The reason for this advantage is the fact that venting for your furnace requires less work in an attic than it does in other locations of your home.
Furnace venting into attic.
Unless you have a slab where the equipment would likely be in the garage venting can go directly through the roof instead of routing through an exterior wall making it much easier to install the duct work along the upper floor.
Now cut 2 inch deep slots.
That is because with an attic installation vents must go down into the thermal envelope to deliver warm air.
Cut 1 inch deep slots a few inches apart along the top side.
Insulation is only so effective in preventing the heat in the house from going up into the attic.
The furnace has a stack which penetrates the flat roof and is about 3 4 above the roof plane.
Attic air is already partly heated from heat in the upper level of your home radiating into the attic despite insulation.
Fresh combustion air for the furnace is coming from the soffit vents from both roofs.
If you re preparing to upgrade an old furnace it might be time to consider moving it back down to the ground level.
However the furnace is in an attic space which has no venting for warm humid air from house from soffits from hot roof etc to escape.
Unwrap the flashing from the vent pipe so you can work with it.
Yes in fact this is the most efficient manner of providing air to your furnace.
Cut enough aluminum flashing from the roll to wrap around the vent pipe with an extra 6 inches to spare.
Bend these newly cut tabs in.