Bay & Bow Window Replacement in Waltham, MA
More Light, More Room
Got an old bay window that's drafty, sagging, or leaking? Or a flat front window that makes the living room feel dark? A new bay or bow window adds light, a few feet of floor space, and real curb appeal.
Top Sash installs bay and bow windows from Marvin, ProVia, and Andersen in Waltham and across Greater Boston. Every unit is custom-built, properly supported, and sealed tight for a Massachusetts winter.
Bay vs. bow: what’s the difference?
Both push out from the wall of your house. The difference is the shape.
Bay window
Usually three windows: a large center window (often a fixed picture window) with two angled side windows. The sides are often double-hung or casement so you can open them. Common angles are 30 or 45 degrees. The result is a crisp, angular nook.
Bow window
Four to six windows of the same size, joined in a gentle curve. Softer look, wider view, more glass. Great on Victorians and on big front rooms.
| Feature | Bay | Bow |
|---|---|---|
| Number of windows | Usually 3 | 4 to 6 |
| Shape | Angular | Curved |
| Projection from wall | More | Less |
| Best on | Colonials, Capes, kitchens | Victorians, large living rooms |
Why homeowners choose bay and bow windows
More light
Light comes in from three or more directions.
More space
The projection adds usable floor space, perfect for a window seat, a reading nook, or plants.
Better views
You see up and down the street, not just straight ahead.
Curb appeal
A bay or bow is often the focal point of the front of a house.
Fresh air
Operable side units (casement or double-hung) let you open them.
The bay and bow windows we install
All three brands we carry make bays and bows. How they arrive on site makes a real difference on install day.
Marvin bays & bows
- Elevate: Ultrex fiberglass outside, wood inside. Bays up to about 7 feet wide, bows up to about 10 feet wide, both about 6 feet tall.
- Ultimate: extruded aluminum outside, wood inside in six species. Custom sizes, grilles, and casing styles for historic homes.
- A window seat and headboard (the finished top panel) come included.
- Side units in casement or double-hung.
ProVia bays & bows
- Comes from the factory fully assembled, as one solid unit. Less carpentry on site means a faster, cleaner install.
- Endure welded vinyl with Sunshield vinyl, so there’s no painting.
- 2-lite, 3-lite, and 4-lite configurations.
- ComforTech glass with warm-edge spacers, up to triple-pane.
- Lifetime limited warranty, transferable to one future owner.
Andersen bays & bows
- 400 Series: wood inside, Perma-Shield vinyl cladding outside.
- E-Series: wood inside, Andersen’s most custom exterior colors.
- Andersen units are joined and finished on site, with the seat, head, and sides built by our carpenters. More labor, but the right choice when you want to match the Andersen windows you already have.
What does a bay or bow window cost?
Bays and bows vary more than any other window. A narrow 50-inch bay and a 120-inch bow are very different projects, so we don’t post one-size-fits-all prices. Here’s what moves the number.
What moves the price
- Size and number of units (a 5-lite bow costs more than a 3-lite bay)
- The brand and series, and whether it arrives assembled
- Operable vs. fixed side windows
- The roof over the bay. Many replacements need the roof rebuilt, insulated, shingled, and flashed.
- Support system and any framing repair
- Interior trim, seat finish, and insulation under the seat
- Turning a flat window into a new bay (a bigger job than replacing an existing bay)
- The building permit
That’s why we come out and look before we quote. Then we match the window to your house, your expectations, and your budget.
Budget tip: if a full bay replacement is more than you want to spend, we can remove the old bay and install a flat unit of three windows instead. That’s often far more affordable.
Making it easier
Why bay windows fail, and how we install them right
Old bays in New England usually fail in the same few ways: water gets in at the roof, the seat sags because it was never properly supported, and cold air pours in through an uninsulated seat and head. A good install fixes all three.
Water at the roof
Water gets in at the roof.
A cap or roof over the bay, flashed into the wall so water runs off, not in.
A sagging seat
The seat sags because it was never properly supported.
A support system, such as cables tied into the framing above or brackets below, so the unit won’t sag over time.
Cold drafts
Cold air pours in through an uninsulated seat and head.
We insulate the seat and headboard, the two spots old bays always leak.
Our bay and bow installation process
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Free consultation
We check the existing bay, the framing, and the roof above it.
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Measure
Bays and bows are measured carefully for projection, angles, and height.
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Order
Custom-built units, about 5 to 6 weeks. Larger custom bows can take longer.
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Permit
Massachusetts requires a building permit for window replacement. We file it with the Waltham Building Department.
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Remove the old unit
Lead-safe containment on any pre-1978 home.
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Support
We install a support system, such as cables tied into the framing above or brackets below, so the unit won’t sag over time.
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Insulate
We insulate the seat and headboard, the two spots old bays always leak.
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Roof and flashing
A cap or roof over the bay, flashed into the wall so water runs off, not in.
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Trim and finish
Interior and exterior trim, caulk, and cleanup. Most bay and bow jobs take 1 to 2 days.
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Walkthrough
We check every sash, show you how it works, and register your warranty.
