Solutions for Rooms Above Garage That Are Too Hot or Too Cold

It’s always nice to discover that your house has a bonus room and that bonus room is usually one that exists above your garage.  Maybe you are thinking about converting it to an office, or a rec room, or maybe renting it out and getting some side cash. Whatever your plans though, you have to get the room’s temperature under control.

If the room above your garage gets too hot or too cold, it probably is not sealed correctly and has poor insulation.  These rooms also usually suffer from inadequate ductwork.  Thermally treating your above-garage room is a straightforward DIY project that can bring your room to a proper temperature balance in no time.

The room above the garage is often overlooked by building contractors looking to lower their costs. Fortunately, fixing the deficiencies is inexpensive and most of the work can be done by a non-professional. We’re going to go over some of the common reasons for temperature fluctuations in your above-garage room and how to fix them.

Common Reasons Your Room Above Your Garage is Too Cold or Hot 

The most common reasons for the room above your garage being too hot or cold are poor insulation, garage door problems, improper sealing, and the lack of HVAC ductwork.  Fortunately, you don’t have to be an expert at home repair – these common culprits are easy to identify.  Let’s go over the things you will need to look for. 

Poor Insulation 

Insulation above the garage isn’t common.  Oftentimes, the original builders of the house will not even bother – after all, it is not a usually lived-in space, and it doesn’t make sense to waste materials insulating the space.  So to save them time and money on insulating a room people won’t live in, they give it the minimum or nothing at all.

To get an idea of why insulation is important, I want you to imagine the difference between coffee in a thermos and coffee in a paper cup.  After an hour, the coffee in the paper cup has cooled, while the coffee in the thermos stays hot because there is a protective shell around it keeping it at the same temperature.

Without insulation, there is no protective shell in the room that keeps the temperature from changing rapidly.  Without insulation, your room is like that paper cup of coffee.  

The Room is not Air Sealed 

Since the room is not made to live in, the above-garage room may not even be airtight.  You’ll know that the room is not air-sealed if you can feel a draft in the room.

An air leak makes it easy for outside air to come in or inside air to get out.  If you slept through that day in science, here’s a quick lesson – temperature seeks to be in balance everywhere, so heat will get transferred from the hotter area or object to the colder area or object.  

That means if the air outside is hot, then your above-garage room is going to heat up to match.  If the outside air is cold, then the warm air in the room is going to escape and leave you freezing.  If your room is not sealed to the outside, you will get the displeasure of suffering the extremes of temperature throughout the day.

You Need Ductwork 

Another thing that builders sometimes fail to do is install enough ductwork above the garage if there is any ductwork.  If the room is not insulated and air-sealed, then chances are that even if there is ductwork, there is not enough.

A room above the garage is often a bit bigger than most of the rooms in your house as well.  If there is any ductwork in there, it is providing the kind of cooling it might do for a smaller bedroom. 

The room above the garage may also be at a distance from your HVAC unit.  This means that this room is going to be the last room in the house to receive any kind of heating or cooling.

Any of these reasons can explain why your above-garage room isn’t being cooled adequately. If your room is suffering from wild temperature swings even in the presence of ductwork, you may need to improve the system or consider alternative heating and cooling methods.

Inadequate Thermal Treatment 

Thermal treatment refers to the overall treatment of the room in preventing the fluctuations in temperature from being too extreme.  Your attic room could very well be ducted, insulated, and sealed against the elements, but still not be adequately thermally treated.

If you are unsure as to why your room is experiencing big fluctuations in temperature, then the best thing to do is to go over each of the common causes of your above-garage room being too hot and assume one of them is not doing an adequate job.  You may discover something that you missed or a defect in the original installation.  

Sometimes, there is no insulation in the floors, meaning that while the room is thermally treated, the garage itself is negating it all.  In this case, you may want to insulate the floors of your room, or go a step further and insulate your garage as well.

