A bay or bow window is cold because it sits outside the insulated wall on three sides, and its weakest points are the box under the seat board and the box above the head. Both are often stuffed with a loose scrap of fiberglass, or nothing, and open to outside air. Insulating and air sealing those cavities and the mull joints fixes a cold, sweating bay seat.
Bays and bows are common on Twin Cities houses built from the 1970s on, and the complaint is always the same: the seat is freezing, the plants die there, and in January there is water or frost on the seat board under the glass. The windows get blamed. Sometimes they deserve it. More often the problem is the carpentry around them. This post walks through each part of the assembly, why it leaks, and how it is fixed.
Why is a bay window colder than the rest of the room?
A bay is a box that projects a foot or two out from the house wall, with glass on three sides, a floor you can sit on and a small roof over it. The wall it replaces was insulated from basement to attic. The bay has to supply its own insulation on the bottom, the top and all three glass faces, in a few inches of framing built by whoever set the window.
The glass faces are windows and insulate like windows: Low-E glass, argon fill, double or triple pane, warm-edge spacers, with a whole-window U-factor on the label. On the Northern ENERGY STAR path that is 0.22 or lower. Glass is always colder than an insulated wall, and a bay has three faces of it, so the room around a bay will never be quite as warm as one with a flat wall. That is expected.
What is not expected is a seat board you cannot sit on and frost on the inside. That comes from the seat cavity and the head cavity, the two boxes that hold the bay up and cap it off.
What is under the seat board?
Pull off the skirt board below a bay and you find a framed box. The top is the seat board and its subfloor; the bottom is a piece of plywood or a soffit panel; between them are the cantilevered joists or brackets holding the whole thing up. That box is the seat cavity.
In the average installation the cavity has a batt of fiberglass laid in it, sometimes only under the center unit, and the bottom panel is nailed on with no sealant. Fiberglass only insulates when air is not moving through it. With the bottom panel open at the seams and the box open at the wall line, outside air washes through the batt all winter and the seat board sits on a cavity at roughly outdoor temperature.
The fix has two parts, and the second matters as much as the first. Insulate the cavity fully, to the depth of the framing, with rigid foam board cut tight and foamed at the edges, or with closed-cell spray foam, which insulates and air seals in one pass. Then air seal the box: sealant or foam at every seam in the bottom panel, at the skirt, at the joint where the box meets the house wall, and inside at the joint between the seat board and the window frames. Loose batts and an open skirt is how the bay was built. Foam and a sealed skirt is how it should have been.
Why does a cold bay seat sweat?
Water on the seat board under the glass in winter is condensation, and it forms for the same reason it forms on a cold drink glass: room air holds moisture, and when it touches a surface colder than its dew point, the moisture comes out of the air onto the surface. The glass is the coldest surface in the room, so it sweats first, and the water runs down the glass onto the seat board.
But an uninsulated seat board is itself below the dew point on cold nights. Room air settles into the bay, cools against the seat, and condenses directly on it, along the joint where the seat meets the window frames and across the whole board on the worst nights. That is why a cold bay seat is wet even when the glass is only lightly fogged, and why the finish on the seat board lifts, the paint stains and the seat board eventually rots at the frame joint.
Warming the seat is the cure. Once the cavity below is insulated and sealed, the seat board runs close to room temperature and the moisture stays in the air. The glass will still fog on the coldest nights if indoor humidity is high, but the seat stops being the thing that sweats. If the seat board has already gone soft at the frame joint, it needs replacing, and the rot is a message about what is under it.
How are the head and the roof insulated?
The top of a bay is the mirror image of the bottom. The head board you see from inside sits under a framed cavity, and above that is the roof cap: a small hip or shed roof, usually shingled or wrapped in copper or aluminum, with flashing where it meets the house wall. Some bays have no separate roof and tuck under the house soffit instead; those still have a head cavity.
