacement windows and siding across the Baltimore-Washington Metro Area.

The short answer, and the one layer that decides it

If the windows and the siding are both coming off, the windows go first. Then the water-resistive barrier. Then the siding.

That order is not a style preference. It exists because of one layer almost nobody reads about on a bid. Behind any lapped cladding sits a continuous sheet, housewrap or felt or a liquid-applied coating, and every piece of flashing at every window has to tie into that sheet the way shingles lap on a roof. Upper over lower, so water running down the back of the siding stays outside that sheet.

Baltimore County puts the requirement on its own roofing and siding permit page. When you replace siding, a water-resistance barrier is to be installed as required by Section R703.1.1 before siding is installed. Flashing is to be installed around all windows and doors and shall extend to the surface of the exterior wall finish or to the water-resistance barrier for subsequent drainage.

Read the second sentence again, because it is the whole argument. The flashing at the window has to reach the barrier. Put the barrier up, cover it with new siding, and replace the windows the following spring, and that flashing now has to reach a barrier buried under a finished wall. Somebody has to cut back into the new cladding, cut into the new barrier, and rebuild a lap that was continuous the day it went on.

A wall sequenced the other way is not doomed. It does mean undoing work that was done correctly in order to do the next piece correctly, and the odds of a clean result drop every time that happens.

How an exterior wall actually sheds water

Siding is a screen, not a waterproof skin. Wind pushes water up under the laps, capillary action drags it along the back of a board, and every fastener is a hole. The assembly assumes some water gets behind the cladding, because it always does.

The Maryland residential code says it plainly. Section R703.1.1 requires the exterior wall envelope to prevent the accumulation of water within the wall assembly by providing a water-resistant barrier behind the exterior cladding and a means of draining to the exterior water that penetrates the exterior cladding. R703.2 requires not fewer than one layer of that barrier over studs or sheathing, continuous behind the veneer, with the upper layer lapped over the lower not less than two inches and joints lapped not less than six inches.

R703.4 is the flashing section, and its first line is the one that matters here. Approved corrosion-resistant flashing shall be applied shingle-fashion in a manner to prevent entry of water into the wall cavity or penetration of water to the building structural framing components. Exterior window and door openings head the list of locations where it is mandatory, alongside wall and roof intersections and the line continuously above all projecting wood trim.

Building Science Corporation's pan flashing information sheet lists the four things a sill pan has to do. Be a continuous waterproof surface with no holes, tears or wrinkles. Have a back dam or a positive slope so water goes outward instead of into the room. Have end dams so water cannot run sideways off the pan into the wall. And lap over the drainage plane beneath the opening.

That fourth one is the sequencing rule stated as a physical fact. The same document is blunt about order: to prevent reverse-lapping at the bottom of the pan flashing, the drainage plane material has to be installed at the bottom of the window before the pan flashing is. It also warns that the pan is entirely subverted if the water it collects cannot drain out, which is what happens when somebody seals the bottom flange of a window to the drainage plane. Jambs and head get sealed. The sill gets left open on purpose.

What goes wrong when the siding goes on first

Say the siding went on last fall over a brand new barrier, and the window order lands in March. There are three ways to handle it and none is as good as having done it the other way.

The first is a pocket window, also called an insert. The new sash and frame drop inside the existing frame, nothing behind the wall gets opened, and the barrier stays untouched. That is the one case where siding-first costs almost nothing, and it carries its own trade-offs, covered below.

The second is full-frame replacement with the wall closed. The old unit, its frame and its sill come out down to the rough opening, so the new unit needs a new pan and new jamb and head flashing. With the siding already on, a crew has to cut the cladding back around the opening, cut into the new barrier, build the pan, and then solve the head. The head is where it falls apart. Head flashing is supposed to tuck up behind the barrier above the window so the barrier sheds onto it. When that barrier is already covered, the choices are to slice a horizontal cut above the head and slide the flashing up under the cut sheet, or to surface-tape over the face of the barrier and call it done. The second one is a reverse lap. It looks finished and it funnels water behind the sheet.

