Cut 1 inch deep slots a few inches apart along the top side.
Furnace venting into attic.
Now cut 2 inch deep slots.
Attic air is already partly heated from heat in the upper level of your home radiating into the attic despite insulation.
The furnace has a stack which penetrates the flat roof and is about 3 4 above the roof plane.
Insulation is only so effective in preventing the heat in the house from going up into the attic.
Unwrap the flashing from the vent pipe so you can work with it.
That is because with an attic installation vents must go down into the thermal envelope to deliver warm air.
Bend these newly cut tabs in.
Yes in fact this is the most efficient manner of providing air to your furnace.
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.
Cut enough aluminum flashing from the roll to wrap around the vent pipe with an extra 6 inches to spare.
You ll start to save money on your heating costs too and your furnace will perform closer to its indicated seer.
You ll see the concentric pipe used.
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.
Fresh combustion air for the furnace is coming from the soffit vents from both roofs.