A roller blind ordered for a bedroom window 30 inches wide arrives with fabric about 28 5/8 inches wide. Nothing went wrong at the factory. On an inside mount, the brackets and the clutch need room at each end of the tube, and one major manufacturer’s measuring guide puts that space at 11/16 inch per side on chain-operated and motorized rollers. Whether those two strips of daylight matter is most of what the choice between inside mount vs outside mount blinds comes down to, along with a number few people think to measure: the depth of the window frame.
Both numbers can be checked with a tape measure before fabric even comes up.
What Inside Mount and Outside Mount Mean
An inside mount puts the brackets inside the window opening, fixed to the top or sides of the frame. The blind sits within the casing and the trim stays visible all the way around.
An outside mount puts the brackets on the wall or on the face of the trim. The blind is made wider and taller than the opening so it overlaps on every side.
The two are ordered differently. For an inside mount you give the exact size of the opening and the factory makes the blind slightly smaller so it runs freely. For an outside mount you give the finished size you want covered, and that is the size you get. A blind made for one mount rarely works for the other, so the mount has to be settled before anyone writes down a width.
How Much Depth an Inside Mount Needs
Depth is the flat surface inside the frame, measured from the front edge of the opening back to the first thing that sticks out: the glass, a crank handle, a lock, a tilt latch. Measure it at the left, centre and right, because frames are rarely the same depth all the way across, and keep the smallest number.
Our window shade measuring guide lists planning depths for the hardware we install:
| Hardware | Approximate depth to plan for |
|---|---|
| Cellular headrail | About 2 to 2 1/2 in |
| Open-roll roller | About 3 in |
| Compact cassette | About 3 1/2 in |
| Fascia | About 3 1/2 to 4 3/8 in |
| Motorized roller | Confirm motor and bracket depth |
| Zebra cassette | Confirm cassette depth before ordering |
Take a window with 2 3/4 inches of flat depth. A cellular blind fits inside it with room to spare. An open-roll roller blind wants about a quarter inch more than the frame offers, so the roll would stand slightly proud of the trim. A compact cassette would project about three quarters of an inch. Some people are fine with a roll that sits partly outside the frame; others see it every time they walk into the room. It is easier to decide at the quote stage than on installation day.
Two product lines need a specific check. Zebra blinds come in a cassette, and cassette depth varies by system. Motorized blinds use different brackets from manual ones, so the depth has to be confirmed for the exact motor.
Depth also has to clear whatever moves. Casement cranks are the usual culprit: the handle sits right where a bottom rail wants to travel. A reverse roll, where the fabric comes off the front of the tube, adds space between the fabric and the window and is sometimes enough. When it isn’t, the blind goes outside the frame.
The Light Gap on an Inside Mount, in Real Numbers
Every inside-mount blind is narrower than the opening it hangs in. How much narrower depends on the product.
On a roller blind, the fabric has to be cut narrower than the mechanism at the top, because the bracket, the clutch or motor head and the idler pin all sit beyond the fabric edge. The manufacturer measuring guides we checked give 9/16 inch per side for a cordless roller and 11/16 inch per side for chain-operated and motorized versions. Another maker’s specification sheet states a minimum of 1/2 inch per side across its roller range, and a retail guide for a chain-operated shade lists about 1 inch on the chain side and 1/2 inch on the other.
Cellular blinds are much tighter. The same retail guide puts their gap at up to 1/4 inch per side, since the fabric runs nearly the full width of the headrail.
Put those figures on that 30-inch bedroom window, and say it is 54 inches tall. A chain-operated roller with 11/16 inch of clearance on each side leaves two strips that add up to about 74 square inches of uncovered glass. A cellular blind at 1/4 inch per side leaves 27. Mount the same roller outside the frame with 3 inches of overlap on each side and the fabric covers the whole opening; the only light left is what slips between the back of the fabric and the wall.
In a living room, 11/16 inch of daylight at each edge rarely bothers anyone. In a bedroom facing a streetlight, or a nursery at nap time, it’s the first thing you see. Our comparison of room darkening and blackout fabrics covers the fabric side of that problem. The mount decides how much of the fabric’s performance reaches the room.
