SUMMARY:  In an enclosed attic space that’s hot despite insulation under the sheathing, is it better to add more insulation under the roof or on the attic “floor” (the other side of the sloped ceiling)?

 

DETAILS:  I’m having a hard time getting good results from insulating an attic ceiling (in CZ-2, where it’s been around 103°F+ outside every day, and the metal roofs gets sun).  The attic space in question is between two skylight wells.  I noticed the walls of the attic space were quite hot, but I didn’t want to insulate those walls because that space is unvented and difficult to vent (don’t ask), so I cut and cobbled a bunch of polyiso under the roof sheathing and on the wall (inside of the exterior wall), but it’s still toasty up there.  Heat camera shows 83-88°F depending on what part of the polyiso I hit.  The camera shows just surface temp and not precisely where the heat is coming from, and heat moves, so a hot panel might not have a hot source behind it.  (Also, I’m wondering whether there’s reflection from the foil facing which is tricking the camera.)  This is when the room temp is 77°F.

I’ve got 5 layers of 1″ polyiso between the rafters and 2 layers of 1″ across the rafters, so that’s a whopping R46 (excluding the rafters).  On the wall I’ve got 2 layers of 1″ between the studs and 1″ over the studs, for R20, code calls for R13.  FWIW, there’s a sheet metal roof.

 

So now I’m wondering about adding insulation to the attic floor, and maybe the attic walls, which would prevent drying to the inside, but what would be trying to dry anyway?  Most of that space is already covered with polyiso.

My goal is to cool the 737sf in that room to 74°F at the hottest part of the day with the existing 13-14,000 BTU of AC.  Right now the room temp rises to about 77°F in the late afternoon, which would be tolerable for me, but for a rental I think I need to offer a 74°F space, especially since even that temp will rise as it’s quite possible the tenants might occasionally want to cook something, use a computer or TV, have friends over, etc.