Master the essential spacer technique to secure your window AC unit over high storm window frames for proper drainage and maximum cooling efficiency.
Getting your home ready for a heatwave often starts with the annual ritual of hauling the cooling unit out of storage. However, if you live in a house with traditional storm windows, that simple task can quickly become a technical puzzle. Learning how to install a window air conditioner with a storm window requires a bit more than just lifting and sliding. The presence of that extra exterior pane and the deep sill it creates means your unit cannot sit flush or drain properly without a few specific adjustments.
This guide will walk you through the entire process, ensuring your home stays cool without damaging your window frames or risking a falling unit. Whether you are dealing with historic wooden sashes or modern aluminum storm frames, these steps will help you achieve a secure, airtight fit.
Understanding the Storm Window Challenge
Before you begin the physical labor, it is important to understand why storm windows complicate air conditioner installation. A standard window air conditioner is designed to sit on a narrow windowsill where the bottom of the unit hooks behind the stool and the top sash lowers to lock it in place.
When you add a storm window into the mix, you create a “well” between the interior window and the exterior storm frame. The outer frame of the storm window is usually higher than the interior windowsill. If you simply place the air conditioner on the inner sill, the back of the unit will hit the high lip of the storm window frame. This tilts the unit forward toward the inside of your house, which is the exact opposite of what you want. Air conditioners must tilt slightly toward the outside so that the condensation—the water pulled from the humid air—can drain outdoors.
Gathering Your Tools and Supplies
Preparation is the secret to a stress-free installation. You do not want to be balancing a heavy appliance with one hand while searching for a screwdriver with the other.
For this project, you will need:
- A tape measure
- A cordless drill with a screwdriver bit
- Wood blocks or specialized spacers (pressure-treated 2x4s or 1x2s work best)
- Weather stripping or high-density foam
- A level
- Scissors or a utility knife
- A helper (highly recommended for safety)
If you are looking for a reliable unit to start with, the Midea U-Shaped Window Air Conditioner is a favorite for many homeowners because its unique design allows the window to close almost completely, though you will still need to follow the spacing steps for your storm frame.
Measuring for the Perfect Fit
Accuracy at the start prevents frustration at the finish. Open your interior window and measure the width of the opening. Compare this to the width of your air conditioner with the side curtains extended.
Next, measure the height of the storm window’s bottom frame. This is the “lip” that you need to clear. You will need to create a base that is at least a quarter-inch higher than this frame so the air conditioner can sit flat and then tilt outward.
Preparing the Window Sill with Spacers
This is the most critical step when learning how to install a window air conditioner with a storm window. To compensate for the height of the storm window frame, you must install a spacer on the interior windowsill.
Cut a piece of wood—such as a scrap 2×4 or a 1×2—to the width of the window opening. Place this wood block on the interior sill, right up against the stool. The goal is to create a new, elevated “ledge” for the air conditioner to rest on. By raising the unit above the storm window’s lip, you allow the back of the air conditioner to hang over the exterior without obstruction.
To ensure a solid foundation, I usually recommend using a Forstner bit to pre-drill small countersink holes if you plan to screw the wood block into the sill, though many people prefer a pressure-fit to avoid permanent holes in historic woodwork.
Securing the Unit and Achieving the Correct Tilt
With your spacer in place, it is time to lift the air conditioner. This is where a helper is invaluable. Carefully lift the unit and set the bottom mounting rail onto the wood spacer you just installed.
Once the unit is resting on the wood, have your helper hold it steady while you check the level. Remember, you want a slight downward pitch toward the outside—usually about half an inch. If the unit is sitting too flat or tilting inward, you may need to add a thinner shim on top of your wood block at the front.
If you prefer a more modern solution than scrap wood, the JEACENT Support Bracket is a heavy-duty exterior support that can help take the weight off the window frame entirely, which is excellent for older homes with storm windows.
Adjusting the Side Curtains and Sealing Gaps
Once the air conditioner is positioned and tilted correctly, lower the interior window sash firmly onto the top of the unit. Most units have a top rail that the window sash should sit behind to prevent the unit from falling outward.
Extend the side curtains to the edges of the window frame. Because you have raised the unit on a block, you might notice that the standard curtains do not perfectly meet the bottom of the sill. This is where weather stripping becomes your best friend.
Fill every gap. Use high-density foam panels or adhesive weather stripping to seal the area around the wood block and the storm window frame. Air leaks not only let in heat but also allow mosquitoes and other pests into your home.
A great tip for a professional finish is to use Frost King Window AC Weatherstripping, which is specifically designed to fill those awkward gaps created by raised installations.
Final Safety Check and Maintenance
Before you plug in the unit and enjoy the cold air, do a final “shake test.” The unit should feel rock-solid. If you are worried about the window being pushed up from the outside, install a small sash lock or a wooden dowel in the window track to prevent the interior window from being opened.
Throughout the summer, keep an eye on the drainage. If you see water dripping from the front of the unit onto your carpet, your tilt is incorrect, or the storm window lip is still blocking the drainage holes. You may need to slightly increase the height of your interior wood spacer.
By taking the time to properly navigate the height of your storm window, you ensure your air conditioner runs efficiently all summer long while protecting your home from water damage and air leaks. Ready your tools, grab a friend, and get that cooling system running right.