How to insulate and vent this garage attic?

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Old 04-13-18, 02:24 PM
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How to insulate and vent this garage attic?

Picture is not of actual site but is basically the same thing except 3 times bigger.
Detached garage with attic (not mine), he wants to finish and condition the attic. New Jersey.

There's no walls on the two sides like the pictures show. The picture looks sort of like half trusses but this attic is just rafters and joists with no diagonal bracing, but it has horizontal collar ties up top which will be used for a ceiling.

I'm adding walls along the two sides.
There will be outlets on all 4 walls every 6 feet, 15" to the bottom of the receptacle box from the floor.

Basically I'm just insulating and sheetrocking it. And adding an insulated door with a bottom sweep.
Can the door be right in front of the steps, or does it need a certain clearance? I've heard it needs a clearance but I've seen 100 newly built homes with basement doors from kitchen to basement which open right to the steps down (Only one step is built past the door). I think what I heard was if it does have a clearance (platform after opening the door before the steps start) that it must be a certain minimum size.

The ground level has a regular garage door which is wood and has warped gaps in it and is drafty and cold.

To insulate this garage attic, do I simply do this?:

Install insulation baffles in the rafters (there are soffit vents), fiberglass batts in rafters, and insulate the horizontal ceiling collar tie framing?
And then insulate the floor joists to keep the heat/cold in the upstairs from loss through the floor. But it will be drafty downstairs, so should I install plastic sheeting or something on the bottoms of the floor joists, or will that lock in moisture in the insulated floor bays?


It's 2x12 joists 16" OC, rafters 2x10 or 2x12 by the way.



Other thing I'm not sure of is, there's a gable vent on one end and an electric fan on the other which is above the collar tie area (above the flat ceiling). He wants a drop ceiling (maybe sheetrock instead but either way). If I don't insulate the ceiling, heat will just rise through it and leave the gable vent, and cold air will come in through the vents. So the ceiling should be insulated.

He wants to add a split unit heat pump for AC/heat which I know nothing about, only thing I'm doing for that is adding a hole to the vinyl exterior if need be for a vent or whatever it needs.

So basically, if I insulate the ceiling and rafters then sheetrock it, should the gable vent and electric fan on opposite gable be removed/sealed over from the exterior to prevent it basically potentially becoming a squirrel hotel and/or letting cold/hot air in?

I realize that in Summer, the gable vent and fan won't be removing heat and this could lead to moisture problems but, in other words, what is done to vent a regular cathedral ceiling home anyway? If the Split unit AC/heat pump has an exhaust fan, maybe it can be set on a thermostat to kick in when the AC isn't on? Should I just leave the gable vent and electric fan alone and let that vent the small dead space between the peak of the roof and the flat ceiling? I'm not even sure if the electric fan works but it's still a vent if not.

Insulating is tricky

Thanks.
 
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Old 04-14-18, 08:03 AM
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Below the knee walls you want solid blocking between the joists


not sure what you mean, but the whole attic floor is already plywood'ed.

I think that yes I should just leave the gable vent and electric fan (maybe just unplug it, yea?) in place so that moisture from heat which builds up on the underside of the roof will be removed via the soffit vents and gable vents.


As for the underside of the joists (the car garage bottom level ceiling), after I fiberglass insulate that, isn't there something I can use besides drywalling and taping the whole thing to prevent draft (and fire spreading)? Or at least something that should be good enough.
I know code requires 5/8 rock taped on garage ceiling in case of a fire the rock is more fireproof than 1/2" plus needs tape to seal all air flow which fire feeds on. Or 3/8-1/2" rock can be used along with silver bubble roll insulation which is fire rated.

Maybe I can just cover the whole underside of the floor joists with the fire proof silver bubble wrap? 5/8 rock would be cheaper but much less work.
 
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