Should You Put a Window AC in Your Converted Attic? The Ultimate Guide
So, you’ve converted your attic into a livable space. Pretty cool, right? It’s a fantastic way to add a bedroom, an office, or a chill-out zone without the headache & expense of a full-on extension. But then summer hits, & you realize you’ve essentially created a cozy, well-decorated sauna. Attics, by their very nature, get HOT. All that heat from the rest of the house rises, & the sun beats down on the roof all day. It's a recipe for a sweltering situation.
The obvious solution seems to be a window air conditioner. It’s relatively affordable, easy to find, & for most rooms, a pretty straightforward installation. But an attic isn’t most rooms. There are a bunch of things you need to think about before you run out to the store & buy the first unit you see. Honestly, it can get a little complicated.
I've been through this myself & have done a TON of research, so I wanted to put together a really comprehensive guide to help you figure out if a window AC is the right move for your new attic space, & if so, how to do it safely & effectively.
First Things First: Why Are Attics So Freaking Hot?
Before we even get into AC units, it's important to understand why your attic feels like the surface of the sun. It’s not just you. No place in your home is likely to get hotter than the attic. They are typically not as well-insulated as the rest of the house because they were originally designed just to be structural support for the roof, not a living area.
Heat rises, so all the warm air from your downstairs living areas makes its way up. On top of that, the roof absorbs a massive amount of solar radiation, heating the space from above. Without proper ventilation & insulation, that heat gets trapped. This is why just sticking any old AC unit in the window might not be enough. You’re fighting a much bigger heat battle than you would be in a first-floor bedroom.
The Big Question: Can You Even Use a Window AC?
The short answer is: YES, you absolutely can. But—and this is a big "but"—it depends on a few critical factors. A window-mounted AC unit can be a great, cost-effective solution for a small to medium-sized attic, but you have to do your homework. Let's break down what you need to consider.
1. Your Windows are EVERYTHING
The type of window you have in your attic is the number one determining factor.
- Standard Double-Hung Windows: If you have these, you’re in luck. This is the type of window that most AC units are designed for. They open by sliding a bottom sash up, creating the perfect space for an AC unit.
- Casement Windows: These windows open outwards with a crank, like a book. Installing a window AC in these is tricky, but not impossible. You can't just plop the unit in. You'll need to use a sheet of plexiglass or plywood to fill the entire window opening, with a cutout for the AC unit itself. There are also special kits you can buy online that use fabric & zippers to create a seal for the exhaust hose of a portable AC, which might be a better option here.
- Sliding Windows: Horizontal sliding windows present a similar challenge to casement windows. You'll need to fill the vertical gap with plexiglass or a custom-built frame. It’s doable, but requires more work & materials.
- Tilted or Skylight Windows: This is where it gets REALLY tough. Installing a traditional window unit in a tilted or skylight window is generally not recommended due to the angle & the difficulty of creating a secure, weatherproof seal. For these types of windows, you’re much better off looking at a portable AC unit or a mini-split system.
2. Structural Support is Non-Negotiable
Window AC units are HEAVY. We're talking 50 to 100 pounds, easily. Just resting that weight on the window sash & frame, especially a vinyl or older wood frame, is a recipe for disaster. The weight can warp, crack, or even break the window.
Here's what you MUST do:
- Use Support Brackets: ALWAYS use support brackets. These are metal arms that attach to the exterior of your house, underneath the AC unit, creating a shelf that takes the majority of the weight off the window itself.
- Level it Properly: The unit needs to be tilted slightly downwards towards the outside (about 3 to 5 degrees). This allows condensation to drain outside instead of into your wall or on your floor. A level can help you get this just right.
- Secure it From the Inside: Use L-brackets to firmly fasten the unit to the window frame from the inside. This prevents it from being pushed out.
NEVER use flimsy materials like books, bricks, or pieces of wood to prop up the unit. These can shift with vibrations & cause the unit to fall.
3. Electrical Supply: Don't Skip This Step
Air conditioners draw a LOT of power. You can’t just plug one into any old outlet with an extension cord.
- Dedicated Circuit: Ideally, your AC unit should have its own dedicated electrical circuit. This prevents you from constantly tripping the breaker that powers the rest of your attic lights & outlets. If you’re using your converted attic as a home office with a computer, printer, & other devices, a dedicated circuit is a must.
- Check the Voltage: Most window units are 115-volt & will work with a standard outlet. Larger units, however, might require a 230-volt outlet, which is a different shape & requires a special circuit. Always check the unit's requirements before you buy.
- No Extension Cords: I'm going to say it again for the people in the back: DO NOT use a standard extension cord. They aren’t rated for the high power draw of an AC unit & can overheat, creating a serious fire hazard. If the AC’s cord isn’t long enough, you need to either move the outlet or get a heavy-duty appliance extension cord that is specifically rated for air conditioners.
Calculating the Right Size AC: The BTU Game
This is where a lot of people go wrong. Bigger isn’t always better. An oversized AC will cool the room down really fast, but it won’t run long enough to effectively remove humidity, leaving the air feeling cold & clammy. A unit that's too small will run constantly, jack up your electricity bill, & never actually cool the room down on the hottest days.
The cooling capacity of an air conditioner is measured in BTUs (British Thermal Units). The general rule of thumb is that you need about 20 BTUs per square foot of living space. So, a 10x15 foot room (150 sq ft) would need a 3,000 BTU unit.
