Curtains vs Blinds for Insulation: What the AERC Numbers Say and Why the Edge Seal Wins

Curtains vs blinds for insulation: the short answer

Neither word tells you anything about insulation. "Blinds" covers a sheer roller and a blackout honeycomb; "curtains" covers a cotton panel on a tension rod and a lined drape that wraps to the wall. What decides heat loss at a window is whether the covering traps a layer of still air against the glass and whether its edges are sealed so room air cannot circulate behind it. Judge by that and the ranking is stable: a cellular shade or a lined, wide, floor-length curtain at the top; roller and roman shades in the middle; slat blinds and shutters near the bottom, because a slat product is a set of gaps by design.

The one certified number sits on the cellular side: the Attachments Energy Rating Council (AERC) publishes U-factors for certified window attachments, and its cellular-shade directory, pulled 2026-09-13, lists 114 products. Those figures anchor what follows; the curtain, blind, and shutter side rests on how each handles air, since no certified rating for them could be fetched.

How does a window covering insulate at all?

Heat leaves through a window three ways, and a covering has to answer all three.

Convection at the glass. Room air touching a cold pane cools, sinks, and pulls warmer air down behind it, a slow loop that carries heat out. A covering that holds a pocket of still air against the glass breaks the loop; still air is the working ingredient in every insulator from a honeycomb cell to a wall cavity.

The edge seal. The pocket only stays still if the edges are closed. Leave the top open and warm air enters there, cools on the glass, and exits at the bottom, a reverse chimney that runs whether or not the covering is thick. That is why the seal matters more than the material: an unsealed thermal curtain with a gap above the rod and a foot of air under the hem is a chimney with a nice fabric, and a plain cellular shade with tight edges is an insulator.

Radiant exchange. A warm room radiates toward a cold pane at night; in summer, sunlight radiates in. An opaque layer blocks the first and a light face toward the glass rejects part of the second.

AERC's directory makes the edge point its own way: every one of the 114 cellular rows carries an identical air-leakage value, so the directory does not separate products on the seal at all. That leaves it where it belongs, on the measurement and the mount.

What does the AERC data actually say about cellular shades?

Every cellular row in the pull is rated on baseline BW-B, a double-pane, clear-glass, non-metal-frame window, so the figures describe a shade on that window, not on your single-pane sash, and U-factors from different baselines cannot be mixed. Lower means less heat passes through.

• All 114 certified cellular shades: U-factor from 0.18 to 0.37, median 0.26.
• Room-darkening cellulars (35 rows): median 0.24.
• Light-filtering cellulars (79 rows): median 0.28.
• Single-cell rows (22): median 0.26. Rows tagged double-cell (8): median 0.275.

The last line runs against the sales pitch: in the certified directory, double-cell does not pull ahead of single-cell. That is not the same as single-cell winning; cell count is inferred from product names, 84 of the 114 rows do not state it, and eight rows is too few to rank on. The honest reading is that the data does not show a second layer of cells buying a lower U-factor, while opacity does: the room-darkening rows, which carry an opaque liner inside the cell, post the lower median. Our single-cell vs double-cell piece covers what the extra layer does buy.

Two caveats travel with the numbers. The pull holds only two manufacturers, Hunter Douglas with 108 rows and Springs Window Fashions, which owns Bali and Levolor, with six; SelectBlinds' house-brand cellulars are not in the directory, and absence is evidence of non-certification, nothing more. And AERC publishes no R-value; any R-value quoted for a cellular shade is the vendor's own number, which is why none appears here.

Which insulates better, a cellular shade or a curtain?

The cellular shade is the only product here with a certified figure; its cells are sealed pockets of still air, and its weakness is the perimeter. An inside mount sits inside the frame, made narrower than the opening so it can travel: on SelectBlinds' cellular measuring chart, most collections take a quarter inch off the headrail and three-eighths off the fabric. That is an air gap on each side plus the gap at the sill, and an outside mount that overlaps the frame closes most of it. SelectBlinds' Premier single-cell light-filtering cellular is the entry version of the idea; for a bedroom where the radiant block matters as much as the air layer, SelectBlinds' Click2Fit no-drill total-blackout cellular adds the opaque liner the room-darkening rows carry. The seller files both under its "Insulating/Energy Efficient" collection.

