The primary reason to insulate any part of the home is to provide a thermal barrier between conditioned heated or cooled interior space and the outdoors or unheated spaces such as an unfinished.
Attic is open in garage.
When the attic becomes part of the home to be heated and cooled open wall gable vents and roof vents are no longer feasible but the underside of the roof the sheathing and rafters can still.
In the garage is an access door to the attic.
The next step is to create the attic.
This insulation resists the heat from the terrace to pass it directly to the living room.
Now i know the way the rats are getting up there is through this very easy to access hole.
I think you should do the reverse one.
Our only attic opening is a very large always open space above wall height in our garage.
You may be able to add extra trusses to the garage in order to secure the attic properly.
Will the garage stay cooler with the trap door closed or open.
Your attic acts as an insulation between the terrace and the living room.
The first thing you need is access to the attic.
Opening your automatic garage door will let cool air and a draft in.
Then prop open the garage s attic access ladder by about 12 inches or enough to let a sizable draft enter your attic.
Using the measurements from your loft ladder kit you should cut a square out of your garage roof.
This will give you access to your attic storage.
Opening your attic access panel will draw that air into the attic and help to push the hot air up there and out through the roof vents assuming you have proper attic ventilation.
Step 2 create an attic.
I have a wireless thermometer set pair that allowed me to do a one day test.
Over a 24 hour period compare the differences in peak temperatures between the outside north in the shade and inside the garage maximum.
As you know cold air settles and hot air rises.
To get up there you just lean a latter against the wall and climb on up.
The garage ceiling has a door to the attic.