Garage Attic

[Sandmannd]

My ceiling is finally up in my garage and I need to get in insulated. I also want to put some flooring up there so we can store some excess things in it. What I plan to do is run 2x4's cross ways across the rafters and then put the floor down. This will give me 8" of space for insulation. Question is how much insulation do I need to blow in? What R value? Is 20 enough, do I need to go to 28? This is going to be a heated garage. 

[Macgyver55]

 I did exactly the same thing.  With 7 inches you end up with approximately R-24. I heat my 1100 sq ft garage full-time all winter (turn it down to 45 at night) and it raised my gas bill less than $25 per month.  My furnace is a Reznor 60,000 BTU output.  I also upgraded to 2 inch thick insulated doors and new weatherstrip on the sides  which I feel is even more important than a few more R-value in the attic.

[rundrave]

do you need to add a vapor barrier or anything? lets say you have a sheetrocked ceiling screwed to the trusses. do you simply just add insulation right on top of the rock and fill in between all of the trusses with insulation and put your flooring on top of that?

[Hoey]

Use (kraft) faced insulation, face down on top of the sheetrock.  Vapor barrier goes against the warm side.

[bobberineyes]

Plenty of ventilation? 

[delcecchi]

Plenty of ventilation? 

Get pretty hot up there in the summer, sun beating down and insulation in the floor.  You will be able to cook baked beans up there. 

[Sandmannd]

Yep, it's been well ventilated and there's a vapor barrier up. Did that when I sheetrocked. 

[Tom7227]

Be sure that you don't store too much stuff up there and create problems.  Standard trusses aren't designed to carry weight.  I don't know how to calculate what the limits are but you should learn a bit about it before you load it up too much unless you bought the heavier trusses with storage in mind.

 

[Sandmannd]

It's all going to be light weight stuff. Basically Christmas stuff, Halloween decorations, stuff like that. Boxes that weight maybe ten lbs each. Spread out so as not to weigh to weigh it down. 

 I did exactly the same thing.  With 7 inches you end up with approximately R-24. I heat my 1100 sq ft garage full-time all winter (turn it down to 45 at night) and it raised my gas bill less than $25 per month.  My furnace is a Reznor 60,000 BTU output.  I also upgraded to 2 inch thick insulated doors and new weatherstrip on the sides  which I feel is even more important than a few more R-value in the attic.

This is good to hear. If I did this I could get away with using 2x4's up there rather than 2x6's and save quite a bit of money. I don't plan on having the heat above 45 unless I'm working out there.

[cavalierowner]

Most truss engineering data I have seen have 10 PSF dead load on the bottom chord. 5/8" sheetrock weighs about 3 PSF and insulation weight is negligible. If you have some 1/2" plywood to set stuff on, that's another 1-1/2 PSF, but use 2 PSF to account for some framing. SO, that leaves about 5 PSF for storage. So if you have a 24" square box (4 square feet), you can put about 20 pounds in that box.

 

Sorry, one of the hazards of being a retired architect and having an unnatural desire to figure stuff out LOL!!!

[delcecchi]

How much safety margin are in those standards?  (retired engineer)

[cavalierowner]

The structural engineers I worked with told me that factor of safety depended on the material, system, etc., and was commonly 2 to 3 times. We could usually get them to go into the factor of safety a bit, but they grumbled a lot! They always told me there is a good reason for factors of safety.