How Do Curtains Reduce Heat Loss

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During winter, heat escapes through windows, driving up heating costs. Curtains create a barrier that reduces heat loss and makes your home more energy efficient. The right curtains can lower your heating bills by up to 25%.

How Do Windows Lose Heat?


There are three ways windows allow heat to escape:

Conduction: Heat flows directly through the glass to the outdoors. Double or triple-paned windows with argon or krypton gas filling reduce conductive heat loss.

Convection: The air against the cold window glass becomes cooler and denser. It sinks to the bottom of the window and forces the warm air against the room side of the window to rise. This creates airflow that carries heat from the room directly through the window.

Radiation: Heat radiates from warm objects to cold surfaces. At night, heat from warm objects in the room radiates through the window glass to the cold outdoors. During the day, heat radiates from the sun through the window glass and warms up the room.

Curtains create a dead air space along the window that acts as an insulator, reducing all three types of heat loss. The still air minimizes convection currents. The fabric blocks radiant heat transfer. Depending on the type of curtains, they also slow conductive heat loss.

How Effective Are Curtains at Reducing Heat Loss?

Studies by the U.S. Department of Energy found that closing tightly-fitting curtains can reduce heat loss by 10-25%. The University of Salford in England found that closing curtains at dusk reduces heat loss by 15-17% and lowering blinds decreases it by 13-14%.

The level of insulation depends on the fabric, color, and fit:

  • Fabrics: Dense fabrics like velvet, tapestry, and brocade insulate better than lightweight fabrics like linen and lace. Insulated linings also boost efficiency.
  • Color: Lighter colors reflect more radiant heat than dark colors. White plastic backings reflect up to 90% of radiant heat.
  • Fit: Eliminate gaps between curtains and walls or window panes. Sealing edges with tape increases insulation. Floor-length curtains insulate better than short ones.

Curtain design also affects insulation:

  • Layers: Sheers covered by opaque curtains create an insulating air gap. Thermal linings or double curtain layers improve insulation further.
  • Pleats & Valances: More fabric overlaps capture more insulating air. Pinch pleats are very effective.
  • Cornices: Pelmets fitted above the curtains minimize air flow and heat loss at the top of windows.
  • Mounting: Tracks mounted very close to windows maximize curtain contact area.

Curtain Fabrics with the Best Insulating Properties

Choose medium to heavy weight fabrics with a tight weave:

  • Velvet: Has a plush pile that traps air. Thermal linings boost insulation. An excellent insulator.
  • Faux Suede: Dense twill-woven fabric with a soft nap that slows convection. Available lined or unlined.
  • Tapestry & Brocade: Tightly woven with intricate patterns. The piles trap air well. Often have fabric linings.
  • Faux Silk & Satin: Tightly woven fabrics with smooth surfaces that reflect radiant heat. Best when lined.
  • Blackout Lining Fabrics: Made of vinyl, plastic, or acrylic-coated materials that block light and radiant heat.
  • Interlining Fabrics: Lightweight knits or woven materials used as curtain linings to add insulation. Cotton flannel and synthetic blends like polyester work well.
  • Insulated Linings: Contain materials that resist heat flow like foam, batting, or laminated films. Boost R-values significantly.

Curtain Color for Heat Retention

Lighter colors reflect radiant heat better than dark colors:

  • White: Reflects 76-90% of radiant heat, depending on material. Excellent for plastic backings.
  • Cream/Light Yellow: Reflects 70-76% of radiant heat. Warmer tone than white.
  • Silver/Light Gray: Reflects 50-75% of radiant heat. Matches cool color schemes.
  • Beige/Tan: Reflects about 60% of radiant heat. Works with warm & neutral décor.
  • Gold/Light Brown: Reflects roughly 55% of radiant heat. Warmer than beige.
  • Red/Orange: Reflects only 25-40% of radiant heat but adds cheerful color.
  • Green/Blue: Reflects 25-35% of heat. Colorful yet still energy efficient.
  • Black: Absorbs almost all radiant heat. Only use blackout linings, not main curtains.