The third is to pull the new siding off around each opening and reinstall it. Vinyl will unlock and go back, mostly. Fiber cement does not come off in reusable condition once it has been blind nailed and the course above locks it down.

There is a fastener problem on top of that. Siding goes on with nails driven into studs, and a crew working around a window flange taped months earlier can drive straight through the tape or the flange without knowing. Fiber cement adds clearance rules a later window makes awkward: the HardiePlank installation instructions call for a quarter inch of clearance between the bottom of the product and horizontal flashing, and say in so many words not to caulk that gap. Add a head flashing to a wall that is already clad and the course above has to be cut down or removed to leave that gap. Usually it gets caulked instead.

A published account of the compromise exists. A Journal of Light Construction field report on replacing windows from the exterior describes a house with clapboard nailed straight to the framing and no barrier at all. The crew relied on the seal between the clapboards and the trim to keep out bulk water, built the sill pan from flashing tape, adhered it about half an inch onto the top of the clapboard below so it would drain, and used a bead of sealant as the back dam. Careful work, documented step by step, and the author is open about what it rests on: sealant and a lap onto cladding, because there was no barrier to tie into.

What the code requires at a window opening

Section R703.4.1 is short and worth knowing before anybody prices your job. Flashing at exterior window and door openings shall extend to the surface of the exterior wall finish or to a water-resistive barrier complying with Section 703.2 for subsequent drainage. Air sealing shall be installed around all window and door openings on the interior side of the rough opening gap.

Then it names the ways you are allowed to get there. First is the fenestration manufacturer's installation and flashing instructions, or, where the window maker is silent, the flashing manufacturer's instructions. Second is the flashing design or method of a registered design professional. Third is other approved methods. And there is a default for when no instructions exist: pan flashing shall be installed at the sill, sealed or sloped to direct water to the exterior wall finish or to the water-resistive barrier, with flashing or protection at the head and sides.

Two things follow for a homeowner. The governing document is usually the window manufacturer's instruction sheet rather than a practice somebody learned twenty years ago, so it is fair to ask for. And the materials carry standards. Self-adhered membranes used as flashing have to comply with AAMA 711, fluid-applied membranes with AAMA 714, and mechanically attached flexible flashings with AAMA 712. A roll of generic tape from the back of the van is not automatically any of those.

Pocket window or full frame, and why it changes the answer

The sequencing argument has a size. It is large for full-frame replacement and small for pocket windows, so settle that question before arguing about order.

A pocket window goes inside the existing frame. The old jambs, sill and exterior trim stay put, and the flashing already there, whatever it is, is the flashing you keep. Nothing is opened, so nothing has to be relapped. The costs are different ones. You lose glass area, sometimes noticeably on an already small window. Anything rotten in the old sill or jamb is now sealed behind a new unit. And if the original flashing was bad, a pocket window does not fix it, it hides it.

Full-frame replacement takes everything out to the rough opening. New pan, new jamb and head flashing, new relationship with the barrier. It is the right call when the existing frames are rotted, when the sill has gone soft, when the opening is out of square enough that an insert would look wrong, and whenever the exterior trim is coming off anyway for new cladding. It is also the version that makes sequence matter, because it is the version that touches the barrier.

What Maryland weather does to a reverse lap

A lap that runs the wrong way is a slow failure, and the local climate is unusually good at finding one.

The 1991 to 2020 normals at BWI record about 93 days a year with a low at or below 32 degrees, and only about 11 days where the temperature never climbs back above freezing. Subtract and you get roughly 83 days a year when the thermometer crosses the freezing line and crosses back. Water sitting in a wet sheathing panel or a saturated sill plate gets that many chances a winter to expand.

The same normals put 45 inches of precipitation across 121 days with measurable rain, about 11 of those dropping an inch or more, plus roughly 29 thunderstorm days concentrated in summer. Wind-driven rain is not a rare event here. It is the normal delivery method.

