The key idea is to level the unit and tilt it slightly outward to help drainage and keep it secure. This simple adjustment can improve efficiency, prevent leaks, and reduce stress on the window frame.
This guide explains how to assess your setup, what tools and materials you’ll need, a step-by-step shimming process for common window and sleeve configurations, plus tips on safety and common pitfalls. It aims to give you practical, up-to-date instructions you can apply to most standard window air conditioners.
Why shimming is important
Shimming stabilizes an air conditioner by ensuring the unit sits level from side to side and has a small outward tilt at the bottom. This tilt promotes proper condensate drainage away from the interior and helps prevent air leaks around the sash. A stable, properly angled unit is less prone to rattling in windy conditions and puts less stress on the window frame and seals.
Tools and materials you’ll need
Gather these items before you start so you can work efficiently and safely.
- Level (torpedo or small laser level) to check horizontal level and tilt
- Shims made of wood, plastic, or composite (typically 1/4" to 1/2" thick)
- Weatherstripping foam or rubber (for sealing gaps around the unit)
- Exterior caulk or spray foam for sealing gaps after shimming
- Anti-tip brackets or window brackets (if recommended by the manufacturer)
- Screws or fasteners appropriate for your window frame
- Tape measure or ruler for precise placement
- Pry bar or flathead screwdriver (for gentle adjustments, if needed)
- Safety gear (gloves and eye protection)
Using sturdy, exterior-grade materials and avoiding compressed or brittle fillers helps maintain a secure installation and protects against weather and movement over time.
Step-by-step shimming for a window air conditioner
Follow these steps to level and tilt your unit while ensuring a solid seal and safe operation. Each step is designed to be practical for typical residential installations.
- Power off and unplug the unit, then move the sash to a position where you can access the opening and support the weight of the AC.
- Insert the air conditioner into the window opening as far as the sash permits. Ensure the front of the unit is aligned with the window frame and that the bottom is resting on the sill or resting against the window frame edge.
- Check the horizontal level along the top edge of the unit. The unit should be level from side to side; the bottom should tilt outward slightly to promote drainage toward the outside. A common target is about 1/4 inch of outward tilt per foot of width, but follow the manufacturer’s guidance if provided.
- Place shims under the bottom edge of the unit at both sides (preferably near the corners) to achieve the desired tilt. Use 1/4" to 1/2" thick shims as needed, and stack cautiously to avoid overfilling and binding the sash.
- Recheck level and tilt with the level. Adjust by adding or removing shims until the unit is securely positioned and properly tilted.
- Secure the unit with anti-tip or window brackets if your installation calls for them or if the manufacturer provides them. Fasten according to the instructions, taking care not to crack the sash or frame.
- Seal any gaps around the sides and top with weatherstripping and exterior caulk or spray foam. Ensure you do not block drainage paths or vents on the unit.
- Trim or remove excess shim material if needed and reseal the exterior to prevent moisture intrusion. Plug the unit back in, power on, and run a test cycle to verify drainage and stability.
After completing these steps, recheck the unit after a few days of use. Temperature changes and wind can affect fit, so a quick recheck can prevent long-term issues.
Shimming considerations for different configurations
Not all installations are identical. Here’s how to adapt shimming for common setups.
Window-unit AC in a standard sash window
Use shims under the bottom corners to create the outward tilt, and secure with brackets if recommended. Maintain a level top edge and avoid blocking the unit’s drainage holes or vents. Seal gaps along the sides and top to minimize drafts and water intrusion.
Through-the-wall or built-in sleeve installations
With sleeves, ensure the sleeve itself is plumb and level. Shims can be placed between the unit and the sleeve interior to adjust tilt, while exterior seals maintain weather tightness. Do not compress insulation or block the sleeve’s drainage channels. Follow the manufacturer’s mounting and sealing instructions closely.
Common mistakes to avoid
Avoid these missteps that can compromise performance or safety.
- Forgetting to unplug the unit before starting work
- Over-shimming, which can bind the sash or put undue stress on the frame
- Blocking or covering drainage holes, vents, or condensate paths
- Skimping on sealing around the unit, leading to drafts or water intrusion
- Using weak or brittle materials that may degrade in heat and weather
- Failing to secure the unit with anti-tip brackets when required
Careful attention to these points helps ensure a stable, efficient, and leak-free installation over time.
Safety and maintenance tips
Stay safe and keep the installation solid over the life of the unit.
- Always unplug before handling the unit or making adjustments.
- Inspect shims and brackets periodically, especially after strong winds or heavy use.
- Use weatherstripping and exterior sealants designed for outdoor exposure to prevent moisture intrusion.
- Do not attempt to shim a unit that is damaged or misaligned beyond simple adjustments; consult a professional if necessary.
Regular checks help catch shifting or wear before it affects efficiency or safety.
Summary
Shimming an air conditioner is a practical way to ensure a level, secure installation with proper drainage. By gathering the right tools, following a careful step-by-step process, and adapting to your specific window or sleeve configuration, you can improve comfort, efficiency, and reliability. Always prioritize manufacturer guidance, secure mounting, and quality sealing to protect your home from drafts and water intrusion.
Can AC cause sinus issues?
Yes, air conditioning can affect people's sinuses for a couple reasons. The first being that cold, dry rooms trigger runny noses the same way going outside in the winter after being in your warm house does. The sudden change in temperature and humidity triggers glands in the nasal membranes to produce mucus.
What is the 3 minute rule for air conditioners?
The 3-minute rule is a simple but important guideline: wait at least three minutes after turning your air conditioner off before turning it back on. That short pause gives the refrigerant pressure in the system time to equalize.
How to shim an AC unit?
Shimming the AC Pad
One of the quickest DIY options is to use plastic or composite shims to level the pad. Shims are small, wedge-shaped pieces that you can insert under the pad to adjust height and create a level surface. Pros: Inexpensive and easy to install; ideal for slight adjustments.
Which AC brand lasts the longest?
Lennox: High-End Performance for the Long Haul
Lennox systems are considered high-end, often lasting 15-20 years. Their energy-efficient models are a favorite among homeowners who want both long-term savings and reliability. Most HVAC companies will frequently recommend Lennox for homes seeking a premium system.