Energy efficiency in Climate Zone 5
A bay is a lot of glass in one spot. In Waltham’s Climate Zone 5 winters, the glass package and the insulation around it matter a lot.
The federal window tax credit ended December 31, 2025. Use Mass Save in 2026.
U-factor
Shows heat loss. Lower is better. ENERGY STAR's Northern-zone standard is 0.22 or lower.
Low-E glass
Reflects heat back into the room.
Argon or triple-pane
Argon gas, or triple-pane glass with krypton, slows heat loss even more.
Insulated seat and head
Stop the cold drafts most old bays have.
Bay and bow windows in Waltham homes
Bays and bows are everywhere in Waltham. You'll see bows on the Victorians around Prospect Hill and Moody Street, bays on Colonials in the Highlands and near Piety Corner, and kitchen bays in the ranches and Capes of Cedarwood and Warrendale. Many of them are original or 1970s to 1980s units with failing seals and uninsulated seats.
Most of these homes were built before 1978. Top Sash is a Massachusetts Lead-Safe Renovation Contractor (LR4694) and an EPA Lead-Safe Certified Firm (NAT-F251059-1), so old trim comes out the safe way.
Why Top Sash
- Licensed: Massachusetts CSL CS-119052, HIC #208527
- Lead-Safe: LR4694 and EPA NAT-F251059-1
- Insured: general liability and workers' comp
- Official ProVia dealer, Andersen Certified Contractor, and experienced Marvin installer
- 10+ years of crew installation experience
- 5.0 stars on Google (formerly VIVA Windows)
Project showcase
Waltham, MA: double-hung to Marvin Elevate bay
A flat double-hung window converted into a new Marvin Elevate bay window.
“Once again everyone did an excellent job.”
★★★★★ Hugh C., Google review
Andersen bay window on a Colonial
A new Andersen bay window with grilles on a yellow Colonial. From inside, the three windows open up the room and add a deep sill.
Bay and bow window FAQs
How much does a bay window cost in Waltham, MA?
Bays and bows vary more than any other window. A narrow bay and a wide custom bow can be very different jobs, and many replacements also need the roof over the bay rebuilt. The size, brand and series, support system, and any framing repair all affect the price. That’s why we come out, look at the opening and the framing, and quote it in writing. If a full replacement is more than you want to spend, swapping the old bay for a flat unit of three windows is often far more affordable. We offer 18 months 0% financing.
What is the difference between a bay and a bow window?
A bay window is usually three windows at sharp angles, a large center window with two angled sides. A bow window uses four to six same-size windows in a gentle curve. Bays project farther and create a defined nook. Bows give a wider, softer view.
Can you replace an old bay window without rebuilding the wall?
Usually, yes. If the framing is sound, we remove the old unit and install a new one in the same opening, with new support, insulation, and flashing. If we find rot in the framing or seat, we’ll show you and repair it before the new unit goes in.
Can I turn a regular window into a bay window?
Yes. This kind of project is called a conversion. We create a new opening and install a new header above it. That’s structural work, so we calculate the header size and get the right documents for the building inspector. We did exactly this on a Waltham home, turning a flat double-hung into a Marvin Elevate bay.
Can you replace a bay window with a flat window?
Yes. That’s a conversion too, and a popular one. Replacing a bay usually means rebuilding its roof, insulating it, and re-shingling and flashing it, which adds up. Removing the bay and installing a flat unit of three windows is often far more affordable, and you still get plenty of light.
What other conversions do you do?
Plenty. For example, a door and a window side by side can become a three-panel sliding door. Any job that changes an opening needs a new header sized for the load and a building permit, and we handle both.
Do bay windows need support?
Yes. A bay or bow is heavy and hangs out from the wall. We install a support system, like cables anchored to the framing above or brackets below, so it won’t sag.
Are bay windows energy efficient?
Modern ones are, when they’re installed right. Choose Low-E glass with argon or triple-pane, and make sure the seat and headboard are insulated. Old bays usually lose most of their heat through those uninsulated areas, not just the glass.
Do bay windows need a roof?
Most do. The roof or cap keeps rain and snow out, and most bay replacements mean rebuilding it. It can be a simple aluminum cap, matching shingles, or a standing-seam copper roof. Flashing it properly into the wall is the key step.
How long does it take to install a bay or bow window?
Most bay and bow replacements take 1 to 2 days on site. The custom unit takes about 5 to 6 weeks to arrive after measuring.
Do I need a permit for a bay window in Waltham?
Yes. Massachusetts requires a building permit for window replacement, and a new or enlarged opening needs structural review too. We always recommend pulling one. Top Sash pulls the permit with the Waltham Building Department.
Can I add a window seat?
Yes. Marvin Elevate and Ultimate bays and bows come with a seat and headboard. We can insulate under it and finish it to match your trim.
More replacement windows
Ready for a new bay or bow window?
Free in-home estimate. We’ll check your framing, show you Marvin, ProVia, and Andersen options, and give you a clear price.
Top Sash Windows & Doors · Waltham, MA · Serving Waltham, Newton, Watertown, Brookline, Belmont, and 15 more towns