Why is the Room Above the Garage So Hot? (5 Effective Ways to Cool It!)

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Why is the Room Above the Garage So Hot in Summer?

Are you disappointed with the “bonus room” you built above your garage? Why is the room above the garage so hot and practically unusable during most of the summer? And to make things worse does it get unbearably cold in winter?

The other rooms on the same floor don’t have this issue. It’s just the room above the garage that gets hot in summer and cold in winter. Why?

Warm air is lighter, so in summer, the garage air rises, heating the room above. This problem worsens if the garage ceiling is poorly insulated or has air leaks.

A garage is more exposed to the elements than any other room in your house, with typically two or three sides of an attached garage exposed, and all sides for a detached garage.

The garage door is the biggest culprit for heat gain or loss. Often made of steel or aluminum, both good heat conductors.

The garage door opens several times during the day. It lets in a huge amount of hot air every time it opens.

Heat is transmitted through conduction, convection, and radiation. An uninsulated garage door excels at all three. Hot air, lighter than cold air, rises and lingers around the garage ceiling drywall.

If the garage ceiling isn’t insulated, heat is transmitted to the room above by conduction. If there are air leaks, it’s transmitted through convection. As a result, the room above the garage gets unusually hot in summer.

How to Cool Bonus Room Above Garage?

Conceptually heat flow is quite similar to water flow. Heat flows from an area with high temperature to an area with low temperature, just like water flows from high ground to low ground.

Think of your home as a water reservoir. When temperatures outside are high, heat flows in and accumulates within the house. HVAC is the way to get the heat out and keep every room in your house at a comfortable temperature.

However, you can not duct your home HVAC to your garage. It is against the building code!

So when temperatures rise and make the bonus room above the garage hot, you can adopt some or all of the ways that I am giving below to cool it.

1. Improve Insulation Above the Garage Ceiling

The insulation between the garage ceiling and the floor of the bonus room above the garage may not be sufficient or may have become less effective with time. This will make the room above the garage hot, as heat from the hot garage gets transferred up.

The least destructive way of increasing the insulation in that space is to use “Injection Foam Insulation”. Injection Foam Insulation is injected into a closed cavity (the joist bay). Holes are drilled into the ceiling drywall and the foam is injected, under pressure, till every available space is filled up.

The entire process is not messy. The holes drilled into the drywall are small and can easily be patched and painted over. Of course injection foam insulation can only be done by experienced contractors who have the expertise and the equipment.

Injection foam not only insulates but also seals all air gaps. The transfer of heat and even the sound to the bonus room above the garage will be reduced significantly.

2. Improve Insulation in the Knee Wall

The insulation in the knee wall is usually totally ignored after construction. Like anything else, it degrades over the years. Poor or inadequate insulation in the knee wall will make the bonus room above the garage hot.

US Department of Energy, Public domain, via Wikimedia Commons

The best way is to pull out the old insulation and replace it with new fiberglass insulation rolls such as Owens Corning R-13 Faced Insulation Roll. To beef up the insulation you can fill up the void space in the knee wall attic with cellulose.

3. Improve Airflow to Bonus Room Above Garage

The bonus room above the garage gets cooled, like any other room in the house by air ducts from the HVAC system. Quite often the problem may be that the airflow into this room is not enough. What about the return-air vent? Is it in working condition and open?

All the work on improving the insulation will make the bonus room above the garage less hot, but it may still not be comfortably cool, without tweaking the airflow from the HVAC system.

4. Make your Garage Cooler

It stands to reason that if your garage is cooler, then the room above the garage would not be so hot. A lot of the heat entering your garage is through the garage door. And a lot of it is radiant heat if your garage faces the sun most of the day.

It takes very little money and effort to insulate the garage door for hot climates. In hot climates, the focus has to be on blocking as much of the solar radiation as possible. This is achieved by sandwiching a layer of air-entrapping material between reflective films.

Here is a product that works both in winter and summer!

Garage Door Insulation Kit (Bundle)

For more information check out my earlier post, The Best Garage Door Insulation For Hot Climates (Helpful Advice!).

5. Add Natural Ventilation

A cool way to ventilate an unconditioned garage is to add vents at the right places to create natural ventilation.

Strategically placed garage wall vents can get the air circulation going naturally. 

A few vents on the higher end of the garage wall (nearer to the roof) or preferably garage roof vents, will let the hot air garage out. A few vents in the wall, just a couple of inches above the floor will pull in the cooler outside air to compensate.

The vents in the wall will not only set up air circulation within the garage but also cool the garage during warm weather.

Garage Wall Vents are the easiest and the cheapest (actually totally free!) way to cool a garage.

Find out more at Why Have Garage Wall Vents? (7 Important Reasons You Need To Know!).

Bottom Line

The bottom line is that improper insulation and inadequate airflow will make the room above the garage hot during summer. The room above the garage gets hot as warm air in the garage moves up and either infiltrates the room or heats it by conduction.

You can take one or preferably all of the following five ways to cool down the room:

1. Improve Insulation Above the Garage Ceiling
2. Improve Insulation in the Knee Wall
3. Improve Airflow from HVAC to the Room Above the Garage
4. Make your Garage Cooler by Insulating the Door
5. Add Natural Ventilation in the Garage using Vents

There is an initial investment cost but there is no maintenance or running cost. All of the five ways are passive and have no running costs.

Thank you very much for reading the post. I do hope you found it informative and useful.