Best Fixes For Your Room Above the Garage Being Too Hot or Cold 

Now that you know what to look for, the next step is to try to fix the problem by increasing the airflow, reinsulating and resealing the room, and as a last resort, replacing the ductwork.  Here is a breakdown of each step you can take and how to do it.

Increase Air Flow 

This is not always possible, but if it is, it is one of the easiest ways to solve the issue.  If you have ductwork with a supply-air vent, check to see if there is a return-air vent.  

You will be surprised how often this does not get included as part of the original build (well, maybe not, since we have already discussed how some installers will cut corners knowing that the above-garage room is unlikely to be lived in). By adding a return vent, you will create airflow which should instantly help cool down or heat your room.

The best part about this fix is that it does not mess with the HVAC system or require you to call a professional to calculate ductwork and system loads.  This is a DIY fix that can be done on a budget, and in combination with other solutions in this article, you will have your room feeling better in no time.

Reinsulate the Room 

If you can’t increase the airflow or the increase isn’t enough, then you’ll need to reinsulate the room.  Remember that not all builders put adequate insulation into the above-garage rooms if insulation was placed in there in the first place.  By adding to your insulation, you can make your room more energy efficient by keeping the temperature steady and less affected by outside influences.

One of the easiest places to start is the floor.  You can drill into the floorboards and fill the cavities in there with cellulose or fiberglass, then patch those holes up.

Next, check the walls and ceiling, and add insulation if need be.  If there is already insulation there, foam board or bubble wrap can help buttress the insulation.  

By insulating your room and making it airtight, you create an ‘envelope’.  This is the barrier to the outside world, and the stronger the envelope of your room, the more comfortable it will be.

Reseal the Room

Before you cover your walls and ceiling with new insulation though, you should see if your room is airtight.  If there are leaks in your walls, windows, and doors, then it is fighting against all that insulation.  This is a very quick and easy remedy to keep your above-garage room’s temperature problems to a minimum.

Try to find the leaks in your room.  An easy way of doing this is lighting an incense stick and using the drift of the smoke to find where the air is either being blown into or out of the room.  Carefully pass the incense stick around the edges of your windows, your doors, and any seams in the walls. 

Once you identify the leaks, it’s time to repair them.  Caulking or sealant the leaks will make your room airtight.  If you have HVAC coming into your room, you may notice a difference right away.

Replace the Ductwork 

If there is not enough duct working (or none at all), then the last step is to add some.  The question is, how much do you need? 

The amount of ducting needed can be found by doing what is called a “Manual J Calculation”, followed by “Manuel D calculations” to calculate the layout, and finally a “Manuel S Calculation” to check if the unit that is powering the cooling system is right for the house.  There is a very simple way of accomplishing this – call an HVAC contractor and ask them to perform it for you.

If you do have to replace the ductwork in your home, unless you are a very skilled DIYer, you may want to consult a professional.  HVAC systems are very specifically calculated and calibrated to provide cooling to a defined area.  

Expanding your HVAC to this room can cause problems elsewhere. To make sure that properly ducting your bonus room and are not overloading your system, consult with an HVAC professional.

When Should You Call a Professional?

Getting your above-garage room to a point where it is no longer suffering from temperature extremes is a fairly straightforward DIY project. Depending on the extent of your improvements, the project is probably not going to be that labor-intensive.

The exception to this is if you need to replace your ductwork. If you are going to make the HVAC work well in your room, it is best to call a professional so that the requirements of your home matched the capacity of your system

You may also want to call a Professional if the situation is more complex than described here. Some situations are more complicated and calling a professional in the beginning will alleviate a headache.  General contractors will typically charge 15% to 20% of the project cost.


Fixing your above-garage room’s temperature issues are easy when you know what to look for and what to do about it.  If getting your HVAC working with your room is a hassle, you should also consider installing a portable window unit or a ductless solution. These solutions will avoid the problem of having to restructure your ducts just to accommodate your bonus room.