Question about insulating garage (framing issue)

[Lakee911]

Just a miscommunication ..

[Dana]

My unprofessional opinion is that blocking would be sufficient. Blown fiberglass or cellulose does not weight very much, even 12" deep. My shop ceiling has tie joists 24" oc and 12" blown fiberglass insulation. There is just 1/2" sheet rock on the underside. If you use 5/8" obs, that should support the insulation with ease. My ceiling has been doing fine for over 20 years.

Cellulose at a foot deep is 1.5-2lbs/ square foot- if the garage is roughly 22x22 you're lookin' at close to a half ton of dead-weight loading for just the insulation alone, added to a comparable amount for the OSB. It's not nuthin'.

It's ~30% more surface area, and would have to be done with batts (or $pray foam) and a ventilation gap to the roof deck is required, but going cathedral-ceiling style and king-pinning the tie-spans to handle the increase in roof load would be easier. If you wanted more R at mininum weight, sheathing the rafters with iso (pick your thickness, figure R6.5/inch) will have the least loading factor. Use thermal-rated foil faced iso to avoid having to add a layer of gypsum or wood sheathing for a thermal barrier (fire code stuff.)

[iminaquagmire]

The more I think about this, the more I don't really want to give up the overhead space. The cathedral ceiling idea has piqued my interest. I hadn't even thought of that. I had considered using rigid foam panels and just insulating the rafters but I didn't want to lose what little heat I had in there to the ceiling. So now what if I just installed collar ties at every rafter and used hurricane ties at the bases of the existing rafters for extra insurance? I'd install the collar ties at no more than 1/3 the length of the rafter up.

[iminaquagmire]

ttt. Anybody have any input on the cathedral ceiling idea in my last post?