BUT, since this is an attic, you need to adjust that number upwards. Here’s a more detailed way to think about it:
- Calculate the Square Footage: Length of the room x Width of the room.
- Find Your Base BTU: Use a standard chart. For example, 150-250 sq ft needs about 6,000 BTUs, while 350-450 sq ft needs around 10,000 BTUs.
- Adjust for Attic Conditions:
- Poor Insulation/Lots of Sun: Increase your BTU needs by 10-15%. Since it's an attic, you can almost guarantee you'll need to do this.
- High Ceilings: If your ceilings are over 8 feet tall, increase the BTU figure by about 10%.
- Occupancy: Add about 600 BTUs for each person over two that will regularly be in the room.
- Kitchen/Electronics: If your attic space has a lot of heat-generating electronics (like a high-end gaming PC), you might want to bump it up a bit more.
There are plenty of online BTU calculators that can help you with this. Taking a few minutes to do this calculation will save you a lot of money & discomfort in the long run.
The Installation Process: A Step-by-Step Vibe
Once you’ve chosen the right unit, it’s time to install it. It's ALWAYS best to have a second person help you. These units are awkward & heavy.
- Prep the Window: Clean the window sill & frame. This is a good time to check for any rot or damage. If the window frame is weak, you need to repair it before you do anything else.
- Assemble the AC: Attach any side panels or mounting rails to the AC unit itself, as per the manufacturer's instructions.
- Install Support Brackets: Securely install the exterior support bracket to the side of your house, right below the window.
- Place the Unit: With your helper, carefully lift the AC unit & place it in the window, centered, with the bottom mounting rail resting behind the window sill.
- Lower the Sash: Carefully lower the top window sash down so it rests firmly behind the top rail of the AC unit.
- Secure Everything: Attach L-brackets to the top of the AC unit & the window sash to lock it in place.
- Fill the Gaps: Extend the side panels to fill the gaps on either side of the unit. Use the foam insulation strips that came with the unit to seal every possible crack & crevice. This is CRITICAL for efficiency. You don't want your precious cold air leaking out.
The Cost of Cool: What to Expect
A window AC unit itself can cost anywhere from $150 for a small, basic model to over $800 for a large, feature-rich one. If you need a professional to install it, which is a good idea if you’re not comfortable with the process, you can expect to pay another $100 to $300. It’s a pretty affordable solution compared to the alternatives.
Thinking Beyond the Window: Other Attic Cooling Options
A window AC is a great choice for many, but it's not the ONLY choice. Depending on your attic's layout, your budget, & your windows, one of these might be a better fit:
Portable Air Conditioners
These are freestanding units on wheels that you can place anywhere in the room. They work by pulling in hot air, cooling it, & venting the now-hot air & moisture out through a hose that you stick in a window.
- Pros: Easy to set up, can be used with almost any type of window (including casement & sliding windows, using a special kit), & you can move it around.
- Cons: They are NOISY because the entire machine is in the room with you. They also take up valuable floor space. Some models have a pan that collects condensation, which you’ll have to empty regularly. And let’s be honest, that big hose isn’t the most attractive look.
Through-the-Wall Air Conditioners
These are very similar to window units, but they are installed directly through an exterior wall.
- Pros: This is a great option if you don't have a suitable window or don't want to block one. They create a more permanent & airtight seal than window units.
- Cons: You have to cut a large, permanent hole in your wall. This is NOT a simple DIY project & will likely require a professional, adding to the cost.
Ductless Mini-Split Systems
This is the high-end option. A mini-split has two main components: an outdoor compressor/condenser & an indoor air-handling unit that is mounted on the wall. They are connected by a small conduit that only requires a three-inch hole in the wall.
- Pros: SUPER quiet & extremely energy-efficient. They provide the best cooling performance by far. They also look sleek & modern.
- Cons: They are, by a significant margin, the most expensive option. The units themselves can cost between $1,500 & $3,000, with installation adding another $500 or more.
Making Your Attic Conversion Worth It
Here's the thing: converting an attic is a brilliant move to increase your home's usable square footage. It adds value & utility. But if it's unusable for half the year because of the heat, you've wasted a lot of effort & money. Properly cooling the space is the final step to making that investment pay off. A comfortable attic office or bedroom can be a huge selling point.
And once you've made your home more spacious & comfortable, you might even start thinking about other ways to optimize your space. That extra storage you thought you lost when you finished the attic? You could find affordable & convenient storage solutions with Prked, which connects you with people who have unused space in their garages, basements, or even closets. It's a clever way to handle overflow without resorting to expensive self-storage units.
Or, if your attic conversion has freed up space elsewhere, like a now-empty garage or driveway spot, you could turn that into a source of passive income. With Prked, you can easily list your unused driveway or garage for rent to drivers looking for convenient parking. It's a fantastic side hustle that helps your neighbors & your wallet.
Final Thoughts
So, can you put a window AC unit in your converted attic? Absolutely. For many people, it's the most practical & budget-friendly way to make their new space comfortable & usable all year round.
Just remember to do it the right way. Check your windows, make sure you have the structural support & electrical capacity, & take the time to calculate the right BTU size for your unique space. Don't be afraid to call in a professional if you're not 100% confident. A little extra planning on the front end will save you from major headaches, high energy bills, & a potentially dangerous situation down the road.
Hope this was helpful! Let me know what you think. Stay cool