The curtain has no certified number, but it has two levers the shade lacks: mass and reach. A drape with a blackout or thermal lining is several fabric layers, and it hangs outside the frame, so it can be made wider and taller than the window and closed against the wall. Hung that way, rod above the frame, panels returning to the wall, hem at the floor, a curtain seals better than an inside-mounted shade ever will. Hung the usual way, on a rod barely wider than the frame with the hem at the sill, it seals nothing. Whether a curtain out-insulates a cellular is decided by the rod, the width, and the length, not the fabric; the best blackout curtains guide works through lined panels. For made-to-measure, TWOPAGES custom curtains lists eight liner options on its homepage, including blackout and thermal, and Curtarra's custom thermal curtains sell a triple-weave lining as the insulating layer. Curtarra publishes its own temperature claims and neither merchant carries an AERC rating, so treat vendor numbers as marketing and the lining menus as the useful part.

Choose the cellular shade when you want insulation that works every time it is down, you can outside-mount it or accept a small side gap, or there is no wall space for drapery. Choose the lined curtain when you can hang it wide, high, and to the floor and will actually draw it, or the window is a sliding door or a wide span where a single shade gets heavy. Skip the curtain for insulation when it will hang at the sill on a frame-width rod; that version is decor.

Roller, roman, slat blinds, shutters: where do they land?

Roller and roman shades are a single layer. A blackout roller stops radiant exchange and adds one air gap, but the top and sides are open unless the line offers side channels. A lined roman does a little better because the folds add layers; blackout roman shades covers those. Both sit behind a cellular on the still-air test, and cellular vs roller shows how much of that is design rather than fabric.

Slat blinds, wood, faux wood, or aluminum, are the weakest insulators in the group. Closed slats overlap at dozens of seams, the route holes for the lift cords are open, there is no seal at any edge, and aluminum adds conduction. They are chosen for light control and looks; wood vs faux wood is the right question for that decision, not this one.

Shutters get asked about most, and the honest answer is that a shutter is a slat product in a fitted frame. The frame is scribed to the opening, so the perimeter seals better than a blind's; the louvers are joints, and closed louvers meet at an angle, not a seal. Against a lined curtain hung to seal, a louvered shutter loses on air and mass; against a cellular it loses on the still-air layer. Choose shutters when you want a hard-wearing, wipeable covering that controls light for decades; skip them for insulation and add a curtain over them if the room is cold.

Solar screens are a summer product and do nothing for winter: the weave is open so you keep the view, and an open weave cannot hold still air. Solar vs roller shades covers that trade.

Does the answer change between winter and summer?

Winter is a retention problem. The room is warmer than the glass, so the covering needs a low U-factor and a closed perimeter, and it only works while closed. Draw at dusk, open sunny south windows by day for free solar gain, and put the sealing effort into the top and bottom, where the loop starts and ends. This is where the room-darkening cellular's 0.24 median and a lined drape's mass pay.

Summer is an interception problem. The heat is sunlight, and once it is through the glass and absorbed by a dark covering it is in the room; a dark curtain in a west window is a radiator by mid-afternoon. The metric is solar heat gain coefficient, and the AERC pull has it: room-darkening cellulars post a median SHGC of 0.18 against 0.32 for light-filtering, the opaque liner again. So prefer a light face toward the glass, a covering that rejects sun over one that soaks it up, and anything outside the glass over anything inside it. A solar screen keeps the view and cuts glare; a light-colored room-darkening cellular blocks more and loses the view; a white-lined curtain sits between. The home office glare guide is the summer version of this article.

When is both the right answer, and when is it overkill?