Curtain Styles that Insulate Windows

Thermal curtains are available in any style. Choose medium to heavy fabrics, include linings, and ensure a good seal with the window for optimal insulation.

  • Rod Pocket Curtains: Slide easily onto curtain rods. Leave minimal gap between rod and window.
  • Tab Top Curtains: Attach to rod with button tabs. Hang tabs slightly overlapping for a tighter fit.
  • Back Tab Curtains: Like tab top but tabs are reversed to hide behind main fabric. Create a smoother look.
  • Grommet Top Curtains: Ring holes slide onto rods. Overlap grommets for tighter fit.
  • Pleated Curtains: Cartridge pleats & pinch pleats allow more fabric overlap for insulation.
  • Curtain Panels: Use two wider panels rather than one for more overlap in center.
  • Thermal Blackout Curtains: Have backing laminated to fabric. Eliminate all light and block radiation.
  • Layered Curtains: Hang sheers and thermal curtains in separate tracks. Adjust for seasonal light and insulation needs.
  • Interlined Curtains: Sew a separate thermal lining to main curtains for added insulation.
  • Lined Curtains: Come with attached lining fabric to boost insulation. Often have blackout properties.

Hanging Thermal Curtains Properly

Proper installation maximizes insulating properties:

  • Cover Entire Window: Curtains should extend beyond window frames to seal small gaps. Allow an extra 2-3 inches overlap on each side.
  • Floor Length: For best coverage, curtains should reach the floor. Puddle curtains extend an extra 2-4 inches onto floor.
  • Tight Overlap at Center: For two curtain panels, overlap extra fabric at the center for a snug closure.
  • Sealing Edges: Use Velcro or double-sided tape to seal edges to window frames and walls. Prevent airflow around cracks.
  • Mount High and Wide: Mount rods 6-12 inches above window and 4-6 inches wider to allow complete window coverage when drawn.
  • Close Fitting: Rod pocket curtains should have less than 1 inch of extra room on rod for a snug, wrinkle-free fit.
  • Pelmets: Mount cornices at top to cover gap between mounting and curtain tops. Further reduce convection.

Maintaining Thermal Curtains

Follow the manufacturer’s care instructions. Some key tips:

  • Regularly open curtains to release trapped moisture and prevent mildew, especially in humid climates.
  • Vacuum both sides frequently using a soft brush attachment.
  • Spot clean stains on fabric-backed curtains with mild detergent and water.
  • Wash foam and plastic-backed curtains by hand with gentle detergent. Line dry. Do not machine wash or dry.
  • Many fabric curtains can be machine washed and dried on delicate settings. Avoid high heat.
  • Dry clean lined curtains and delicate fabrics. Request non-immersion cleaning to avoid backing damage.
  • Steam or press curtains gently on fabric side only. Do not iron plastic backings.
  • Wash and completely dry curtains before off-season storage to inhibit mildew.

Replace Insulating Curtains Every 5-7 Years

Curtain fabrics break down over time. Fibers weaken and insulative properties decrease. Lining adhesives can also fail after several years. Check curtains occasionally for small cracks or holes, especially along folds and edges. Drafts entering through deteriorated spots defeat the insulating purpose. Replace thermal curtains every 5-7 years to maintain optimal heat retention.

Insulate Windows Year-Round with Curtains

Curtains make an attractive and affordable way to reduce heating and cooling costs in any season. Follow these best practices:

  • Winter: Close thermal curtains at dusk to minimize heat loss through windows overnight. Open south-facing curtains during sunny days to capture free solar warmth.
  • Summer: Close blackout-lined curtains on sunny windows to block heat gain from direct sunlight. Open at night to release stored heat.
  • Spring & Fall: Adjust curtains to capture or block solar heat as needed. Close at night for insulation.
  • Use sheers on their own during very warm weather to let breezes flow through while preserving privacy.
  • Layer curtains and adjust them daily based on sunlight, temperature, season, and room use.

With the right curtains installed correctly, you can watch your energy bills drop without sacrificing interior style and natural light. Consult a window treatment specialist if unsure which products are best for your climate and windows. Investing in quality thermal curtains will pay off for years through lower utility costs.

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