Gap sizes differ from one hardware system to the next, so ask for the figure for the exact blind being quoted.
Inside Mount vs Outside Mount Blinds, Compared
| Factor | Inside mount | Outside mount |
|---|---|---|
| Where the brackets go | Inside the frame, at the top or sides | On the wall or the face of the trim |
| Size you order | The exact opening; the factory makes the blind slightly smaller | The finished size you want; made as ordered |
| Frame depth needed | About 2 to 4 3/8 in, depending on hardware | None, but at least 2 in of flat surface above the opening for brackets |
| Light at the edges | Roller: roughly 1/2 to 1 in per side. Cellular: up to 1/4 in | Covered by the overlap |
| Trim and casing | Stay visible | Partly or fully covered |
| Handles and cranks | Must clear inside the frame | The blind hangs in front of them |
| Out-of-square openings | Gaps widen where the frame runs out | The overlap hides the variation |
| Suits | Deep frames, good trim, blinds layered behind curtains, cellular blinds | Shallow frames, bedrooms, doors, crank windows |
Plenty of houses end up with both. A deep living-room window with painted casing takes an inside mount, and the bedroom down the hall, same builder and same window series, gets an outside mount because someone there sleeps past sunrise.
How Much Overlap an Outside Mount Needs
Our measuring guide suggests 1.5 to 3 inches of overlap on each side for most rooms. The low end suits a kitchen or a living room, where the aim is a tidy edge and reasonable privacy. Bedrooms deserve the full 3 inches, which is also the figure the manufacturer and retail guides we checked recommend for light control.
Height works the same way. The brackets need at least 2 inches of flat, solid surface above the opening or on the face of the trim. One retail guide adds a minimum of 2 1/2 inches to the height for them, and starting above the opening also keeps the rolled-up blind clear of the glass when it’s raised. At the bottom, run the blind to the sill. Where there is no sill, match the overlap you used at the top.
For the 30 by 54 inch bedroom window, that works out to a blind 36 inches wide, mounted 2 1/2 inches above the opening and dropping to the sill: 36 by 56 1/2 inches.
Two details catch people out. First, on a roller blind the fabric is narrower than the hardware on an outside mount too, so ask whether the width on the quote is the fabric or the full bracket-to-bracket measurement. Second, a blind that covers 3 inches of casing on each side may leave a thin strip of wider casing showing beyond the fabric. Decide whether the blind should cover the trim completely or only the glass, and say so when the measurements go in.
Which Mount Keeps More Heat In?
Natural Resources Canada estimates that windows, doors and skylights can account for up to 35 percent of a house’s total heat loss, and its guidance on upgrading windows and exterior doors notes that curtains and blinds help reduce radiant heat loss during the heating season.
The best-documented product is the cellular blind. In a U.S. Department of Energy study run at a test house in Knoxville, Tennessee, from December 2019 to May 2020, cellular shades cut heating energy by up to 20 percent and combined heating and cooling energy by up to 15 percent compared with bare windows. The department’s summary credits well-fitted shades in particular and describes how they work: air is trapped inside each cell and in the space between the shade and the glass. Laboratory simulations cited in the same summary found that cellular shades saved more energy than every other shading type modelled. Knoxville winters are far milder than Ottawa’s, so the percentages show direction only; they don’t predict a heating bill here.
Neither source ranks inside mount vs outside mount blinds on heat loss, and we haven’t found a published test that does. The mechanism points to fit. The less air that can move around the edges of a blind, the better the trapped layer holds. A cellular blind mounted inside the frame, with gaps of a quarter inch or less, is about as close-fitting as an ordinary blind gets. A roller blind inside the frame has wider gaps, and outside the frame it covers the whole opening but stands off the wall by the depth of its brackets. If warmth is the priority, an inside-mounted cellular blind is the place to start.
A snug blind has one side effect in a Canadian winter. The same federal guidance points out that curtains and blinds are not airtight and may cause window condensation problems in cold weather. Our guide to blinds and window condensation explains how to manage it.