The head cavity has the same problem as the seat: a loose batt or nothing, open at the soffit or the wall line, so wind moves through it. Because warm air rises, it also collects the warmest, moistest room air through any gap in the head board or trim, and that air condenses on the cold underside of the roof sheathing. Frost inside the roof cap in February, dripping in March, is a common finding when a bay head is opened up.
The fix is the same as the seat: fill the cavity with rigid foam or closed-cell spray foam, and air seal the head board to the window frames and the wall so room air cannot get into the cavity at all. Then deal with the roof from outside. The step flashing or apron where the bay roof meets the siding is where these leak. The cap should have a kickout or counterflashing at the wall, sealed laps, and no exposed nail heads. A drip cap over the head trim of the individual units keeps water off the head joint.
On a bay with no roof, tucked under the eave, the head cavity often connects to the vented soffit above. Block the soffit end of the cavity with foam board and seal it, or the head board is a lid on a cold box.
What about the mull joints between the units?
A bay is usually three windows: a picture or fixed unit in the center and two casements or double-hungs angled out at the sides. A bow is four to six narrower units on a gentle curve. Where each unit meets the next is a mull joint, a vertical seam between two frames set at an angle, covered by a mull post or a trim cover inside and out.
Factory-assembled bays come with the mulls sealed and reinforced. Site-built bays often have mull joints that are two frames screwed together with a strip of trim over the gap. Wind finds that gap. Inside, you feel it as a cold line down the mull cover; on a bad night you can see frost on the trim.
Fixing a leaky mull means removing the interior mull cover, filling the joint between the frames with low-expansion foam or backer rod, sealing both faces, and putting the cover back. Outside, the mull cover or post should be caulked to both frames with a sealant that stays flexible. Casements that pull into a compression gasket leak less air at the sash than double-hungs, and that shows up most on the angled flankers, which take the wind.
- Cold line down the interior mull cover: unsealed joint between frames. Pull the cover, foam, seal, replace.
- Frost on the mull cover itself: cover is a thin, uninsulated strip over an open gap. Same fix, and consider an insulated cover.
- Draft at the flanker sash but not the mull: worn casement gasket or pile seal on a double-hung. Replace the weatherstripping.
- Water stain at the base of the mull: exterior mull cover caulk failed, or the head flashing above is dumping water on it.
Do the windows themselves matter?
Yes, once the boxes are done. If the seat and head cavities are insulated and sealed and the mulls are tight, what is left is the glass and frames. Three faces of glass in a Minnesota winter is a lot of glass, so a bay is one of the places triple-pane earns its price: the glass runs warmer, the seat is warmer, and the plants on it live.
The glass package should be a Low-E coating on the inside of the outer pane, argon or krypton fill, a warm-edge spacer so the bottom edge of each pane does not frost, and a whole-window U-factor on the label at or under the ENERGY STAR Northern figure of 0.22 on the prescriptive path. The frames should be foam-filled or multi-chamber vinyl, composite, fiberglass or clad wood; bare aluminum frames on an older bay are a cold bridge and frost on their own.
Each unit also meets the seat board, the head board and its neighbors, and every one of those joints needs low-expansion foam in the gap and an interior air seal at the trim line. A bay with new triple-pane glass and an empty seat cavity is still a cold bay.
How do you tell where the cold is coming from?
Spend a cold, windy evening working slowly around the bay with your hand. Moving air is a leak; still cold is missing insulation. An incense stick shows the leaks better than a hand, and an infrared thermometer shows missing insulation as a cold patch on the seat or head board.