A cellular inside the frame and a lined curtain outside it are two layers doing different jobs: the shade supplies the still-air layer at the glass, the curtain supplies mass, the radiant block, and the perimeter seal the shade cannot reach from inside the frame. The cellular closes the middle and the curtain closes the edges, the stack our bedroom blackout strategy uses for darkness.

Choose both when the window is single-pane or drafty, faces north in a cold climate, is a sliding door you use every day, or belongs to the bedroom that never warms up. Skip the second layer when the window is a modern double-pane unit in a mild climate, the room is rarely heated or cooled, the window is small, or the curtain would hang at the sill anyway. Then the second layer costs more than it returns, and the same money spent weatherstripping the sash does more; on a leaky frame, the frame is the problem.

Decision table: which covering for which job?

Only the cellular rows carry certified data. Cost tiers are in words on purpose: SelectBlinds prices were read at the configurator's default size on a sale day and move with promotions, and the shutter and ready-made tiers are market position, not a fetched figure. Of the products linked here, the single-cell light-filtering cellular is the entry tier, the no-drill blackout cellular a tier up, and the Premier Custom Drapes the top.

Which covering for which room?

• Bedroom: a room-darkening cellular first; add a lined curtain if the room is cold or the street is bright. The best cellular shades roundup ranks candidates.
• Living room with a picture window or patio door: drapery, because a wide span is where a curtain's reach beats a shade's weight; wide window and patio door coverings has the vertical-cellular alternative.
• Home office: the fight is glare, not heat; a light-filtering cellular or a solar screen, with a lined curtain only if the room runs cold.
• Rental: a no-drill cellular in the frame and a curtain on a tension or bracket rod; drill-free blind options has the hardware.
• The cold north room: both layers, an outside-mounted shade under a floor-length lined curtain, after checking the sash weatherstripping.

How do you measure and mount for the seal?

Inside mount. SelectBlinds' cellular measuring page has you measure the width at the top, middle, and bottom and use the smallest, then the height at the left, center, and right and use the largest, rounding width down and height up to the nearest eighth. Then stop: "We will make all the necessary deductions based on the measurements you provide during checkout. Do not make your own deductions." Its chart lists the deduction per collection, a quarter inch off the headrail and three-eighths off the fabric for most cellular rows, with a stated tolerance of plus or minus an eighth. Every seller applies its own deductions and they differ, so give the opening and let the factory cut. That deduction is your air gap, and where the seal matters it argues for the other mount. Inside vs outside mount walks the trade-offs; the glossary defines inside mount and outside mount.

Outside mount. Here the factory makes exactly what you order: "We will make your product to the measurements you give us, and we won't make changes or additions." SelectBlinds asks for at least two and a half inches of flat surface, then the largest of three width measurements plus at least two inches on each side, four in total, and at least two inches added at the top and the bottom. The page adds "For more light coverage and privacy, add even more to the measurement," and the same overlap that blocks light is the one that closes the air path.

Curtains. Rod wider than the frame on both sides and mounted high, ideally at the ceiling; enough fullness to return to the wall at the outer edges; length to the floor, not the sill. A cornice or a wrap-around rod closes the top, the inlet of the loop and the gap most people leave. SelectBlinds' Premier Custom Drapes are made to the width and length you give, and the seller files the line under both its blackout and its insulating collections. The curtains and drapery picks list alternatives, and the cellular shades page does the same for the other layer.

The honesty that goes with a custom order. Made-to-size shades and drapes are cut to your numbers and are generally not returnable, so the measuring net is the seller's remake policy, not a refund. SelectBlinds' FIT Protection covers a corrected size within two inches per side, claimed within 90 days of purchase, one remake per product and four per household for life, with remake shipping charges applying. Its standard warranty is three years and an oversize fee of $85 applies over 90 inches wide. Bali's lifetime warranty runs as long as the original purchaser owns the product, with five years on draperies and fabric cornices and valances. Policy text is perishable; read the current page before ordering.