Then summer humidity gets a turn. Maryland sits in the mixed-humid zone, climate zone 4A, everywhere except Garrett and Allegany counties out west. Such a wall has to dry outward in winter while you heat and inward in summer while you cool. A wall that is slightly wet and can dry both ways usually survives. A wall taking on water behind the barrier at every storm does not.

Symptoms arrive late and in the wrong place. Paint failing on interior drywall below a window. A stain at the bottom corner of a casing that returns every March. Soft sheathing found during the next job, two feet to the side of the opening, where water tracked down a stud before it soaked in.

When doing only one of them is the right call

Plenty of houses need siding and do not need windows, and plenty are the reverse. Pretending otherwise is a sales move rather than an assessment.

Siding alone makes sense when the windows are recent, the insulated glass seals are intact with no fogging between the panes, the frames are square and sound, and the units carry a legible performance label. Windows in that condition have years left, and tearing them out to serve a sequencing principle is waste. What the job still has to handle is the connection. When the old cladding and trim come off, the existing window flanges get exposed, and the new barrier has to lap into them correctly. On the sill that is usually manageable. At the head it depends on whether the existing head flashing can be reached and relapped, or whether the new barrier has to be cut and tucked above it. Ask that specifically, by window type, before the first board comes off.

Windows alone makes sense on brick. A solid masonry or brick veneer wall has no lapped cladding to remove and no housewrap to integrate with, and the window detail lives at the lintel and the sill instead. Pocket windows into sound frames are the other clean case, since nothing behind the wall gets disturbed. Single failures belong here too. One fogged insulated glass unit, one sill gone soft under a south-facing window with no overhang. Replacing one or two units is maintenance and does not commit you to anything else.

What pushes an exterior project toward the high end is rarely the cladding itself. It is the number of roof-to-wall intersections needing step flashing and kickouts, how much trim and sheathing turns out to be rotted once the wall is open, a second story and the access it needs, and whether the house is old enough to bring lead rules into it. A one story rancher with simple gables and sound sheathing sits at the low end of the same scope.

Remodel Now installs replacement windows and siding on houses throughout Baltimore County and the wider Baltimore-Washington Metro Area, including Towson and the postwar neighborhoods around it where the original windows and the original cladding tend to reach the end of their lives a decade apart. To talk through which part of your exterior actually needs doing first, the number is (301) 798-4696.

Who issues the permit, and who never looks at your flashing

This is where the jurisdiction matters, and a statewide answer would be wrong.

  • Residential siding replacement. Baltimore County: No permit unless the property is in a designated historic district or on the Landmarks List. Baltimore City: Not on the exempt list, so an alteration requires a permit. Montgomery County: Listed under work that most likely does not need a permit. City of Rockville: Exempt primarily for in-kind replacement of the outermost exterior covering layer.
  • Window replacement in the same opening. Baltimore County: No window or door category appears on the county's permit application list. Baltimore City: Not on the exempt list; alterations require a permit, ordinary repairs generally do not. Montgomery County: No permit where the opening size and the net clear opening dimensions do not change. City of Rockville: Reviewed by the city's own Inspection Services Division, not the county.
  • Changing the size of an opening. Baltimore County: An alteration to an existing dwelling, so a permit is required. Baltimore City: Permit required. Montgomery County: Falls outside the no-permit language. City of Rockville: Falls outside the in-kind exemption.
  • Work on a historic property. Baltimore County: Permit required for any work affecting the exterior appearance. Baltimore City: Separate historic review applies. Montgomery County: Exterior work on historic property requires a permit. City of Rockville: Certificate of approval review through the city.
  • Where the application goes. Baltimore County: Permits, Approvals and Inspections in Towson. Baltimore City: Housing and Community Development. Montgomery County: Permitting Services in Wheaton. City of Rockville: Inspection Services at Rockville City Hall.

The Baltimore County siding exemption and its historic district exception both sit on that roofing and siding permit page, while enlarging an opening lands on the county's addition and alteration page. The county publishes 410-887-3953 for deciding whether a given project needs a permit.

Baltimore City is a separate jurisdiction with a separate code. Its list of work exempt from permit covers replacing roof shingles and roof tarring on the building side, and that is the extent of the exterior exemption.

