Windows are a weak point in any building's thermal envelope. They allow heat to escape in winter, let heat in during summer, and often have drafts around the frames. Renters can't replace windows, but several effective improvements don't require permanent changes.
Understanding Window Heat Transfer
How Heat Moves Through Windows
- Conduction: Heat passes directly through the glass
- Radiation: Infrared heat radiates through the glass
- Air leaks: Conditioned air escapes around frames and seals
- Solar gain: Sunlight heats the space through the glass
Winter vs. Summer
Windows work against you in both seasons:
- Winter: Heat escapes through glass and around frames
- Summer: Heat enters through glass from direct sun
Window Treatments
Curtains and Drapes
The right curtains help in both seasons:
- Thermal curtains: Insulated backing reduces heat transfer
- Blackout curtains: Block light and provide some insulation
- Heavy drapes: More mass means more insulation
- Proper installation: Should extend beyond window frame and reach floor
Using Curtains Effectively
- Winter nights: Close curtains to reduce heat loss
- Winter days: Open south-facing curtains for solar gain
- Summer: Close curtains on sun-facing windows to block heat
- Summer nights: Open curtains if outside is cooler (for ventilation)
Cellular (Honeycomb) Shades
- Air pockets provide excellent insulation
- Available in various cell sizes (more cells = more insulation)
- Work well for both heating and cooling seasons
- Some designs are cordless and renter-friendly
Window Insulation Film
Shrink Film Kits
Plastic film applied to the interior window frame:
- Creates an insulating air pocket
- Applied with double-sided tape
- Shrinks tight with a hair dryer
- Temporary—removes in spring without damage
- Prevents operation of the window while installed
Static Cling Films
Films that adhere to the glass itself:
- Some provide insulation
- Some reflect heat (reduce summer heat gain)
- Removable and reusable
- Varying degrees of visibility impact
Draft Sealing
Air leaks around windows waste energy year-round—see our draft sealing guide for details.
Common Window Draft Sources
- Around the window frame where it meets the wall
- Between sash and frame on operable windows
- Around window air conditioners
- Through cracked or damaged weatherstripping
Renter-Friendly Sealing
- Removable weatherstripping
- Rope caulk (press in, remove later)
- Draft snakes along windowsills
- Foam tape for temporary sealing
Window AC Units
If you have window air conditioners:
- Remove units in winter if possible (they're poorly insulated)
- If units stay installed, cover with insulated AC covers
- Seal gaps around the unit with foam or weatherstripping
- Indoor covers provide additional insulation
Working with Landlords
For more substantial window improvements:
- Report broken seals, cracked glass, or damaged frames
- Note if windows don't close properly
- Document drafts—landlords may be required to address
- Some landlords will allow tenant-installed improvements
The Layered Approach
The most effective window treatment combines multiple layers: seal drafts around the frame, add insulation film on the glass, and use thermal curtains. Each layer adds improvement, and together they can significantly reduce heat transfer through even old, single-pane windows.