Is it worth insulating the roof deck when my HVAC is in the attic?

The contractor isn't necessarily lying about the HVAC fighting the temps, but they are defintely underselling the importance of air sealing before you do anything else. If you just blow in loose-fill or spray the roof without sealing up every single wire penetration, plumbing stack, and top plate, you're still going to have massive heat loss. Air moves through insulation like a filter; it doesn't stop the flow of warm air escaping your living space into that attic.

I went through this exact scenario last year with a $500 winter bill. I opted for the floor insulation because the price gap for spray foam is massive and takes a decade or more to break even on energy savings alone. But before I put a single bag of cellulose down, I spent two days with cans of spray foam and a caulk gun. You have to pull back whatever is there and seal every gap where air can leak from the house into the attic. If you have recessed lights that aren't IC-rated, get covers for them. This is the only way to actually stop the 'chimney effect' pulling heat out of your rooms. For your HVAC unit, don't just leave it exposed. You can build a simple enclosure using **rigid foam board** and foil tape to create a thermal break around the plenum and the main unit. Also, make sure your ductwork is sealed with mastic—not just duct tape—and then wrap it in R-8 insulation. It’s alot cheaper than foaming the whole roof deck and accomplishes about 80% of the same goal.

If you go the floor route, make sure you have proper ventilation. You need soffit vents and a ridge vent to keep the air moving so you don't trap moisture. If you block those soffits with loose-fill, you’re asking for mold issues down the road. Use baffles at the eaves to keep the channel open. My bill dropped by nearly 40% just by air sealing and hitting R-60 on the floor, and I didn't have to spend $15k on spray foam.

Oh wow, 40% savings just from air sealing and the floor?? That is acutally insane! I'm just starting to look into all this for my new place and I'm super pumped to get my hands dirty. I keep hearing about this 'stack effect' and it's blowing my mind how much air just vanishes trough the ceiling. Quick question though—since you mentioned the rigid foam board for the HVAC, has anyone ever tried using those reflective space blankets on the attic door? My brother says they work wonders but he's also kind of a conspiracy theorist lol.

I've seen folks try the bubble wrap stuff on ducts and it's a waste of time. Back in the 70s, we just threw more fiberglass batts over everthing and called it a day, but that didn't help the unit one bit. If you're going the floor route, don't overlook the attic hatch. Most people leave a giant hole right there. I built a 'coffin' box out of scrap plywood and glued some thick foam to the top. When that door is shut, it's sealed tight. Also, watch out for those old whole-house fans; they're basicly a wide-open window to the sky if you don't build a cover for winter.

Spot on about the attic hatch 'coffin' box. That's usually the biggest thermal bridge left in the room. One thing a lot of guys forget when building those is to add a weatherstripping gasket around the perimeter where the box meets the frame. Without a compression seal, you’re still getting air bypass right at the gap. I used some simple adhesive rubber D-strips on mine and it made a noticeable difference in the draftiness of the hallway below.

Quick update — I decided to go with the floor insulation and focus heavily on air sealing like you guys suggested. I spent a weekend sealing every wire penetration and built a weatherstripped 'coffin' box for the hatch. My bill already dropped by $180! Glad I didn't shell out for the roof spray; air sealing was definitely the real game changer here.