# How to insulate and vent this garage attic?



## Nealtw (Jun 22, 2017)

Below the knee walls you want solid blocking between the joists and the floor is insulated from below and the garage will need 5/8 drywall taped and fill on the ceiling.,
Better to build knee walls out of 2x6 for more insulation. The sloped ceiling wants air chutes.
https://www.homedepot.ca/en/home/p.22-inches-x-48-inches-10-pieces.1000136854.html?eid=PS_GOOGLE_HD+%7C+E-Comm_E-Comm+%7C+Shopping+%7C+BBQ+-+FR_BBQ_aud-306143177453la-295272882166&gclid=Cj0KCQjwqsHWBRDsARIsALPWMENpRDkNvGv3tl4Y6TRt-yZt7kWjA_0j5Pmqxcip_701VjA7txXYfUEaAlDDEALw_wcB
They allow air to flow from the soffets to the attic above the insulation. And yeas you insulate the flat ceiling section. Heat rises to the top and escapes the vent in or near the roof and pulls cooler air from the soffets. Usually you have ridge vents or box vents in the roof near the peak.


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## gunner66 (Jan 3, 2017)

_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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## Bud9051 (Nov 11, 2015)

Do your vent area calculations as the net free area of gable vents is often not impressive. Electric fans are usually controlled by temperature and do not help with moisture in winter.

Link below on capes that illustrate the blocking below the kneewall. 

http://www.finehomebuilding.com/2012/09/06/two-ways-to-insulate-attic-kneewalls

I doubt the "silver bubble roll insulation which is fire rated" will pass inspection or perform as advertised. You need to install what your local code requires. You are pulling permits and getting inspections, y/n?

Bud


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## gunner66 (Jan 3, 2017)

thanks bud, now I see what people were referring to about insulating the knee wall. Originally I was going to do like 'bring the attic inside' example in the link you showed and just insulate the entire rafter and joist. but it'll maybe be cheaper to insulate only around the actual room (and then put blocking and foam board to seal the cut ends of the fiberglass).


rafters will get insulation baffles starting at top of knee wall and ending at dead space mini triangular attic which will be created once the flat ceiling is insulated, thus allowing gable vents vacuum roof underside heat out from the soffit vents. 

Then FiberGlass bats in the rafters sealed on bottom end with foam board/wood and caulk so that soffit vent doesn't flow into the cut end of FG insulation. 

Knee wall gets FG insulation.
Floor gets blocked /foam board block, and caulked ,,, to contain FG insulation area only between span within both knee walls (only insulating under the room). 

The flat attic ceiling (collar ties right under the roof ridge) will get FG insulation then a drop ceiling. 

Should I put impermeable plastic sheeting over the FG insulation above the drop ceiling so air doesn't just pass right through the drop ceiling? I like that, unlike if drywalled, the electric gable vent fan can be accessed from a drop ceiling by removing FG insulation, and if a hole has to be cut in plastic sheeting and patched also that not a big deal. 

Last thing I'm stuck on is if there's a way to get 1 hour fire rating on the garage ceiling besides 5/8 drywall (or 1/2 rock over silver bubble wrap class 1 rated Reflectix brand or similar I've seen used and passed inspection).

There's no like thick silver foil roll rated for 1 hour I can just roll out and tack on?? Thus also eliminating draft. I'm not sure the time rating for Reflectix. It just says Class A/Class 1 Fire Rating. If I doubled it up, it might equal an hour but then I guess even if I overlapped each run and sealed with silver duct tape that the tape isn't fire resistant.


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## Bud9051 (Nov 11, 2015)

No drop ceiling.
No foil bubble wrap.
Standard fire rated drywall.
Standard attic hatch, insulated and air sealed.
I don't care for attic fans.
If access will be from below, more fire regulations.
All construction should be permitted and inspected.
Ceiling below could have other options but you need to discuss that with your local code official.
Electrical should be permitted and inspected, same for plumbing if any.

Bud


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## gunner66 (Jan 3, 2017)

It's against code to put a drop ceiling with the "attic" and paper faced fiberglass above it because it's not fire rated drywall?

Similar to the picture, the 'attic' would just be the tiny 2 foot area above the rafter collar ties. Would want access to the gable fan by just removing a drop ceiling tile while standing on a ladder and move the fiberglass out of the way. (and cutting a hole and then patching it if any air barrier sheet is placed on the face of the fiberglass because as questioned, heat and AC might just be lost through the drop ceiling).

edit: ok I see what you mean, just a 'hatch' not a full fold up ladder. Does that hatch need to be fire proof too? Hence no drop ceiling?Premade fireproof access panels are tiny and cost like 400 bucks. 
He wants drop ceiling though. Not possible?


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## gunner66 (Jan 3, 2017)

i dont get why codes are so concerned about a possible fire in a garage spreading to an above living space, I mean of course I understand and if I were building my own house and if in theory it didn't need codes, I'd still 5/8 rock the ceiling and walls, but what I'm saying is like how come basements don't require anything on the ceiling? I read only %2 of house fires start in the garage. Basements have gas furnices etc and unfinished areas have dusty floors where dust can blow onto pilot lights under water heaters for one example. 

I think I read it's something to do with gas and flamible paints etc are often stored in a garage but this is kind of like saying you need an inspector to make sure you keep your canola oil in the pantry in a fireproof box or something.

Anyway, as far as getting the 1 hour garage fire rating with something besides 5/8 rock, maybe something that rolls out and tacks-on fast, I don't know of anything and no one else replied with one. So it'll have to be 5/8 rock.


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