| What you notice | Likely cause | Fix |
|---|---|---|
| Seat board cold to the touch across its whole width | Seat cavity uninsulated or wind-washed | Fill and air seal the seat cavity; seal the skirt |
| Water or frost on the seat board, not just the glass | Seat below dew point; same cause as above | Insulate below; lower indoor humidity; replace a soft seat board |
| Draft at the joint between seat and window frames | No foam or sealant where frames meet the seat | Pull the stool or apron trim, foam, seal, reinstall |
| Cold line down the interior mull cover | Unsealed mull joint between units | Remove cover, foam and seal the joint, replace |
| Head board cold; frost or drips at the top in late winter | Head cavity empty or open to soffit; roof cap condensing | Insulate and seal the head cavity; block the soffit end |
| Water stain on head board or top of mull after rain or thaw | Roof cap flashing or drip cap failed | Reflash the cap at the wall; add or replace drip cap |
| Cold at the flanker sash only | Worn weatherstripping on the angled units | Replace gaskets or pile; check the lock pulls tight |
| Whole bay cold, glass frosts at the edges, boxes are fine | Old glass, no Low-E, aluminum spacer or frame | Replace the units with Low-E, argon, warm-edge glass |
What JMB Builders does
JMB Builders is a licensed Minnesota residential building contractor (BC809005) based in Zimmerman, working across the Twin Cities metro. We open up bay seats and heads, insulate and seal them, replace rotted seat boards, tighten mull joints and re-trim the inside. When the units themselves are done, we replace them. Send photos of the seat, the head and the outside of the bay and we will tell you which parts need work. Call 763-204-2099 or send photos through the site.
- Bay window interior trim: seat board, head board and jambs — jmbbuildersmn.com/services/window-trim/bay-window-trim-out
- Rotted window sill or seat board inside? Stool replacement and what the rot means — jmbbuildersmn.com/services/window-trim/interior-window-sill-rot-replacement
- Drip cap installation above windows and doors — jmbbuildersmn.com/services/exterior-trim/drip-cap-installation
- Insert and full-frame replacement windows, including bay and bow units, measured and ordered by us — jmbbuildersmn.com/services/full-service
- Send three photos per window, written quote within one business day.
Frequently asked questions
Why is my bay window seat so cold?
The seat board sits on a framed box that hangs outside the house wall, and in most bays that box holds a loose scrap of fiberglass or nothing, with an unsealed skirt panel underneath. Outside air moves through it all winter and the seat runs near outdoor temperature. Filling the cavity with rigid or spray foam and sealing every seam warms the seat.
How do you insulate under a bay window?
Remove the skirt or bottom panel from outside, or the seat board from inside, and fill the cavity to the full depth of the framing with rigid foam board foamed at the edges or with closed-cell spray foam. Then seal every seam: the bottom panel, the skirt, the joint at the house wall and the joint where the seat board meets the window frames inside. Insulation without air sealing does not fix a bay.
Why does my bay window get condensation on the seat?
Room air holds moisture, and when it touches a surface colder than its dew point the moisture condenses on it. An uninsulated seat board gets cold enough on winter nights to be that surface, so water forms on the seat itself and not only on the glass. Insulating the cavity under the seat brings the board close to room temperature and stops the sweating; lowering indoor humidity helps the glass.
Does the roof of a bay window need insulation?
Yes. The head board inside sits under a cavity that is usually empty and often open to the soffit or to outside air at the roof cap. Warm room air leaks up into it and condenses or frosts on the cold roof sheathing. Fill the head cavity with foam, air seal the head board to the frames and wall, block any connection to the vented soffit, and make sure the roof cap flashing at the house wall is sound.
What is a mull joint on a bay window?
It is the vertical seam where two of the window units meet at an angle, covered by a mull post or trim cover inside and out. On factory-built bays it is sealed and reinforced; on site-assembled bays it is often two frames screwed together with a strip of trim over the gap, and it leaks air. The fix is to remove the interior cover, foam and seal the joint, and re-caulk the exterior cover to both frames.
Should I replace a bay window or insulate it?
Insulate first if the glass units are sound: clear, not fogged, with Low-E glass and frames that are not bare aluminum. Most cold-bay complaints come from the seat and head cavities and the mull joints, not the glass. Replace the units when they are single-pane, have failed seals, have aluminum frames that frost, or when the seat and head framing has rotted badly enough that the bay is being rebuilt anyway.
