A while back i wrote about the incompatibility of putting an atmospheric combustion furnace in a sealed attic.
Builder sprayed insulation all over furnace in attic.
If you don t have a crawl space or a basement then it can be a challenge to place a furnace on the ground floor of the structure.
Most often the attic is sealed by installing spray foam insulation at the roofline thus bringing the attic inside the building enclosure and turning it into conditioned space directly or indirectly.
Any existing batt or roll insulation in the attic should have the facing against the attic drywall floor or no facing at all.
You might not receive the same air movement benefits if the unit is in a garage either.
A lot of builders and homeowners are going with spray foam insulation because of the airtightness benefits.
Gable vents and soffits are the big culprits here.
If the air is not piped in directly to the furnace the attic space would be considered the plenum.
A case of incompatibility.
Batt or rolled insulation or blown loose fill insulation fiberglass or cellulose can be installed on top of old insulation.
The attic is the worst place to install hvac equipment and duct work.
If the furnace is a fuel burning one be sure you provide air for combustion.
I found instead a vintage 3 ton ac unit with 1 rigid insulation taped all over it and insulated flex duct snaking all over the poorly insulated attic.
She asked me to go into the attic to find the roof leaks or where the snow is blowing in through the soffits.
In a proper spray foam job a contractor doesn t just spray foam all over the top of your attic and go home.
But then the questioner mentioned that the spray foam contractor had intentionally left big holes to the outside by not sealing the gable vents.
This puts the furnace in at least semi conditioned space.
If this house has a basement then both handlers or furnaces should be installed there.
They also need to seal off areas of air leakage to the outdoors.
The attic floor needs an insulation value of r30 40.
The rigid insulation was saturated from condensation.
A plenum is the source of combustion air.
If the builder says that will cost more then use the money you would have spent on the foam insulation and attic enclosures to do this.
Otherwise humidity will quickly creep into your attic.
Insulation should be rolled out perpendicular to the joists and unfaced rolls should be used.