Insulation question/confirmation)

[Lakeozark]

I am closing in on finishing the Insulation and getting ready for drywall.

As you can see I have a gable vent on both sides of the garage. My plan is to drywall from the eaves up the the Truss supports and then straight across. This will leave a "gap" in the center of the ceiling between the drywall and the insulated roof. Kind of like this: //----\\
(My initial idea was exposing the cross members and dry walling around them and direct to the ceiling truss'...but that is a PIA)

Since I will have that 1 1/2' to 2' gap below the ridge, I assume this will be somewhat like an "attic" - although unusable. Should I skip the insulation on the roof in that area and just put down insulation on top of the drywall?
Or Should I keep going and complete the ceiling with insulation and leave that area open for ventilation?
I am in Central Missouri so it is HOT in the Summer and COLD in the winter. I don't believe it is required by code to do a vapor barrier.
My concern is that the roof insulation is pointless if there is cold air going under it and I will have some heat loss going thru the drywall and into the "vented attic" space.
OR option #3 - Completely close off the Gable vents and have no air flow....

 

[Eds Garage]

The way you have it setup I think you'll want the insulation against the drywall with a really tight vapor barrier. Any warm moist air will condense againt that exposed metal roofing in cold weather. You will also want soffit vents with air traveling along the underside of the roof to the small attic space. I would probably close off the gable vents and instead install a ridge vent to allow moisture laden air to quickly escape.

 

[bczygan]

A couple of things. First, lets get our terms straight. You have rafters, with a ridge board and collar ties. Each pair of rafters is self supporting. There is no ridge beam cutting the load on each rafter in half and taking the other half and transmitting it to columns. This means each rafter is carrying twice the load it might. Also, since you have collar ties rather than trusses or ceiling joists, the rafter depth is partly responsible to resist the outward force trying to spread the top of the walls. This also means the connection of the collar ties to the rafters is critical. I don't know what the species of all the members are, or the strengths or loads or spans, so I can't tell you what the deflections will be. I don't know if the collar ties will be adequate for supporting drywall and lights or any storage above them. I would really NOT store anything above. Personally I would run some ceiling joists at 2'o.c. at the top plate of the wall and install insulation above them and drywall below. But if you want to maximize ceiling height then your plan will work with a couple of changes. First, you need more of the collar ties than you have. They need to be no more than 2'o.c., so install new ones between each existing one. Second, you need a 1" airspace above the insulation in each and every cavity between the rafters. That airspace needs to extend from the eave vent to the attic space you are enclosing without interruption. The easiest way to accomplish this is to use a propavent continuous in each rafter bay. http://www.dspinspections.com/atticventing.htm
I would be sorely tempted to add a couple of roof vents in this attic space as well. Use Visqueen on the warm side of the entire ceiling and walls as a vapor barrier.

 

[Lakeozark]

Wow, I had to re-read your post like 3 times to try and get my mind wrapped around that.

I think I now understand what you are saying - other than what the deflection might be or if that is seriously important?
I have 1" of airspace in all cavities going from the eaves to the top ridge vent so I think I am good there.
I will definitely not be storing anything up there - but now you have me a little concerned about not being able to hang drywall due to weight.
the rafters are 14' long 2X12's if that helps.
My thought was running "nailers" perpendicular to the collar ties for added support for the drywall. Since the majority of those ties are 4' OC.
Maybe I should re-think and install them every 24" OC?

Thank you for you input - I have actually learned a lot and the link was helpful as well. As you can tell, I am not very literate on construction terminology or building anything other than cars....

 

[bczygan]

Yes, do install new collar ties in between each existing tie, so you have them on every pair of rafters to tie the rafters together and triangulate them into a trussed condition. Be careful to properly fasten them. Also provide rows of 2"x4" blocking perpendicular to the ties and between them at 24"o.c. so you have a level 2'x2' grid to nail your drywall to. Also provide 2x4 blocking between rafters so you have support for drywall joints. Run your drywall horizontally and stagger the joints. If you use insulation with a Kraft paper facing, slash it before installing the Visqueen vapor barrier to avoid double vapor barriers. I would put at least R38 (Or more) in the ceiling and as much as I could fit in the rafter spaces while maintaining the 1" space. Carefully consider your lighting and power runs (and any other runs like communication or security or fire alarm cable or computer or intercom or speaker wires) before you close this area up. Put covered junction boxes for future light and power needs. Consider bracing or blocking for fastening hose reels and heaters and any other things that might be suspended.

 

[Lakeozark]

thanks again. Got all my collar ties in today, now complete at 24" centers.
Glad I asked before I went to drywalling - or I probably would have had issues down the road!