Windows Where the Mount Is Already Decided
Doors
French doors have very shallow glazing frames and a handle in the way, so a blind there is fixed to the face of the door and sized to clear the handle. A sliding patio door usually gets an outside-mounted blind above the frame, often split in two so the panel over the door you walk through can stay up while the other comes down.
Openings that are out of square
Measure the width at the top, middle and bottom. One retail measuring guide draws the line at half an inch: if the three readings differ by more than that, the blind may not run properly and the light gaps will be larger than expected, and an outside mount is the better choice. Check this first in older houses, where frames have had decades to settle.
Tile, metal and concrete
A bathroom window tiled right into the opening, or a condo window with a metal frame and a concrete header, changes the question from which mount looks better to where a screw is allowed to go. In a condo, the answer may be in the declaration; our guide to condo window covering rules covers what to look for before drilling.
Deep frames with trim worth showing
Inside mount, nearly always. The blind disappears into the opening, the casing frames it, and curtains can still hang in front on their own rod.
Shallow basement frames and hopper windows that tilt inward are a common case where outside mount wins. See blinds for basement windows for a worked sizing example.
One Safety Check Before Either Mount
Canada’s corded window covering regulations came into force in 2021 and apply to custom-made blinds as well as off-the-shelf ones. Health Canada’s window covering safety page tells households to securely attach any tension device supplied with a blind so a child can’t get the cord around their neck, and to follow the manufacturer’s installation instructions. In practice, a chain-operated blind needs a solid spot for that device beside the window whichever way it is mounted: the frame on an inside mount, the wall or trim on an outside one. A motorized or cordless blind has no chain to anchor.
A Quick Way to Decide
- Measure the flat depth of the frame at the left, centre and right. Keep the smallest reading.
- Compare it with the hardware table above. If the frame is short by more than you can live with seeing, go outside.
- Look for cranks, handles and locks in the path of the fabric.
- Measure the width at the top, middle and bottom. More than half an inch of difference points to an outside mount.
- Ask how dark the room has to get. For a bedroom or nursery, that means a roller blind outside the frame with 3 inches of overlap, or a cellular blind inside it.
If all five answers favour the frame, mount inside and enjoy the trim.
Frequently Asked Questions
Is inside mount or outside mount better for blackout?
Outside mount, in most bedrooms. An inside-mounted roller blind leaves roughly 1/2 to 1 inch of open glass down each side however opaque the fabric is, and an outside mount with 3 inches of overlap covers it.
How much depth do I need for inside mount blinds?
About 2 to 2 1/2 inches for a cellular headrail and about 3 inches for an open-roll roller blind, going by the planning depths in our measuring guide. Cassettes and fascias need 3 1/2 inches or more. A shallower frame can sometimes still take an inside mount with the hardware projecting past the trim.
How far should outside mount blinds overlap the window?
Plan on 1.5 to 3 inches per side, and use the full 3 inches in bedrooms. Above the opening, allow 2 1/2 inches or more for the brackets.
Can I switch from an inside mount to an outside mount later?
Rarely with the same blind. An inside-mount blind is made narrower than the opening, so hung outside the frame it would leave the edges of the glass exposed. That’s why the choice between inside mount vs outside mount blinds belongs at the measuring stage, before the order goes in.
Do outside mount blinds make a window look bigger?
They can. A blind that extends past the opening on each side and starts above it reads as a larger window, which helps a small or awkwardly placed one. The casing disappears behind the fabric, though.
Which mount is better for insulation?
A cellular blind mounted inside the frame, where its side gaps are a quarter inch or less, is the closest fit. We found no published test that compares the two mounts directly.
Can I mix inside and outside mounts in one house?
Yes, and it’s common. Keep the mount consistent where windows sit side by side in one room, and let bedrooms and doors differ from the living areas.
Bring the Measurements, or Let Us Take Them
If you’re still weighing inside mount vs outside mount blinds for a particular room, three depth readings and three widths per window are enough to start. Send them with a photo through our request a quote form, or book an in-home consultation in Ottawa through our services page, where we measure each frame and check hardware depth against it before anything is ordered. Montréal customers are served with French-language support. In the meantime, browse the full range of custom blinds, or contact us about a window that doesn’t fit any of the cases above.