Montgomery County's home improvements guidance lists siding, and windows and doors without changing the opening size or the net clear opening dimensions, among work that most likely does not need a permit. Exterior work on a historic property is on the other list.

Rockville is the trap. It sits in Montgomery County and issues its own permits. The city's administrative bulletin on the 2021 code amendments says the exemption language for roofing and siding replacements was edited to clarify that it applies primarily to in-kind replacement of the outermost exterior covering layer. A job that also replaces the barrier or the sheathing underneath is not that, and the office to ask is the city's Inspection Services Division.

Now the uncomfortable part. Across most of this region a straightforward residential siding job needs no permit, which means no inspector ever sees the barrier, the laps, the pan or the head flashing. The requirements in R703.1.1, R703.2 and R703.4 still apply. There is no checkpoint between the crew and the finished wall, and Baltimore County printing the barrier and flashing language on a page for work that mostly does not require a permit tells you how often it gets skipped.

That absence is the real reason sequence matters. A bathroom gets a rough inspection that forces hidden work into daylight. On an exterior, the only person who will ever know how the head of your third window was lapped is the installer who did it.

The energy code, and the sticker on the glass

Siding is quiet under the energy code. Windows are not.

Maryland enforces the 2021 energy code statewide, in effect since May 2023, and it has a section written for replacement units. Section R503.1.1.1 says that where some or all of an existing fenestration unit is replaced with a new product, including sash and glazing, the replacement has to meet the applicable U-factor and solar heat gain coefficient for the climate zone. Where more than one unit is going in, an area-weighted average across all of them is an accepted way to show compliance.

For climate zone 4 other than marine, which covers Maryland outside the two western counties, the prescriptive table sets the fenestration U-factor at 0.30 and the glazed fenestration solar heat gain coefficient at 0.40. Both numbers appear on the NFRC label stuck to the glass of any tested unit. Look at the label on the sample before the order goes in, not on the units sitting in your driveway.

The same section lists alterations that do not have to meet new construction requirements as long as building energy use does not go up. Storm windows over existing fenestration are on it, as is surface-applied film on single-pane glazing where the code is not requiring the assembly to be replaced anyway. If the frames are sound, those are legitimate options rather than a dodge.

Lead paint changes the exterior job, not just the dust

A house built before 1978 brings a federal rule into both projects, and on an exterior it is visible to the whole street.

Anyone paid to disturb painted surfaces on a pre-1978 home works under the EPA Renovation, Repair and Painting Rule, which EPA administers directly in Maryland. The work practice standards are specific about exteriors. All doors and windows within 20 feet of the renovation get closed, including on floors below at the same horizontal distance. The ground gets covered with plastic sheeting extending 10 feet beyond the perimeter of the surfaces being worked on, or far enough to catch falling debris, whichever is greater. Where the work comes within 10 feet of a property line, vertical containment or equivalent precautions are required so dust does not reach the neighbor.

Three practices are off the table. Open-flame burning or torching of painted surfaces is prohibited. Power sanding, grinding, planing or blasting painted surfaces is prohibited unless the machine has a shroud or containment system with a HEPA vacuum collecting dust at the point of generation. A heat gun may only run below 1,100 degrees Fahrenheit. Pulling siding and pulling windows out of an older house are the operations that language was written around.

What to ask before you sign anything

The questions that separate a good exterior bid from a bad one are not about the brand of siding.

Ask what order the work happens in, and what laps over what at the head of each window. You want to hear the barrier shedding onto the head flashing, the head flashing shedding onto the face of the window, and the sill left open to drain. If the answer involves taping over the face of the housewrap at a window head, that is a reverse lap described out loud.

Ask whose installation instructions govern. The code's first route is the window manufacturer's own flashing instructions, so the sheet exists. Asking to see it tells you whether anyone has read it.

Ask what the sill detail is, by name. Pan flashing with a back dam and end dams, corners folded rather than cut, bottom flange left unsealed so the pan can drain. Caulk across the bottom of a window is the opposite of that.

Ask who handles the roof-to-wall intersections and whether kickouts are going in where a roof terminates against a wall. That intersection concentrates more water than anywhere else on the house.

Ask for the Maryland Home Improvement Commission license number and check it before you sign, and ask whether the firm is EPA lead-safe certified if the house predates 1978.

A wall is a set of overlapping layers that has to be built in one direction. Windows, barrier, cladding. Once it is closed, the only evidence of whether it was done right is a stain that shows up four winters later, somewhere that is not directly under the window that caused it.

Sources

Frequently Asked Questions

Should I replace my windows or my siding first?

Windows first, then the water-resistive barrier, then the siding, if both are coming off. The flashing at every window opening has to lap into the barrier behind the cladding, and the Maryland residential code requires that flashing to extend to the exterior wall finish or to the barrier for subsequent drainage. Build the barrier and the siding first and that lap has to be rebuilt by cutting into finished work. The exception is a pocket window, which goes inside the existing frame and never touches the barrier.

Can I replace siding without touching the windows?

Yes, and it is often the right call when the windows are recent, the insulated glass seals are intact, the frames are square and the units carry a legible performance label. The siding crew still has to integrate the new barrier with whatever flashing those windows already have. The sill is usually straightforward. The head is the question worth asking about specifically, because the new barrier has to shed onto the head flashing rather than be taped over the top of it.

Do I need a permit to replace siding in Maryland?

It depends on the jurisdiction, and there is no single statewide answer. Baltimore County does not require a permit for residential siding unless the property sits in a proposed or designated historic district or on the Landmarks List. Montgomery County lists siding among work that most likely does not need a permit. Baltimore City does not include siding on its exempt list, so an alteration requires one. The City of Rockville issues its own permits and exempts siding primarily for in-kind replacement of the outermost exterior covering layer.

Do I need a permit to replace windows?

Usually not for a like-for-like swap inside the existing opening, and usually yes once the opening changes. Montgomery County exempts windows and doors where the opening size and the net clear opening dimensions do not change. Baltimore County does not list a window or door category among its permit types, and it publishes 410-887-3953 for confirming whether a specific project needs one. Enlarging or relocating an opening is an alteration to an existing dwelling and does require a permit.

What is a pocket window, and when does it make sense?

A pocket window, also called an insert, is a new sash and frame installed inside the existing window frame. The old jambs, sill and exterior trim stay in place, so nothing behind the wall is opened and the existing flashing is untouched. That makes it the one case where replacing siding first costs nothing in water management. The trade-offs are a loss of glass area and the fact that any rot in the old sill, or any bad flashing already there, gets sealed in rather than fixed.

Is the siding itself the waterproof layer?

No. Lapped cladding is a screen. Wind drives water up under the laps, capillary action pulls it along the back of a board, and every fastener is a hole. The code requires a separate water-resistive barrier behind the cladding plus a means of draining water that gets past it, with not fewer than one layer applied over studs or sheathing and lapped so the upper course covers the lower. Baltimore County restates the requirement on its roofing and siding permit page.

Do replacement windows have to meet the energy code?

Yes. Maryland enforces the 2021 energy code statewide, and its replacement fenestration section applies when some or all of an existing unit is replaced, including sash and glazing. For climate zone 4 other than marine, which covers Maryland outside Garrett and Allegany counties, the prescriptive table sets the fenestration U-factor at 0.30 and the glazed fenestration solar heat gain coefficient at 0.40. Both numbers appear on the NFRC label. An area-weighted average across multiple units is an accepted way to comply.

What does lead paint add to a siding or window job?

On a house built before 1978, anyone paid to disturb painted surfaces works under the federal Renovation, Repair and Painting Rule, which EPA administers directly in Maryland. On exteriors that means closing all doors and windows within 20 feet of the work, covering the ground with plastic sheeting extending 10 feet beyond the perimeter of the surfaces being worked on, and adding vertical containment where the work comes within 10 feet of a property line. Torching painted surfaces is prohibited, as is uncontrolled power sanding or grinding.

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